Review of pool boiling enhancement with additives and nanofluids Int. J. Heat Mass Transf. (IF 3.458) Pub Date : 2018-04-05 Gangtao Liang, Issam Mudawar
Enhancement of nucleate pool boiling by modifying fluid properties has drawn considerable attention in recent years. This paper provides a comprehensive review of published literature concerning enhancement methodologies of surfactant and polymer additives, and nanofluids. Each method is discussed in detail in terms of measured impact on the nucleate boiling heat transfer coefficient and critical heat flux (CHF), mechanisms proposed for any heat transfer enhancement, and predictive models. It is shown that adding surfactant to base liquid shifts the nucleate boiling region of the boiling curve towards lower surface superheats, thereby promoting earlier boiling incipience and increasing the nucleate boiling heat transfer coefficient, but the heat transfer merits of polymer addition are polymer specific. Despite significant enhancement in CHF with most nanofluids, there are many contradictory findings concerning influence of nanofluids on nucleate boiling heat transfer coefficient. These contradictions are the result of many complex influences of base liquid, nanoparticles, and initial surface roughness. Despite the potential heat transfer benefits of nanofluids, there are several serious practical concerns that must be considered carefully before deploying nanofluids in practical cooling applications.
Review on the measurement and calculation of frost characteristics Int. J. Heat Mass Transf. (IF 3.458) Pub Date : 2018-04-05 Mengjie Song, Chaobin Dang
As a common physical phenomenon, frost deposition is inevitable and always has significant negative effects on several industry fields, such as aerospace, aviation, and heating, ventilation, air conditioning, and refrigeration. To accurately predict and control a frosting–defrosting cycle, there is a need to understand the interrelated heat, mass, and momentum transport phenomena within the frost and at the air–frost interface, which is a moving boundary condition. Consequently, during the past several decades, there has been a continuous effort to advance the understanding and modeling of frost formation on cold surfaces on the basis of experimental, semi-empirical, theoretical, and numerical approaches. To provide an overview of the analytical tools for scholars, researchers, product developers, and policy designers, a review and a comparative analysis of the available literature on frosting characteristics, correlations, and mathematical models are presented in this study. The mechanisms of the frost formation process and its influence will be first introduced, followed by the presentation of methods for the measurement of the frost layer thickness and the frosting rate. Then, the frost characteristics, including the accumulation, the density, the thermal conductivity and morphology, and the heat and mass transfer coefficients, will be summarized. The existing gaps in the research works on frost will be identified, and recommendations will be offered as per the viewpoint of the present authors. Finally, the conclusions of this study will be given.
Tribological behaviour of the steel/bronze friction pair (journal bearing type) functioning with selective mass transfer Int. J. Heat Mass Transf. (IF 3.458) Pub Date : 2018-04-05 Filip Ilie
Effect of CNT coating on the overall thermal conductivity of unidirectional polymer hybrid nanocomposites Int. J. Heat Mass Transf. (IF 3.458) Pub Date : 2018-04-05 M.K. Hassanzadeh-Aghdam, M.J. Mahmoodi, J. Jamali
The role of carbon nanotube (CNT) coating on the carbon fiber (CF) surfaces in the effective thermal conductivities of the unidirectional polymer hybrid nanocomposites is investigated by a newly presented multi-stage micromechanical method. The constructional feature of the hybrid nanocomposite is that randomly oriented CNTs grown on the CF surfaces. For simulating, a new version of the semi-empirical Halpin-Tsai (H-T) model is appropriately coupled with an analytical unit cell micromechanical model developed in the present research. The model captures the influences of the CNTs random dispersion, waviness, length, diameter, volume fraction and the CNT/polymer interfacial thermal resistance and also the CF cross-section shape parameters. The predicted results for the thermal conductivities of fibrous composites and polymer nanocomposites containing CNTs are verified with the available experimental data and a very good agreement is found. The results show that the longitudinal thermal conductivity of CF-reinforced hybrid nanocomposites is not affected by the CNTs coating. However, the nanocomposites transverse thermal conductivities are significantly enhanced over those of the conventional fibrous composites without the CNTs coating. An improvement in the nanocomposites transverse thermal conducting behavior can be observed with (i) increasing the CNTs volume fraction and length (ii) using straight CNTs and (iii) forming a perfect bonding interface.
Numerical simulation of natural convection in a horizontal enclosure: Part I. On the effect of adiabatic obstacle in middle Int. J. Heat Mass Transf. (IF 3.458) Pub Date : 2018-04-05 Jae Ryong Lee
In this study, an effect of a three-dimensional obstacle of natural convection in a horizontal enclosure was discussed. Geometry which was taken into account was horizontal enclosure with unit aspect ratio and length of π along spanwise direction. The enclosure was heated from the bottom wall, and then was cooled down from above. An obstacle was located in the middle of the enclosure to examine its effect. A three-dimensional solution was obtained using Chebyshev spectral multi-domain methodology for different Rayleigh number at which the thermal behavior was evolved from a steady state to a chaotic pattern. As the geometry was elongated in conjunction with periodic boundary conditions to allow lateral freedom for the convection cells, longitudinal geometry along spanwise direction was discretized through a Fourier series expansion with a uniform mesh configuration. An adiabatic obstacle played a different role in determining the thermal behavior: No-slip condition of the surface of the obstacle disturbed the overall plume behavior in terms of the momentum transfer, whereas the adiabatic boundary condition did not influence significantly in terms of energy transfer. At a low Rayleigh number, thermal behavior in three-dimensional enclosure showed steady invariant solution along spanwise direction which is identical to two-dimensional result. With increasing buoyant force, spanwise invariance of longitudinal roll cell was collapsed and three-dimensional mode was obtained following flow regime transition. After undergoing periodically oscillatory phase, a chaotic flow transition occurred. At a high Rayleigh number, three-dimensional thermal plume oscillates freely in elongated geometry and consequently yields higher heat transfer rate. In addition, the thermal flow field was captured by visualizing the three-dimensional vortical structure. The chaotic three-dimensional flow behavior was quantitatively examined by obtaining the turbulent statistics.
Non-uniform ground-level wind patterns in a heat dome over a uniformly heated non-circular city Int. J. Heat Mass Transf. (IF 3.458) Pub Date : 2018-04-05 Yifan Fan, Yuguo Li, Shi Yin
Experimental investigation of heat transfer characteristics and wall pressure distribution of swirling coaxial confined impinging air jets Int. J. Heat Mass Transf. (IF 3.458) Pub Date : 2018-04-05 Burak Markal
In the present study, swirling coaxial confined impinging turbulent air jets issuing from a novel designed nozzle is studied experimentally. Heat transfer characteristics and pressure distribution on the impingement plate are analyzed. Experiments have been conducted at different dimensionless nozzle-to-plate distances (H/D = 0.5, 1.0, 1.5, 2.0 and 2.5) and dimensionless flow rates (Q∗ = 0.25, 0.50 and 0.75) for a constant total flowrate of 1.33 × 10−3 m3 s−1 (80 L/min). The results show that the flowrate ratio improves the uniformity of the heat transfer through the impingement surface and increases the average Nusselt number. Also, the intensity of convective heat transfer is shown to enhance significantly with decreasing nozzle-to-plate distance. With regards to the pressure distribution, subatmospheric regions occur on the impingement plate. Contribution of swirl is also compared against the pure circular impingement jet condition (Q∗ = 0.0).
(Semi-)analytical solution of Luikov equations for time-periodic boundary conditions Int. J. Heat Mass Transf. (IF 3.458) Pub Date : 2018-04-05 R. Pečenko, N. Challamel, T. Colinart, V. Picandet
The paper addresses the problem of coupled heat and moisture transfer in porous materials with the time-periodic boundary conditions applied. The solution of Luikov equations , which describe coupled heat and moisture transfer, is presented. Laplace transform is used, where some terms of the inverse Laplace transform ought to be solved by Gaussian quadrature, meaning that the solution is semi-analytical. The time-periodic boundary conditions are applied to simulate the humidity and temperature oscillations of natural environment. Therefore, the proposed solution is appropriate to evaluate the distribution of moisture and temperature within the porous material exposed to everyday natural cycles. The paper presents convergence tests, validation of semi-analytical solution and application to different building materials are presented in the paper.
Liquid film thicknesses of oscillating slug flows in a capillary tube Int. J. Heat Mass Transf. (IF 3.458) Pub Date : 2018-04-05 Young Jik Youn, Youngbae Han, Naoki Shikazono
Liquid film thickness is an important parameter for predicting boiling and condensation heat transfer coefficients in a microscale slug flow. In the present study, liquid film thickness of oscillating slug flow in a capillary tube is experimentally investigated under adiabatic condition. Laser focus displacement meter is used to measure the initial liquid film thickness. Circular tube with inner diameter of 1 mm is used for the test tube, and water and ethanol are used as working fluids. Measurement is carried out using a capillary tube with one open end and the other connected to a stepping motor. Driving frequency is ranged from 1 to 10 Hz at equivalent slug stroke of 31.7 mm and 51.2 mm. As the frequency and equivalent slug stroke are increased, the liquid film thickness deviates from that in the steady condition and becomes thinner or thicker under flow acceleration or deceleration, respectively. The empirical correlations for the initial liquid film thickness under acceleration and deceleration conditions proposed in the previous study (Youn et al., 2015, 2016) well predict the liquid film thicknesses of the oscillating flows within 15% accuracy.
