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  • Macroscopic networks of thermal conduction: Failure tolerance and switching processes
    Int. J. Heat Mass Transf. (IF 3.458) Pub Date : 2018-01-10
    Jin Shang, Ruizhe Wang, Chen Xin, Gaole Dai, Jiping Huang

    The world is so wonderful and colorful thanks to the analytic discontinuities and singularities of phase transitions. Here we propose a class of macroscopic networks of thermal conduction that combine regular networks with macroscopic thermally conductive systems. We find that their phase transition phenomena (variation tendency) cannot be explained simultaneously by existing theories of networks, percolation or effective media. We report the bond-free property of these networks and the associated three switching processes caused by the geometric property of bonds, and we reveal the effect of single-point connection. Also, we propose some potential applications including thermal diodes. Our results are confirmed by finite-element method simulation and experiment. This work offers different insights into the theories of networks, percolation and effective media. It also provides a different method to design thermal metamaterials and manipulate thermal conductivities.

    更新日期:2018-01-15
  • Numerical investigation of spacer effects on heat transfer of supercritical fluid flow in an annular channel
    Int. J. Heat Mass Transf. (IF 3.458) Pub Date : 2018-01-10
    Yao Xiao, Jinsong Pan, Hanyang Gu

    This paper conducts a numerical investigation of spacer effects on heat transfer of supercritical R-134a flows in a vertical annular channel. The simulations are conducted with SST k-ω turbulent model in Fluent 15.0. The investigation range is in the normal and improved heat transfer region at supercritical pressure. The results show that the spacer end enthalpy has remarkably influence on the spacer effects at supercritical pressure, which is different from that at subcritical pressure. In the liquid-like region, the enhancement effectiveness of spacers increases with increasing spacer end bulk enthalpy. However, in the in the gas-like region, the enhancement effectiveness of spacers decreases with increasing spacer end bulk enthalpy. The enhancement effectiveness reaches a peak value near the pseudo-critical enthalpy. For the parameters sensitivity, besides the blockage ratio, spacer end enthalpy and dimensionless distance to the spacer end, the flow parameters and local enthalpy also have significant influences on the spacer effects. Mechanism analysis shows that the characteristic of the mechanism of disruption of boundary layer is the main cause for the appearance of peak value of enhancement effectiveness near the pseudo-critical region at supercritical pressure. The HTC ratio of bare channels can be used to help the prediction of the HTC ratio in the corresponding spacer downstream.

    更新日期:2018-01-15
  • Stability analysis of transmission tower foundations in permafrost equipped with thermosiphons and vegetation cover on the Qinghai-Tibet Plateau
    Int. J. Heat Mass Transf. (IF 3.458) Pub Date : 2018-01-10
    Lei Guo, Zhongqiu Zhang, Xinbin Wang, Qihao Yu, Yanhui You, Chang Yuan, Yanli Xie, Tingtao Gou

    During the construction of the ±400 kV direct current power transmission line (DCPTL), frozen blocks were backfilled into the foundation pits in permafrost regions because of the lack of backfilling materials and other problems, but this resulted in less compact backfilled soils. To ensure the stability of the tower foundations, a large number of thermosiphons were installed. This study discusses the threat to the stability of tower foundations of water infiltration along the large voids created within the backfilled soils by the use of frozen blocks, and quantifies the efficacy of a combination of thermosiphons and vegetation cover in enhancing tower stability, based on field collected data from January 2011 to April 2017. The results indicate that the cooling effects of thermosiphons caused a large amount of net heat removal from the foundation soils, even during the first operational year of the foundation, while foundation soils without thermosiphons exhibited net heat input during the same period. Ponding in the pits and downward infiltration of water obviously work to warm the foundation soils, and can result in the settlement of the tower footings, threatening the tower stability. The combination of thermosiphons and vegetation cover is shown to effectively cool the foundation soils and to reduce the settlement of the footings, thus ensuring the continued tower stability. This study also shows that the backfilling of frozen blocks should be avoided in the current climate conditions, even though effective cooling measures like thermosiphons are used because the downward infiltration of water along the large voids between the frozen backfilled blocks can’t be totally prevented. If the current degradation of the frozen state of the backfilled soils continues, the infiltrated water, which has been frozen at the bottom of the backfill, will begin to thaw again, threatening the stability of the tower foundations.

    更新日期:2018-01-15
  • Shear-driven two colliding motions of binary double emulsion droplets
    Int. J. Heat Mass Transf. (IF 3.458) Pub Date : 2018-01-10
    Xiangdong Liu, Chengyao Wang, Yuanjin Zhao, Yongping Chen
    更新日期:2018-01-15
  • Numerical study on novel airfoil fins for printed circuit heat exchanger using supercritical CO2
    Int. J. Heat Mass Transf. (IF 3.458) Pub Date : 2018-01-10
    Xinying Cui, Jiangfeng Guo, Xiulan Huai, Keyong Cheng, Haiyang Zhang, Mengru Xiang

    The Printed Circuit Heat Exchanger (PCHE) has attracted a lot of attentions recently due to its high efficiency and compactness, and the airfoil fin has excellent thermal–hydraulic performance compared with the zigzag channel in PCHE. Two novel fins were proposed in this study based on the configuration of NACA 0020 airfoil fin, in order to further improve the performance of airfoil fin PCHE. The thermal–hydraulic performance and the enhancement mechanism of the fins were numerically investigated using supercritical CO2 as the working fluid. The results showed that one of the novel fins has better comprehensive performance and less entransy dissipation than the NACA 0020 airfoil fin used in PCHE, since the j factor of this novel fin is 2.97–6.15% larger, and the pressure drop is 0–4.07% less than the NACA 0020 airfoil fin under the selected conditions. And the staggered arrangement and the proper shape of fins could reduce the effect of boundary layer and improve the thermal–hydraulic performance effectively. In addition, the enhancement mechanism of the novel fin PCHE could be explained with field synergy principle very well.

    更新日期:2018-01-15
  • Heat transfer and friction factor of nanodiamond-nickel hybrid nanofluids flow in a tube with longitudinal strip inserts
    Int. J. Heat Mass Transf. (IF 3.458) Pub Date : 2018-01-10
    L. Syam Sundar, Manoj K. Singh, Antonio C.M. Sousa

    The heat transfer and friction factor of a new kind of nanodiamond-nickel (ND-Ni) hybrid nanofluids flow in a tube with and without longitudinal strip inserts were studied experimentally. The hybrid ND-Ni nanoparticles were synthesized using in-situ and chemical coprecipitation method and characterized with different techniques. The bulk quantity of hybrid ND-Ni nanofluids were prepared and used for heat transfer and friction analysis. The experiments were conducted in the Reynolds number range from 3000 to 22,000, for the volume concentrations of 0.1% and 0.3%, and for different aspect ratios (AR = 1, 2 and 4) of the longitudinal strip inserts. The Nusselt number is enhanced by 35.43% (without insert) and further enhanced by 93.30% with longitudinal strip insert (AR = 1) for 0.3% volume concentration of nanofluid and at Reynolds number of 22,000 compared to water data. The friction factor penalty of 1.12-times without inserts and further penalty of 1.248-times with longitudinal strip insert (AR = 1) at 0.3% volume concentration of nanofluid and at Reynolds number of 22,000 compared to water data. The obtained experimental Nusselt number of hybrid nanofluids was compared with that of other kind of hybrid nanofluids available in the literature. The Nusselt number and friction factor correlations were developed based on the experimental data.

    更新日期:2018-01-15
  • Numerical investigation of droplet spreading and heat transfer on hot substrates
    Int. J. Heat Mass Transf. (IF 3.458) Pub Date : 2018-01-10
    Yongpan Cheng, Fan Wang, Jinliang Xu, Dong Liu, Yi Sui
    更新日期:2018-01-15
  • A fractal method to calculate the permeability for compressible gas flow through a porous restrictor in aerostatic bearings
    Int. J. Heat Mass Transf. (IF 3.458) Pub Date : 2018-01-10
    Hailong Cui, Yang Wang, Min Zhang, Wei Wang, Chunjiang Zhao

    In this study, a fractal method was applied to calculate the permeability for compressible gas flow through a porous restrictor in aerostatic bearings. A mathematical permeability model without any empirical parameters was established according to the fractal theory, gas conservation equations and Darcy's Law. The box-counting method was applied to determine the pore area fractal dimension and tortuosity fractal dimension. The effects of the scanning magnification and image size on the calculation results of the fractal dimension were also investigated. The porosity characteristics of the scanning images have obvious self-similarity, and the diameters of the maximum pore and minimum pore satisfy the fundamental condition of fractal analysis. Magnification ranges of 50–100× × and 1–2k× × were appropriate for calculating the permeability of ceramic and graphite porous restrictors, respectively. Image pixels of 512×512 512 × 512 were sufficient to ensure the accuracy of permeability calculation. In addition, an experimental platform was constructed to measure the permeability of ceramic and graphite porous restrictors. The differences between the calculation results and the experimental data for both ceramic and graphite porous restrictors were less than 10%, which demonstrated that the permeability for compressible gas flow through a porous restrictor in aerostatic bearings can be accurately predicted by the fractal calculation method.

    更新日期:2018-01-15
  • On the oscillatory modes of compressible thermal convection in inclined differentially heated cavities
    Int. J. Heat Mass Transf. (IF 3.458) Pub Date : 2018-01-10
    Marcello Lappa, Tomislav Gradinscak

    We consider the case of a tall differentially heated cavity (containing air) for which the buoyancy flow is in the so-called boundary-layer regime (displaying thin regions featured by strong temperature gradients adjacent to the vertical walls and thermally stratified fluid in the centre). By using a numerical method able to account for compressibility and variable viscosity effects, we investigate the response of such a system to the application of temperature differences in a range of characteristic numbers for which the resulting flow is expected to be time-periodic or highly time-dependent. More precisely, the problem is examined by allowing both the Rayleigh number and the inclination (θ) of this configuration with respect to the vertical direction to span relatively wide intervals (1.8 × 105 ≤ Ra ≤ 3 × 106, −90° ≤ θ ≤ 90°). Insights into the influence of variable thermodynamic properties are obtained by comparing such numerical results with equivalent simulations based on the classical Boussinesq approximation. The evolution of the considered system towards a fully turbulent state is also discussed to a certain extent.

