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  • CFD simulation of heat and mass transfer through cylindrical Zizania latifolia during vacuum cooling
    Heat Mass Transfer (IF 1.551) Pub Date : 2019-08-26
    Ning Wang, Ankang Kan, Zipei Huang, Jiaxi Lu

    Abstract As the Zizania latifolia has a delicious taste, high nutritional value and rich moisture, it is easy to rot and deteriorate due to the high moisture content, resulting in short shelf life. In order to maintain quality of Zizania latifolia, the vacuum cooling process of cylindrical vegetables is experimentally studied. Considering the transient heat transfer and mass conservation, a mathematical model of heat and mass transfer in the vacuum cooling process of cylindrical vegetables is established. Meanwhile, the temporal trends of total system pressure, temperature distribution, evaporation rate of water, weight loss and moisture content of Zizania latifolia are simulated by CFD software. The experimental data are compared with the CFD simulation results. It is found that the differences of the temperature between the simulation and the experiments are 4.7%. The amount of water evaporated from the Zizania latifolia by simulation is 5.5% during the whole vacuum cooling, while the tested water loss rate is 9.8%, the maximal deviation of weight loss is within 4%. The simulated result of 83.5% after a cooling time of 850 s, gives a reasonable agreement with the experimential moisture content of 86.7% in the same cooling period. The results show that the CFD simulation values agreed well with the experimental data. It is significant for the study of the water transfer characteristics of cylindrical vegetables tissue and the optimization of the vacuum cooling process of the food industry.

    更新日期:2020-01-22
  • Experimental and numerical investigation of heat transfer characteristics of jet impingement on a flat plate
    Heat Mass Transfer (IF 1.551) Pub Date : 2019-08-17
    Joseph Issac, Dushyant Singh, Saurabh Kango

    Abstract In this present work, round jet impingement on a heated flat plate at constant heat flux is analysed experimentally and numerically. Experiments have been performed at various Reynolds numbers (Red = 10,000 to 25,000) and at four different nozzles to plate spacing (h/d = 4, 6, 8 and 10). Different RANS turbulence model namely, k-ω SST, Realizable k-ε, RNG k-ε and ν2f were used to validate the numerical results with experimental results. The results of various turbulence models were analysed for varying inlet turbulent intensities and eddy viscosity ratios. It has been observed that the inlet turbulent intensity and eddy viscosity ratio are significant for the accurate prediction of realistic results.

    更新日期:2020-01-22
  • Influence of single and multiple coupling factors on the stability of paraffin-based nanofluids
    Heat Mass Transfer (IF 1.551) Pub Date : 2019-08-07
    Jiemei Liu, Ning Wang, Yawei Song, Bin Yang

    Abstract Nanofluids have been introduced to improve the thermal properties of phase-change materials. However, stability problems caused by agglomeration and deposition have greatly restricted the use of nanofluids. In this work, paraffin-based nanofluids were prepared under the dual coordination of magnetic stirring and ultrasonic oscillation, the influence of ultrasonic power and time, base fluid volume, particle concentration and type on fluid stability was studied by single factor method, and the influence degree of the above four factors on fluid stability was studied by fractional factor design method. The following conclusions were drawn from the experiment: (1) With the increase of ultrasonic power, the relative absorbance increases within the allowable range of the experimental instrument, and the maximum improvement is 3.06%.(2) The optimal ultrasonic time for nanofluids preparation is 2 h. (3) The sample volume under the optimal state is 45 ml. (4) With the increase in concentration, the absorbance per concentration decreases, and the reduction range becomes slow. (5) The stability of the nanofluid is also related to the characteristics of the particle surface. (6) The coupling effect of ultrasonic power - concentration (PC) has the most significant effect on the overall absorbance change. This study provides a clear improvement direction and basis for the preparation of highly stable nanofluids.

    更新日期:2020-01-22
  • Experimental investigation of cooling performance with graphene based nano-fluids in a vehicle radiator
    Heat Mass Transfer (IF 1.551) Pub Date : 2019-08-15
    Ferhat Kılınç, Ertan Buyruk, Koray Karabulut

    Abstract The cooling performance of a vehicle radiator was experimentally investigated using pure water, graphene oxide (GO) and graphene nano ribon (GNR) nano-fluids in this paper. Three different fluid inlet temperatures (36, 40 and 44 °C) and four different flow rates (0.6, 0.7, 0.8 and 0.9 m3 h−1) were performed in the experiments. GO and GNR nano-fluids were obtained at 0.01 and 0.02% vol. concentrations by means of pure water as a base fluid. In order to determine the heat transfer enhancement, the experimental datas were compared as overall heat transfer coefficients (U) for pure water and nano-fluids. The mean enhancement values of U for all temperatures were obtained as 5.41% and 26.08% for 0.01% and 0.02% vol. concentrations of GO/water nano-fluid and 15.62% and 20.64% for 0.01% and 0.02% vol. concentrations of GNR/water nano-fluid, respectively.

    更新日期:2020-01-22
  • Performance and emissions of pre-mixed and post-mixed combustion systems with a casting aluminum-silicon alloy (CASA) condensing gas boiler
    Heat Mass Transfer (IF 1.551) Pub Date : 2019-08-30
    Fengguo Liu, Longfeng Zheng, Rui Zhang, Aibin Yan, Meng Shao

    Abstract Condensing gas boilers are widely employed for their high heat efficiency, which attributes to their ability to use the recoverable sensible heat and latent heat in flue gas. In this paper, the optimal excess air rate of a novel condensing gas boiler is investigated experimentally. With the change of excess air rate, the pre-mixed and post-mixed systems were used to study the performance, the thermal efficiency and emissions of the boiler. The CO emission reduces firstly then increases regardless of which premixed system, when the excess air rate is from 1.15 to 1.4. The NOx emission decrease from 64 ppm to 10 ppm and the efficiency decreases from 64 ppm to 19 ppm as the excess air rate increases from 1.15 to 1.4. The efficiency decreases from 102.05% to 98.7% and the efficiency decreases from 100.9% to 97.6% as the excess air rate increases from 1.15 to 1.35. Considering both NOx emission and thermal efficiency, the optimum excess air rate is 1.23. With the change of heat load, the emission, the fan speed and the thermal efficiency of the two system were compared. The concentration of CO increases from 7.5 ppm to 65.5 ppm when the pre-mixed system heat load changes from 8 kW to 24 kW. The efficiency of post-mixed system decreases from 106.5% to 97.5% and the efficiency of pre-mixed system decreases from 106.9% to 96.8% as the heat load increases from 9 to 26 kW. The variation of thermal efficiency and condensate volume with various heat load in the post-mixed system with different return water temperature are also studied. The efficiency decreases from 107% to 98.5% as the heat load increases from 9 to 24 kW at the inlet of return water is 30 °C. And the vapor condensation ratio ω decreases from 105% to 97.5% as the heat load increases from 9 to 24 kW at the inlet of return water is 40 °C. In summary, post-mixed system is more suitable for the development of small condensing boilers. The lower the return water temperature and heat load, the higher the thermal efficiency of boiler.

    更新日期:2020-01-22
  • Performance evaluation of mass transport enhancement in novel dual-channel design of micro-reactors
    Heat Mass Transfer (IF 1.551) Pub Date : 2019-08-17
    Jing Li, Hui An, Agus P. Sasmito, Arun S. Mujumdar, Xiang Ling

    Abstract Various design configurations of semi-T-shaped dual-channel micro-reactors were numerically examined for their laminar mass transport performance in heterogeneous catalytic combustion of methane and air. One single-channel and five dual-channel configurations (i.e., parallel, divergent, convergent, zig-zag, and curved configurations) were investigated with a two-dimensional computational fluid dynamics model. These innovative design configuration were compared in terms of CH4 utilization, pressure drop, CO/CO2 ratio, catalyst utilization, and a performance index at various Reynolds numbers. Dual-channel micro-reactors were found to enhance mass transport due to the well mixed flow and the increased reaction contact area. By suitably modifying the dual-channel layout angle and shape, recirculation zones can be formed within the reactor which increase CH4 utilization. However, the improved conversion rate is achieved at the cost of high pressure drop. The parallel dual-channel design provides the highest conversion per unit pressure drop over the range of the Reynolds numbers studied. For Reynolds numbers of 20 and 40, compared to the single-channel micro-reactor, divergent, convergent and curved channel designs yield higher conversion per unit pumping power. However, further increase of Reynolds number (i.e., 60, 80, and 100) deteriorates their performance due to the significantly increased pressure drop and shorter residence time.

    更新日期:2020-01-22
  • Heat conduction in a shell with gas-phase deposition of material on its surface
    Heat Mass Transfer (IF 1.551) Pub Date : 2019-08-03
    Georgy N. Kuvyrkin, Inga Yu. Savelyeva, Alexander V. Zhuravskii

    Abstract The development of additive technologies such as gas-phase deposition provides the possibility of applying various coatings with different properties. The study introduces a model of heat conduction which relies on the specific character of heat and mass transfer in chemical gas-phase deposition on a curvilinear surface with a linear change in the curvature over the shell thickness. A difference scheme was constructed with use the interpolation method, and the computational solution of the problem was found. The approximation and the stability of difference schemes were investigated, and examples of numerical calculations for various materials were given.

    更新日期:2020-01-22
  • A meshless procedure for analysis of fluid flow and heat transfer in an internally finned square duct
    Heat Mass Transfer (IF 1.551) Pub Date : 2019-08-29
    Jakub Krzysztof Grabski

    Abstract Application of the method of fundamental solutions in combination with the global radial basis function collocation method for analysis of fluid flow and heat transfer in an internally finned square duct is presented in the paper. Fluid flow problem is solved using the modified method of fundamental solutions. After that, the average fluid velocity and product of friction factor and Reynolds number can be determined. Heat transfer problem in the fluid is governed by a nonlinear equation with linear boundary conditions. The Picard iteration method is employed in the paper in order to transform the nonlinear problem into a sequence of inhomogeneous problems. At each iteration step, the general solution is obtained using the modified method of fundamental solutions and the particular solution is obtained using the global radial basis function collocation method. When the iteration process is stopped, the Nusselt number can be determined.