Mathematical model for thermal behavior of lithium ion battery pack under overcharge Int. J. Heat Mass Transf. (IF 3.458) Pub Date : 2018-04-05 Chuang Qi, Yanli Zhu, Fei Gao, Kai Yang, Qingjie Jiao
An overcharge model of lithium ion battery pack was built by coupling the electrochemical model with thermal abuse model. The pack consists of three fully-charged batteries, each of which has a capacity of 10 Ah, using Li[Ni1/3Co1/3Mn1/3]O2 as the positive electrode. The three batteries in the pack were juxtaposed, and only the middle one was overcharged. The influences of current, convection coefficient and gap between batteries on the thermal runaway propagation were studied. The results of temperature and voltage obtained from the models were validated experimentally, and they were agreed well with the experimental data with the relative error within 6%. The results showed that the onset temperature of thermal runaway of the charged battery increased with an increase in the current, while the temperatures for the other two decreased. The temperature rate of the charged battery changed little when the convection coefficient was greater than 40 W/m2 K. The clamp of lithium ion battery pack had an important effect on the thermal runaway propagation. The occurrence of thermal runaway propagation was depended on whether there was the existence of clamp when the battery gap exceeded 5 mm.
Direct simultaneous reconstruction for temperature and concentration profiles of soot and metal-oxide nanoparticles in nanofluid fuel flames by a CCD camera Int. J. Heat Mass Transf. (IF 3.458) Pub Date : 2018-04-05 Guannan Liu, Dong Liu
A reconstruction model based on inverse radiation analysis is presented to determine the temperature and concentration distributions of soot and metal-oxide nanoparticles in nanofluid fuel sooting flames using radiative intensities received by a CCD camera. The combined method consisting of the least-square QR decomposition (LSQR) algorithm and one dimensional searching was adopted to solve the inverse problem. Influences of ray number, wavelength combination, measurement error and metal-oxide nanoparticle concentration on the reconstruction accuracy were studied in details. The reconstructed results illustrated that the temperature distribution and soot concentration fields can be accurately retrieved, even with the measurement signal to noise ratio (SNR) as low as 39 dB, whereas the metal-oxide nanoparticle concentration field estimation process was more easily influenced by the measurement error and the practical metal-oxide nanoparticle concentrations. The proposed reconstruction method here is effective and robust for simultaneously retrieving the temperature distribution and concentration fields of soot and metal-oxide nanoparticles, even with noisy data.
The research on the heat source characteristics and the equivalent heat source of the arc in gaps Int. J. Heat Mass Transf. (IF 3.458) Pub Date : 2018-03-27 Kai Liu, Fan Yang, Shaohua Wang, Bing Gao, Chong Xu
The discharging process causes the multiple physical field coupling problem, which make it complicated to analyze the heat effect of discharging arc. The heat effect of the arc can be divided two parts including the heat effect of the arc column and the heat effect on the cathode surface. Therefore, this paper proposes an equivalent heat source to be equivalent to the two parts heat effect of the arc and simplifies the process for thermal analysis of the arc. Firstly, this paper analyzes the heat source characteristics of the arc based on MHD model. According to the heat source characteristics of the arc, this paper proposes respectively using ellipsoid heat source and Gaussian surface heat source to be equivalent to the two parts of the heat effect of the arc. Then the equivalent heat source is used to calculate the temperature distribution of tip-plane electrode under discharging. Comparing with the numerical results in MHD model, the error between the two models is within 0.5%, which proofs that the equivalent heat source can be used to equivalent to the heat effect of the arc. Moreover, this paper discusses the method to determine the parameters of the equivalent heat source. Finally, an experiment is carried out to verify the accuracy of the MHD model and the equivalent heat source of the arc.
Thermal diode using controlled capillary in heterogeneous nanopores Int. J. Heat Mass Transf. (IF 3.458) Pub Date : 2018-03-27 Tadeh Avanessian, Gisuk Hwang
The development of a compact, efficient, reliable thermal diode is crucial to improve advanced thermal management efficiency and controllability, and to enable brand new applications such as thermal logic gates and computers. In this study, we examine a nanoscale and efficient capillary-controlled thermal diode mechanism in Ar-filled Pt-based heterogeneous nanoporous structures, using Grand Canonical Monte Carlo (GCMC) simulation combined with Non-Equilibrium Molecular Dynamics (NEMD) simulation at the temperature range of 70–150 K and the pressure of 1.66 atm. Results show that the large thermal conductivity contrast between the controlled adsorption and capillary states using the structural heterogeneity (nanopillars on only one surface) and/or material heterogeneity (two different materials for nanogap surfaces) allows for the maximum thermal rectification ratio, Rmax ∼ 140 with minimal hysteresis under the cyclic operating temperatures −40 K < ΔT < +40 K. It is also found that the material heterogeneity is equivalent to the structural heterogeneity for minimizing the hysteresis in adsorption-capillary transition, but the heat flux across the nanogap with the material heterogeneity reduces due to weaker Ar-solid interaction. The coupled structural-material heterogeneity for the capillary-driven thermal diode is also discussed. The obtained results pave pathways for advanced thermal management systems such as thermal transistors, thermal logic gates, and computers.
Experimental study on boiling heat transfer of a self-rewetting fluid on copper foams with pore-density gradient structures Int. J. Heat Mass Transf. (IF 3.458) Pub Date : 2018-03-27 Leping Zhou, Wei Li, Tengxiao Ma, Xiaoze Du
Light-weight and high-surface-area metal foams used in phase change heat transfer may suffer flow resistance from the porous matrix and cause boiling deterioration. To alleviate the flow resistance, metal foams with pore-density gradient was proposed and significant enhancement of pool boiling heat transfer was achieved for fluids such as water and refrigerants. In this work, a self-rewetting fluid (aqueous n-butanol solution) was used for boiling on copper foams with pore-density gradient structures formed by using several layers of foam covers. The experimental results show that, comparing with the one-layer foam, the bubble departure phenomenon was substantially attenuated due to the largely increase of pore density and hence the bubble moving resistance when using a two- or three-layer foam structure. However, the increase of pore density can enhance the pool boiling of water when the foam thicknesses are the same due to more active cavity sites being formed in a denser metal foam. While the enhancement for the solution is not obvious especially for that in the foam structure with higher pore density and heat transfer deterioration may emerge at high heat fluxes, the boiling heat transfer of the solution can generally be enhanced by using the 110 ppi foam and its gradient structures as compared to the polished surface. This provides new insight into enhancing the boiling heat transfer utilizing both the surface properties formed in the pore-density gradient structure and the unique interfacial properties of the self-rewetting fluids.
Effect of non-condensable gas on pressure oscillation of submerged steam jet condensation in condensation oscillation regime Int. J. Heat Mass Transf. (IF 3.458) Pub Date : 2018-03-24 Weichao Li, Zhaoming Meng, Jianjun Wang, Zhongning Sun
The pressure oscillation is an important characteristic of direct contact condensation of steam in subcooled water. Many experimental works have been performed on pure steam submerged jet condensation. However, the effect of non-condensable gas content on the characteristics of the pressure oscillation is not yet fully understood. So, present paper aims to investigate the effect of air mass fraction on the characteristics of the pressure oscillation. Experimental results show that: for pure steam jets, the pressure oscillation dominant frequency decreases with the rise of water temperature and nozzle diameter. While it increases with the rise of steam mass velocity, which is consistent with the most of previous research results. The pressure oscillation intensity increases with the rise of water temperature, steam mass velocity and nozzle diameter. For air-steam mixture gas jets, the effect of water temperature and steam mass velocity on pressure oscillation characteristics is the same as pure steam jets. The pressure oscillation dominant frequency rapidly decreases with the rise of air mass fraction. However, air mass fraction has a complex effect on pressure oscillation intensity. As the air mass fraction increases, the pressure oscillation intensity rapidly increases at first, then slowly decreases and then slowly increases. In addition, new correlations for pressure oscillation dominant frequency and intensity are developed. The predicted results agree well with the experimental results.
Novel measurement of receding wicked liquid responsible for critical heat flux enhancement Int. J. Heat Mass Transf. (IF 3.458) Pub Date : 2018-03-26 Hong Hyun Son, Namgook Kim, Sung Joong Kim
In-situ hydrodynamic behavior of wicked liquid comes from interfacial dynamics at triple contact line, resulting in receding motion around expanding dry spot. We here introduce a new and creative technique of wicking experiment adopting an external pressure source equivalent to bubble nucleation pressure in order to investigate the receding behavior of wicked liquid. On the various types of surface morphology including smooth, nanostructure, nanoporous, and microstructure, it was clearly observed that wicked liquid receded from expanding dry area except for a smooth surface. The receding velocity was slower at microstructure, nanoporous, and nanostructure, in order. Clearly this result provides a hydrodynamic evidence of smaller dry area size and contact line length on microscale structure than on nanoscale structure. Moreover, the diameter of dry area showed a linear relation with CHF enhancement that indicates smaller diameter of dry area is more effective to delay irreversible expansion of dry spots. This novel observation is expected to provide reliable analysis of contact line dynamics with CHF enhancement on wicking-dominant surfaces.
Nanofluid flow and heat transfer in a microchannel with interfacial electrokinetic effects Int. J. Heat Mass Transf. (IF 3.458) Pub Date : 2018-03-26 Qingkai Zhao, Hang Xu, Longbin Tao
The behaviour of microchannel flow of a nanofluid between two parallel flat plates in the presence of the electrical double layer (EDL) is investigated in this paper. The problem is formulated based on the Buongiorno nanofluid model with the electrical body force due to the EDL being considered in the momentum equation. As one of the highlights of the present investigation, the unphysical assumption introduced in previous studies often leading to the discontinuities of flow field that the electrostatic potential in the middle of the channel has to be equal to zero is eliminated. In addition, the inappropriate assumption that the pressure constant is treated as a known condition is also rectified. The new model is developed with the governing equations being reduced by a set of dimensionless quantities to a set of coupled ordinary differential equations. Based on the analytical approximations, the influences of various physical parameters on the flow field and temperature field, and the important physical quantities of practical interests are analysed and discussed in detail.