    更新日期:2018-01-15
  • Experimental study on mixing and stratification of buoyancy-driven flows produced by continuous buoyant source in narrow inclined tank
    Int. J. Heat Mass Transf. (IF 3.458) Pub Date : 2018-01-10
    Tao Du, Dong Yang, Haibin Wei, Zhongjie Zhang

    In this study, a series of experiments are performed to investigate the mixing and stratification of buoyancy-driven flows in a narrow inclined tank. The buoyancy-driven flow is produced by continuously releasing dense brine into an inclined tank filled with fresh water. Two distinct mixing regimes are identified. In one of the regimes, fresh water enters the tank because of the negative pressure induced by the brine plume. In the other regime, the fresh water inflow is mainly caused by the stack effect. A light attenuation technique is used to measure the density stratification and distribution in the tank. The effects of the buoyant source volume flow rate, source buoyancy flux, and source location on the mixing and stratification are investigated. The results indicate that the mixing between the buoyant and ambient fluids is strengthened by the increase in the source volume flow rate and the height difference between the source location and the lower end of the tank. However, the current downstream of the source becomes more stratified as the source buoyancy flux increases. A dimensionless parameter, λ λ , is proposed to evaluate the overall mixing intensity in the tank. As λ λ increases, the flow changes from a bidirectional one to a unidirectional one. The evolution of reduced gravity along the longitudinal direction is also investigated. The results indicate that fresh water is entrained into the brine layer if the thickness of the brine layer is less than the tank height. Otherwise, the reduced gravity remains constant along the longitudinal direction of the tank.

    更新日期:2018-01-15
  • Enhanced safety of geologic CO2 storage with nanoparticles
    Int. J. Heat Mass Transf. (IF 3.458) Pub Date : 2018-01-11
    Harpreet Singh, Akand Islam

    Some methods have been developed to detect leakage of CO2 from its desired storage domain, but that is not sufficient to prevent and mitigate a leak. Two techniques have been proposed to prevent the migration of buoyant CO2 from the storage domain by expediting mixing of CO2 with the brine and mitigate risk of its leakage risk. These two methods are injection of CO2 pre-mixed with brine, and injection of CO2 with nanoparticles (NPs). The former has been studied to some extent, however, understanding of the latter is very limited. Unlike the application of NPs in hydrocarbon recovery, its use to enhance safety of CO2 storage is a fairly unexplored topic that can have important benefits for the safety of the storage process. Also, the use of NPs for subsurface application in general is compromised for its cost. We investigate how NPs produced from low-level nuclear waste can be added with injected CO2 to enhance the mixing of CO2 with brine, which can mitigate leakage risk of CO2. We numerically investigate the effect of adding NPs from nuclear waste with the CO2 and show that it enhances the mixing of CO2 with in-situ brine in saline aquifers that mitigates the risk related to buoyancy and high mobility of CO2. Additionally, we examine the effect of reservoir heterogeneity on mixing of CO2 in reservoir brine when it is injected with NPs. The results show that: (i) addition of NPs to CO2 leads to higher mixing, (ii) the discrete shape of CO2 concentration in brine tends to diffuse and become smooth as the heterogeneity of the medium increases, and (iii) the impact of heterogeneity is more pronounced than the fraction of NPs on mixing.

    更新日期:2018-01-15
  • Hydrodynamic effects on the aggregation of nanoparticles in porous media
    Int. J. Heat Mass Transf. (IF 3.458) Pub Date : 2018-01-11
    Ngoc H. Pham, Dimitrios V. Papavassiliou

    The aggregation of spherical nanoparticles as they propagate through porous media is explored using lattice Boltzmann simulations and tracking of the trajectoties of individual particles. The porous media are modeled as periodic arrays of spheres in different packing configurations. The effects of interparticle interactions on particle aggregation are treated through a single aggregation probability upon particle collision. Fast aggregation is represented by an aggregation probability with values close to one, and slow aggregation is represented by lower aggregation probability. An algorithm that accounts for the different time scales between hydrodynamics and Brownian motion is devised and validated. It is found that primary particle size, initial particle concentration, injection flow rate, and aggregation probability significantly impact the dynamics of the aggregation. Calculations of the transient mean size of the aggregates show that the aggregation rate is high near the entrance of the porous medium, yet dramatically decreases when moving farther downstream. The reason is that the growth of larger aggregates slows the aggregation process. It is also found that the bigger aggregates are formed in the pore space near the pore matrix surface, where particle residence time is long enough to allow the aggregates to grow. An empirical correlation based on measurable parameters is proposed for the prediction of the mean aggregate size.

    更新日期:2018-01-15
  • Hybrid LES/DNS of turbulent forced and aided mixed convection to a liquid metal flowing in a vertical concentric annulus
    Int. J. Heat Mass Transf. (IF 3.458) Pub Date : 2018-01-11
    L. Marocco

    In the present study fully-developed turbulent forced and mixed convection heat transfer to a liquid metal flowing upwards in a concentric annulus is numerically investigated by means of Large Eddy Simulation (LES). The inner-to-outer radius ratio is 0.5. The Reynolds number based on bulk velocity and hydraulic diameter is 8900, while the Prandtl number is set to a value of 0.026. A uniform and equal heat flux is applied on both walls. Three different buoyancy strengths are considered, corresponding to onset of turbulence reduction, maximum impairment and recovery condition on the inner wall while recovery and enhancement develop on the outer wall. Due to the difference between thermal and hydrodynamic turbulent scales in liquid metals it is shown that with the same grid resolution a LES is performed for the flow field and at the same time a “thermal“ Direct Numerical Simulation (DNS) for the temperature field. From a detailed analysis of the two-point correlation functions of velocity and temperature fluctuations it emerges that a streamwise extent of 25δ δ and 40δ δ (being δ δ the half gap width) is necessary for forced and mixed convection, respectively, while a quarter of circumference is enough in azimuthal direction for this radius ratio, Reynolds and Prandtl number. Comparison of the forced convection flow field with available DNS simulations shows very good agreement. Nusselt numbers evaluated from the few available literature correlations for liquid metals flowing in an annulus give unsatisfactory results, mainly on the inner wall. The mixed convection results are thoroughly analyzed and discussed in terms of friction factor, Nusselt number, first and second order statistics, budgets of turbulent kinetic energy and budgets of temperature variance. The obtained data are also useful for validating Reynolds-Averaged turbulence models. Moreover, simulations with two coarser grids at the condition of maximum turbulence reduction are also compared with the reference results of the fine LES. It results that when turbulence is impaired the grid resolution in circumferential direction can be strongly coarsened.

    更新日期:2018-01-15
  • 更新日期:2018-01-15
  • Effects of continuous wavy ribs on heat transfer and cooling air flow in a square single-pass channel of turbine blade
    Int. J. Heat Mass Transf. (IF 3.458) Pub Date : 2018-01-11
    Longfei Wang, Songtao Wang, Fengbo Wen, Xun Zhou, Zhongqi Wang

    A wavy rib with the simple structure is proposed for the internal cooling channel in turbine blade, in order to enhance heat transfer and reduce pressure loss. The heat transfer performance and flow characteristics of a single-pass, stationary channel with wavy ribs are studied by numerical method. The channel width is 12.7 mm, with an aspect ratio of 1. Four major geometric parameters of wavy rib, including rib height (he = e−3e), rib round radius (r  = 0–5 mm), rib angle (α α = 20–55°) and rib thickness (th = 0.5e−2e) are investigated in optimal design. Furthermore, the potential high-performance wavy ribs are explored as well. The investigated Reynolds number is 10,000–40,000. Meanwhile, the typical 45° V-shaped ribs which have high heat transfer improvement are selected as the reference scheme of wavy rib. Both the rib height (e) and rib width are 1.58 mm, and the rib pitch P/e is 10. From the results, flow behavior of cooling air is presented, then heat transfer performance and thermal performance of channel walls, including ribbed walls and side walls, are analyzed. The results show that the rib height, rib round radius and rib angle have great impact on heat transfer and flow of channel, while the influence of rib thickness is relatively small. Heat transfer performance and pressure penalty are positively correlated with rib height and rib angle, while negatively correlated with rib round radius. As a result, wavy ribs with high rib height and large rib round radius perform better in increasing heat transfer and decreasing pressure drop. The diversion effect of wavy ribs on cooling air is responsible for friction penalty reduction. Due to the advantages of saving space and reducing pressure loss, small rib angle is beneficial to improve the performance of wavy ribs as well. In comparison with 45° V-shaped ribs, high-performance wavy ribs induce that ribbed wall Nu/Nu0 Nu / Nu 0 and ribbed wall area improve by 7–37% and 28–52%, respectively, without friction loss increasing. It is indicated that the wavy rib appears to be an effective method of heat transfer improvement in internal cooling channels.

    更新日期:2018-01-15
  • Experimental and numerical study about local heat transfer in a microchannel with a pin fin
    Int. J. Heat Mass Transf. (IF 3.458) Pub Date : 2018-01-11
    Yingying Wang, Jeong-Heon Shin, Corey Woodcock, Xiangfei Yu, Yoav Peles

    Local single-phase flow heat transfer downstream a single pin fin in a microchannel was experimentally and numerically studied. Three distinct flow regimes, depending on the Reynolds number, were characterized, namely: laminar flow with steady wake, laminar flow with unsteady wake, and turbulent flow. Local temperature measurements with high spatial resolution were obtained by incorporating an array of micro resistance temperature detectors (RTDs) (∼55 µm × 55 µm) on the internal microchannel surface. Local surface temperatures were related to the flow structures under different flow regimes. An enhanced local heat transfer coefficient at the trailing edge of the wake region downstream the pillar was observed. It is believed to be a result of vortex shedding and large-scale flow mixing triggered by flow instability at high Reynolds number. The numerical model enabled a full conduction/convection conjugate analysis of the entire system including heat conduction within the solid substrates and heat losses to the surrounding environment. Local heat transfer coefficient downstream the pin fin at each Reynolds number was obtained.