    更新日期:2020-01-22
  • Experimental and computational analysis of a row of three co-swirling impinging flames
    Heat Mass Transfer (IF 1.551) Pub Date : 2019-08-01
    Parampreet Singh, Ratna Kishore Velamati, Subhash Chander

    Abstract Detailed flow field and heat transfer characteristics developed in a row of three co-rotating dual swirling impinging flames have been investigated experimentally as well as numerically. Impingement heat transfer and pressure distribution has been studied experimentally at different separation distances and inter-jet spacings. Inverse heat conduction procedure (IHCP) has been used for estimating heat fluxes on the front side of impingement plate. Turbulence induced mixing results in strong interactions amongst adjacent flames causing deflections of inner flames of the burners situated at sides of central burner. Numerical simulation predicted formation of asymmetric recirculation zones for side flames. Symmetric interactions taking place for central flame produced two equal recirculation lobes for central flame. Behavior of central inner flame has been observed to be dependent on the value of inter-jet spacing used. Suppression of central inner flame tend to occur at higher inter-jet spacings due to recirculating products. Impingement pressure distribution is observed to be consistent with the heat flux distribution. Averaged heat fluxes registered at the impingement plate due to the central flame are higher in magnitude to those pertaining to each of the side flames.

    更新日期:2020-01-22
  • Analytical solution of Newtonian nanofluid flow in a tapered artery based on a consistent couple stress theory
    Heat Mass Transfer (IF 1.551) Pub Date : 2019-08-05
    Fatemeh Karami, Afshin Ahmadi Nadooshan, Yaghuob Tadi Beni

    Abstract Reduction of the stenosis in arteries is one of the most important applications of nanoparticles in medicine. Thus, investigating nanoparticles in stenosed arteries is a subject attracting so much attention from the researchers nowadays. However, considering the small size of arteries, it seems that non-classical theories demonstrate better results when compared to the classical Navier-Stokes theory. The present paper aims at investigating the Newtonian nanofluid flow through divergent and convergent, non-tapered and tapered arteries with mild stenosis according to the consistent couple stress theory. Concentration, temperature, and velocity profiles, as well as the impact of various parameters such as the height of the stenosis, the shape of the stenosis curve, Brownian distribution parameter, thermophoresis distribution parameter, Darcy number, and the length scale were calculated for all the three geometries. Results demonstrate that an increase in the height of the stenosis and an increase in the Darcy coefficient, respectively, lead to a decrease and an increase in the axial velocity in all three artery geometries. Moreover, an increase in the impact of the length scale leads to a decrease in the axial velocity. The impact of the length scale on the velocity profile shows the significance of the length parameter on the flow within the geometries with small sizes which is not present in the classical Navier-Stokes theory.

    更新日期:2020-01-22
  • Kinetics of ozonation and mass transfer of pharmaceuticals degraded by ozone fine bubbles in a plant prototype
    Heat Mass Transfer (IF 1.551) Pub Date : 2019-08-02
    Surabhi Patel, Rohit Agarwal, Subrata Kumar Majumder, Papita Das, Pallab Ghosh

    Abstract The kinetics of degradation of pharmaceutical drugs is studied by ozonation in a fine bubble system. In the present experimental work, fine bubbles of ozone were used for degradation of pharmaceutical drugs soluble in water. The ozonation of pharmaceutical drugs by ozone bubbles was found to be very effective at high pH and high ozone flow rates. The apparent reaction rate constant was estimated at different pH of the solution. The axial variation of ozone concentration in solution in the reactor is analyzed by dispersion model. The volumetric mass transfer coefficients based on the ozonation of pharmaceutical drugs by fine bubble based are also enunciated in this present work. The results may be useful for removal of drug waste in water treatment process by advanced oxidation techniques.

    更新日期:2020-01-22
  • New Empirical Models for Condensation on Banks of Plain Tubes
    Heat Mass Transfer (IF 1.551) Pub Date : 2019-08-03
    Mudather Ibrahim Mudather Zeinelabdeen, Muhammad Sajid Kamran, Adrian Briggs

    Abstract New empirical models for evaluating heat-transfer coefficients for condensation on banks of tubes were developed using an experimental database of over 4000 individual data points for 7 different condensing fluids and 13 different tube bank configurations. New empirical models within each of the free and forced convection flow regimes for both of the steam and non-steam fluids separately were obtained, including the relevant dimensionless groups that may affect the heat transfer during the condensation. Two models within the free-convection flow regime for the non-steam were proposed due to large volume and variation of data in this regime. While comparing the results of the new empirical models with the recent database; it was proposed that the overestimation for some of the non-steam data of Shah (1978 and 1981) was due to the presence of the non-condensable gas (air) within the flowing fluid. In terms of predicting the tube bank thermal performance and condensation of flowing fluid, the results of the newly developed models were seen to be the most reliable and accurate when compared with the existing models mentioned in this paper. An overall agreement was seen with the existing experimental database within an average absolute of the errors of about 15%. For steam data, the present models were seen to yield the most accurate results for the inline and staggered tube banks when compare with other empirical and analytical models. However, for non-steam fluids (R-12, R-21 and R-113 only) flowing through staggered tube bank, Honda et al. (1989) model is recommended.

    更新日期:2020-01-22
  • Effect of water vapor generation on cell polarization in active area of anode-supported solid oxide fuel cells
    Heat Mass Transfer (IF 1.551) Pub Date : 2019-08-26
    Jungmyung Kim, Ehtesham Ali, Minwoo Kim, Hyungtae Lim, Heesung Park

    Abstract In this paper, a three-dimensional computational fluid dynamics model based on the finite volume method is developed for an anode-supported solid oxide fuel cell with an active area of 1.54 cm2. The numerical model including electrochemical reaction has been compared with experimental data obtained under the same conditions to verify the reliability of the code, air and fuel concentrations at the anode and cathode of the cell were shown and analyzed according to the three fuel utilization rates. The calculated current density of the SOFC has been affected by the flow velocity and gas concentration due to water vapor generation. The electrochemical reaction generates water vapors at the reaction sides which impedes gas diffusion. We have shown the streamlines of gas flow distracted by increased water vapor concentration. Consequently, the operating conditions of SOFC should be determined by considering the effects of water vapor generation for the practical applications. We have verified the water vapor generation in SOFC which significantly affects the performance.

    更新日期:2020-01-22
  • Thermodynamic irreversibility and conjugate effects of integrated microchannel cooling device using TiO 2 nanofluid
    Heat Mass Transfer (IF 1.551) Pub Date : 2019-08-07
    G. Narendran, N. Gnanasekaran, D. Arumuga Perumal

    Abstract Thermal management is highly essential for the latest electronic devices to effectively dissipate heat in a densely packed environment. Usually, these high power devices are cooled by integrating micro scale cooling systems. Most of the works reported in the literature majorly concentrate on microchannel heat sink in which the characteristics of friction factor and enhancement of heat transfer are analyzed in detail. However, due to the advent of compact electronic devices a crucial investigation is required to facilitate an amicable environment for the neighboring components so as to improve the reliability of the electronic devices. Henceforth, in the present study a combined experimental and numerical analysis is performed to provide an insight to determine the performance of a copper microchannel integrated with aluminium block using TiO2 nanofluid for different particle configurations. Needless to say, the present study, which also focuses on entropy generation usually attributed to the thermodynamic irreversibility, is very much significant to design an optimum operating condition for better reliability and performance of the cooling devices.

    更新日期:2020-01-22
  • Preparation, characterization and thermal properties of PCM nanocapsules with polystyrene/nano garaphen oxide shell for energy storage
    Heat Mass Transfer (IF 1.551) Pub Date : 2019-08-19
    S. Valizadeh, M. Ehsani, M. Torabí Angaji

    Abstract This “study deals with fabrication, physico-chemical characterizations and thermal properties of AP25 nanocapsules as organic PCM (phase change materials) for TES. The influence of reaction temperature and stirring rate on the properties of phase change materials nanocapsules was studied using RSM method. The FESEM and TEM micrographs show that nanocapsules are uniform of spherical shape with core–shell structure and particle size. TEM results revealed that it is successfully acted as another protective screen to protect paraffin from leaking. The nanocapsules with phase change latent heat of 141.8 J/g at 27.4 C have a great potential for TES”.

    更新日期:2020-01-22
  • Nanofluid flow by a permeable stretching/shrinking cylinder
    Heat Mass Transfer (IF 1.551) Pub Date : 2019-08-17
    N. C. Roşca, A. V. Roşca, I. Pop, J. H. Merkin

    Abstract The purpose of this paper is to theoretically investigate the steady nanofluid flow due to a permeable stretching/shrinking cylinder using the mathematical nanofluid model proposed by Buongiorno. The effects of the stretching/shrinking parameter as well as the suction and curvature parameters are studied. A similarity transformation is used to reduce the governing partial differential equations to a set of nonlinear ordinary (similarity) differential equations which are then solved numerically using the function bvp4c from Matlab for different values of the governing parameters. It is found that the solution is unique for stretching case; however, multiple (dual) solutions exist for the shrinking case. A closed form analytical solution is also obtained when the curvature parameter α = 0. For α small (α ≪ 1) dual solutions exist for both stretching and shrinking cylinder, while for α large (α ≫ 1) dual solutions are found only for the shrinking cylinder(λ < 0).

    更新日期:2020-01-22
  • Effect of helical perforated twisted tape parameters on thermal and hydrodynamic performance in heat exchanger circular tube
    Heat Mass Transfer (IF 1.551) Pub Date : 2019-08-09
    Rajesh Maithani, Anil Kumar

    Abstract Effect of helical perforated twisted tape parameters like diameter ratio (dR/DI), relative pitch ratio (PPT/LT), perforation index (PA/TA) and Reynolds number(Re) on the thermal and hydrodynamic performance of heat exchanger circular tube experimentally analysed. Experimental data pertinent to air flow and heat transfer is generated and thermal hydrodynamic performance is determined for different sets of helical perforated twisted tape insert and flow parameters. The highest value of thermal hydrodynamic is observed at dR/DI of 0.65, PPT/LT of 0.086 and PA/TAof 10%. The maximum value of the thermal hydrodynamic performance was found to be 2.12 for the range of parameters examined.