Total hemispherical apparent radiative properties of the infinite V-groove with specular reflection Int. J. Heat Mass Transf. (IF 3.458) Pub Date : 2018-03-26 Rydge B. Mulford, Nathan S. Collins, Michael S. Farnsworth, Matthew R. Jones, Brian D. Iverson
Multiple reflections in a cavity geometry augment the emission and absorption of the cavity opening relative to a flat surface in a phenomenon known as the cavity effect. The extent of the cavity effect is quantified using apparent absorptivity and apparent emissivity. Analysis of complicated thermal systems is simplified through application of apparent radiative properties to cavity geometries. The apparent radiative properties of a specularly-reflecting, gray, isothermal V-groove have been derived analytically, but these results have not been validated experimentally or numerically. Additionally, the model for apparent absorptivity of an infinite V-groove subjected to partial illumination in the presence of collimated irradiation is not available. In this work, the following existing models for a specularly-reflecting V-groove are collected into a single source: (1) the apparent absorptivity of a diffusely irradiated V-groove, (2) the apparent emissivity of an isothermal V-groove and (3) the apparent absorptivity of a V-groove subject to collimated irradiation with full-illumination. Further, a new analytical model is developed to predict the apparent absorptivity of an infinite V-groove subject to collimated irradiation with partial-illumination. A custom, Monte Carlo ray tracing solver is used to predict the apparent radiative properties for all cases as a means of numerical verification by comparing the ray tracing data with the results from the new model in this work and the previously existing models. For diffuse irradiation, the analytical model and ray tracing data show excellent agreement with an average discrepancy of 4.4 × 10−4, verifying the diffuse-irradiation analytical model. Similar agreement is found for collimated irradiation, where the full and partial illumination models indicate average discrepancies of 4.9 × 10−4 and 4.6 × 10−4 when compared with ray tracing data.
Buoyancy-driven flow of nanofluids in an inclined enclosure containing an adiabatic obstacle with heat generation/absorption: Effects of periodic thermal conditions Int. J. Heat Mass Transf. (IF 3.458) Pub Date : 2018-03-22 Sameh E. Ahmed, Hillal M. Elshehabey
The buoyancy-driven heat transfer enhancement and fluid flow of nanofluids inside inclined enclosures in the presence of heat generation/absorption effect are investigated in this paper. The bottom and top walls of the enclosure are thermally insulated, while both side walls are considered to have sinusoidal distributions of thermal boundary conditions. Two cases are considered; an enclosure containing a cold obstacle and the other one contains a cold circular cylinder. The governing equations are converted to dimensionless forms and then solved using Galerkin finite element method. Effects of the key-parameters, namely, Rayleigh number, nanoparticle volume fraction, cavity inclination angle, heat generation/absorption parameter, amplitude parameter, phase angle, size and position of the inner shapes on the contours of streamlines and isotherms as well as average Nusselt number are examined. It is found that an enclosure with a square obstacle enhances the heat transfer rate with a higher rate comparing with the circular cylinder case. Also, Considering an inclined cavity gives rate of heat transfer greater than the horizontal/vertical cavity. In addition, regardless the inner shape, the average Nusselt number is an increasing function of nanoparticle volume fraction, amplitude parameter and phase angle.
Approximate, analytical procedure for rectangular annular fins by accommodating the Cauchy–Euler equation Int. J. Heat Mass Transf. (IF 3.458) Pub Date : 2018-03-22 Antonio Campo, Agustín M. Delgado-Torres
An approximate, analytical treatment is presented for the rectangular annular fin by transforming the complicated modified Bessel equation of zero order into a rudimentary Cauchy-Euler equation. The essential step in the computational procedure revolves around a simple manipulation of the radial coordinate that sets up a variable coefficient in the third term of the modified Bessel equation of zero order. In the third term, the radial variable will be replaced by the mean radius of the inner and outer radius, whereas the radial variable prevails in the first and second terms. This action paves the way to the easier Cauchy-Euler equation. For a collection of rectangular annular fins of interest in engineering applications, approximate, analytical temperature distributions and heat transfer rates (via the fin efficiency) written in terms of two binomials demonstrate excellent quality levels in all cases. Additionally, relative error distributions are presented in detailed manner using as the baseline cases the classical exact, analytical temperature distributions and heat transfer rates expressed in terms of the complicated modified Bessel functions of first and second kind.
Quantitative measurements of nanoscale thin frost layers using surface plasmon resonance imaging Int. J. Heat Mass Transf. (IF 3.458) Pub Date : 2018-03-22 Chan Ho Jeong, Dong Hwan Shin, Vinaykumar Konduru, Jeffrey S. Allen, Chang Kyoung Choi, Seong Hyuk Lee
This study reports the presence of a nanoscale thin frost layer. During the frosting process, the surface plasmon resonance (SPR) imaging method can be used to overcome conventional optical limits and quantify this layer. The research outlined here also provides quantitative thickness measurement of the thin frost layer via a proposed calibration method based on the measured SPR intensity. The SPR system established in this study consists of a 50 nm gold-coated BK7 cover glass, a prism, a light source, a polarizer, a lens and a filter for the collimated light of a 600 ± 5 nm wavelength, and a CCD camera. The SPR angle of the ice phase is 72°, which corresponds to the ice refractive index of 1.307. The gold-glass specimen is cooled from room temperature (23 ± 1 °C) to −4.0 ± 0.8 °C by using a thermoelectric cooler to maintain the relative humidity of 20 ± 3% (at the room temperature). As a result, it is found that the nanoscale thin frost layer between the frozen condensates exists on the surface. Also, the present study yields the spatial distribution of reflectance that is associated with the frost layer thickness, indicating that the local information about thin frost layer thickness can be obtained through this SPR imaging method. It is found that the SPR imaging method enables successful capture of the depthwise spatial variations of the thin frost layer, showing that the frost layer was grown over time as a result of the de-sublimation of water vapor.
Heat transfer and fluid flow characteristics of a pair of interacting dual swirling flame jets impinging on a flat surface Int. J. Heat Mass Transf. (IF 3.458) Pub Date : 2018-03-22 Parampreet Singh, Subhash Chander
Experimental and numerical studies have been conducted to investigate the flow field and heat transfer characteristics of a pair of dual interacting swirling flames impinging on a flat surface. Commercial computational fluid dynamics (CFD) code (FLUENT®) has been used to simulate the interacting isothermal swirling impinging jets. Inverse heat conduction procedure (IHCP) has been used to calculate the impingement heat fluxes from the surface temperatures captured by Infra-red camera. Effect of separation distance (H/Dh = 2.5, 4, 6 and 8) and inter-jet spacings (S/Dh = 4, 6, 8 and 10) have been studied at various Reynolds numbers (Re(o) = 7000, 9000, 11000, 13,000 and Re(i) = 700, 1000, 1300) under stoichiometric conditions. Strong interactions between adjacent dual swirling flames result in high heat transfer due to increased mixing and turbulence in the interaction region. The inner non-swirling flames are seen to deflect towards interacting side due to asymmetric interactions. Numerical simulation predicted this deflection to be primarily due to large recirculation bubble developed from asymmetric interactions. Tilted cross-flow, emerging from interaction region has been observed due to momentum exchange taking place between cross-flow and swirling flames (jets). Area weighted average of local heat flux and relative deviation from averaged value has been calculated at various H/Dh and S/Dh. High average heat fluxes are obtained at smallest H/Dh and S/Dh. It has been concluded that for a system of burners considered for the present study, H/Dh = 2.5 and S/Dh = 8 is the optimum configuration on the basis of minimum relative deviation.
Mixing improvement induced by the combination of a micro-ramp with an air porthole in the transverse gaseous injection flow field Int. J. Heat Mass Transf. (IF 3.458) Pub Date : 2018-03-22 Lang-quan Li, Wei Huang, Li Yan, Shi-bin Li, Lei Liao
A new injection strategy combined with a micro-ramp and an air porthole is proposed in this paper, and the properties of the transverse gaseous injection flow field with such injection strategy have been investigated simultaneously. The numerical approach employed in the current study has been validated against the two-dimensional and three-dimensional experimental data in the open literature, and it can be used with confidence to investigate the influence of the air porthole aspect ratio and the distance between the air porthole and the fuel orifice on the transverse injection flow field with the combination of a micro-ramp and an air porthole. The obtained results predicted by the three-dimensional Reynolds-average Navier–Stokes (RANS) equations coupled with the two equation k-ω shear stress transport (SST) turbulence model show that the mixing performances of the transverse gaseous injection flow fields vary under different conditions, and a transverse injection flow field with short mixing length, low stagnation pressure loss and ideal fuel penetration depth has been achieved by adding the combination of an optimized micro-ramp with a proper air porthole, i.e. Case 6–8, and its mixing length decreases considerably by 14.26 mm on the basis of Case c, even shorter than the mixing length of Case a by 2.86 mm. However, its total pressure loss increases when compared with Case c, and its stagnation pressure loss is 2.7 percent smaller than Case a. Further, the hydrogen distribution on the flat plate of Case 6–8 is much less than that of Case a and Case b. Additionally, it is found that the mixing enhancement mechanism of the air jet is different from that of the micro-ramp. The micro-ramp enhances the mixing process between the fuel and air by inducing large-scale vortices, while the air porthole enhances the mixing process by seeding lots of air into the fuel boundary layer, as well as fuel plume.
Cascade-like and cyclic heat transfer characteristics affected by enclosure aspect ratios for low Prandtl numbers Int. J. Heat Mass Transf. (IF 3.458) Pub Date : 2018-03-23 Bin Wang, Tien-Mo Shih, Xiwen Chen, Richard Ru-Gin Chang, Chen-Xu Wu
For low Prandtl-number fluids, heat-transfer characteristics under the influence of the aspect ratio for buoyancy-driven recirculating flows in rectangular enclosures (with left hot, right cold, top/bottom insulated walls) behave differently from those for high-Pr fluids. At Ra=106 Ra = 10 6 and Pr=0.025 Pr = 0.025 , as the enclosure widens, time-averaged and hot-wall-spatially-averaged heat transfer first decreases, then cascades downward. Locally, heat transfer peaks at a few locations, and these peaks travel cyclically as time elapses. For completeness, the present study serves as a sequel of a previous high-Pr investigation.