    更新日期:2018-01-15
  • Theoretical and experimental research of thermal conductivity of silver(Ag) nanowires in mesoporous substrate
    Int. J. Heat Mass Transf. (IF 3.458) Pub Date : 2018-01-11
    Jing Li, Yanhui Feng, Xinxin Zhang, Ge Wang

    Thermal conductivities of Ag nanowires with different sizes and temperatures were studied theoretically. The lattice and electronic thermal conductivities of Ag nanowires include contributions from interface scattering and grain boundary scattering. While the molecular dynamic simulation was used to get the contribution of interface scattering to the lattice thermal conductivity of Ag nanowires along the axial direction, a model derived from Boltzmann transport equation and the Wiedemann-Franz relation were applied to calculate electronic thermal conductivities of Ag nanowires along the axial direction. And then coupling with the lattice and electronic thermal conductivities, the effective thermal conductivity of Ag nanowire along the axial direction was obtained. Furthermore, Kubo linear-response formalism was used to approximately predict the thermal conductivity in the plane perpendicular to the axis of the Ag nanowire. It turns out that effective thermal conductivities of Ag nanowire in the r-θ plane and along the Z direction show a great anisotropy. Finally, by measuring the thermal conductivities of Ag/SBA-15, the thermal conductivity of a single Ag nanowire can be obtained by using PWDM model, and the theoretical thermal conductivities of Ag nanowire were compared with the experimental results.

    更新日期:2018-01-15
  • Effect of two isothermal obstacles on the natural convection of nanofluid in the presence of magnetic field inside an enclosure with sinusoidal wall temperature distribution
    Int. J. Heat Mass Transf. (IF 3.458) Pub Date : 2018-01-11
    Ahmad Hajatzadeh Pordanjani, Akram Jahanbakhshi, Afshin Ahmadi Nadooshan, Masoud Afrand

    In this study, the effect of magnetic field on the natural convection of Al2O3-water nanofluid inside a square enclosure with isothermal obstacles and sinusoidal wall temperature distribution has been numerically studied. The sidewalls were subject to sinusoidal boundary conditions, while the top and bottom walls were insulated. Two isothermal heat sources were implemented within the enclosure at the same distance from the center. The governing equations were transformed into the algebraic form using finite volume method and were then simultaneously solved using the SIMPLE algorithm. The proposed model by Vajjha was used to calculate the coefficient of thermal conductivity by taking Brownian motion of particles into account. In this study, the effect of Rayleigh number, aspect ratio, Hartmann number, direction change of applied magnetic field, and the volumetric percentage of nanoparticles was investigated. The results indicated that by increasing the Hartmann number, the fluid velocity as well as the Nusselt number decreased at all volumetric fractions of nanoparticles. An increase in the volumetric fraction of nanoparticles increased the Nusselt number, so that at a nanoparticle concentration of 6%, the mean Nusselt number increased by 9.04% compared to that of the base fluid. Moreover, the Nusselt number increased by increasing the magnetic field angle and the Rayleigh number, while it decreased as the aspect ratio was increased.

    更新日期:2018-01-15
  • Molecular dynamics simulation on moisture diffusion process for drying of porous media in nanopores
    Int. J. Heat Mass Transf. (IF 3.458) Pub Date : 2018-01-12
    Yuejin Yuan, Jin Zhang, Dong Wang, Yingying Xu, Bhesh Bhandari

    Aiming at the problem that the mechanism of the moisture transfer in the nanopores of porous media has not been researched clearly. In this work, the molecular dynamics model of moisture diffusion in nanopores of porous media was established and the moisture diffusion process was analyzed by the molecular dynamics (MD) method. The Kelvin effect of water diffusion in nanopores of porous media was simulated and the model was validated by drying of potato slices. The results of simulation and verification showed that the molecular dynamics model could simulate and calculate the moisture diffusion coefficient of potato slices well. The maximum relative error of the diffusion coefficient between the simulation and experimental results is 8.6%. During the drying process, the temperature, pore size, wall roughness and the phase area fraction have important effects on the water diffusion in the nanopores. With the increase of the temperature, pore size and phase area fraction, and with the decrease of the wall roughness, the diffusion coefficients of porous media drying increased accordingly.

    更新日期:2018-01-15
  • Optimizing fin design for a PCM-based thermal storage device using dynamic Kriging
    Int. J. Heat Mass Transf. (IF 3.458) Pub Date : 2018-01-09
    Mike Augspurger, K.K. Choi, H.S. Udaykumar

    A key challenge in the development of a practical thermal storage device (TSD) is the low thermal conductivity of common phase change materials (PCM). This low conductivity impedes both heat input and extraction. The most common solution is to use conductive metal fins to spread heat through the device. However, optimizing the effectiveness of the container and the fin arrangement is difficult due to the large number of potential design parameters. This paper develops a strategy to make simulation-based optimization process affordable and accurate. First, numerical techniques are designed to accurately and efficiently compute heat and mass transport in a variety of geometries without generating grids to conform to each geometry. This facilitates rapid prototyping and mitigates the expense of individual simulations. Second, a pre-screening process identifies the independent variables with the largest and most nonlinear effect on the objective function in the optimization process, thus narrowing the parameter space. Finally, a dynamic Kriging-based optimization approach constructs a multidimensional response surface using sparse input datasets; the response surface is then used to identify an optimal design. The combination of the above three strategies is shown to result in an approach that can aid in the design of optimal thermal storage devices that rely on a mixture of PCM and metal fins.

    更新日期:2018-01-10
  • Flow boiling of R245fa in a microgap with staggered circular cylindrical pin fins
    Int. J. Heat Mass Transf. (IF 3.458) Pub Date : 2018-01-09
    Pouya Asrar, Xuchen Zhang, Craig E. Green, Muhannad Bakir, Yogendra K. Joshi

    In this study, flow boiling of refrigerant R245fa is investigated in a microgap of height 200 μm populated with a staggered pin fin array of diameter 150 μm and spacing 200 μm. For heat fluxes up to 498 W/cm2, mass flux values up to 7896 kg/m2 s, and inlet temperatures of 13 °C and 18 °C, average two-phase heat transfer coefficient up to 60 kW/m2 K are measured. High speed flow visualizations at frame rate of 2229 fps elucidate the flow boiling patterns inside the microgaps, including bubbly and foggy that are generated in the pin finned area. Surface temperatures are measured for heat fluxes up to 498 W/cm2 which enable determination of heat transfer characteristics.

    更新日期:2018-01-10
  • VIV study of an elastically mounted cylinder having low mass-damping ratio using RANS model
    Int. J. Heat Mass Transf. (IF 3.458) Pub Date : 2018-01-09
    Niaz B. Khan, Zainah Ibrahim, M. Ijaz Khan, T. Hayat, Muhammad F. Javed

    This study addresses vortex-induced vibration analysis by an elastic rigid cylinder oscillating free in cross flow direction subject to low mass damping ratio. Two-dimensional Reynold-Averaged Navier-Stokes equations (RAN) for this purpose are simulated. Analysis is carried out for range of reduced velocity = 2–16 which corresponds to Reynolds number 1700–14,000. The mass-ratio is 2.4 and mass-damping ratio 0.0013. Previously, similar studies have been performed numerically but are limited to achieve maximum cylinder response. In the current study, comparatively higher value of maximum amplitude of cylinder is computed. However delay in transition from ‘upper branch’ to ‘lower branch’ is noticed. In addition to maximum amplitude, other hydrodynamic coefficients are also discussed. The results extracted from current simulations are compared with previous experimental studies in literature.

    更新日期:2018-01-10
  • Energy analysis of droplet jumping induced by multi-droplet coalescence: The influences of droplet number and droplet location
    Int. J. Heat Mass Transf. (IF 3.458) Pub Date : 2018-01-09
    Fuqiang Chu, Zhiping Yuan, Xuan Zhang, Xiaomin Wu

    Self-propelled droplet jumping phenomenon during condensation on superhydrophobic surfaces has various engineering applications such as heat transfer enhancement, hotspot cooling and anti-icing/frosting. Since most of the droplet jumping phenomena are induced by multi-droplet rather than two-droplet coalescence, understanding the influences of droplet number and multi-droplet location on the droplet jumping is essential. In this work, droplet jumping induced by multi-droplet coalescence was simulated using the volume of fluid (VOF) method with various coalesced droplet number and locations considered. The simulations showed visualized droplet morphology evolutions during the droplet jumping processes. The changes of several energies, such as surface energy, kinetic energy and jumping energy, were also discussed with the energy conversion efficiencies calculated. The results show that increasing the coalesced droplet number is advantageous for surface energy releasing and the energy conversion efficiency from the surface energy to the jumping energy also increases with increasing coalesced droplet number. Coalescence of droplets with more concentrated location distribution has weaker oscillation during the jumping process, along with less oscillatory kinetic energy, indicating that concentrated droplet distribution is conductive to increase the energy conversion efficiency from the surface energy to the jumping energy.