    更新日期:2020-01-22
  • Heat transfer improvement in simulated small battery compartment using metal oxide (CuO)/deionized water nanofluid
    Heat Mass Transfer (IF 1.551) Pub Date : 2019-08-02
    N. Ahmed Bin-Abdun, Z. M. Razlan, S. A. Bakar, C. H. Voon, Z. Ibrahim, W. K. Wan, M. J. M. Ridzuan

    Abstract Improving the heat transfer coefficient of working fluids is essential for achieving the best performance of manufacturing systems. As a replacement of conventional working fluids, nanofluids have a high potential for improving this heat transfer coefficient. However, nanofluids are seldom implemented in actual systems, and several factors should be considered before actual application. Accordingly, this study investigated the thermophysical properties and heat transfer rate of CuO/deionized water nanofluid with and without sodium dodecyl sulfate (SDS) surfactants. Three different volumetric concentrations of the nanofluid were prepared using a two-step preparation method. The experimental steps were divided into two phases: static and dynamic. In these experiments, the thermophysical properties of the prepared nanofluids and the heat transfer coefficient were measured using an apparatus designed based on an actual heat exchanger for a lithium ion polymer battery compartment. The effects of flow rate and surfactants on the heat transfer rate of the nanofluids with varying volumetric concentrations of 0.08%, 0.16%, and 0.40% were analyzed. The results indicate that the heat transfer rate increases considerably as the flow rate increases from 0.5 L/min to 1.2 L/min and with the presence of surfactants. The highest heat transfer rate was obtained at a 0.40% volumetric concentration of CuO/deionized water nanofluid with SDS surfactant.

    更新日期:2020-01-22
  • A study of heat transfer through the heavy plate thickness under multi-slit jet impingement
    Heat Mass Transfer (IF 1.551) Pub Date : 2019-09-02
    Bingxing Wang, Yue Xia, Zhixue Liu, Zhaodong Wang, Guodong Wang

    Abstract The influence of cooling models and moving velocity on the temperature variation and cooling rate through the thickness of a metal plate is experimentally investigated. In this investigation, the cooling process is divided into four stages: a starting stage (I), a rapid cooling stage (II), a slow cooling stage (III) and a stopping stage (IV). Based on the curves, cooling rate, temperature difference, the heat transfer coefficient and center temperature curves are discussed. The results indicate that the cooling model and moving velocity influence the heat transfer on the surface, and affects the cooling rate through the thickness by changing the temperature difference. When the flow rates of nozzles 1, 2 and 3 are set to be V1, V2 and V3 respectively, the highest heat transfer is observed when V1 > V2 > V3. Under this cooling model, the maximum temperature difference occurs at the time of transition from the rapid cooling stage (II) to the slow cooling stage (III). In the slow cooling stage (III), increased moving velocity improves the synchronization of the heat transfer and decreases the maximum heat flux. As well, the speed of motion affects the internal heat conduction and cooling rate by affecting surface heat transfer.

    更新日期:2020-01-22
  • Experimental and numerical investigation on shell-and-tube exhaust gas recirculation cooler with different tube bundles
    Heat Mass Transfer (IF 1.551) Pub Date : 2019-08-21
    Lin Liu, Tong Shen, Lin Zhang, Hao Peng, Suolong Zhang, Weigang Xu, Shi Bo, Songzheng Qu, Xiaomeng Ni

    Abstract The heat transfer characteristics of shell-and-tube exhaust gas recirculation (EGR) coolers with different tube bundles were studied by experiment and numerical simulation. The overall heat transfer coefficient and shell-side pressure drop were determined. The influences of tube spacing, baffle form, baffle arrangement and number on the overall heat transfer coefficient and shell-side pressure drop were investigated. The maximum differences between the numerical and experimental results are approximately 4.1% for overall heat transfer coefficient and 3.3% for shell-side pressure drop, respectively. The results indicate that the overall heat transfer coefficient of EGR cooler with wave fin arrays internally finned tubes is 1.3~1.7 times than that of EGR cooler with longitudinal plate-rectangle internally finned tubes. And the comprehensive heat performance is more reasonable when tube spacing and baffle number are equal to 12 mm and 9, respectively. It is confirmed that the trisection ellipse helical baffle is superior to the segmental baffle, but the influence of improving overall thermal performance by setting baffles is limited for small EGR cooler.

    更新日期:2020-01-22
  • Analytical solution of coupled heat and mass transfer equations during convective drying of biomass: experimental validation
    Heat Mass Transfer (IF 1.551) Pub Date : 2020-01-18
    Abdelghani Koukouch, Ilias Bakhattar, Mohamed Asbik, Ali Idlimam, Belkacem Zeghmati, Ahmed Aharoune

    Abstract In this work, analytical solutions of the thin-layer and the Luikov models were proposed. For the thin-layer model, the moisture content and temperature distribution inside a biomass, assimilated to a parallelepiped material, are uniform whereas the Luikov model considers the sample as a porous medium. Analytical solutions of the Luikov model equations were obtained using Hermite’s zero-order approximation. These analytical solutions and those of the thin-layer model equations were applied to forced convection drying of two types of biomass: raw olive pomace (ROP) and deoiled olive pomace (DOP). It has been shown that the Luikov model is in good agreement with the experimental results except for the case where the sample has a thickness of 0.5 cm. In addition, thermal properties of ROP and DOP have been determined experimentally. On the other hand, a brief parametric study has been conducted and simultaneous effects of the dimensionless numbers: Luikov (Lu < <1), Kossovitch (Ko) and Posnov (Pn), on the drying process are highlighted. Thus, the increase of Luikov number accelerates the drying process and for specific couples (Lu and Ko) or (Lu and Pn), a remarkable change of dimensionless average water content with respect to dimensionless average temperature is observed, at the beginning of the drying process.

    更新日期:2020-01-21
  • Modeling of thermal performance of multilayer protective clothing exposed to radiant heat
    Heat Mass Transfer (IF 1.551) Pub Date : 2020-01-18
    Adam K. Puszkarz, Waldemar Machnowski, Anna Błasińska

    Abstract Studies presented in this paper concern wide issue of thermal comfort of protective clothing. The Computer Aided Design (CAD) software tools to analyze thermal insulation of multilayer textile assembly used in thermal protective clothing were applied. First, 3D geometry and morphology of a real textile assembly was modeled. In the designed model different scales of resolution were used for individual layers, ranging from a homogenized nonwoven fabrics model to mapping the geometry of yarns in woven fabrics model. Next, the finite volume method to estimate thermal insulation properties of this assembly, when exposed to heat radiation, was used. Finally, the simulation results were verified experimentally using method described in EN ISO 6942. On the basis of both simulation and experimental results obtained for the multilayer textile assembly, protective clothing parameters directly affecting the ability to protect against heat, were determined. Correlating simulated and experimental values of these parameters were obtained, which may indicate that applied software can be an effective tool in analyzing thermal properties of newly designed multilayer functional clothing.

    更新日期:2020-01-21
  • Investigation of the effect of temperature on Tunisian clay product during drying process
    Heat Mass Transfer (IF 1.551) Pub Date : 2020-01-18
    Nouha Jhider, Mohamed Bagané

    Abstract The aim of this work was to evaluate the influence of drying temperature on the convective drying process of clay sample as well as on the effective diffusivity. The study was performed on Tunisian raw-clay product in here simple spherical geometry with initial moisture content of 0.35 kg water/kg d.b. Air-drying characteristics were investigated in a laboratory convective dryer for a temperature range varying from 30 to 60 °C at a constant air velocity of 1,44 m/s with an air humidity ratio of 50%. Four analytical solutions based on Fick’s and Crank’s diffusion equations were proposed to describe the drying process. The first and the second equations neglected shrinkage effect while the two other equations consider it as a fundamental parameter. According to the obtained results, it was proved that the increase in air temperature increases the drying rate, as well as the effective diffusion coefficients. Based on the analytical solutions used in this study, it was concluded that the model including the shrinkage effect gives the lowest value of the effective diffusion coefficient.

    更新日期:2020-01-21
  • Development of new construction material for thermal insulation of building based on aggregate of waste paper and cement
    Heat Mass Transfer (IF 1.551) Pub Date : 2020-01-17
    Bouchaib Mandili, Mohamed Taqi, Hamid Chakir, Omar Douzane, Mohamed Errouaiti

    Abstract In this work, a new method of bricks making based on waste paper has been developed for the thermal insulation of buildings. This method involves converting paper waste into aggregates and combining them with a Portland cement binder. A physical and thermal characterization of the Aggregate of Waste Paper (AWP) is carried out, followed by a structural study by Scanning Electron Microscopy (SEM). Six Paper Brick Types are made with the mass dosage variation of the composite. After drying the bricks in the open air for 28 days, experimental investigations were carried out to determine the density, the water absorption, the compressive and flexural strength, and the propagation speed of ultrasonic waves. In addition, a SEM analysis combined with an Energy-Dispersive X-ray spectroscopy analysis (EDX) is performed to examine the adhesion between the AWP and the cement particles. The characterization results of the aggregates showed a low density and thermal conductivity amounting to170kg/m3 and 0.06 W/m.K, respectively, and the analysis of their microstructure by SEM demonstrated that their structure is composed of randomly distributed fibers. Moreover, the mechanical properties of the paper bricks are more resistant, since the compressive strength varies between 3.43 and 6.43 MPa, and their thermal properties are lower compared to the conventional bricks, from 0,0851 to 0,0978 W/m.K. Therefore, these findings clearly indicate that the newly made bricks represent an innovative solution for thermal insulation in buildings that can also be used as a structural material.

    更新日期:2020-01-21
  • Effect of synergistic interaction between hydrogen inductions with Simarouba glauca -diesel blend for CI engine application
    Heat Mass Transfer (IF 1.551) Pub Date : 2020-01-15
    Mathanraj Vijayaragavan, Ganapathy Subramanian Lalgudi Ramachandran

    The study is mainly to investigate the use of straight Simarouba oil as an alternate in the DI compression ignition engine and also the effect of hydrogen induction. Simarouba fuel is used as pilot fuel and hydrogen is used as secondary fuel which is inducted in inlet manifold by small modification in the CI engine. The mass share of hydrogen induction is fixed to maximum limit with the use of a manometer and for safety two flame arresters are used to avoid backfire in the hydrogen fuel line. The maximum hydrogen mass share percentage for diesel, B20, and B40 are 12.20%, 13.05% and 10.88% is fixed in full load condition. In maximum mass share, the BTE at full load conditions for diesel, B20, and B40 are 34.80%, 33.10%, and 29.65%. The obtained value of B20 at 13.05% mass share is nearly equal to the diesel value (0% mass share of hydrogen). The major drawback of increasing the mass share of hydrogen will increase nitric oxide emission due to the high heating value of hydrogen. The nitric oxide is increasing around 18% in the maximum mass share of hydrogen for B20 compare with diesel (0% mass share of hydrogen). But, there is a decreasing trend for other emissions like CO, HC, and smoke. This may be due to the absence of a carbon molecule in hydrogen fuel. The blended fuel B20 with maximum mass share obtain a better result than diesel with 0% mass share of hydrogen.