Numerical and experimental investigation on thermal shock failure of Y2O3-coated CVD ZnS infrared windows Int. J. Heat Mass Transf. (IF 3.458) Pub Date : 2018-03-23 Xing Liu, Jiaqi Zhu, Jiecai Han
Infrared transparent windows on aircraft and missiles can be subjected to extreme aerothermodynamics, which can cause thermal shock failure. Finite element analysis and oxygen–propane flame jet impingement tests were performed to investigate the thermal shock failure of an yttrium oxide-coated chemical vapor deposition (CVD) ZnS infrared window. Good agreement was achieved between the simulation and experimental results, which indicated that thermal shock failure occurs under high temperature differences and thermal stresses. The temperature and stress in the samples increased rapidly in a few seconds and then trended to be stable. The center area of the window surface failed most easily because the maximum temperature and stress both occurred in this area. No delamination of the Y2O3 films occurred during the thermal shock, which indicated good adhesion between the Y2O3 films and CVD ZnS substrate. In the experiment, the center area of the specimen surface was damaged in the form of pits and line cracks.
A new rate-transient analysis model for shale gas reservoirs coupled the effect of slip flow and surface diffusion Int. J. Heat Mass Transf. (IF 3.458) Pub Date : 2018-03-22 Yanan Miao, Xiangfang Li, Yunjian Zhou, John Lee, Zheng Sun, Yucui Chang, Shan Wang, Chenhong Hou
Forecasting production in shale gas reservoirs accurately has been of growing interest in the industry. Horizontal wells with multiple fractures are commonly utilized to develop shale reservoirs, which indicates that the dominant flow regime observed will be linear flow for several years. Until now, it has been widely recognized that the rate-transient data analysis is the most efficient approach to estimate rate, where it appears as a straight line on the plot of normalized pressure vs. square root of time in linear flow. However, the traditional square-root-of-time plot may result in overestimation of reservoir properties and will not allow us to forecast production with confidence in shale gas reservoirs. In this paper, a new analytical methodology is put forward to analyze the rate-transient data from fractured wells in shale gas reservoirs producing at a constant flowing-pressure, which incorporates both slip flow/Knudsen diffusion of bulk gas and surface diffusion of adsorbed gas directly into the model. These flow mechanisms cannot be well described by traditional models. Depending on flow discrepancies from conventional reservoirs, the modified pseudo-pressure and pseudo-time equations to account for these critical transport mechanisms are constructed. In addition, a new procedure for rate-transient data analysis applying the proposed model is presented in details, which is reliable and easy to utilize. The novel approach is validated against numerically simulated cases and field observations. Comparisons between the new approach and traditional method are conducted by a number of test cases. The results demonstrate that the newly developed model dramatically eliminates the inaccuracy of production forecast and provides a more reliable estimated ultimate recovery (EUR). This work should provide an efficient guidance to assist analysts in evaluating hydrocarbon production accurately in shale gas reservoirs.
Experimental investigation of hydronic air coil performance with nanofluids Int. J. Heat Mass Transf. (IF 3.458) Pub Date : 2018-03-22 Roy Strandberg, Debendra Das
The objective of this study is to experimentally characterize and compare the performance of a nanofluid comprised of Al2O3 nanoparticles with 1% volumetric concentration in a 60% ethylene glycol/40% water (60% EG) solution to that of 60%EG in a liquid to air heat exchanger. The test bed used in the experiment was built to simulate a small air handling system typical of that used in HVAC applications. Previously established empirical correlations for thermophysical properties of fluids were used to determine the values of various parameters (e.g. Nusselt number, Reynolds number, and Prandtl number). The testing shows that the 1% Al2O3 nanofluid generates a marginally higher rate of heat transfer than the 60% EG under certain conditions. At Re = 3000, the nanofluid produced a rate of heat transfer that was 2% higher than that of the 60% EG. The empirically determined Nusselt number associated with the convection in the coil tubing for the nanofluid follows the behavior predicted by the Dittus-Boelter correlation (R2 = 0.97), while the empirically determined Nusselt number for the 60% EG follows the Petukhov correlation similarly (R2 = 0.97). Pressure loss and hydraulic power for the nanofluid were higher than for the base fluid over the range of conditions tested. The exergy destroyed in the heat exchange and fluid flow processes were between 9% and 12% lower for the nanofluid than the base fluid over the tested range of Reynolds numbers.
Innovative analytic and experimental methods for thermal management of SMD-type LED chips Int. J. Heat Mass Transf. (IF 3.458) Pub Date : 2018-03-22 Hosung Jang, Jae Hwa Lee, Chan Byon, Byeong Jun Lee
In this study, we propose innovative analytic and experimental methods for thermal management of SMD-type LED chips: a geometry optimization algorithm of natural convective heat sinks together with a novel technique for estimation of the LED surface temperature. An analytic algorithm for the optimal design of the LED heat sink is proposed. By using this algorithm, the optimal fin configuration and corresponding thermal performance of the heat sink can be readily predicted according to the inputted base plate dimensions, ambient condition, heat dissipation rate, and LED chip distributions. In addition, a novel experimental technique for an accurate measurement of the LED junction temperature is proposed based on infrared thermometry and an isothermal chamber with an observation hole. The LED junction temperature is also measured using T3ster method, and the results are compared with those from the aforementioned infrared thermometry and analytic procedure. The proposed analytic and experimental results are shown to agree with each other well. The present analytic model is well validated by experimental results, and can be widely utilized for designing the cooling system related to various LED products.
Thermodynamic optimization for an air-standard irreversible Dual-Miller cycle with linearly variable specific heat ratio of working fluid Int. J. Heat Mass Transf. (IF 3.458) Pub Date : 2018-03-22 Zhixiang Wu, Lingen Chen, Yanlin Ge, Fengrui Sun
This paper establishes an air-standard irreversible Dual-Miller cycle (DMC) model with the specific heat ratio (SHR) of working fluid (WF) linearly varying with its temperature. Because the specific heat (SH) of WF varies with combustion reaction in actual internal combustion engine (ICE), the SHR of WF should be a function of temperature but not a constant. In order to accurately reflect the practical characteristics of DMC engine, performance of DMC with linearly variable SHR, and with heat transfer (HT) loss, friction loss (FL) and other internal irreversible losses (IILs) is analyzed and optimized by applying finite-time thermodynamics. Analytical formulae of the power output (P P ), efficiency (η η ), entropy generation rate (EGR) and ecological function (E E ) are derived. Relationships among P P , η η , E E and compression ratio are obtained via numerical calculations. Effects of the design parameters, cycle temperatures and linearly variable SHR of WF on P P , η η and E E are investigated. Performance differences among the DMC and its simplified cycles, including Otto cycle (OC), Dual cycle (DDC) and Miller cycle (OMC) are compared. Performance characteristics of the DMC with different optimization objective functions (OOFs) are analyzed. The results indicate that the maximum power output (MP MP ), maximum efficiency (MEF MEF ) and maximum ecological function (ME ME ) of the DMC are superior to those of OC, DDC and OMC, and optimizing E E is the best compromise between optimizing P P and optimizing η η . The presented results may be helpful to optimize the performance of practical DMC engines.
A computational and experimental study of thermal energy separation by swirl Int. J. Heat Mass Transf. (IF 3.458) Pub Date : 2018-03-21 B. Kobiela, B.A. Younis, B. Weigand, O. Neumann
When compressed air is introduced into a tube in such a way as to generate a strong axial vortex, an interesting phenomenon is observed wherein the fluid temperature at the vortex core drops below the inlet value, while in the outer part of the vortex, the temperature is higher than at inlet. The most familiar manifestation of this phenomenon is known as the Ranque-Hilsch effect, and several alternative explanations for it have been proposed. In this study, we present an analysis of the heat transfer mechanism underlying this phenomenon, based on consideration of the exact equation governing the conservation of the turbulent heat fluxes. The outcome is a model that explicitly accounts for the dependence of the heat fluxes on the mean rates of strain, and on the gradients of mean pressure. These dependencies, which are absent from conventional closures, are required by the exact equation. To verify the model, an experimental investigation of flow in a swirl chamber was conducted, and the measurements were used to check the model’s performance as obtained by three-dimensional numerical simulations. Comparisons between predictions and measurements demonstrate that the new model yields predictions that are distinctly better than those obtained using conventional closures.
The thermal and hydrodynamic behaviour of confined, normally impinging laminar slot jets Int. J. Heat Mass Transf. (IF 3.458) Pub Date : 2018-03-20 Andrew Sexton, Jeff Punch, Jason Stafford, Nicholas Jeffers
Jet impingement cooling is an effective means of inducing high convective heat transfer coefficients for applications such as the cooling of gas turbine surfaces, drying of textiles, and the tempering of metal and glass. Slot jets can be used for applications which require cooling over a line or strip, and these can be realized at the micro-scale if size is constrained. In this paper the application of these jets is of specific interest for high-density photonic integrated circuits (PICs), which can generate device-level heat fluxes as high as 1 kW / cm 2 . In this context, an understanding of the thermal and hydrodynamic behaviour of low Reynolds number, submerged, confined and normally impinging slot jets is currently unavailable in the literature and would be beneficial for the design of micro-scale cooling systems. This investigation utilized the isoflux heated foil technique and Particle-Image Velocimetry (PIV), to measure the single-phase convective heat transfer and velocity fields associated with confined liquid slot jets. The aspect ratios of the jets were varied from 1 to 8, with a constant hydraulic diameter, in order to determine the influence of aspect ratio on the measured parameters. The study was carried out over a jet exit Reynolds number range of 100–500, and a fixed confinement ratio of H/Dh=1 H / D h = 1 . The results showed that, enhancements of up to 68% in area-averaged heat transfer could be achieved through increasing slot jet aspect ratio, with a corresponding 12% reduction in head loss coefficient. The appearance of off-center peaks in the velocity and Nusselt number distributions were also observed. These peaks were concluded to be as a result of the stagnation zone fluid dynamics and local flow entrainment.