    更新日期:2018-01-10
  • Analysis and predictive modeling of nanofluid-jet impingement cooling of an isothermal surface under the influence of a rotating cylinder
    Int. J. Heat Mass Transf. (IF 3.458) Pub Date : 2018-01-08
    Fatih Selimefendigil, Hakan F. Öztop

    In this paper, numerical study and thermal prediction for a nanofluid jet impingement cooling of an isothermal hot surface with an adiabatic rotating cylinder were performed. Finite volume method was used for the solution of resulting governing equations along with the boundary conditions. Influence of various pertinent parameters such as Reynolds number (between 100 and 400), angular rotational velocity of the cylinder (between −0.1 and 0.1), horizontal location of the cylinder (between 0 and 3.75w) and solid particle volume fraction (between 0 and 0.04) on the fluid flow thermal characteristics were examined. It was observed that cylinder rotation and its location affect the cooling performance of the hot surface. It can be used as control element for heat and fluid flow. At the highest angular rotational speed as compared to motionless cylinder case, average Nusselt number reduces by about 20.16% % for clockwise rotation. Solid particle addition to the base fluid affects the variation of first and secondary peaks in the Nusselt number along the hot wall. At the highest solid when the cylinder is away from the inlet slot and average Nusselt number enhancement is by about 8.08% % at the highest volume fraction. An efficient modeling strategy was developed based on proper orthogonal decomposition and radial basis neural networks for thermal predictions. Accurate and fast results were achieved as compared to high fidelity computational fluid dynamics simulation results.

    更新日期:2018-01-09
  • Two-probe calibration integral equation method for nonlinear inverse heat conduction problem of surface heat flux estimation
    Int. J. Heat Mass Transf. (IF 3.458) Pub Date : 2018-01-08
    Hongchu Chen, Jay I. Frankel, Majid Keyhani

    In this paper, a new two-probe surface heat flux calibration integral equation method (CIEM) is proposed for resolving nonlinear inverse heat conduction problem possessing temperature-dependent thermophysical properties and an unknown time-varying back surface boundary condition. Compared to the one-probe CIEM, the two-probe CIEM can be applied to a more general class of one-dimensional inverse heat conduction problems that can be used for accurately predicting the front surface heat flux without any restrictions on the back surface boundary condition. Temperature-dependent thermophysical properties and probe locations are not specified a priori but are implicitly accounted through a calibration campaign. The final mathematical formulation involves resolving a Volterra integral equation of the first kind for the unknown surface heat flux. A first kind Chebyshev expansion possessing undetermined coefficients, is used for approximating the introduced property transform function. Next, three well-defined calibration tests are used for estimating the undetermined coefficients associated with the Chebyshev expansion. A time sequential study of the coefficients is shown to yield the optimal truncation of the series expansion. A future-time method is applied for stabilizing the ill-posed first kind Volterra integral equation. Phase plane and cross-correlation analyses are employed for estimating the optimal regularization parameter (i.e., the future-time parameter) from a spectrum of chosen values. The effectiveness of the proposed approach is verified by numerical simulation using stainless steel as the host material. The calibration-based or “parameter free” inverse approaches significantly reduce time and costs in the resolution of the surface condition as there is no need to experimentally determine the thermophysical properties, probe locations, and probe characteristics. These system properties are implicitly included in the proposed calibration procedure. Many applications, especially in aerospace sciences where exotic materials are often used, do not have replicable properties among samples. Conventional inverse methods rely on accurately specifying the input parameters for forming the predictions. These “parameter required” methods are solely based on numerical algorithms and thus their predictions are strongly relied on accurate input of parameters and measured probe data.

    更新日期:2018-01-09
  • Numerical simulation of flow patterns and the effect on heat flux during R32 condensation in microtube
    Int. J. Heat Mass Transf. (IF 3.458) Pub Date : 2018-01-08
    Chunxu Wu, Junming Li

    Based on the fluid volume approach, a transient numerical model for the condensation heat transfer and flow features in a microchannel is proposed. The flow condensation of R32 in a circular microtube with 0.1 mm diameter was studied. Four typical flow patterns, annular, injection flow, slug flow and bubbly flow are simulated along a two-dimension calculational domain successively. The numerical mode is verified by the experiments from the literature. The numerical results discern that the increase of mass flux, wall temperature and saturation temperature affect the detachment point of vapor slug further toward the outlet with higher occurrence frequency, which can be attributed to the higher Weber number and Capillary number of tail vapor core respectively. The local heat flux and wall shear stress will decrease along the flow direction overall, and tend to be constant in the single phase liquid area. However, there exit some rebounds and oscillations of the local heat flux and wall shear stress during injection and slug flow. The transient oscillations of wall shear stress can induce fluctuations and even waves in the annular flow upstream, which can be concluded as “surface tension force affecting upstream”, is proposed based on the minimum potential energy theory. This mechanism could be a supplement for the traditional theory of “flow pattern transition at high mass flux being induced by fluctuations which grow up while flowing downstream”.

    更新日期:2018-01-09
  • Improving temperature uniformity of a lithium-ion battery by intermittent heating method in cold climate
    Int. J. Heat Mass Transf. (IF 3.458) Pub Date : 2018-01-08
    Zhiguo Lei, Yuwen Zhang, Xueguo Lei

    The charge-discharge performances of lithium-ion batteries in hybrid electric vehicles (HEVs) and pure electric vehicles (EVs) decline rapidly at low temperatures. Many heating methods have been proposed to improve low-temperature performance, but these heating methods require long heating time and lithium-ion batteries at the end of heating have poor temperature uniformity. Recently, a self-heating lithium-ion battery (SHLB) has been proposed to recover charge-discharge performances of lithium-ion batteries at low temperature in short time. However, temperature uniformity of a lithium-ion battery heated by SHLB is also poor. A three-dimension heating finite element model is established in this work to analyze temperature gradient of a lithium-ion battery heated by SHLB heating method in detail, and intermittent SHLB heating method is proposed. For the intermittent SHLB heating method, a lithium-ion battery is heated for some time and stopped heating for some time instead of continuing heating. Through simulation analysis and comparison, heating for 0.1 s and stopping heating for 0.3 s is proposed to decrease the temperature gradient, and temperature difference is decreased from 10–11 K to 2–3 K.

    更新日期:2018-01-09
  • A numerical model for predicting distributions of pressure and temperature of superheated steam in multi-point injection horizontal wells
    Int. J. Heat Mass Transf. (IF 3.458) Pub Date : 2018-01-08
    Fengrui Sun, Yuedong Yao, Xiangfang Li, Guozhen Li, Zheng Sun

    In order to make full use of the advantages of superheated steam (SHS), SHS injection in multi-point injection wells (MPIW) is proposed in this paper. Firstly, a mathematical model comprised of SHS flow model in inner tubing (IT) or long tubing (LT) and annulus, transient heat transfer model in oil layer is established. Secondly, type curves of SHS flow in MPIW is obtained by finite difference method on space and the iteration technique. Then, the effect of injection temperature on distributions of thermophysical properties of SHS in MPIW is discussed in detail. Results show that: (a) When the heat exchange between IT and annulus is taken into consideration. SHS temperature in IT has a decrease while SHS temperature in annulus has an increase. (b) With the help of MPIW, heating effect at both heel and toe points of the horizontal wellbores can be enhanced. (c) While the increase of SHS temperature certainly benefits the formation heating effect through the increase of both SHS temperature and superheat degree, the following decrease of SHS pressure in annulus will lead to the decrease of SHS absorption rate. This paper unravels some intrinsic flow characteristics of SHS in MPIW, which has a significant impact on the optimization of SHS injection parameters and analysis of heat transfer law in MPIW.

    更新日期:2018-01-09
  • Bubble nucleation in superhydrophobic microchannels due to subcritical heating
    Int. J. Heat Mass Transf. (IF 3.458) Pub Date : 2018-01-06
    Adam Cowley, Daniel Maynes, Julie Crockett, Brian D. Iverson

    This work experimentally studies the effects of single wall heating on laminar flow in a high-aspect ratio superhydrophobic microchannel. When water that is saturated with air is used as the working liquid, the non-wetted cavities on the superhydrophobic surfaces act as nucleation sites and allow air to effervesce out of the water and onto the surface when heated. Previous works in the literature have only considered the opposite case where the water is undersaturated and absorbs air out the cavities for a microchannel setting. The microchannel considered in this work consists of a rib/cavity structured superhydrophobic surface and a glass surface separated by spacers. The microchannel is 60 mm long by 14 mm wide and two channel heights of nominally 183 μm and 366 μm are explored. The superhydrophobic side is in contact with a heated aluminum block and a camera is used to visualize the flow through the glass side. Thermocouples are embedded in the aluminum to record the temperature profile along the length of the channel. Temperatures are maintained below the boiling temperature of the working liquid. The friction factor-Reynolds product (fRe) is obtained via pressure drop and volumetric flow-rate measurements. Five surface types/configurations are investigated: smooth hydrophilic, smooth hydrophobic, superhydrophobic with ribs perpendicular to the flow, superhydrophobic with ribs parallel to the flow, and superhydrophobic with ribs parallel to the flow with several breaker ridges perpendicular to the flow. The surface type/configuration has a significant impact on the mass transport dynamics. For surfaces with closed cell micro-structures, large bubbles eventually form and adversely affect fRe and lead to higher temperatures along the channel. When degassed water is used, no bubble nucleation is observed and the air initially trapped in the superhydrophobic cavities is quickly absorbed by the water.

    更新日期:2018-01-07
  • Experimental study on evolutions of temperature and height of waterfilm during static flash
    Int. J. Heat Mass Transf. (IF 3.458) Pub Date : 2018-01-06
    Dan Zhang, Qingzhong Yang, Ting Liang, Junjie Yan

    The temperature and height of waterfilm were a pair of coupled characteristic parameters indicating the instant heat/mass transfer during static film flash of pure water. Their evolutions and interactions were studied through experiments with initial height of waterfilm ranging between 0.15 and 0.3 m, initial temperature between 80.2 and 93.0 °C, initial pressure of vacuum chamber between 0.013 and 0.061 MPa. Results suggested both height and decrease rate of temperature of waterfilm experienced a rapid increase to peak value and a gradual decline after that. Considering most of heat transfer happened within waterfilm, volumetric evaporation coefficient was introduced as the total heat flux transferred through all bubble interfaces within unit volume of waterfilm under unit superheat. This coefficient increased significantly with rising volumetric void fraction of waterfilm. Except vapor generation, mass transfer during static flash also included the overflowing of vapor. Its overflowing speed increased with initial height of waterfilm. According to these results, a model was set up to calculate the evolutions of temperature and height of waterfilm from given evolution of saturation temperature corresponding to the pressure of flash chamber.