    更新日期:2020-01-15
  • The relationship between thermal conductivity and electrical resistivity of silty clay soil in the temperature range − 20 C to 10 C
    Heat Mass Transfer (IF 1.551) Pub Date : 2020-01-14
    Qiang Sun, Chao Lyu, Weiqiang Zhang

    With increasing engineering projects carried out in cold regions, the analysis and evaluation of the thermal conductivity and electrical resistivity of frozen soil have become important considerations in engineering construction and theoretical research. Through experiments performed on silty clay, the effects of the initial water content and soil temperature on the thermal conductivity and electrical resistivity of silty clay soil were analyzed. Theoretical models were also formulated for the soil thermal conductivity and electrical resistivity. The results showed that the variations in the thermal conductivity and electrical resistivity can be discussed as three stages: the freezing prophase, freezing metaphase and freezing anaphase. The electrical resistivity increases rapidly in the temperature range − 2 °C to −6 °C. The thermal conductivity increases rapidly from −2 °C to −4 °C and then decreases slightly from −4 °C to −6 °C due to the development of frost heave cracks in samples with higher water contents (15%, 18% and 20%). However, the thermal conductivity of samples with a water content of 10% continues to increase under these conditions. At the same temperature, the thermal conductivity increases linearly with water content in general. The electrical resistivity decreases approximate linearly with water content from 10 °C to −4 °C and increases with increasing water content from −4 °C to −20 °C. The thermal conductivity is inversely proportional to electrical resistivity from 10 °C to −4 °C and is proportional to electrical resistivity from −10 °C to −20 °C. The results of the experiments performed on silty clay soil verify the soundness of the proposed model for the thermal conductivity and electrical resistivity of unfrozen and frozen soil.

    更新日期:2020-01-15
  • Effect of axial conduction in integral rough friction stir channels: experimental thermo-hydraulic characteristics analyses
    Heat Mass Transfer (IF 1.551) Pub Date : 2020-01-11
    Sheetal Pandya, Shubhankar Gurav, Gaurav Hedau, Sandip K. Saha, Amit Arora

    Integral mini-channels fabricated in the metal substrate by friction stir channeling process are different from conventional mini-channels due to irregular shape, and large surface roughness. The effect of axial wall conduction on heat transfer in such mini-channels is significant, which could limit their utilisation as cooling channels in heat transfer based applications. Hence, in this study, we present an experimental analysis of the thermo-hydraulic performance of a friction stir channel to identify the axial wall conduction effect using de-ionized water as working fluid. The thermo-hydraulic performance parameters are analysed in the range of Reynolds number of 500 to 2100. The local wall temperature distribution along the length of the mini-channel is found to be non-uniform due to the significant axial wall conduction effect. The distinct pattern of numerical heat flux distribution is used as evidence to confirm the presence of axial wall conduction effect which is implied by the local wall temperature distribution. The higher average heat transfer and flow characteristics compared to the theoretical predictions for conventional mini-channels are observed as a result of simultaneously developing flow and the presence of surface roughness.

    更新日期:2020-01-13
  • On correction factors in thermal-hydraulic correlations for compact fin-tube bundles
    Heat Mass Transfer (IF 1.551) Pub Date : 2020-01-10
    Karl Lindqvist, Erling Næss

    Abstract Heat exchanger designers need reliable thermal hydraulic correlations to optimize heat exchanger designs, particularly when a compact volume is desired. Current correlations for circular fin-tube bundles typically employ a flow velocity dependent term and a number of correction terms to account for geometry variations and other flow conditions. This paper evaluates five phenomena that influence thermal-hydraulic performance and discusses them in relation to current models for heat transfer and pressure drop. The varied parameters are the direction of heat flow (gas heating or gas cooling), the fin type (annular or helically wound), the fin efficiency, the fin pitch, the number of streamwise tube rows and the inlet turbulence level. A validated numerical model is used to produce Nusselt and Euler numbers for more than 30 helically wound fin-tube geometries in a staggered equilateral triangle layout. A few selected geometries are further analyzed in detail. Results indicate that a large gas-to-tube temperature difference, a low fin pitch or a high thermal effectiveness can lead to situations where some of the current models for heat transfer, pressure drop or fin efficiency perform poorly. Specifically, pressure drop is shown to be 25% higher for helical fin-tubes under gas heating conditions than for equivalent annular fin-tubes under gas cooling conditions, which is not considered by most correlations. Fin efficiencies can, further, be in error by more than 20%, which can partially be attributed to flow passing outside the fin diameter. More data is, however, needed to characterize these effects thoroughly in the general case.

    更新日期:2020-01-11
  • Numerical investigation on effects of fuel tube diameter and co-flow velocity in a methane/air non-premixed flame
    Heat Mass Transfer (IF 1.551) Pub Date : 2020-01-09
    Mohsen Nasiri Soloklou, Ali Akbar Golneshan

    In this paper, the effects of variations in the fuel tube diameter and co-flow velocity in the combustion chamber on the non-premixed laminar flame are investigated. Methane gas, as a fuel, and the dry air, as an oxidizer. The size of the combustion chamber is constant and, by changing the fuel tube diameter and co-flow velocity, changes in the numerical values of temperature, velocity, density, and concentration of the species of reactants and products in the combustion chamber are evaluated. A finite volume method (FVM) with staggered grids is used for numerical solution. Equations of continuity, momentum, energy, ideal gas state and kinetic equations with thermodynamic and thermochemical information of chemical species are solved using numerical method of SIMPLE. The convective terms are discretized using Power Law scheme (PLS).The calculations are carried out using Dryer and Glassman’s three-stage chemical kinetics. Variable under relaxation factor dependent on temperature has been used to handle the solving chemical kinetic equations. Initially, the results of calculations are compared with the experimental and numerical results of other researchers, which show an acceptable agreement.. The results show that increasing the diameter ratio reduces the length of the flame. With the large ratio of the diameters, location of the combustion’s maximum temperature is at the chamber entrance and for the small diameter ratios, its location moves to nearly outlet of the chamber. In addition, the reduction of the ratio of the diameters increases the flame lift-off. Also the results show that the optimal of diameters ratio is 0.6 in order to prevent the lift-off flame and return the flame to inlet opening of combustion chamber. Also increasing the fuel tube diameter, increases the amount of oxygen due to the return flow formation and decreases the volumes of water vapor and carbon dioxide in the centerline of the combustion chamber. The flame length attains the maximum possible value with respect to diameter ratio of 0.6 at inlet air velocity of 0.3 m/s. In addition, it is shown that increasing the air velocity increases the total flame lift-off and flame length until the air velocity reaches the value of around 0.3 m/s and by increasing the air velocity more than 0.3 m/s, the total flame lift-off and flame length decreases.

    更新日期:2020-01-11
  • Experimental research on dynamic concentration distribution for combustible dust based on ultrasonic-electric hybrid detection
    Heat Mass Transfer (IF 1.551) Pub Date : 2020-01-03
    Yan Zhang, Wenzhong Lou, Hongjun Wang, Mingru Guo, Shenghua Fu

    Abstract The concentration distribution of combustible dust determines thermal intensity distribution during an explosion. Current measurements for dust concentration have their particular limitations. Targeting this, we proposed an “ultrasonic-electric” hybrid detection system and a fusion model. We deployed 12 of the ultrasonic-electric hybrid systems in orthogonal arrays to comprehensively observe the clouds. First, the ultrasonic-electric hybrid detection systems obtained concentration data in real time, and those data were calculated by fusion model. Then, the clouds and their concentrations changing with time were depicted. We analyzed those trends and found certain patterns in them. Our approach can provide a fast, accurate way to detect concentrations of dynamic and complex dust. Finally, the corresponding relationship between the dust concentration distribution and its explosive heat intensity distribution is obtained. The results show that the thermal distribution of combustible dust at a concentration of 20-120 g/m3 is proportional to the concentration. This is important for preventing dust explosions and reducing the thermal intensity of explosions.

    更新日期:2020-01-04
  • Natural convection energy recovery loop analysis, part I: energy and exergy studies by varying inlet air flow rate
    Heat Mass Transfer (IF 1.551) Pub Date : 2020-01-03
    Yulin Ma, Mohammad Ali Fazilati, Ahmad Sedaghat, Davood Toghraie, Pouyan Talebizadehsardari

    Abstract The natural convection energy recovery loop is analyzed experimentally in different airflow rates. The system was introduced previously as a prototype of the standalone air conditioning system and its transient and steady performance was verified. As a new generation of energy recovery tool between the building return and fresh air, the system is rated from the viewpoints of energy and exergy. Different forms of effectiveness and 2nd law efficiency are studied and their values extracted for inlet airflow rates of 2–6 m3/h. The results show that the prominent factor that controls the system behavior is the concentration ratio from which the solution free- motion originated. The maximum sensible, latent, and total effectiveness of the system are 0.23, 062, and 0.54 respectively and are for the airflow rate of 2m3/h. It is confirmed that the number of a transfer unit (NTU) and capacity ratio (Cr*) are not independent and are varied oppositely to each other. By increasing the airflow rate, the mass flow rate and heat capacity rate of desiccant solution increase more than that of air streams. For flow rates less than 3.5m3/h, external heat transfer is just enough to induce natural motion of desiccant and in this way the loop performance is similar to a forced convection energy transfer loop which exchanges heat and moisture only between the air streams.

    更新日期:2020-01-04
  • Experimental investigation on forced convection heat transfer of ferrofluid between two-parallel plates
    Heat Mass Transfer (IF 1.551) Pub Date : 2019-07-11
    Milad Valitabar, Masoud Rahimi, Neda Azimi

    Abstract This study presents an experimental investigation on forced convection heat transfer of ferrofluid between two parallel-plates in the presence of a static magnetic field (SMF). The heat transfer between two parallel-aluminum plates is studied, which heat source with a constant heat flux is applied on the bottom plate. The process of heat transfer is examined for DI-water and ferrofluid in the absence and the presence of the magnetic field. The heat transfer characteristics at the different flow rates, magnet distance from the test section (d = 2–80 mm) and nanoparticle volume fractions (ϕ = 0.25–2%wt) are compared to those of pure water. The results depicted that the heat transfer coefficient (h) and Nusselt number (Nu) of ferrofluid are higher than DI-water. In addition, the results show that applying SMF could enhance the convective heat transfer rate and it decreased by an increase in d that means the decrease in the magnetic field strength. The increase in the nanoparticle volume fraction leads to higher heat transfer enhancement. The maximum value of the heat transfer coefficient and Nusselt number are achieved for SMF with d = 2 mm and ϕ = 1% wt.