Keyhole dynamic thermal behaviour in K-TIG welding process Int. J. Heat Mass Transf. (IF 3.458) Pub Date : 2018-03-20 ZuMing Liu, ShiYu Chen, Shang Liu, Zhen Luo, JunRui Yuan
Fully penetrated keyhole welding process can be achieved by employing the free burning arc in Tungsten Inert Gas (TIG). The interaction between the heat source and weld pool determines the dynamic behaviour of the keyhole and the resultant weld quality. To determine the energy transfer behaviour, in this paper, the dynamic keyhole behaviour was imaged with the vision system when the heat source was changed by adjusting the welding current or welding speed in real time. It was found that (1) keyhole exit positional and dimensional parameters were all changed as a response to the welding speed changing; (2) keyhole exit size parameters changed much faster than the positional parameters did when the welding current was suddenly decreased or increased; (3) a sharp pulse emerged in the position curve of the keyhole exit center point after the welding current was suddenly changed. The behaviour mechanism was analyzed in view of dynamic changing of the force field and heat field in the arc-keyhole-weld pool system.
Selectively enhanced near-field radiative transfer between plasmonic emitter and GaSb with nanohole and nanowire periodic arrays for thermophotovoltaics Int. J. Heat Mass Transf. (IF 3.458) Pub Date : 2018-03-20 Haitong Yu, Yuanyuan Duan, Zhen Yang
To design a nano-gap thermophotovoltaic device with selectively enhanced radiative transfer above the cell’s bandgap, this work theoretically investigated the near-field radiative transfer from a plasmonic Drude emitter to a nanostructured GaSb absorber, with a finite-thickness surface layer of nanowire or nanohole arrays, across a 200 nm vacuum gap. The Fourier Modal method (FMM) is used to rigorously characterize the radiative transfer involving diffractive periodic structures. The results showed that the added nanostructure, especially nanowires, effectively and selectively enhanced the near-field radiative transfer above the bandgap, with a maximum of three times the spectral radiative heat flux when compared to the unstructured GaSb case. By considering periodic structures in two dimensions, this work revealed the difference between the nanowire and nanohole absorbers in manipulating of the radiative heat flux, showing that the nanowire array offers largely enhanced radiative heat transfer compared with the nanohole arrays with similar geometric parameters, which cannot be quantitatively characterized by effective medium theories even though the structural size is much smaller than the studied wavelength. The results proved that nanohole and nanowire structures can be used to significantly enhance the power and efficiency of a nano-gap thermophotovoltaic device, for which the equivalent of anti-reflection structures of the semiconductor cells have seldom been studied.
Measured and predicted upward flow boiling heat transfer coefficients for hydrocarbon mixtures inside a cryogenic plate fin heat exchanger Int. J. Heat Mass Transf. (IF 3.458) Pub Date : 2018-03-20 Rixin Li, Jinping Liu, Jiayao Liu, Xiongwen Xu
Boiling of hydrocarbon mixtures inside plate fin heat exchangers (PFHEs) is most prevalent in many cryogenic processes of petrochemical plants. However, there is very little experimental data and a lack of prediction methods regarding boiling mixtures in PFHEs. This paper established an experimental setup that utilized a single-stage cryogenic cycle to measure the boiling heat transfer coefficients (HTCs) for hydrocarbon mixtures inside a perforated PFHE. The components of the mixtures include methane, ethylene, propane and isobutene. Twenty-seven sets of experiments were conducted under various operating conditions and 261 data points of the boiling HTC were obtained. The experimental conditions cover the mass fluxes of 3.69–19.38 kg m−2 s−1, pressures of 1.35–5.22 bar and vapor qualities of 0.05–0.77, which are representative of a wide range of actual industrial conditions. The corresponding heat fluxes only range from 55.7 to 3837.3 W m−2. The experimental boiling HTC varies from 21.4 to 1055.7 W m−2 K−1, and it is a strong function of the heat flux. Moreover, twelve existing correlations were assessed by using the present experimental data. The results show that the HTCs predicted by Chen type correlations were not consistent with the experimental results, and a new correlation developed from mini-channel correlations was recommended.
Experimental and numerical modeling of heat transfer in directed thermoplates Int. J. Heat Mass Transf. (IF 3.458) Pub Date : 2018-03-20 Imane Khalil, Ryan Hayes, Quinn Pratt, Christopher Spitler, Daniel Codd
We present three-dimensional numerical simulations to quantify the design specifications of a directed thermoplate expanded channel heat exchanger, also called dimpleplate. Parametric thermofluidic simulations were performed independently varying the number of spot welds, the diameter of the spot welds, and the thickness of the fluid channel within the laminar flow regime. Results from computational fluid dynamics simulations show an improvement in heat transfer is achieved under a variety of conditions: when the thermoplate has a relatively large cross-sectional area normal to the flow, a ratio of spot weld spacing to channel length of 0.2, and a ratio of the spot weld diameter with respect to channel width of 0.3. Experimental results performed to validate the model are also presented.
Measurement and modeling of thermal conductivity of graphene nanoplatelet water and ethylene glycol base nanofluids Int. J. Heat Mass Transf. (IF 3.458) Pub Date : 2018-03-20 Yuguo Gao, Haochang Wang, Agus P. Sasmito, Arun S. Mujumdar
Three graphene nanoplatelet (GNP) nanofluids with different base fluids, viz. ethylene glycol (EG), deionized water (DW), and EG/DW (1:1) were prepared and characterized. The stability of GNP nanofluid was analyzed. Thermal conductivity was tested over the temperature range −20 °C to 50 °C. A new model is proposed for the effective thermal conductivity of the GNP nanofluid considering Brownian motion, length, thickness, average flatness ratio and interfacial thermal resistance of GNP, and it was compared with Maxwell, H-C and Chu models. The maximum thermal conductivity enhancement of EG, EG/DW (1:1) and DW based nanofluid is 4.6%, 18% and 6.8% respectively. Interestingly, the thermal conductivity of EG based GNP nanofluids does not show appreciable enhancement. The thermal conductivity enhancement of EG/DW (1:1) GNP nanofluid is greater than that of pure EG GNP nanofluid. In particular, the enhancement ratio at subzero temperature is larger than that at higher temperatures. The new model and Chu model are in agreement with the experimental data, and the new model is more rational for the GNP nanofluids. The new model shows that the influence of Brownian motion of GNP on thermal conductivity is significant at higher temperatures, higher concentration and smaller nanoparticles.
Ferrofluids for heat transfer enhancement under an external magnetic field Int. J. Heat Mass Transf. (IF 3.458) Pub Date : 2018-03-20 N. Gan Jia Gui, C. Stanley, N.-T. Nguyen, G. Rosengarten
Overheating of power electronic devices has become a significant issue due to their continued miniaturization and increased heat flux that needs to be dissipated. Ferrofluids (magnetic nanofluids) have been shown to have higher thermal conductivity than their base aqueous or oil based fluids due to the solid magnetic nanoparticles that make up the ferrofluid. This allows higher convective heat transfer rates and, importantly, the ability to externally effect the flow using a magnetic field. In this paper, we focus on material characterization of ferrofluids and measurement of heat transfer rates for single-phase ferrofluidic forced convective flow in microchannels. We show that heat transfer properties of the flow are enhanced with the use of ferrofluids and that the material make-up of the ferrofluid affects these properties. In this paper, we argue that generally, convective heat transfer rates for ferrofluids are increased by increasing the solid volume concentration of magnetic particles (∼0.2–0.4%). Interestingly, increasing magnetic flux was shown to decrease heat transfer enhancement. This was due to a reduction in the thermal conductivity of the bulk fluid caused by magnetic nanoparticles being drawn out of the isotropic mixture and becoming pinned to the channel wall in the region of strongest magnetic field. We show that there is good correlation between both theory and experimental visualization of this phenomenon.
A novel approach to temperature-dependent total emissivity estimation based on isothermal cooling Int. J. Heat Mass Transf. (IF 3.458) Pub Date : 2018-03-20 Zujun Peng, Weihua Xie, Songhe Meng, Weizhen Pan, Shanyi Du
This paper proposes a novel approach to estimate the temperature-dependent total emissivity based on the isothermal cooling process. The realizability of the ideal isothermal cooling phase has been verified through the finite-element model, even for specimens with low thermal conductivity. During the cooling process, the temperature difference of the specimen can be less than 0.8%. Moreover, based on the temperature curve of isothermal cooling, nonlinear total emissivity can be calculated using the inverse method over a broad temperature range, e.g., 500–1200 K, and the relative errors will be less than 8.83% with virtual test data. For a real-world application, we introduce the Savitzky-Golay (SG) filter to weaken the effect of noise on computation precision and successfully gain the smooth temperature-dependent total emissivity.
Experimental investigation on the characteristic of jet break-up for butanol droplet impacting onto a heated surface in the film boiling regime Int. J. Heat Mass Transf. (IF 3.458) Pub Date : 2018-03-20 Chunze Cen, Han Wu, Chia-fon Lee, Fushui Liu, Yikai Li
In view of the importance of atomization, droplet breakup induced by high Weber number and vapor bubble during droplet impacting onto a solid and heated surface has been studied extensively, but the understand on jet break-up phenomenon in the film boiling regime is still lacking. Thus, the work is trying to study the dynamic of jet break-up of n-butanol, a potential alternative biofuel for internal combustion engine, under the influence of Weber number. During experiment, a high-speed camera, set at 512 × 512 pixels, 10,000 fps, and 20 μs exposure time was used to visualize the droplet impacting behavior. The droplet falling height was set from 1 cm to 9 cm with interval of 0.5 cm, with corresponded Weber number around from 6.94 to 102.12. The heated surface temperature was set at 250 °C, to ensure the impact locates at the film-boiling regime. The results show that the jet break-up is dominated by Rayleigh-Plateau instability, the secondary droplet is formed through contraction of symmetric unstable surface waves since the waves are clearly observed on the jet column. Under tested wall conditions, the jet break-up only takes place when the Weber number of butanol droplet is around from 14.34 to 89.13. Within the jet break-up regime, the number of separated droplets first increases then decreases with Weber number increasing, and reaches the maximum at We = 65. The length of jet break-up first increases slowly then decreases rapidly and then increases with Weber number. While, the time of jet break-up first decrease then increase with Weber number and also reaches the minimum at We = 65.20. In addition, the timing of jet break-up is fitting well with the theory of Rayleigh instability.