    更新日期:2018-01-07
  • A review of boiling heat transfer and heat pipes behaviour with self-rewetting fluids
    Int. J. Heat Mass Transf. (IF 3.458) Pub Date : 2018-01-04
    Yanxin Hu, Kaixin Huang, Jin Huang

    The self-rewetting fluid is the liquid which have an anomalous surface tension increasing with temperature when the temperature exceeds a certain value. This particular property can cause the subcooled liquid to be drawn towards the heated surface if a dry patch appears, thus making it possible for the self-rewetting fluids to become promising working fluids. This paper presents an overview of the recent developments of the research on boiling heat transfer using self-rewetting fluids. Thermophysical properties, pool and flow boiling experiments and heat pipe applications of self-rewetting fluids have been reported in this paper. It can be found that the use of self-rewetting fluids in a wide range of applications appears promising. However, further detailed and valuable theoretical and numerical investigations are necessary for us to better understand the boiling phenomenon of self-rewetting fluids.

    更新日期:2018-01-06
  • Parametric study on the heat transfer of a blunt body with counterflowing jets in hypersonic flows
    Int. J. Heat Mass Transf. (IF 3.458) Pub Date : 2018-01-04
    Jinghui Guo, Guiping Lin, Xueqin Bu, Lizhan Bai, Yanmeng Chao

    A quantified parametric study for the heat transfer acting on a hypersonic blunt body with counterflowing jets is presented. Three-dimensional turbulent Navier-Stokes equations are solved to simulate freestream-jet interactive flowfields. The freestream and jet controlling parameters are treated as input sources of variation, and a point-collocation non-intrusive polynomial chaos (NIPC) method is utilized to quantify the variations in the output surface heat flux and total surface heat load acting on the blunt body by identifying the maximum and minimum of surrogate response values predicted by the NIPC. All of the sample cases are confirmed to form steady jet structures. Furthermore, through a global sensitivity analysis, Sobol indices evaluated by the NIPC, are used to rank the contributions of each input parameter to the variation in output quantities of interest. It is found that the designed upstream injection of baseline case effectively reduces the heat transfer to body surface compared with the no-jet case. The variations of input parameters induce remarkable variations of output heat flux and total heat load. The sensitivity analysis indicates that the jet-to-freestream total-pressure ratio is the top contributor to variations in heat flux, followed by the freestream Mach number. The jet total temperature is mainly important on the front part of forebody, while the contributions of jet Mach number and freestream temperature slightly increase downstream. The freestream density has the smallest effect. The sensitivity of total heat load to input parameters coincides with that of heat flux. This parametric study is expected to illustrate the significance of flow-controlling parameters to the heat transfer over blunt body and provide insight for aerothermal management by using counterflowing jets in hypersonic flows.

    更新日期:2018-01-06
  • Novel radial pulsating heat-pipe for high heat-flux thermal spreading
    Int. J. Heat Mass Transf. (IF 3.458) Pub Date : 2018-01-04
    Brian Kelly, Yoshikazu Hayashi, Yoon Jo Kim

    As technology becomes increasingly miniaturized, extremely localized heat dissipation (so called hot-spots) leads to the challenge of how to keep these devices from overheating. Heat dissipation from advanced power and military electronics is expected to be on the order of 1 kW/cm2, while conventional cooling techniques can only cool up to 10 W/cm2 with forced air convection cooling and 500 W/cm2 with advanced microchannel liquid cooling. In the present study, we propose and investigate a novel radial pulsating heat-pipe (RPHP), which is tailored for effective “spreading of heat” from a local high heat-flux heat source. An experimental system for RPHP was constructed with a 110 mm diameter circular brass plate with 1 mm depth and 1 mm width primary channels. The primary channels are enclosed using a polycarbonate cover that is equipped with an internal working fluid charging port. The diameters of the boiling chamber (or evaporator section) and the condenser section were 10 mm and 60 mm, respectively. Thermocouples were installed to measure the temperatures of RPHP surface and the working fluid. The pressure of the fluid in the boiling chamber was measured using an absolute pressure transducer. The measured data was used to evaluate the thermal performance of the RPHP in terms of convective heat transfer coefficient and thermal resistance with respect to working fluid fill ratio and power input. The study showed that the system was effective at spreading locally concentrated heat; in the study the heater temperature was dropped by 23 °C compared to that of pure heat conduction through the RPHP body in 30 W heater power case.

    更新日期:2018-01-06
  • Heat and mass exchange and feasible mode sets of irreversible separation processes
    Int. J. Heat Mass Transf. (IF 3.458) Pub Date : 2018-01-04
    Alexandr I. Balunov, Anatoliy M. Tsirlin

    The article deals with the separation systems’ mean boundary from the perspectives of finite-time thermodynamics. We obtained the lower-bound estimates of the minimum energy input for the separation systems that have a specified separation. It is shown that these inputs do not tend toward zero while the concentration of one of the separated mixture’s components tends to one. Separation systems using heat power have ultimate performance. The article provides examples of the obtained estimates’ application.

    更新日期:2018-01-06
  • The influence of interface contaminated degree on the wake characteristics of a spherical bubble at moderate Reynolds number under the condition of isothermal flow
    Int. J. Heat Mass Transf. (IF 3.458) Pub Date : 2018-01-04
    Yang Fei, Mingjun Pang

    The wake flow characteristics of a spherical bubble against the interface contaminated degree are deeply investigated at moderate Reynolds number (Re = 25–200) based on the 3D rear stagnant cap model. The interface contaminated degree is controlled by changing the magnitude of the cap angle, for θ = 0–180°. It is found that the vortex size does not change obviously when θ = 0–67.5°; when θ > 67.5°, it decreases until it is equal to zero when θ > 135°. The variations of vortex size and separation angle are related to the deceleration of fluid near the lee face of the bubble. The vortexes display the 3D characteristic due to the skip of pressure coefficient component in the flow direction at the leading edge of the stagnant cap when θ = 45°, 67.5°, 112.5° and 135°.

    更新日期:2018-01-06
  • Thermal switch using controlled capillary transition in heterogeneous nanostructures
    Int. J. Heat Mass Transf. (IF 3.458) Pub Date : 2018-01-05
    Tadeh Avanessian, Gisuk Hwang

    The development of a nanoscale thermal switch is a crucial step toward advanced thermal management systems including future thermal logic gates and computers. This study demonstrates a new nanoscale thermal switch mechanism using controlled, morphological transition from adsorption to capillary state in a novel gas-filled nanostructure, i.e., a nanogap with controllable nanoposts on one surface only. The degree of thermal switch, S, at given gas pressures are predicted using Ar-filled Pt-based nanostructures and Non-Equilibrium Molecular Dynamics (NEMD) simulation combined with Grand Canonical Monte Carlo (GCMC) simulation. It is found that S increases by increasing the height of the nanoposts and temperature difference across the nanostructure, and decreasing the interpost spacings, with the maximum degree of switch, Smax ∼ 45 and ∼ 170 for ΔT = 10 K and 60 K, respectively, for the nanogap size of 5 nm. It is also observed that a stronger solid-fluid surface interaction results in a wider switch operating temperature window.

    更新日期:2018-01-06
  • An experimental, computational and flow visualization study on the air-side thermal and hydraulic performance of louvered fin and round tube heat exchangers
    Int. J. Heat Mass Transf. (IF 3.458) Pub Date : 2018-01-05
    Abdulkerim Okbaz, Ali Pınarbaşı, Ali Bahadır Olcay, Muharrem Hilmi Aksoy

    The aim of this study is to determine heat transfer and pressure drop characteristics in different louvered fin geometries for manufacturing of commercial louvered fin and round tube heat exchangers. Numerical simulations were carried out for various louver angles, louver lengths (pitches), fin pitches and frontal air velocities. The heat transfer and pressure drop characteristics of the louvered fin and round tube heat exchangers, Colburn and friction factors, were respectively normalized with Colburn and friction factors of the flat plate fin and round tube heat exchangers operating under the same conditions and they were presented as the relative Colburn factor j∗ and the relative friction factor f∗. Thermal & hydraulic performance was presented as JF∗. Temperature and local Nusselt number contours, and streamline patterns were provided to reveal the mechanisms behind the heat transfer enhancement. Among different heat exchangers for which heat transfer and pressure drop characteristics were obtained, one was chosen to manufacture a real size heat exchanger. Flow visualization studies were also conducted with a PIV system in an open water channel to determine whether the flow structure is louvered directed or not. The louvered fin heat exchanger tested in the PIV system was a five times scaled up model of the real size louvered fin heat exchanger and made from a transparent plexiglas material. PIV results were presented and evaluated based on streamlines and velocity vectors. Furthermore, a numerical analysis was performed using exactly the same dimensions and conditions of the model tested in the PIV system. The comparison between numerical and experimental results was done to validate the numerical model. Consequently, the performance of the fabricated real size heat exchanger was tested at different air velocities in a wind tunnel in a conditioned room. The experimental results were compared with numerical analyses and found to be compatible with each other. Finally, thermal and hydraulic performance of the louvered fin and round tube heat exchanger was compared with a wavy fin and round tube heat exchanger with identical size and specifications. It was found that the thermal and hydraulic performance of the louvered fin and round tube heat exchanger is higher than that of the wavy fin and round tube heat exchanger. The Colburn factor j, friction factor f and JF of the louvered fin and round tube heat exchanger are higher about 16.8–7%, 19.9–8.2% and 10–4.3% than that of the wavy fin and round tube heat exchanger depending on the Reynolds number, respectively.