    更新日期:2020-01-04
  • Nozzle exit conditions and the heat transfer in non-swirling and weakly swirling turbulent impinging jets
    Heat Mass Transfer (IF 1.551) Pub Date : 2019-07-26
    Muhammad Ikhlaq, Yasir M. Al-Abdeli, Mehdi Khiadani

    Abstract Investigations have been conducted into turbulent impinging jets but the exact flow dynamics and mechanisms leading to the observed heat transfer distributions at the impingement plane remain outstanding. In particular, use of different swirl generators (vanes, twisted inserts) means the role of varying inflow conditions (at the nozzle exit plane x/D = 0) should be studied to resolve its role on the observed convective heat transfer trends. The present paper studies axisymmetric turbulent weakly swirling (S = 0.31) jets (D = 40 mm) impinging onto a heated plate. Parameters varied include inflow conditions and the effects of impingement distance (H/D = 2, 4, and 6). The Reynolds Averaged Navier Stokes (RANS) equations are used to model the jets using the k-kl-ω turbulence model, which is benchmarked against other models. Three azimuthal () velocity profiles at a Reynolds (Re) number of 24,600 are used at the nozzle exit plane: Uniform (UP), Solid Body Rotation (SBR), and Parabolic Profiles (PP). The start of the wall jet region, designated through elevated levels of turbulent kinetic energy correlates well with the widely observed first peak in Nu distribution. This is however extremely sensitive to the imposition of any swirl, with the application of even weak swirl (S = 0.31) minimally modifying flow dynamics (in the upstream jet region) and leading to recirculation zones stabilized at the impingement plane. This occurs in near-field impingement (H/D) for some inflow conditions (S031-UP), but not others thereby highlighting the significance varied nozzle and swirl generation methods on trends observed in the literature. The imposition of elevated levels of turbulence at the nozzle inflow (x/D = 0) appreciably modifies the heat transfer distribution, particularly in far-field impingement (H/D = 6).

    更新日期:2020-01-04
  • Single phase flow of nanofluid including graphite and water in a microchannel
    Heat Mass Transfer (IF 1.551) Pub Date : 2019-07-09
    Oğuzhan Yıldız, Özgen Açıkgöz, Güldem Yıldız, Mustafa Bayrak, Ahmet Selim Dalkılıç, Somchai Wongwises

    In this study, convective heat transfer performance of a nanofluids containing graphite is studied in an industrial microchannel. In the experiments, initially, to prepare nanofluids at the volume fraction values of 0.5, 1, 1.5, 2%, distilled water has been employed as the base liquid. To provide sedimentation and stabilization of nanofluids in distilled water, Cetyltrimethylammonium bromide (CTAB) is utilized as surfactant. Thermophysical properties of nanofluids such as thermal conductivity, dynamic viscosity, and specific heat are determined experimentally. Furthermore, by building an experimental setup, in the temperature range of 20–30 °C and with temperature intervals of 2 °C, performance experiments are carried out in a microchannel of which hydraulic diameter is 1.6 × 10−3 m. Additionally, experiments have been conducted using nanofluids at different volumetric rates from 1 to 7 l min−1, heat fluxes from 100 to 1100 W, and volume fractions from 0.5 to 2%. Measuring heat flux, temperature, and flow rate, outcomes such as convective heat transfer coefficient, Reynolds number, and Nusselt number are calculated. The validation process of the experimental results has been performed by plotting the figures of Nusselt numbers vs Reynolds ones, and heat transfer coefficient vs supplied heat considering distilled water and nanofluids having various volumetric proportions. Regarding with the performance of nanofluids against distilled water under similar operating conditions, some proportional positive increase are acquired. Using outcomes attained from experiments, new correlations for Nusselt number have been derived with the R2 values around 0.96, and afterward by means of those correlations experimental data have been compared with those in the literature. A large number of measured and calculated data are given in the paper for other researchers to validate their theoretical models.

    更新日期:2020-01-04
  • Experimental study on modified specific energy, temperature field and mechanical properties of Xuzhou limestone irradiated by fiber laser
    Heat Mass Transfer (IF 1.551) Pub Date : 2019-07-19
    Yijiang Wang, Yali Shi, Jinyi Jiang, Guoqing Zhou, Zegui Wang

    Abstract This paper reports the experimental investigation of the modified specific energy, mechanical properties and temperature field of limestone rock that was irradiated by continuous wave fiber laser at different irradiation powers. Results show that the specific energy decreases nonlinearly from 17 kJ/cm3 to 11 kJ/cm3 and rate of perforation increases from 0.2 mm/s to 0.65 mm/s with laser power increasing from 600 W to 1000 W. The modified specific energy which is used to evaluate rock excavation efficiency is observed to decrease monotonously from 0.27 kJ/cm3 to 0.068 kJ/cm3 with increasing laser power. Higher laser power results in larger surface temperature and temperature gradient. The rate of rock surface temperature irradiated by laser with power of 1000 W reaches 14 °C/ms within 140 ms. The temperature gradient, with largest value of 1400 °C/mm near to the sample axis, decreases sharply with radius. The high solution 3D X-ray microanalyser (XRM) images show that the rock perforation is a V-shaped hole and obvious cracks penetrate the whole rock sample. The extremely rough surface and well-developed cracks for irradiated samples are observed through scanning electron microscope (SEM). X-ray diffraction (XRD) results also show that the main minerals of rock after irradiation are quite different from that of original sample. The maximum reduction rate of compressive strength of irradiated rock with laser power of 1000 W is 60%, and the maximum increasing rate of peak strain of irradiated rock with laser power of 800 W is 65%.

    更新日期:2020-01-04
  • Mixing of a confined two-layer stratified liquid by a bubble plume
    Heat Mass Transfer (IF 1.551) Pub Date : 2019-07-19
    Xiaofei Xu, Xin Dong, Le Gao, Fengxia Liu, Wei Wei, Zhijun Liu

    Abstract Bubble plumes are often encountered in numerous industrial applications where the gas-liquid two phase interaction is used to promote the mixing of the liquid phase. The current work is focused on the investigation of the mixing behavior of bubble plumes produced by a circular source in confined two-layer stratified fluids. In analogy to the study of linear stratification, two non-dimensional parameters PN_EQ (=\( {N}_{EQ}^3{H}^4/{Q}_Bg \), where NEQ is the equivalent linear buoyancy frequency, H is the water depth, QB is the gas flow rate, and g is the gravity acceleration) and MH (=\( {Q}_Bg/\left(4\pi {\alpha}^2{u}_s^3H\right) \), where α is the entrainment coefficient and us is the slip velocity) are selected to investigate the flow patterns and mixing efficiency in two-layer stratified fluids. The equivalent linear buoyancy frequency NEQ of the two-layer stratification is proposed to calculate the non-dimensional parameter PN_EQ. The flow structure of a bubble plume in two-layer stratified fluids is introduced in detail. The correlations of the non-dimensional parameters with the bubble plume properties, such as the flow patterns, initial destratification height and mixing efficiency, are obtained. The results show that correlations with the non-dimensional parameter PN_EQ can be successfully used to predict the bubble plume behavior in a confined two-layer stratified fluids.

    更新日期:2020-01-04
  • The transpiration characteristics and heat dissipation analysis of natural leaves grown in different climatic environments
    Heat Mass Transfer (IF 1.551) Pub Date : 2019-07-12
    Kai Xu, Chuanmao Zheng, Hong Ye

    Because vegetation is one of the typical backgrounds on land, it is necessary to investigate the transpiration characteristics and heat dissipation processes of the leaves grown in different natural environments to optimize the thermal infrared camouflage performance of the bionic materials for countering the thermal infrared detection. Considering ambient temperature and relative air humidity as the dominant environmental factors, daily transpiration rates of various kinds of leaves grown in Wuxi (summer and winter, moderate relative air humidity) and Xishuangbanna (summer, high relative air humidity) were measured respectively, and a thermophysical model was established to investigate the heat dissipation processes of the leaves. Results show that the average daily transpiration rates of the leaves are 1.42, 0.42 and 0.92 kg/m2∙day in Wuxi summer, Wuxi winter and Xishuangbanna summer respectively, indicating that the daily transpiration rates vary significantly with different environments. In Wuxi summer and winter, the daily transpirative heat flow accounts for approximately 55.8% and 24.3% of the total heat dissipation of the Cinnamomum camphora leaves respectively, revealing the significant effect of the transpirative heat transfer on the temperature of the leaves grown in the environment of high ambient temperature and the weakened effect in the environment of low ambient temperature. In Xishuangbanna summer, the daily transpirative heat flow accounts for approximately 37.3% of the total heat dissipation of the Ailanthus leaf, which is significantly lower than that in Wuxi summer, indicating that relative air humidity also dominates the effect of transpirative heat transfer on the temperature of the leaves.

    更新日期:2020-01-04
  • Experimental study on adsorption characteristics of SAPO-34 zeolite and silica gel in vacuum condition
    Heat Mass Transfer (IF 1.551) Pub Date : 2019-08-01
    S. Y. Li, C. X. Du, Z. X. Yuan

    Using the gravimetric method the adsorption characteristics of SAPO-34 zeolite and silica gel to water vapor in vacuum condition have been experimentally studied. Comparisons of the result in vacuum condition to the result in moist air condition were carefully conducted. It was found that the equilibrium adsorption concentration of the two materials in vacuum condition was higher than the equilibrium concentration in moist air under the same condition. Also, the adsorption time to reach the equilibrium state was greatly shortened in vacuum cases. The maximum time shortening was up to 48.1% for the SAPO-34 zeolite and 38.8% for the silica gel as compared to the time of the moist air cases. With an anti-S type of the adsorption isotherm curve the concentration change of the SAPO-34 zeolite was insensitive in the middle pressure range, while the concentration change of the silica gel was more sensitive in the middle pressure range with a S type isotherm curve. The existing Freundlich equation and S-B-K equation did not fit well the adsorption isotherm of the SAPO-34 zeolite and the silica gel. Therefore novel correlations were proposed and they fitted the experimental data with much better accuracy.