A simple efficient method of nanofilm-on-bulk-substrate thermal conductivity measurement using Raman thermometry Int. J. Heat Mass Transf. (IF 3.458) Pub Date : 2018-03-20 Vladimir Poborchii, Noriyuki Uchida, Yoshinobu Miyazaki, Tetsuya Tada, Pavel I. Geshev, Zhandos N. Utegulov, Alexey Volkov
In contrast to known Raman-thermometric measurements of thermal conductivity (k) of suspended Si nano-membranes, here we apply Raman thermometry for k measurement of mono- and nano-crystalline Si films on quartz, which is important for applications in thermoelectricity and nanoelectronics. Experimentally, we measure linear dependence of the laser-induced Raman band downshift, which is proportional to the moderate heating ΔT, on the laser power P. Then we convert the downshift to ΔT and determine the ratio ΔT/P. The actual power absorbed by the film is calculated theoretically and controlled experimentally by the reflection/transmission measurement. Then we calculate ΔTcalc/P for arbitrary film k assuming diffusive phonon transport (DPT). Film k is determined from the condition ΔT/P = ΔTcalc/P. We show that this method works well for films with thickness h > Λ, where Λ is phonon-mean-free path, even for low-k films like nano-crystalline Si and SiGe. For h < Λ, despite ballistic phonon transport contribution, this approach works when the in-plane DPT dominates, e.g. in Si films on quartz with h ≥ 60 nm. We also show that the influence of thermal boundary resistance on the determined k is negligible at this condition. The proposed method is simple and time efficient, as dozen of films can be examined in one hour.
Mechanistic model to predict frequency and amplitude of Density Wave Oscillations in vertical upflow boiling Int. J. Heat Mass Transf. (IF 3.458) Pub Date : 2018-03-20 Lucas E. O'Neill, Issam Mudawar
Modeling of two-phase flow transient behavior and instabilities has traditionally been one of the more challenging endeavors in heat transfer research due to the need to distinguish between a wide range of instability modes systems can manifest depending on differences in operating conditions, as well as the difficulty in experimentally determining key characteristics of these phenomena. This study presents a new mechanistic model for Density Wave Oscillations (DWOs) in vertical upflow boiling using conclusions drawn from analysis of flow visualization images and transient experimental results as a basis from which to begin modeling. Counter to many prior studies attributing DWOs to feedback effects between flow rate, pressure drop, and flow enthalpy causing oscillations in position of the bulk boiling boundary, the present instability mode stems primarily from body force acting on liquid and vapor phases in a separated flow regime leading to liquid accumulation in the near-inlet region of the test section, which eventually departs and moves along the channel, acting to re-wet liquid film along the channel walls and re-establish annular, co-current flow. This process was modeled by dividing the test section into three distinct control volumes and solving transient conservation equations for each, yielding predictions of frequencies at which this process occurs as well as amplitude of associated pressure oscillations. Values for these parameters were validated against an experimental database of 236 FC-72 points and show the model provides good predictive accuracy and capably captures the influence of parametric changes to operating conditions.
Investigation of subcooled and saturated boiling heat transfer mechanisms, instabilities, and transient flow regime maps for large length-to-diameter ratio micro-channel heat sinks Int. J. Heat Mass Transf. (IF 3.458) Pub Date : 2018-03-20 Seunghyun Lee, V.S. Devahdhanush, Issam Mudawar
This study investigates the interfacial behavior and heat transfer mechanisms associated with flow boiling of R-134a in a micro-channel test module. The test module features 100 of 1 × 1 mm2 square micro-channels. Large length of micro-channels used (609.6 mm) is especially important to capturing broad axial variations of both flow and heat transfer behavior. The fluid is supplied to the test module in subcooled state to enable assessment of both the subcooled boiling and saturated boiling regions. The study employs a combination of temperature measurements along the test module and high-speed video to explore crucial details of the flow, including dominant flow regimes, flow instabilities, and downstream dryout effects. It is shown that, unlike macro-channel flows, where flow regimes can be clearly demarcated, flow regimes in micro-channels are associated with transient fluctuations that are induced by flow instabilities. The dominant flow behavior and associated dryout effects are characterized with the aid of a new transient flow regime map and a dryout map, respectively. Two sub-regions of the subcooled boiling region, partially developed boiling (PDB) and fully developed boiling (FDB), are examined relative to dominant interfacial and heat transfer mechanisms, and a previous correlation is identified for accurate prediction of the heat transfer coefficient for both PDB and FDB. The saturated boiling region is shown to consist of three separate sub-regions: nucleate boiling dominated for qualities below 0.3, combined nucleate and convective boiling for qualities between 0.3 and 0.5, and convective boiling dominated for qualities above 0.5. Above 0.5, dryout effects begin to take effect, causing a gradual decline in the heat transfer coefficient followed downstream by a more severe decline. A previous correlation is identified for prediction of the heat transfer coefficient in the saturated boiling region.
Rarefied cylindrical Couette flow: Shear stress and energy flux maxima Int. J. Heat Mass Transf. (IF 3.458) Pub Date : 2018-03-20 A.A. Abramov, V.Yu. Alexandrov, A.V. Butkovskii
We consider energy and momentum transfer between surfaces in the rarefied cylindrical Couette problem. We show that in contrast to the continuum mode, in the free molecular and transient regimes the dependences of the energy and momentum fluxes transferred to the surface on the velocity of the rotating surface have maxima. This effect occurs due to the difference between the energy and momentum transfer mechanism in the continuum, transient and free molecular modes. The influence of the rarefaction on the energy and momentum transfer in the transient regime is studied using the direct simulation Monte-Carlo (DSMC) method. We show that at some surface velocities the shear stress and energy flux transferred to the surface have maxima as functions of the Knudsen number.
The transient start-up process of natural draft dry cooling towers in dispatchable thermal power plants Int. J. Heat Mass Transf. (IF 3.458) Pub Date : 2018-03-20 Peixin Dong, Xiaoxiao Li, Zhiqiang Guan, Hal Gurgenci
A one-dimensional (1-D) transient cooling tower model is presented that can be used to simulate the start-up process of natural draft dry cooling towers (NDDCTs). The model simulates the behaviour of a NDDCT following a step increase in the heat exchanger temperature. The start-up process is analysed in two successive stages. In the first stage, the dominant mechanism is natural convection operating through generation and propagation of hot plumes rising from the heat exchanger surface. An understanding of different phases of plume development based on scaled analysis helps to predict the air flow development in this first stage. In the second stage, the air flow is driven by the draft caused by the difference in the inside and outside densities. The cooling tower system air flow development in the second stage is simulated through a quasi-steady state solution of the well-known draft equation. The simulation is repeated for three different input temperatures. The results show that the higher the input temperature, the higher is the inlet air velocity and shorter the start-up process. The results are validated against data from the commissioning tests of the University of Queensland natural draft cooling tower Gatton test rig. This study aims to help fill the knowledge gap in understanding the NDDCT start-up process. This understanding is important to the employment of NDDCTs in future thermal power plants operating in dispatchable mode.
A lattice Boltzmann study on dendritic growth of a binary alloy in the presence of melt convection Int. J. Heat Mass Transf. (IF 3.458) Pub Date : 2018-03-20 Dongke Sun, Yong Wang, Hongyao Yu, Qingyou Han
A multi-relaxation-time (MRT) lattice Boltzmann (LB) based model is utilized to simulate the dendritic growth with melt convection in solidification of alloys. It models melt convection by the MRT-LB equation and solute transport by a conservation equation with a pseudo-potential function. The D2Q9 lattice vectors are proposed to describe interface advancement in the liquid-solid transition. Effects of undercoolings, interface curvature and preferred growth orientation are incorporated into the model implicitly. After model validation, dendritic growth under several conditions of pure diffusion and melt convection was numerically investigated, and the solidification entropies were proposed to quantitatively characterize the solidification system. The result shows that the growth behavior, microstructure formation and solute segregation are significantly influenced by melt convection. The solidification entropies reflecting complexity of the solidification system are useful to characterize dendritic growth and solute segregation. This work offers a potential solution for studies of microstructure evolution in solidification of alloys.
Decoupling analysis on the variations of liquid velocity and heat flux in the test of fouling thermal resistance Int. J. Heat Mass Transf. (IF 3.458) Pub Date : 2018-03-20 Chao Shen, Yuan Wang, Zilong Zhao, Yiqiang Jiang, Yang Yao
Fouling deposit is a common issue on the heat transfer surface caused by the impurity of working water. Many researchers conducted experimental test to study the relationship between the fouling thermal resistance and operating parameters, such as water quality, tube geometry, and liquid velocity, targeting at developing the accurate correlation of fouling thermal resistance on heat transfer tubes. The accurate test of fouling thermal resistance is critical for investigators. In fouling test, with the fouling deposit on the internal surface, both the liquid (water) velocity through the tube and the heat flux of the test tube deviated automatically. Although testers usually tried to adjust the water velocity and heat flux back to the original point, it is hard to be realized, thus the water velocity and heat flux deviated somehow inevitably. In fact, the variations of water velocity and heat flux would cause the change of overall thermal resistance of test tubes, which should be separated from the change caused by fouling deposit. This process could be named as “decoupling”. This paper analyzed the effect of deviations of water velocity and heat flux on the test results of fouling resistance quantitatively based on experimental test, and a decoupling method and formulas were developed. One set of accelerated fouling test was conducted and result shows the fouling resistance with decoupling and non-decoupling had a maximum difference of 0.000002124 m2 K/W for tube 1, and 0.000002363 m2 K/W for Tube 2, 0.000001316 m2 K/W for tube 3.