    更新日期:2018-01-06
  • Dependence of submerged jet heat transfer on nozzle length
    Int. J. Heat Mass Transf. (IF 3.458) Pub Date : 2018-01-05
    Barak Kashi, Herman D. Haustein

    In this work the influence of nozzle length in submerged jet impingement heat transfer was studied by validated direct numerical simulations, in the laminar flow regime. With the purpose of examining the entire range of nozzle lengths and 500⩽Re⩽2000 500 ⩽ Re ⩽ 2000 , other effects were reduced by setting a low nozzle-to-heater spacing (H/D = 3) and ideal, undisturbed inlet conditions. While developing pipe-flow is well-known, this parametric study characterized in detail short and intermediate nozzle flow regimes, affected by a separation bubble at the sharp-edged inlet. It is found that the maximal (centerline) jet velocity first decreases with increasing effective nozzle length, Z = L/(D ⋅ Re), to a minimum at Z∗≈0.0015 Z ∗ ≈ 0.0015 , beyond which it increases as in developing pipe-flow. For Z

    更新日期:2018-01-06
  • Numerical study on condensation heat transfer and pressure drop characteristics of ethane/propane mixture upward flow in a spiral pipe
    Int. J. Heat Mass Transf. (IF 3.458) Pub Date : 2018-01-05
    Shulei Li, Weihua Cai, Jie Chen, Haochun Zhang, Yiqiang Jiang

    Spiral wound heat exchanges (SWHE) has been the most widely used in large-scale liquid natural gas (LNG) plants. However, few studies have been focused on the condensation heat transfer and pressure drop for hydrocarbon mixture refrigerant in SWHE tube side. In this paper, the condensation heat transfer and pressure drop characteristics for ethane/propane mixture upward flow in a spiral pipe were numerically investigated. The numerical model was established and verified based on the existing experimental results and new flow pattern observation experiments. It discussed the variation trends of void fraction, frictional pressure drop, heat transfer coefficient and heat and mass transfer resistance with various parameters, such as, vapor quality, mass flux, heat flux, saturation pressure and inclination angle. Comparing with the existing correlations, the results showed that Steiner’s correlation, modified Fuchs’s correlation, Boyko’s correlation could better predict the void fraction, frictional pressure drop, film heat transfer coefficient with mean absolute deviation of 6.08%, 10.67% and 13.06%, respectively. Meanwhile, modified Silver approach was used to modify the mixed effects of ethane/propane mixture on heat transfer. The study provides some constructive instructions to understand the condensation of zeotropic mixtures in the spiral pipe, and is helpful in designing more effective SWHE.

    更新日期:2018-01-06
  • Flow characteristics of gaseous flow through a microtube discharged into the atmosphere
    Int. J. Heat Mass Transf. (IF 3.458) Pub Date : 2018-01-05
    Chungpyo Hong, Goku Tanaka, Yutaka Asako, Hiroshi Katanoda

    Flow characteristics for a wide range of Reynolds number up to turbulent gas flow regime, including flow choking were numerically investigated with a microtube discharged into the atmosphere. The numerical methodology is based on the Arbitrary-Lagrangian-Eulerian (ALE) method. The LB1 turbulence model was used in the turbulent flow case. Axis-symmetric compressible momentum and energy equations of an ideal gas are solved to obtain the flow characteristics. In order to calculate the underexpanded (choked) flow at the microtube outlet, the computational domain is extended to the downstream region of the hemisphere from the microtube outlet. The back pressure was given to the outside of the downstream region. The computations were performed for adiabatic microtubes whose diameter ranges from 10 to 500 μm and whose aspect ratio is 100 or 200. The stagnation pressure range is chosen in such a way that the flow becomes a fully underexpanded flow at the microtube outlet. The results in the wide range of Reynolds number and Mach number were obtained including the choked flow. With increasing the stagnation pressure, the flow at the microtube outlet is underexpanded and choked. Although the velocity is limited, the mass flow rate (Reynolds number) increases. In order to further validate the present numerical model, an experiment was also performed for nitrogen gas through a glass microtube with 397 μm in diameter and 120 mm in length. Three pressure tap holes were drilled on the glass microtube wall. The local pressures were measured to determine local values of Mach numbers and friction factors. Local friction factors were numerically and experimentally obtained and were compared with empirical correlations in the literature on Moody’s chart. The numerical results are also in excellent agreement with the experimental ones.

    更新日期:2018-01-06
  • Influence of core scale permeability on gas production from methane hydrate by thermal stimulation
    Int. J. Heat Mass Transf. (IF 3.458) Pub Date : 2018-01-05
    Yongchen Song, Yangmin Kuang, Zhen Fan, Yuechao Zhao, Jiafei Zhao

    The hydrate dissociation process involves heat transfer in the decomposing zone, multi-phase fluid flow during gas production, and the intrinsic kinetics of hydrate dissociation. The potential impact of laboratory-scale permeability on hydrate exploitation from hydrate-bearing sediments was predicted from a previously developed and verified two-dimensional axisymmetric model. We herein continue the previous work to investigate the influence of core-scale hydrate sediments’ permeability on gas production by the thermal stimulation method. The results show that the gas production in relatively low permeability reservoirs proceeded at a faster rate, requiring less time to complete the dissociation process, although an optimal permeability was associated with the fastest gas production. In addition, with the temperature continuously increased, the dissociation front displaced from the boundary wall to the core axis along the radial direction. In a lower permeability system, however, the hydrate dissociation process at the zone opposite the outlet valve was delayed. Due to the varying processes associated with hydrate dissociation, the overall thermal conductivity declined faster at an earlier stage in sediments of high permeability as compared with sediments of lower permeability. Furthermore, the effects of boundary heat transfer were more significant for low permeability systems.

    更新日期:2018-01-06
  • Heat reduction by thermal wave crystals
    Int. J. Heat Mass Transf. (IF 3.458) Pub Date : 2018-01-05
    A-Li Chen, Zheng-Yang Li, Tian-Xue Ma, Xiao-Shuang Li, Yue-Sheng Wang

    Non-Fourier heat conduction models assume wave-like behavior does exist in the heat conduction process. Based on this wave-like behavior, thermal conduction controlled in a one-dimensional periodical structure, named thermal wave crystal, has been demonstrated through both theoretical analysis and numerical simulation based on the Cattaneo-Vernotte (CV) heat-conduction model. The transfer matrix method and Bloch theorem have been applied to calculate the complex dispersion curves of thermal wave propagating in thermal wave crystals. And the temperature responses are obtained by using the FDTD method. The results show that the band gaps with pronounced heat reduction do exist in non-Fourier thermal transfer process because of the Bragg scattering. The mid-gap frequency is well predicted analytically based on the Bragg scattering mechanism. Finally, the key parameters determining the band gaps are presented and discussed. This study shows the potential applications of these materials in heat isolation and reduction.

    更新日期:2018-01-06
  • Analytical modeling of oscillatory heat transfer in coated sorption beds
    Int. J. Heat Mass Transf. (IF 3.458) Pub Date : 2018-01-03
    Hesam Bahrehmand, Mehran Ahmadi, Majid Bahrami
    更新日期:2018-01-05
  • High anisotropy metamaterial heat spreader
    Int. J. Heat Mass Transf. (IF 3.458) Pub Date : 2018-01-03
    Ahmed Hamed, Sidy Ndao

    The ability to manipulate heat flow has numerous promising benefits in thermal energy applications such as in heat spreader design. This paper introduces a novel thermal functional metamaterials heat spreader with high anisotropy. The metamaterial heat spreader consists of alternating layers of copper/VHTC and PDMS thin films. The effects of the number of bilayers, bilayers’ thermal conductivity ratio, and thermal spreader thickness on the metamaterial lateral heat spreading have been investigated numerically. In comparison to a simple copper heat spreader, results show significant enhancements of the lateral heat spreading with both the copper and VHTC based metamaterial heat spreaders.

    更新日期:2018-01-05
  • Atomization and surface heat transfer characteristics of cryogen spray cooling with expansion-chambered nozzles
    Int. J. Heat Mass Transf. (IF 3.458) Pub Date : 2018-01-03
    Xin-Sheng Wang, Bin Chen, Zhi-Fu Zhou

    Cryogen spray cooling (CSC) is commonly applied in laser dermatology to protect the epidermis from thermal damage. Many efforts have attempted to improve the cooling capacity of CSC, among which the use of expansion-chambered nozzles is an effectively simple method with considerable potential. This study examined the influences of the expansion-chambered nozzle structure, including the ratios of inlet nozzle diameter to discharge nozzle diameter and of chamber diameter to discharge nozzle diameter on R134a and R404A spray cooling. Fifteen transparent expansion-chambered nozzles with the expansion chamber aspect ratio of 1.0, chamber diameter to discharge nozzle diameter ratios of 5.0–10.0, and inlet nozzle diameter to discharge nozzle diameter ratios of 0.6–1.4 were tested. The internal flow pattern inside the expansion chamber, external spray pattern, and surface heat transfer characteristics of cryogen spray using different nozzles, including the straight-tube nozzle, were investigated. The structure of the expansion chamber was found to have an important effect on the spray patterns and cooling characteristics. The spray radius obviously decreased when the expansion-chambered nozzles were used, and the spray pattern became narrower as the ratio of chamber diameter to discharge nozzle diameter increased. By contrast, the increase in ratio of two nozzle diameters enlarged the spray radius. Surface temperature and heat flux with different nozzles and cryogens showed a similarity, and correlations of surface temperature and heat flux were proposed. The introduction of expansion-chambered nozzles could effectively improve the spray cooling capacity. The minimum average surface temperature during the fully developed spray period could be reached for both R134a spray and R404A spray by an expansion-chambered nozzle with a chamber diameter to discharge nozzle diameter ratio of roughly 5.0 and an inlet nozzle diameter to discharge nozzle diameter ratio of roughly 0.6.