    更新日期:2020-01-04
  • Experimental study on differences of heat and mass flux on opposite side of a 10-nm nanoporous ceramic membrane tube
    Heat Mass Transfer (IF 1.551) Pub Date : 2019-07-22
    Haiping Chen, Boran Yang, Chao Ye, Yijun Feng, Xiangsheng Li

    Abstract A nanoporous ceramic membrane tube (CMT) is able to extract water vapor and latent heat from gas mixture, such as flue gas exhausted from thermal power plant, because of its excellent permselectivity and heat-exchange capacity. However, due to the effect of the gravity force on condensation film formation and condensate permeation process, the water and heat-recovery performance on top and bottom parts of a horizontal CMT may be distinct. In this study, a corresponding experiment was performed to verify and research the phenomenon by installing six equidistant and symmetrical measuring points at top and bottom parts of the experimental CMT module, respectively. According to the experimental data, the local temperature of cooling water on top (Tc,l,t) and bottom (Tc,l,b) parts of CMT module exhibited a distinct changing tendency, and the maximum temperature difference (ΔTc) could approach 2.68 °C. Typically, the effects of gravity force on condensation film was negligible, while the permeation capacity of condensate was influenced by gravity force apparently. Moreover, most of condensate (92.24% maximally) was generated during the normal condensation process, merely a small proportion of condensate was produced by the capillary condensation process. This paper is useful for horizontal membrane type gas-water technology improvement.

    更新日期:2020-01-04
  • Numerical investigation of heat transfer enhancement and fluid flow characteristics in a microchannel heat sink with different wall/design configurations of protrusions/dimples
    Heat Mass Transfer (IF 1.551) Pub Date : 2019-07-25
    Muhammad Mohib Ur Rehman, Taqi Ahmad Cheema, Faraz Ahmad, Ahmad Abbas, Muhammad Sohail Malik

    The convective cooling associated with Microchannel heat sink (MCHS) devices for electronic components with high power density is a recent topic of cutting-edge research. However, thermal improvement with minimum degradation in hydrodynamic characteristic by extending the effective heat transfer area of different walls of MCHS is still a major challenge. In this regard, the heat transfer enhancement and fluid flow behavior of MCHS with protrusions, dimples and their different wall, geometric and design combinations are numerically studied in the present study. The wall configurations considered for the present analysis includes: Base wall protrusions/dimples (BWP/D), Side wall protrusions/dimples (SWP/D) and all walls protrusions/dimples (AWP/D). While for design configurations, the (AWP/D-Aligned), (AWP/D-staggered) and all wall protrusions and dimples mix (AWPD-Mix) cases are considered. The governing equations are discretized and solved across the computational domain using commercial computational fluid dynamics code with three-dimensional conjugate laminar flow model. The numerical model is then validated with experiment and theory in the literature and reasonable agreement in the results of average Nusselt number (Nuavg) and apparent friction factor (fapp) are observed. The effect of geometric parameters i.e. protrusion/dimple’s diameter (Dfr = 200 - 230 μm) and Pitch (Sfr = 400 - 1200 μm), operating parameters i.e. Reynolds number (Re = 100–1000) and Heat flux (qw = 50–100 W/cm2) on the heat transfer and fluid flow characteristics are examined to provide a better physical understanding of the energy management. The results indicate that the addition of protrusions/dimples to different walls of MCHS significantly improves the heat transfer with reasonable increase in pressure drop. The fluid flow pattern with the addition of the protrusions/dimples to different walls is improved through better mixing and lower pumping power augmentation to transport the same heat load than Straight MCHS. The favorable configuration along with geometric and operating parameters in terms of better thermal and hydrodynamic performance is suggested based on thermal enhancement factors (ƞ) and entropy generation rates (\( {\dot{\mathrm{S}}}_{\mathrm{gen}} \)). Among the proposed wall configurations, AWP demonstrates superior thermal performance by resulting in maximum improvement of 82% in ƞ compared to BWP configuration. When compared to the straight MCHS, AWP-aligned MCHS achieved maximum enhancement of 115% in Nuavg at the cost of 152% higher fapp at same operating conditions of Re = 1000 and qw = 100 W/cm2 for design configurations. Furthermore, the outcomes of this study is expected to provide some important guidelines for the future experiments on such MCHS devices for energy saving and management.

    更新日期:2020-01-04
  • Optimization and advance thermodynamic analysis of dual stage Co 2 power cycle combined to gas turbine
    Heat Mass Transfer (IF 1.551) Pub Date : 2019-07-12
    Hosaien Khosravi, Gholamreza Salehi, Masoud Torabi Azad

    Abstract This paper presents a thermodynamic and economic modeling of Rankine cycles (RCs) that use waste exhaust energy of a gas turbine in the power plant of Sirri Island in Iran based on energy, exergy and economic concepts. In addition, the exergoeconomic and advanced exergy analyses were performed. This cycle is composed of a supercritical CO2 Rankine and a transcritical CO2 cycle. Two objective functions, including the most rate of net power generation of the Rankine cycle (as a thermodynamic criterion) and the lowest total purchased equipment cost (as an economic criterion), are considered simultaneously. This model is constructed on the basis of the energy, exergy and economic analysis according to the Total Revenue Requirement (TRR) method. After validating the model, the advanced exergy analysis is performed to split exergy destruction rate into endogenous, exogenous, avoidable and unavoidable parts in order to provide detailed information about the potential improvement of the system components. The final results indicate that the net electric power output of the Rankine cycle and the total purchased equipment cost become 2.31 (MW) and 301473 (US$), respectively. Also, the exergy efficiency and the total exergy destruction of the cycle reach 48.4% and 8757.4 (KW), respectively.

    更新日期:2020-01-04
  • Examining the curvature dependency of surface tension in a nucleating steam flow
    Heat Mass Transfer (IF 1.551) Pub Date : 2019-07-22
    Ehsan Amiri Rad, Saeed Naeemi, Behzad Davoodi

    Many researches have been performed on the phenomenon of vapor condensation at the hypersonic stages of the steam turbine. This phenomenon consists of nucleation and droplet growth consequently. One of the most important parameters affecting the nucleation and droplet growth is surface tension, which is often assumed to be equal to the surface tension of a planar interface. The nucleated droplets have the nano-scale size at the beginning of the process and they will not be bigger than macro-scale after the growth. On the other hand, there are many reports about the dependency of surface tension on the curvature of the surface in such a small size. In order to evaluate the effects of curvature dependency on the surface tension, three famous models, proposed by Tolman, Benson and Rasmussen, were incorporated with nucleation theory and their effects on the wet steam flow were investigated. The results revealed that the dependency of the surface tension on the droplet radius significantly improved the accuracy of the predicted droplet radius. Also, it was shown that Tolman’s theory have offered the best results.

    更新日期:2020-01-04
  • The effect of magnetic nano-fluids (Fe 3 O 4 ) on the heat transfer enhancement in a pipe with laminar flow
    Heat Mass Transfer (IF 1.551) Pub Date : 2019-07-11
    Anggito P. Tetuko, Silviana Simbolon, Tri G. Sitorus, Reggy Zurcher, Rizki K. Hadi, Eko A. Setiadi, Candra Kurniawan, Masno Ginting, Perdamean Sebayang

    Abstract In this research, the effect of magnetic nano-fluids (Fe3O4) on the heat transfer enhancement in a pipe with laminar flows (Re numbers of 171, 228 and 285) were examined. Different heats (15.75 and 20.6 W) were applied to an acrylic pipe to analyse the effect of magnetic nano-fluids for transferring the heat and reducing the wall temperature on a non-metal material pipe. Variations of nano-particles (volume percentage) were used in the nano-fluids solution: 2, 3 and 4%, respectively. The magnetic nano-fluids (Fe3O4) were prepared from 32.5 g of FeCl3.6H2O (ferric chloride) and 12.7 g of FeCl2.4 H2O (ferrous chloride) by using a co-precipitation method. The material characterisations using XRD and FE-SEM confirmed that Fe3O4 single phase occurred and shown that the average size diameters of the nano-particles are within a range of 20-40 nm. The experimental results suggested that the cooling capability can be enhanced by adding magnetic nano-particles controlled by permanent magnet and increasing the heat transfer in the magnetic nano-fluids system. The aggregation of the magnetic nano-particles following the magnetic field applied from the permanent magnet increases the convection heat transfer from the heating source to the nano-fluids, and thus reducing the wall temperature of the acrylic pipe.

    更新日期:2020-01-04
  • Experimental study and analytical modeling of thermosyphon loop for cooling data center racks
    Heat Mass Transfer (IF 1.551) Pub Date : 2019-07-16
    Chayan Nadjahi, Hasna Louahlia-Gualous, Stéphane Le Masson

    Abstract This paper presents experimental study and analytical model for a passive two-phase thermosyphon loop designed to remove directly the hot air flow inside a data center. The cooling loop with separated lines working without electricity is tested under different conditions. A mini-channel parallel-flow evaporator and a finned-tubes condenser are used to transfer heat from the data center to outside environment. Experimental tests are conducted with different n-pentane fill ratios. According to these tests, the cooling performance is showed and the refrigerant distribution is specified. The proposed steady-state model is based on the combination of the thermal and hydraulic models of the two-phase flow inside the loop. It well predicts the experimental results such as the mass flow rate and working fluid pressure drops. As investigated by the model, the most parameters influencing the cooling capacity are: the working fluid, the condenser tubes number and the outside temperature.

    更新日期:2020-01-04
  • Convective heat transfer on a flat target surface impinged by pulsating jet with an additional transmission chamber
    Heat Mass Transfer (IF 1.551) Pub Date : 2019-07-20
    Chan Tang, Jing-Zhou Zhang, Yuan-Wei Lyu, Xiao-Ming Tan

    A series of experimental tests are performed for the pulsating jet impingement heat transfer by varying the Reynolds number (5000 ≤ Re ≤ 15000), operation frequency (5 Hz ≤ f ≤ 40 Hz) and dimensionless nozzle-to-surface distance (2 ≤ H/d ≤ 10) while fixing the duty cycle as DC = 0.5. The maximum uncertainty in the measurement of Nusselt number is estimated to be about ±7%. Meanwhile, numerical simulations are performed to demonstrate the instantaneous flow field of the pulsating jet impingement. Particular attention is paid to examine the influence of transmission chamber on the pulsating jet impingement heat transfer. The results show that by using an additional transmission chamber, the pulsating jet impingement heat transfer is enhanced. For example, the circumferentially-averaged Nusselt number around the stagnation point (x/d ≤ 2) is increased up 8%~16% by the adding of a transmission chamber in related to the baseline case under Re = 10000 and H/d = 6. Due to the presence of transmission chamber, the exiting jet velocity profile at the orifice outlet is varied. In related to the baseline case, the transmission chamber makes the time-averaged ejecting velocity in the central zone increase but decrease in the edge zone. As the peak velocity in the central zone of orifice outlet is effectively increased by the use of transmission chamber, the jet impinging velocity approaching to the target surface is also strengthened, resulting in a stronger jet impingement in the vicinity of the stagnation point.