Tuning capillary penetration in porous media: Combining geometrical and evaporation effects Int. J. Heat Mass Transf. (IF 3.458) Pub Date : 2018-03-20 Mingchao Liu, Jian Wu, Yixiang Gan, Dorian A.H. Hanaor, C.Q. Chen
A new radial integration polygonal boundary element method for solving heat conduction problems Int. J. Heat Mass Transf. (IF 3.458) Pub Date : 2018-03-20 Miao Cui, Hai-Feng Peng, Bing-Bing Xu, Xiao-Wei Gao, Yuwen Zhang
A new approach, radial integration polygonal boundary element method (RIPBEM), for solving heat conduction problems is presented in this paper. The proposed RIPBEM is a new concept in boundary element method (BEM), which would be of great flexibility in mesh generation of complex 3D geometries. Due to the characteristic of arbitrary shapes of polygonal elements, conventional shape functions are insufficient. Moreover, the resulted surface boundary integrals cannot be directly evaluated by the standard Gauss quadrature. To solve these problems, general shape functions for polygonal elements with arbitrary number of nodes are given. To generally and numerically calculate the resulted surface integrals, the radial integration method (RIM) is employed to convert the surface boundary integrals into equivalent contour line integrals of the polygonal elements. As for 3D domain integrals, they are transformed to equivalent line integrals using RIM twice. This methodology can explicitly eliminate strong singularities. Several numerical examples are given to show the effectiveness and the accuracy of the proposed polygonal boundary element method for solving heat conduction problems.
Tuning thermal conductivity of porous graphene by pore topology engineering: Comparison of non-equilibrium molecular dynamics and finite element study Int. J. Heat Mass Transf. (IF 3.458) Pub Date : 2018-03-20 H. Ghasemi, A. Rajabpour, A.H. Akbarzadeh
Tuning thermal conductivity of porous graphenes has attracted much interest in the thermal management of nanoelectronics devices due to the promising multifunctional properties of engineered nanomaterials. To explore the potential of tuning thermal properties of monolayer porous graphenes in multiple scales, non-equilibrium molecular dynamics (NEMD) and finite element method (FEM) are implemented to manipulate their thermal conductivity and temperature distribution by the engineering of pore topology. Results indicate that the thermal conductivity of porous graphenes can be significantly lower than a pristine graphene. The thermal conductivity reduction is attributed to phonon scattering at the boundaries of defects described by the phonon density of states analysis. It is found that the thermal conductivity and the temperature distribution of a porous graphene can be desirably tuned by the simultaneous engineering of relative density, pore topology, and pore orientation. Then, the effect of unit cell periodicity on the thermal conductivity of periodic porous graphenes, called phononic graphene or graphene metamaterial, is explored. Finally, comparing the results of continuum mechanics approach through the implementation of FEM and NEMD simulation presents the advantages of NEMD for predicting the thermal conductivity of engineered porous graphenes with characteristic length of lower than 50 nm.
Thermal management of electronics: An experimental analysis of triangular, rectangular and circular pin-fin heat sinks for various PCMs Int. J. Heat Mass Transf. (IF 3.458) Pub Date : 2018-03-20 Hafiz Muhammad Ali, Muhammad Junaid Ashraf, Ambra Giovannelli, Muhammad Irfan, Talal Bin Irshad, Hafiz Muhammad Hamid, Faisal Hassan, Adeel Arshad
This study implies experimental investigation for optimization of heat transfer in electronic integrated circuits using close packed phase change materials (PCMs) filled pin-fin heat sinks. The aim of this study is to find the most efficient pin-fin configuration filled with optimum PCM to extend the operating range of electronic circuits. The experimental methodology is based upon variation of pin-fin configurations in rectangular, round and triangular cross-sections. Each configuration is allowed a pin-fin volumetric percentage of 9%. For analysis using PCM a volume fraction of 90% is maintained and six PCMs with different thermo-physical properties (varying melting temperatures, latent heats and heat capacities) are selected. These include paraffin wax, RT-54, RT-44, RT-35HC, SP-31 and n-eicosane. Moreover, the power levels mimicking heat input range between 5 W and 8 W. The resulting information is analyzed for the performance of a heat sink with and without PCM. Besides that, PCM ascendency is manipulated in terms of operational time, enhancement ratios, Stefan number and storage ratio. The outcomes suggest that triangular pin-fins are found to be the most effective pin-fin configuration for heat transfer both with and without PCM.
Computational investigation of film cooling and secondary flow on turbine endwall with coolant injection from upstream interrupted slot Int. J. Heat Mass Transf. (IF 3.458) Pub Date : 2018-03-20 Qingzong Xu, Qiang Du, Pei Wang, Jun Liu, Guang Liu
High pressure turbine vane surface and endwall regions are extensively cooled through discrete holes and leakage flow from combustor-turbine interface gap. For making better use of the limited amount of the leakage flow, this paper describes numerical investigation of endwall film cooling performance of a two-dimensional cascade with upstream interrupted slot injection. The geometry of the vane, size of the slot and mainstream parameters are all taken from a real engine high pressure turbine. The effects of varying blowing ratio, location of the upstream slot and coolant incidence angle on cooling effectiveness are studied. The ranges of the studied parameters are: blowing ratio 1.0, 1.7, 1.86; slot location Z/Cax = −0.1, −0.2, −0.3; coolant incidence angle −10°, 10°, 20°. The calculations are completed by solving three-dimensional Reynolds-Averaged Navier-Stokes (RANS) equations with shear stress transport (SST) k-ω turbulence model, meanwhile, the turbulence model was validated by comparing the calculated results with the experiment data. The calculated results show an important influence of blowing ratio and axial position of the interrupted slot on film cooling effectiveness. Cooling Effectiveness is increased with increasing blowing ratio and decreasing distance between the slot and leading edge of the vane. To compare with BR = 1.7 cooling effectiveness can be improved significantly in the condition of BR = 1.86. Slot location of Z/Cax = −0.1 provides much higher cooling effectiveness than the rest two locations. The physical mechanism of the improvement is that in the condition of high blowing ratio high momentum coolant flow helps to reduce the strength of horseshoe vortex and therefore to limit its negative effect on the cooling effectiveness. The high momentum coolant jet impinges on leading edge of the vane and climbs the surface of it. The coverage of the coolant at the leading edge endwall and pressure side endwall junction becomes better. The contours of effectiveness for different slot location are similar to those of different blowing ratio. The momentum of the coolant jet increases with the slot moving close to the leading edge of the vane and creates similar effectiveness contour patterns of raising blowing ratio. The effect of varying coolant incidence angle on the cooling effectiveness is weaker than blowing ratio and slot location. Average cooling effectiveness is increased about 14% when the angle changed from 10° to 20°.
Direct Numerical Simulation of an air-filled differentially heated square cavity with Rayleigh numbers up to 1011 10 11 Int. J. Heat Mass Transf. (IF 3.458) Pub Date : 2018-03-20 Frederic Sebilleau, Raad Issa, Sylvain Lardeau, Simon P. Walker
A set of Direct Numerical Simulations in a heated square cavity invoking the Boussinesq approximation was carried out at Rayleigh numbers ranging between 108 10 8 and 1011 10 11 and Prandtl number of 0.71. The three dimensional configurations studied represent an infinitely deep cavity, thus corresponding to a statistically two-dimensional flow with an imposed temperature varying linearly on the horizontal walls. In such configuration, the Rayleigh number, and therefore turbulence intensity, is the highest ever reached. The database presented herein includes first and second order statistical moments as well as full Reynolds stresses, turbulent heat fluxes and temperature variance budgets. The latter are extremely rare for buoyancy driven flow configurations and are therefore believed to be valuable to the turbulence modelling community. The analysis of the data collected thus focuses on aspects of relevance to the Reynolds averaged modelling of such flows. The effect of increasing the Rayleigh number on the flow statistics, Nusselt number predictions and thermal stratification is investigated. The most important aspect influencing the behaviour of the budgets was found to be the displacement of the position of the maximum of temperature variance towards the inner zone of the boundary layer. Such difference in behaviour between the thermal and velocity boundary layers introduces regions of negative production in the budgets that tend to increase with the Rayleigh number. The production of turbulence by buoyancy is also found to be of the same order of magnitude as other budget terms at all Rayleigh numbers.
Effects of half spherical bulges on heat transfer characteristics of CPU cooled by TiO2-water nanofluids Int. J. Heat Mass Transf. (IF 3.458) Pub Date : 2018-03-20 Cong Qi, Ning Zhao, Xin Cui, Tiantian Chen, Jinding Hu
TiO2-water nanofluids are prepared by a two-step method, and the stabilities of nanofluids are studied by sedimentation observation method and transmittance method respectively. Related experimental system for studying the heat transfer characteristics of CPU cooled by nanofluids is established, and the reliability of the experimental system is researched. The heat transfer and flow characteristics of CPU cooled by TiO2-water nanofluids are experimentally investigated. The effects of half spherical bulges arrangement modes (aligned and staggered arrangement) on the heat transfer enhancement of CPU are discussed. It is found that the surface temperatures of CPU cooled by nanofluids with aligned arrangement and staggered arrangement can be reduced by 10.5% and 12.5% at best compared with base fluid respectively. Also, the effects of nanoparticle mass fractions (ω = 0.1%, 0.2%, 0.3%, 0.4% and 0.5%) and Reynolds numbers (Re = 98, 185, 274, 323, 365, 400, 478, and 548) on the thermo-hydraulic performances are analyzed. It is found that nanofluids with ω = 0.4% show the best heat transfer performance, and the arrangement model and nanoparticle mass fraction have little effect on the resistance coefficient at high Reynolds number (Re > 365). Lastly, the comprehensive performances of nanofluids in the CPU are investigated.