    更新日期:2018-01-05
  • Heat transfer enhancement in suddenly expanding annular shear-thinning flows
    Int. J. Heat Mass Transf. (IF 3.458) Pub Date : 2018-01-03
    Khaled J. Hammad

    Heat transfer enhancement in suddenly expanding annular pipe flows of Newtonian and shear-thinning non-Newtonian fluids is studied within the steady laminar flow regime. Conservation of mass, momentum, and energy equations, along with the power-law constitutive model are numerically solved. The impact of inflow inertia, annular-diameter-ratio, k, power-law index, n, and Prandtl numbers, is reported over the following range of parameters: Re = {50, 100, 150}, k = {0, 0.5, 0.7}; n = {1, 0.8, 0.6}; and Pr = {1, 10, 100}. Heat transfer enhancement downstream of the expansion plane, i.e., Nusselt numbers greater than the downstream fully developed value, Nu/Nufd > 1, is only observed for Pr = 10 and 100. In general, higher Prandtl numbers, power-law index values, and annular-diameter-ratios, result in more significant heat transfer enhancement downstream of the expansion plane. Heat transfer augmentation, for Pr = 10 and 100, increases with the annular-diameter-ratio. For a given annular-diameter-ratio and Reynolds numbers, increasing the Prandtl number from Pr = 10 to Pr = 100, always results in higher peak Nu values, Numax, for both Newtonian and shear-thinning flows. All Numax values are located downstream of the flow reattachment point, in the case of suddenly expanding round pipe flows, i.e., κ κ  = 0. However, for suddenly expanding annular pipe flows, i.e., κ = 0.5 and 0.7, Numax values appear upstream the flow reattachment point. For Pr = 10 and 100, shear-thinning flows display two local peak Nu/Nufd values, in comparison with one peak value in the case of Newtonian flows. The highest heat transfer enhancement, Numax/Nufd ≈ 5, is observed at κ = 0.7, n = 0.6, and Pr = 100.

    更新日期:2018-01-05
  • Nonlinear convection regimes in superposed fluid and porous layers under vertical vibrations: Positive porosity gradients
    Int. J. Heat Mass Transf. (IF 3.458) Pub Date : 2018-01-03
    E.A. Kolchanova, N.V. Kolchanov

    We investigate the onset of average convection and its nonlinear regimes in a single-component fluid layer overlying a fluid-saturated porous layer. A heated from below cavity with a superposed fluid and a porous medium undergoes high-frequency and small-amplitude vertical vibrations in the gravitational field. Porosity of the medium decreases linearly with depth at a positive porosity gradient. Thermal vibrational convection equations are obtained by the averaging method and solved numerically. The shooting method, Galerkin method and finite-difference method are applied. It is shown that for small vibration accelerations, a convective flow is generated as short-wave rolls in the fluid layer overlying a porous medium. The heat flux undergoes abrupt changes as the supercriticality increases. It is due to the fluid flow penetrating into pores. A magnitude of the jump grows with the growth of vibration intensity. For sufficiently large vibration accelerations, the average convection is excited in the form of long-wave rolls that penetrate both layers. Here, the Nusselt number is 2–3 times higher than its value in the static gravity field.

    更新日期:2018-01-05
  • Saturated pool boiling enhancement using porous lattice structures produced by Selective Laser Melting
    Int. J. Heat Mass Transf. (IF 3.458) Pub Date : 2018-01-03
    K.K. Wong, K.C. Leong

    Pool boiling heat transfer of saturated FC-72 under atmospheric pressure was studied for porous lattice structures fabricated using the Selective Laser Melting (SLM) technique. The substrates possess repeating geometry of octet-truss unit cell and were varied with unit cell sizes of 2.0 mm, 3.0 mm and 5.0 mm and structure heights of 2.5 mm, 5.0 mm and 10.0 mm. In comparison with a plain surface, the porous structures show significant enhancement in nucleate boiling heat transfer coefficients and delay of Critical Heat Flux (CHF). The enhancement is attributed to the increased surface area, increased nucleation site density and capillary-assisted suction of the porous structure. The porous structure allows sustained liquid replenishment which delayed the hydrodynamic choking and CHF significantly. The best performing substrate with the 3-mm unit cell size and 5-mm structure height has an average nucleate boiling heat transfer coefficient of 1.35 W/cm2·K, which is 2.81 times that of the plain surface at 0.48 W/cm2·K. Heat transfer mechanisms are proposed for the different heat flux levels of the porous structures based on visual observations. The boiling patterns are classified as low, mid, high and very-high heat flux levels. At high heat flux level, two separate modes of stable and unstable boiling patterns are observed. For the stable boiling pattern, there are distinct bubble departure and liquid replenishment pathways, thus allowing a good convection flow. However, for the unstable boiling pattern, there is major liquid–vapor counter-flow, which disrupts the orderly liquid replenishment pathway.

    更新日期:2018-01-05
  • Regulating solar absorptance for remedying thermal asymmetry of a roadway embankment
    Int. J. Heat Mass Transf. (IF 3.458) Pub Date : 2018-01-04
    Xingui Zhang, Li-E Yan, Yinghong Qin

    Roadways in permafrost regions are laid upon embankments because permafrost stratum is highly sensitive to the thermal disturbance at the ground surface. The south-facing slope of the embankment is exposed to longer insolation than the north-facing one annually, differing the solar absorption of the side slopes and thus developing differential settlement across the embankment. A simple, economical strategy is to reduce the solar absorptance of the southern side slope to a critical level such that the solar absorption of both slopes is equalized. However, it is unknown this critical absorptance of a specific embankment and it is also unknown if this critical absorptance can be achieved by using existing techniques. This study mimics the solar radiation falling on the side slopes of embankments with different configurations, and subsequently it estimates the solar absorption of the side slopes. We found that in sunny weather when the thermal asymmetry of an embankment is maximum, reducing the southern side-slope absorptance from 0.80 to 0.40 can eliminate the thermal asymmetry. By analyzing the solar absorptance of a rough surface such as a crushed-rock layer, we can ascertain that this critical absorptance can be achieved by coating the side slope with nonwhite high-reflective pigment.

    更新日期:2018-01-05
  • Transient and steady state heat transport in layered materials from molecular dynamics simulation
    Int. J. Heat Mass Transf. (IF 3.458) Pub Date : 2018-01-04
    Chenhan Liu, Weiyu Chen, Yi Tao, Juekuan Yang, Yunfei Chen

    In this paper, both transient and steady state heat transfer in graphite are investigated with molecular dynamics (MD) simulations. The simulation results demonstrate that elastic anisotropy controls heat transfer in the transient state, which makes the outermost isothermal surface of temperature distribution similar to the phonon group velocity surface that has a shape of very flat ellipse in layered materials. In steady state, with the help of phonons engaging in sufficient scattering, the basal plane phonon modes determine the thermal conductivity along all directions except those very close to the cross plane. Our simulation results confirm that the classical theoretical model can accurately predict the thermal conductivity along arbitrary directions in layered materials.

    更新日期:2018-01-05
  • Effects of spontaneous nanoparticle adsorption on the bubble-liquid and bubble-bubble interactions in multi-dispersed bubbly systems – A review
    Int. J. Heat Mass Transf. (IF 3.458) Pub Date : 2017-12-20
    Yang Yuan, Xiangdong Li, Jiyuan Tu

    Robust predictive models of dynamic bubbly systems of nanoparticle-liquid mixtures are vital to the design and assessment of relevant industrial systems. Previous attempts to model bubbly flows of dilute nanofluids using the classic two-phase flow models were unsuccessful although the apparent hydrodynamic properties of the dilute nanoparticle-liquid mixtures were only negligibly different to those of their pure base liquids. Emerging studies demonstrated that when bubbles exist in the mixture, nanoparticles tend to spontaneously aggregate at the bubble interface, forming a layer of “colloidal armour” and making the bubble interface partially rigid and less mobile. The colloidal armour also significantly modifies the characteristics of the bubble-liquid and bubble-bubble interactions. Therefore, it was proposed that the key job when developing a predictive model based on classic two-phase flow models is to re-formulate the bubble-liquid and bubble-bubble interactions. However, the adsorption of nanoparticles in dynamic bubbly systems has rarely been studied. The lack of mechanistic understanding has severely hindered the model development. Therefore, this study reviews the common findings yielded from experimental and numerical investigations reported in literature, with the aim to clarify the critical points to address when modelling bubbly flows containing nanoparticles using the classic two-fluid and MUSIG models.

    更新日期:2017-12-21
  • Experimental evidence of the impact of radiation coupling on binary scaling applied to shock layer flows
    Int. J. Heat Mass Transf. (IF 3.458) Pub Date : 2017-12-19
    G. de Crombrugghe, O. Chazot, T.J. McIntyre, R. Morgan

    Binary scaling is a similitude law used to study the aerothermodynamics of hypersonic vehicles in ground-based high-enthalpy facilities. It enables the duplication of shock layer flows in the vicinity of the stagnation point, including binary chemistry and nonequilibrium processes, over length scales that are practical for experimental testing. It is built on the assumption of a flow devoid of radiation coupling, which drastically narrows down the envelope of flows for which binary scaling can be used. Indeed, if it is strong enough, radiative heat transfer will cause a substantial amount of energy to leak out of the shock layer to the free-stream and other flow regions. As demonstrated in this paper, the strength of that coupling will increase as the length-scale of the flow increases, impacting other flow properties such as its chemical composition or temperature. The resulting impact on macroscopic features of the flow are for example a reduction of the wall heat flux (both convective and radiative) or of the shock standoff distance. These side impacts are identified with experimental measurements on the shock standoff distance in an expansion tube with CO2–N2 CO 2 – N 2 mixture flows representative of the venusian atmosphere over cylinders.