    更新日期:2020-01-04
  • Measurement of heat transfer coefficient and pressure drops in a compact heat exchanger with lance and offset fins for water based Al 2 O 3 nano-fluids
    Heat Mass Transfer (IF 1.551) Pub Date : 2019-07-25
    Ranganayakulu Chennu, Vasudeva Rao Veeredhi

    Compact Heat Exchangers (CHEs) are widely used in various applications in thermal fluid-systems including aerospace due to their compactness and light weight coupled with high effectiveness. Among the different types of CHEs for Aircraft Environmental Control System (ECS) and Avionics cooling applications, a cross-flow CHE with Lance & Offset fin is of special interest because of their high heat rejection capability. In the present work, the heat transfer coefficient of water based Al2O3 nano-fluid (NF) flowing in a Compact Plate-fin Heat Exchanger having Lance & Offset fins is studied experimentally. As part of experimental investigation a test setup is designed and developed. Experiments are conducted in the range of mass flux from 50 to 350 kg/s m2 (100 ≤ Re ≤ 600) at room temperature as well as at higher temperatures up to 70 °C. In this study, Al2O3 volume concentration in water is varied from 0.255% to 0.51%. The variation of heat transfer coefficients and pressure drops of Al2O3 nano-fluid with respect to water are presented in graphical form for different mass fluxes. The heat transfer coefficient gets enhanced in low NF temperature regions (between 30 °C-45 °C) up to 25% for 1%NF and up to 35% for 2%NF, where as there is only a marginal improvement at higher operating temperatures (between 60 °C-75 °C). The uncertainty analysis is carried out for heat transfer coefficient based on sensors specification.

    更新日期:2020-01-04
  • Effect of throttling on the two-phase flow stability in an open natural circulation system
    Heat Mass Transfer (IF 1.551) Pub Date : 2019-07-10
    René Manthey, C. Schuster, W. Lippmann, A. Hurtado

    Low pressure natural circulation systems fit the concept of passive safety systems as useful feature to bring a nuclear power plant to a safe and stable state during a postulated accident without active systems. The operation is characterized by low mass fluxes and passing the known unstable two-phase region from activation to a continuous heat removal in the stable two-phase region. The occurring mass flow oscillations and the condensation induced water hammer phenomenon limited the performance of heat removal. These effects are highly undesirable because they may effect destruction of entire apparatus. The GENEVA test facility as an open natural circulation system represents the containment cooling condenser of the KERENATM reactor concept in the main dimensions. Initiated by the steam supply, the mass flow results from induced heat flux and is limited only by the internal pressure drops: the local, the frictional, the acceleration and the gravitational pressure drop across the flow path. The influence of the flow resistance on its unstable two-phase region and their oscillatory behavior are experimentally studied in detail. Its variation is summarized in stability maps plotted in the parameter plan of the heat flux and the subcooling number at the inlet of the heated section. Thereby, the dynamical evolution of the boiling boundary indicates the initiation of two-phase flow oscillations and the condensation induced water hammer phenomenon. The increasing local pressure drop at the inlet of the heated section reduces the velocity amplitudes but a clear reduction of the pressure surges could not be determined although these should be more pronounced due to decreasing buoyancy and increasing back flow.

    更新日期:2020-01-04
  • Effects of dimples’ arrangement style of rough surface and jet geometry on impinging jet heat transfer
    Heat Mass Transfer (IF 1.551) Pub Date : 2019-07-31
    Nevin Celik

    Abstract The enhancement of heat transfer from the impinging jets can be reason of tens of parameters. In earlier studies, researchers have done numerous experiments or numerical runs to show the effect of each parameter individually. However, in recent decade some new methodologies, such as Design of Experiment (DoE) and Analysis of Variance (ANOVA) have been used to achieve more comparative parameter analysis and to optimize the number of experiments (or numerical runs). In this study, DoE and ANOVA methods are applied to an experimental impinging jet study. The jet geometry and roughness of impingement plate are the main considered parameters of this study. Beside them, effects of jet-to-surface distance (H) and radial distance (r) on the target surface are analyzed. The data runs are performed for a constant jet Reynolds number 20,000. The Taguchi DoE method is applied to the study in order to design the experiments. Totally 18 experiments are run base on the orthogonal array of L18 (16) (33), and effects of each design parameter on heat transfer is found out by ANOVA. As a result, it is concluded that, the highest effect on Nusselt number is observed to be the radial distance (88%), while surface roughness has the effect in percentage of 8%. The contribution of jet geometry and jet-to-surface distance are much lower, as being 3% and 1%, respectively.

    更新日期:2020-01-04
  • Analyzing the universality of the dimensionless vibrating number based on the effective moisture diffusivity and its impact on specific energy consumption
    Heat Mass Transfer (IF 1.551) Pub Date : 2020-01-02
    L. Meili, H. Perazzini, M.C. Ferreira, J.T. Freire

    Using vibro-fluidized bed dryers (VFB) is an alternative to perform drying with less specific energy consumption compared to fluidized bed dryers (FB). Finding an optimum combination of the dimensionless vibrating number (Γ) with the drying conditions is a key factor to analyze the viability of VFB towards energy efficiency. In the present study, the performance of a VFB was investigated by correlating the drying kinetics of porous particles with the specific energy consumption to obtain an optimum drying condition. Experiments were carried out under different operating conditions and three combinations of vibration amplitude (A) and frequency (F) chosen to yield a constant value of Γ. The impact of the operating parameters was analyzed by effective moisture diffusivity (Deff,G), estimated by fitting the diffusive model to the experimental data. It was found that vibration intensifies Deff,G and is the preponderant effect in the convective mass transfer. Different values of Deff,G were found for the same Γ obtained under the tested combinations of A and F, which indicates that this parameter cannot be used alone as a single descriptor of the vibration energy. Using a high temperature and a gas velocity exceeding the minimum fluidized velocity combined to a higher value of A and a lower value of F enhanced drying, as this combination yielded the highest Deff,G. This same combination of high A and low F associated to low values of temperature and gas velocity enhanced the energy performance. The results also showed that only Γ is not an adequate parameter to perform energy analysis of VFB.

    更新日期:2020-01-02
  • Photosynthesis as a thermodynamic cycle
    Heat Mass Transfer (IF 1.551) Pub Date : 2019-12-21
    Nabil Swedan

    Thermodynamics of photosynthesis has been a subject of interest to the scientific community; it is, therefore, addressed in this paper. This work reveals that traditional thermodynamic relationships may be used to calculate and project photosynthesis. Solar energy is required for the chemical reaction of green matter production. When the size of the green matter expands, less solar energy is received by the surroundings and more chemical energy is stored in plants and vegetation. If everything else is the same, the increase in the chemical energy produced is equal to the decrease in the heat of the biosphere and vice versa. Photosynthesis expansion is thus equivalent to heat transfer from the biosphere to the green matter. Plants surrounding air may be assumed as a heat reservoir at air dry bulb temperature, Tdb. The colder air enclosed by the space of the green matter may be assumed as a cold reservoir at air wet bulb temperature, Twb, and photosynthesis may be represented by a Carnot engine cycle. The thermal efficiency of the cycle is equal to 1-(Twb/Tdb)0.5. If everything else is the same, the difference, Tdb-Twb, is a limiting factor of terrestrial photosynthesis. Based on this understanding, equations to predict growth of the green matter and tree diameter are derived and validated based on observations. Other findings include photosynthesis global average thermal efficiency is between 0.61% and 0.72%, and seasonal greening is nearly 0.80%. Neglecting deforestation, surface greening trend with climate change is between 0.23% and 0.28% annually.

    更新日期:2019-12-21
  • Effects of the cone angle on the stability of turbulent nonpremixed flames downstream of a conical bluff body
    Heat Mass Transfer (IF 1.551) Pub Date : 2019-12-20
    Alper Ata, I. Bedii Ozdemir

    The effects of a stabilizer and the annular co-flow air speed on turbulent nonpremixed methane flames stabilized downstream of a conical bluff body were investigated. Four bluff body variants were designed by changing the outer diameter of a conically shaped object. The co-flow velocity was varied from zero to 7.4 m/s, while the fuel velocity was kept constant at 15 m/s. Radial distributions of temperature and velocity were measured in detail in the recirculation zone at vertical locations of 0.5D, 1D and 1.5D. Measurements also included the CO2, CO, NOx and O2 emissions at points downstream of the recirculation region. Flames were visualized under 20 different conditions, revealing various modes of combustion. The results evidenced that not only the co-flow velocity but also the bluff body diameter play important roles in the structure of the recirculation zone and, hence, the flame behavior.

    更新日期:2019-12-20
  • Modeling of mass transfer and hydrodynamic investigation of H 2 S removal from molten sulfur using porous Sparger
    Heat Mass Transfer (IF 1.551) Pub Date : 2019-12-20
    F. Tari, S. Zarrinpashne, M. Shekarriz, A. Ruzbehani

    Empirical and theoretical aspects of non-catalytic molten sulfur degassing as one of the vital actions in Claus Unit have been investigated in this article. A laboratory bubble column has been set up to study the hydrodynamic behavior of gas-liquid system. Values of gas holdup were also compared with predictions of some correlations in the case of application of porous spargers. The mathematical model of degassing was also developed considering reaction of H2S with sulfur molecules in the molten sulfur and generation of H2Sx species. The experimentally-measured parameters of gas holdup and bubble size were used in the mathematical model. Other parameters including Henry’s law constant of H2S-liquid sulfur system and reaction rate constants were obtained from published formulas presented by Marriott and Ji, respectively. The obtained results were compared to the empirical data of non-catalytic degassing performed in the laboratory setup. Reasonable compatibility was observed between the model-derived and experimental results. The results showed a fast removal of dissolved H2S within few minutes, followed by very slow removal of H2Sx through its chemical conversion to H2S and its purging by sweep gas. The novel gas holdup profile and images presented in this article show interesting features of hydrodynamic behavior of molten sulfur.