Quantification of heat and mass transport limitations in adsorption heat exchangers: Application to the silica gel/water working pair Int. J. Heat Mass Transf. (IF 3.458) Pub Date : 2018-03-20 Jens Ammann, Patrick Ruch, Bruno Michel, André R. Studart
Developing strategies to reduce mass and heat transport limitations is one of the most important challenges in adsorption heat exchanger technology. Due to the strong coupling of mass and heat transport in these systems, it is difficult to determine the individual transport limitations quantitatively. In order to find an optimal design where heat and mass transport are balanced, a quantitative method that enables a direct comparison of these two transport phenomena is needed. In the present work, a novel experimental approach to discriminate between mass and heat transport is proposed based on the measurement of adsorbent temperature, heat exchanger surface temperature and vapor pressure. The methodology is applied to micro/mesoporous silica spheres arranged in a monolayer or bilayer loose grain configuration or in a monolayer configuration adhesively bonded to the substrate. While the monolayer configuration exhibits balanced heat and mass transport, we find that the bilayer and the thermally enhanced configurations are limited by heat and mass transport, respectively. The application of the proposed methodology to compare heat and mass transport limitations in other industrially-relevant adsorbent materials should greatly aid the design of more efficient adsorption heat exchangers for a wide range of applications.
Analysis of temperature simulation in downhole reaction chamber of hydrothermal jet drilling Int. J. Heat Mass Transf. (IF 3.458) Pub Date : 2018-03-20 Zehao Lyu, Xianzhi Song, Gensheng Li, Yu Shi, Yu Liu
Hydrothermal jet is an alternative drilling method for the exploitation of oil and geothermal energy in deep hard formations. For the application of this novel technology, the successful generation of hydrothermal jet is very important. This paper focuses on investigating applications of different reaction, turbulence and radiation models to the supercritical water oxidation process in downhole reaction chamber of hydrothermal jet drilling. The objective is to identify the pros and cons of each model and determine a set of models that are the most appropriate for the reaction. Simulation models are tested and optimized through two different operating conditions. Simulation results are compared with experimental data. Results show that the entire space of the reaction chamber is in a high temperature state using the laminar finite rate model. The finite rate model is suitable for the simulation compared with other reaction models discussed. The Magnussen constant A and B in the finite rate model can be modified to be 7 and 0.5 to further reduce the error. In addition, the high temperature areas in k-omega model and SAS model are more concentrated, while they are more uniform in RNG k-epsilon model and standard k-epsilon model. The RNG k-epsilon model and DO or DTRM are the most appropriate turbulence and radiation models through comparison. Results in this paper can provide implications for the reaction simulation of hydrothermal jet drilling.
Flow boiling of HFE-7100 in silicon microchannels integrated with multiple micro-nozzles and reentry micro-cavities Int. J. Heat Mass Transf. (IF 3.458) Pub Date : 2018-03-20 Wenming Li, Jiaxuan Ma, Tamanna Alam, Fanghao Yang, Jamil Khan, Chen Li
Flow boiling of dielectric fluids in microchannels is one of the most desirable cooling solutions for high power electronics. However, the flow boiling of dielectric fluids is hindered by their unfavorable thermophysical properties. Specifically, without precooling dielectric fluids, it is challenging to promote critical heat flux (CHF) due to its high vapor density, low surface tension and the resulted superior wettability. In this study, each side wall of a five-parallel silicon microchannel array was structured with an array of microscale reentry cavities and four micronozzles bypassed by an auxiliary channel. The present microchannel configuration aims to significantly enhance CHF of HFE-7100 flow boiling by improving global liquid supply using auxiliary channels and micrononozzles as well as by sustaining liquid film using capillarity induced by reentry cavity array. Equally important, these structures can promote nucleate boiling at low heat flux, generate intense mixing, and promote thin film evaporation at high heat flux, resulting in high flow boiling heat transfer rate. Flow boiling of HFE-7100 in the present microchannel configuration is characterized with mass flux ranging from 231 kg/m2 s to 1155 kg/m2 s. The effective two-phase heat transfer coefficients (HTCs) are ranging from 6 kW/m2 K to 117 kW/m2 K. Compared to the four-nozzle plain-wall microchannels, for example, the effective HTC and CHF can be substantially enhanced up to 208% and 37%, respectively, without escalating pressure drop at a mass flux of 462 kg/m2 s. Compared to plain microchannels with inlet restrictors, CHF is considerably enhanced up to 70% with a reduction of pressure drop ∼82% at a mass flux of 1155 kg/m2 s. Significantly reduced pressure drop is achieved by integrating bypass and the enhanced confined bubble removal. A peak CHF value of 216 W/cm2 is achieved at mass flux of 2772 kg/m2 s in the present microchannel configuration with inlet temperature at room temperature.
Theoretical and experimental investigation of the dynamic behaviour of a standing-wave thermoacoustic engine with various boundary conditions Int. J. Heat Mass Transf. (IF 3.458) Pub Date : 2018-03-20 Geng Chen, Lihua Tang, Brian R. Mace
This paper investigates the dynamic behaviour of a one-dimensional, standing-wave thermoacoustic engine with various boundary conditions. The thermoacoustic engine is composed of an acoustic tube and a ceramic stack with two heat exchangers. The tube has one end open and the other end closed with a deformable plate whose boundaries are constrained. Theoretical models are developed based on the acoustic wave equations and the linear theory of thermoacoutics and validated by experiment. With the validated theoretical models, a parametric study is performed to investigate the influence of the boundary conditions at both ends of the tube. It is found that acoustic radiation at the open end and the presence of the deformable structure at the closed end not only influence the onset temperature and frequency of the system, but also affect the acoustic field along the tube. The developed theoretical models and experimental method in this study provide useful guidelines for the design of thermoacoustic engines for potential energy harvesting systems.
Performance evaluation of different bipolar plate designs of 3D planar anode-supported SOFCs Int. J. Heat Mass Transf. (IF 3.458) Pub Date : 2018-03-20 Deepra Bhattacharya, Jayanta Mukhopadhyay, Nayan Biswas, Rajendra Nath Basu, Prasanta Kumar Das
The performance of Solid Oxide Fuel Cells (SOFCs) is highly sensitive to the fluid dynamics, the interfacial areas, and the residence time of the gases. These parameters are primarily dictated by the geometry of the channels carrying the fuel and the oxidant. However, not many investigations have been made to study the effect of bipolar plate designs on cell performance. We report a detailed comparative study of the performance characteristics of straight and serpentine channel geometries. Simulations of these two channels have been made taking into account fluid flow through the channels and the porous electrodes, multicomponent diffusion, heat transfer, charge transfer reaction kinetics and electrodynamics. Performance of each channel has been compared to in-house experimental data. Extensive parametric analyses have been carried out to evaluate the dependence of cell performance on fuel and air flow rates. Favourable operating ranges of hydrogen and air feeds have been estimated analytically taking into account fuel utilisation, cell temperature, channel pressure drops, and current density. It has been shown that serpentine geometries offer remarkably more uniform distribution of ionic current density, and significantly higher power output and fuel utilisation compared to straight channel geometries. However, these are accompanied by a penalty of pressure drop. This analysis can provide a useful guideline for selecting the channel geometry.
Supersonically sprayed nanotextured surfaces with silver nanowires for enhanced pool boiling Int. J. Heat Mass Transf. (IF 3.458) Pub Date : 2018-03-20 Hong Seok Jo, Tae Gun Kim, Jong-Gun Lee, Min-Woo Kim, Hyun Goo Park, Scott C. James, Jeehoon Choi, Sam S. Yoon
Rapid production of nanoscale-textured surfaces for microscale devices is important for commercial applications. In this study, we introduce a commercially viable method to fabricate nanotextured surfaces used in pool-boiling heat-transfer applications. Silver nanowires were supersonically sprayed onto copper substrates with good adhesive strength. The coating method required little time and could be adapted for roll-to-roll processing. The fabricated nanotextured surfaces showed a significantly increased critical heat flux (CHF) and effective heat transfer coefficient (heff), as evidenced by the release of numerous bubbles from nanotextured nucleation sites during pool-boiling. The silver nanowires were well connected either by self-sintering or due to the fusion induced by supersonic impacts with the copper substrate. The thickness of the coated layer could be controlled by the number of spray sweeps/passes and the optimal thickness for maximizing CHF and heff was identified. The nanotextured surfaces were characterized by scanning electron microscopy and by bubble formation and release as visualized with a charge-coupled device camera.
A new model to determine the thermal conductivity of fine-grained soils Int. J. Heat Mass Transf. (IF 3.458) Pub Date : 2018-03-20 Jun Bi, Mingyi Zhang, Wenwu Chen, Jianguo Lu, Yuanming Lai
In the study, a three-parameter model was presented to calculate the thermal conductivity at full range of degree of saturation (S) for fine-grained soils based on the Fredlund and Xing model and the normalized thermal conductivity method. Three parameters (a, b, c) of the new model are determined by two equations and a measured point at a certain S. Two equations were obtained by the correlations between the parameters a, b, c and the basic properties of 30 Canadian soils by regression analysis. Moreover, the relationship between the thermal conductivity and S at full range of S was defined as the thermal conductivity curve (TCC) and was divided into 3 regions for the improvement of the calculation result. According to the sensitivity analysis, it is found that the calculated TCC is the most reliable when a point is measured in the Region 2. In addition, the new calculation model was also verified by 6 Chinese soils, suggesting the new model could present a good calculation result (R2 = 0.97) for the TCC.
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