    更新日期:2017-12-20
  • Nucleation model for mesopore-confined water freezing kinetics
    Int. J. Heat Mass Transf. (IF 3.458) Pub Date : 2017-12-19
    Marcin Koniorczyk, Dalia Bednarska, Magdalena Nowosielska, Jacek Rynkowski

    Cyclic freezing of water is the main reason of microstructure damages in porous materials exploited in cold climate. It is usually assumed that water freezes much faster than temperature changes, therefore the kinetic effects are neglected. Nevertheless, in the reality such assumption might be often violated. In this paper the kinetics of water freezing in mesopores of two silica gels with dominant pore diameter equal 11 and 36 nm has been investigated. To obtain the freezing rate functions the differential scanning calorimetry test with multiple cooling rate program has been performed. The activation energy is calculated using both, differential and local integral methods. It is found that the activation energy is proportional to the reciprocal temperature. The most suitable kinetic model has been selected by means of two methods: the classical procedure recommended by ICTAC KC and the linear model-fitting method. The former method indicates that two models are appropriate: the empirical Šesták–Berggren model and physical Johnson-Mehl-Avrami model. The SB model demonstrates high accordance with the experimental data, whereas the JMA model allows to approximate the experimental data with high accuracy only if the activation energy is calculated using differential approach. The application range of linear model-fitting method has been extended with the fractional numbers for JMA model parameter. The best correlation between experimental data and theoretical model is obtained for the non-integer values of JMA parameter. It indicates that nucleation and nuclei growth for both silica gels are of fractional dimensions.

    更新日期:2017-12-20
  • A coupled mathematical model of oxygen transfer in electroslag remelting process
    Int. J. Heat Mass Transf. (IF 3.458) Pub Date : 2017-12-18
    Xuechi Huang, Baokuan Li, Zhongqiu Liu
    更新日期:2017-12-19
  • Defrosting behavior and performance on vertical plate for surfaces of varying wettability
    Int. J. Heat Mass Transf. (IF 3.458) Pub Date : 2017-12-19
    Hisuk Kim, Guangri Jin, Jaehyeon Jeon, Kwan-Soo Lee, Dong rip Kim

    The defrosting behaviors and performances of super-hydrophilic, bare, and super-hydrophobic surfaces were experimentally investigated along different frost layer densities on a vertical plate. The defrosting behavior can be divided into three types based on the size of the water permeation layer. The defrosting behaviors of bare and super-hydrophilic surfaces were similar, whereas the super-hydrophobic surface behaved differently by allowing the frost layer to be easily removed from its surface. Defrosting performance was evaluated based on defrosting time and water retention ratio. Within the low frost layer density range, defrosting time did not depend on surface characteristics. However, as the density of the frost layer increased, defrosting time increased in the bare and super-hydrophilic surfaces, whereas in the super-hydrophobic surface, that time tended to decrease rather than increase. Water retention ratio was highest on the super-hydrophilic surface and lowest on the super-hydrophobic surface in all frost layer densities. Therefore, the defrosting performance of a super-hydrophobic surface was outstanding over a wide range of frost layer densities.

    更新日期:2017-12-19
  • Effects of variable particle sizes on hydrothermal characteristics of nanofluids in a microchannel
    Int. J. Heat Mass Transf. (IF 3.458) Pub Date : 2017-12-19
    Tehmina Ambreen, Man-Hoe Kim

    This study investigates the influence of nanoparticle size on the heat transfer and pressure drop characteristics of nanofluids for laminar forced convection in a microchannel subjected to constant heat flux. Aqueous nanofluids containing spherical shaped particle dispersions of Al2O3 Al 2 O 3 and TiO2 TiO 2 , have been simulated by employing discrete phase model (DPM) for a range of ten particle sizes 20–200 nm. Analysis has been carried out by considering two particle weight concentrations (0.1% and 2%) at Reynolds number of 1000, 1200 and 2000. Results demonstrate that for constant nanofluid compositions and flow conditions, convective heat transfer and friction factor are in inverse association with the particle diameter. With the reduction in particle size, the heat transfer coefficient of nanofluids escalates because of particles’ enhanced effective particle surface area and uniform distribution along the channel radial direction. However, this improvement in heat transfer coefficient is compensated by undesirable increase in pressure drop as a consequence of higher viscosity. The variation in hydrothermal characteristics of nanofluids with particle diameter is more significant at higher particle concentration. The maximum heat transfer and friction factor difference of 11% and 20% respectively has been observed between particle sizes 20 nm and 200 nm for the particle concentration of 2%.

    更新日期:2017-12-19
  • A highly accurate backward-forward algorithm for multi-dimensional backward heat conduction problems in fictitious time domains
    Int. J. Heat Mass Transf. (IF 3.458) Pub Date : 2017-12-19
    Yung-Wei Chen

    This paper proposes highly accurate one-step backward-forward algorithms for solving multi-dimensional backward heat conduction problems (BHCPs). The BHCP is renowned for being ill-posed because the solutions are generally unstable and highly dependent on the given data. In this paper, the present algorithm combines algebraic equations with a high-order Lie-group scheme to construct one-step algorithms called the backward fictitious integrate method (BFTIM) and the forward fictitious integrate method (FFTIM). First, the original parabolic equation is transformed into a new parabolic equation of an evolution type by introducing a fictitious time variable. Then, the numerical integration of the discretized algebraic equations must satisfy the constraints of the cone structure, Lie-group and Lie algebra at each fictitious time step. Finally, the algorithms with the minimum fictitious time steps along the manifold of the Lie-group scheme approach the true solution with one step when given an initial guess. In addition, this paper provides a strategy to determine the initial guess, which is the reciprocal relationship of the initial condition (IC) and the final condition (FC). More importantly, the IC and FC can be recovered by the BFTIM and FFTIM according to the relation between the IC and FC, even under large noisy measurement data. Five numerical examples of the BHCP are tested and numerical results demonstrate that the present schemes are more effective and stable. In general, the numerical implementations of the BFTIM and FFTIM are simple and have one-step convergence speeds.

    更新日期:2017-12-19
  • Uncertainty quantification for modeling pulsed laser ablation of aluminum considering uncertainty in the temperature-dependent absorption coefficient
    Int. J. Heat Mass Transf. (IF 3.458) Pub Date : 2017-12-19
    Yeqing Wang, Getachew K. Befekadu, Hongtao Ding, David W. Hahn

    In this paper, an extension of the result of Wang et al. (“Modeling pulsed laser ablation of aluminum with finite element analysis considering material moving front,” Int. J. Heat & Mass Transfer, 113, 1246–1253, 2017) concerning the problem of uncertainty quantification for pulsed laser ablation (PLA) of aluminum is considered, when the source of uncertainty is due to an inherent randomness of the temperature-dependent absorption coefficient. In particular, we use a generalized polynomial chaos (gPC) method to incorporate the parameter uncertainty for the temperature-dependent absorption coefficient within the representation of the laser heat conduction phenomena. Furthermore, numerical simulation studies for the PLA of aluminum, with nanosecond Nd:YAG 266 nm pulsed laser, that demonstrate the proposed gPC predictions are presented. Finally, a sensitivity study is performed to identify whether small changes in the lower and/or upper parameter values of the absorption coefficient provide the most variance in the thermal and ablation responses.

    更新日期:2017-12-19
  • Modeling heat and mass transfer during ground freezing taking into account the salinity of the saturating fluid
    Int. J. Heat Mass Transf. (IF 3.458) Pub Date : 2017-12-19
    A. Rouabhi, E. Jahangir, H. Tounsi

    In geotechnical engineering applications, the modeling of artificial ground freezing is primarily aimed at predicting the extent of frozen zone around the cooling sources. This modeling could be more or less complex not only according to the material’s texture and the hydro-geological context but also to the salt concentration of the saturating fluid. Through a thermodynamically consistent framework, a fully coupled heat and mass transfer formulation considering the salinity effect was derived. This formulation was intended to capture the most relevant phenomena of ground freezing encountered in geotechnical applications. Particular attention was given to the phase change problem where appropriate simplifying assumptions were made in order to make the proposed methodology easier to apply in field applications. The proposed approach was validated by means of freezing-thawing laboratory tests, carried out on specimens initially fully saturated with sodium chloride solutions at various concentrations. Good agreement was obtained between the measured and predicted results.

    更新日期:2017-12-19
  • Thermo-hydraulic analysis of multi-row cross-flow heat exchangers
    Int. J. Heat Mass Transf. (IF 3.458) Pub Date : 2017-12-19
    Cheen Su An, Man-Hoe Kim

    This paper presents thermal hydraulic analysis of the cross-flow finned tube heat exchangers for an out-door unit in residential air-conditioning and heat pump applications. Performance of heat exchangers affect significantly the system energy efficiency and size of the air-conditioning and heat pumps. The Navier-Stokes equations and the energy equation are solved for the three dimensional computation domain that encompasses multiple rows of the fin-tube heat exchangers. Rather than solving the flow and temperature fields for the outdoor heat exchanger directly, the fin-tube array has been approximated by the porous medium of equivalent permeability, which is estimated from a three dimensional finite volume solution for the periodic fin element. This information is essential and time-effective in carrying out the global flow field calculation which, in turn, provides the face velocity for the microscopic temperature-field calculation of the heat exchanger. The flow field and associated heat transfer for a wide range of face velocity and fin-tube arrangements are examined and the results are presented compared with experimental data. The predicted pressure drop and heat transfer rate for various inlet velocities are in excellent agreement with the measured data.

    更新日期:2017-12-19
Some contents have been Reproduced with permission of the American Chemical Society.
Some contents have been Reproduced by permission of The Royal Society of Chemistry.
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