    更新日期:2019-12-20
  • Experimental investigation of the heat transfer and pressure drop characteristics of SiO 2 /water nanofluids flowing through a circular tube equipped with free rotating swirl generators
    Heat Mass Transfer (IF 1.551) Pub Date : 2019-12-18
    Chaiwat Jumpholkul, Lazarus Godson Asirvatham, Ahmet Selim Dalkılıç, Omid Mahian, Ho Seon Ahn, Dong-Wook Jerng, Somchai Wongwises

    The heat transfer and pressure drop characteristics of SiO2/water nanofluids flowing through a horizontal circular stainless steel tube equipped with free rotating swirl generators (FRSGs) at the entrance of a test tube are reported in this article. The experimental studies were performed with a Reynolds number ranging from 3500 to 13,000; volume concentrations of 0.5, 1, and 2 vol%; and inlet temperatures of 25, 30, and 35 °C, where a constant heat flux was imposed on a test tube. FRSGs made from aluminum, with three different twisted angles (30°, 60°, and 90°) and a length of 2.3 cm, were located at the entrance of the test tube. The results indicated that FRSGs had significant effects on heat transfer and pressure drop characteristics for the flow of both water and nanofluids in the tube. The Nusselt number increased with increasing volume concentration, inlet temperature, and Reynolds number. However, the friction factor of nanofluids flowing through FRSGs was significantly greater than that of water when the Reynolds number was lower than 6000. This is because FRSGs do not rotate under the given conditions. A maximum efficiency index of 1.57–1.7 was obtained when using SiO2/water nanofluids with FRSGs at a volume concentration of 2 vol%, inlet temperature of 35 °C, and Reynolds number greater than 7000. On the other hand, the efficiency index value was lower than unity when the Reynolds number was lower than 7000 and with volume concentrations of 0.5 and 1 vol%. No significant differences were found among twisted angles of FRSGs on heat transfer, pressure drop, and efficiency index.

    更新日期:2019-12-18
  • Multi-walled carbon nano-tubes for performance enhancement of thin film heat flux sensors
    Heat Mass Transfer (IF 1.551) Pub Date : 2019-12-17
    Akash Jadhav, Ravi Peetala, Vinayak Kulkarni

    Surface heat transfer measurement is an important aspect in many research problems. Thin film heat flux sensor (TFHFS) is mostly considered in such situations for heat flux measurement due to its quick response and high accuracy. In the present studies, multi-walled carbon nanotubes (MWCNTs) are mixed with platinum while making the TFHFSs. Such addition is noticed to increase the sensitivity and decrease the temperature coefficient of resistance (TCR) of the thin film sensors. Improved sensitivity by 151% and 119% for Macor and Quartz sensors has led to increase in strength of the temperature response of the sensors during dynamic calibration experiments. Though heat flux recovery is seen to have encouraging agreement for all the sensors, sensitivity enhancement is noticed to be more prominent and advantageous for Macor based sensors. Present studies recommend adequately finished substrate for better adhesion. Further, use of MWCNTs is advisable especially for low heat flux measurement and also for Macor substrate since either situation demands the higher sensitivity to increase the output response.

    更新日期:2019-12-18
  • Experimental analysis of condensation heat transfer and frictional pressure drop in a horizontal circular mini channel
    Heat Mass Transfer (IF 1.551) Pub Date : 2019-12-17
    Tejendra Patel, A. D. Parekh, P. R. Tailor

    The present study aims to investigate the effect of working fluid, mass flux, vapour quality and saturation temperature on local condensation Heat Transfer Coefficient (HTC) and local Frictional Pressure Drop (FPD) in a horizontal circular mini-channel. The refrigerants HFO-1234yf and HFC-R134a have been used as a working fluids. The mass flux was varied from 200 to 800 kg/m2 s whereas two distinct saturation temperatures were used: 35 °C and 40 °C. The experimental analysis was carried out using horizontal circular single port mini-channel of hydraulic diameter 1 mm. During experiment, inlet condition of refrigerant was maintained to be saturated vapour. The results show that HTC and FPD increases with increase in vapour quality and mass flux whereas decreases with increase in saturation temperature. The experiment results of present study and previously published results are compared with various predictive models. Models which show minimum deviation from experimental results were used to develop new modified model to predict HTC with MARD of ± 15 % and FPD with MARD of ± 10 %.

    更新日期:2019-12-18
  • Dynamics of obstructed droplet breakup in microfluidic T-junction based on diffuse interface method
    Heat Mass Transfer (IF 1.551) Pub Date : 2019-12-17
    Jian Chang, Jiejin Cai

    Gas-liquid two phases flow is common in microfluidics but still needs more study. Precise but efficient controlling the droplets volumes is important for subsequent reaction or processing. In this paper, the diffuse interface method and boundary condition equations are introduced at the beginning. Then, to validate the present phase-field model, three kinds of simulation results are shown and fitted well with the experiments. It indicates that this method can be used to simulate the droplet deformation process in the T-junction. Afterwards, the details about the three kinds of droplet behaviors are presented and due to the instability of droplet breakup with tunnels, we mainly focus on the droplet with t obstruction for the steady breakup process. By analyzing the sensitivity about capillary number and static contact angle, we find that they both have great influence on the breakup process. The fitted curve under a 45 degree contact angle agree well with the experiment of Fu et al. Chem Eng Sci 66(18) 4184-4195, [9] which verifies the accuracy of simulation again. Next, we fit the relation between scaled critical droplet length and the Ca number and the static contact angle. At last, the critical neck width observed in the experiment is discussed based on the minimum surface energy principle as supplementary to improve this two-dimensional formula.

    更新日期:2019-12-18
  • A novel spray cooling device based on a dual synthetic jet actuator integrated with a piezoelectric atomizer
    Heat Mass Transfer (IF 1.551) Pub Date : 2019-12-17
    Wei He, Zhenbing Luo, Xiong Deng, Zhixun Xia

    To solve the heat dissipation problem of electronic equipment, a novel spray cooling device based on a dual synthetic jet actuator integrated with a piezoelectric atomizer (DSJAPA) is proposed and designed. The flow field, spray characteristics and cooling performance of DSJAPA are investigated experimentally. The experimental results show that the spray angle increases with the increase of driving voltage, and it can achieve the maximum of 74°. Under a certain driving voltage, there exists an optimum driving frequency making the spray angle reach to the maximum. The spray flow field is investigated by droplet image velocimetry (PIV) system. The results show that the spray droplet velocity can be accelerated to about 22.5 m/s by the high-speed ejection of the jet. With the increase of dual synthetic jet velocity, the dense spray with low velocity changes into the dilute spray with high velocity. A dual synthetic jet actuator can improve the cooling capability of spray, reduce the temperature non-uniformity of surface (from 32 °C in spray cooling to 18 °C in DSJAPA cooling) and enlarges the direct impingement range (from (−30 mm, 25 mm) in spray cooling to (−60 mm, 57 mm) in DSJAPA cooling).

    更新日期:2019-12-18
  • A parametric fin structure design study for cooling aerospace electro-mechanical actuators with high-speed axial fans
    Heat Mass Transfer (IF 1.551) Pub Date : 2019-12-17
    Osama Mesalhy, Cody Rath, Daniel Rini, John Kizito, Quinn Leland, Louis Chow

    Despite their advantages compared to hydraulic actuators, electric actuators are prone to overheating due to their high heat dissipation. So, developing reliable cooling systems for electric actuators is a crucial task, especially for aerospace applications which require the fulfillment of high safety requirements. In this paper, an air-cooling system utilizing wing bay air is investigated. An axial fan sucks air to flow through a shrouded-fin surface attached to the motor housing. A CFD model is developed to study the heat transfer and air flow processes over the finned surface. The relation between the fan pressure jump with the volumetric flow rate of a high speed SUNON fan at different rotational speeds and ambient pressures is measured in a fan loop and incorporated in the model. To validate the CFD results, a test rig consisting of a finned surface brazed on a heated aluminum cylindrical block and attached to a fan is built. The predicted and measured temperatures at different locations in the aluminum block show good agreement when the numerical simulation is performed using the k-ω-SST turbulence model. The average discrepancy of the predicted and measured steady state temperature differences (T-T∞) reduces from 1.6 °C for the k-ε Realizable model to 0.31 °C for the k-ω SST model. Numerical simulation is performed to predict the effect of fin shape, fin number and fin thickness on the cooling performance of the fin structure. The results show that the straight plate fin configuration outperforms the offset-strip and corrugated fin. Also, it is found that there is an optimum value for the fin number, and this optimum fin number changes with the fan rotational speed and ambient pressure. Reducing the solidity of the fin structure by reducing the fin thickness results in improving the thermal performance when the fan operates at a low ambient pressure (0.2 atm). By comparing all data, it is found that the straight fin structure with 110 fins and fin thickness 0.2 mm is the optimum one. For fan speed of 12,000 rpm, this structure can restrict the thermal resistance between 0.17 °C/W at 0.2 atm and 0.047 °C/W at 1.0 atm.

    更新日期:2019-12-17
  • Heating of a water droplet on inclined transparent polydimethylsiloxane (PDMS) surface
    Heat Mass Transfer (IF 1.551) Pub Date : 2019-12-16
    Abdullah Al-Sharafi, Bekir S. Yilbas, Haider Ali

    Heat transfer from replicated hydrophobic and transparent PDMS surface to a water droplet is considered. The laser texturing of alumina surface is carried out to obtain hydrophobic surface and later the textured surface is replicated by polydimethylsiloxane (PDMS) via a solvent casting method. An experiment is carried out to assess the water droplet pinning on the replicated inclined PDMS surface while keeping the replicated surface at uniform temperature (308 K). The flow and temperature fields inside the inclined water droplet are simulated in line with the experimental conditions. The flow velocities inside the droplet are validated through the data obtained from the particle imaging velocimetry (PIV). It is found that the velocity predictions agree well with the PIV data. The droplet heating results in a circulation cell inside the droplet due to the Marangoni and the buoyancy currents. Increasing inclination angle of the PDMS replicated surface enhances the maximum velocity inside the droplet, which is more pronounced for the large size droplets. The Nusselt and the Bond numbers increase with the inclination angle of the PDMS surface.

    更新日期:2019-12-17
  • Performance of a Linde-Hampson refrigerator operating from –120 °C to –60 °C with optimised R14-hydrocarbon mixtures exhibiting vapour-liquid-liquid equilibria
    Heat Mass Transfer (IF 1.551) Pub Date : 2019-12-16
    Sunil Jerome, G. Venkatarathnam

    Single stage Linde-Hampson refrigerators that operate with R14-hydrocarbon mixtures can be used in place of traditional two/three stage vapour compression cascade refrigerators to provide refrigeration below–60 °C. The exergy efficiency of Linde-Hampson refrigerators can be increased by using compositions that exhibit vapour-liquid-liquid-equilibrium at low temperatures. The main aim of this work is to present the theoretical performance of a Linde-Hampson refrigerator operating with optimal R14-hydrocarbon mixtures from −120 to −60 °C. The results show that constant temperature refrigeration, however, cannot be provided above −80 °C with these mixtures. The use of nitrogen as an additional component below −100 °C is also investigated.

    更新日期:2019-12-17
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