Comparison and Analysis of Heat Transfer in Aluminum Foam Using Local Thermal Equilibrium or Nonequilibrium Model

Aluminum foams are favorable in modern thermal engineering applications because of the high thermal conductivity and the large specific surface area. The present study aims to investigate an application of porous aluminum foam by using the local thermal equilibrium (LTE) and local thermal nonequilibrium (LTNE) heat transfer models. Three-dimensional simulations of laminar flow (porous foam zone), turbulent flow (open zone), and heat transfer are performed by a computational fluid dynamics approach. In addition, the Forchheimer extended Darcy's law is employed to evaluate the fluid characteristics. By comparing and analyzing the average and local Nusselt numbers, it is found that the LTNE and LTE models can reach the same Nusselt numbers inside the aluminum foam when the air velocity is high, meaning that the aluminum foam is in a thermal equilibrium state. Besides, a high interfacial heat transfer coefficient is required for the aluminum foam to reach a thermal equilibrium state as the height of the aluminum foam is reduced. This study suggests that the LTE model can be applied to predict the thermal performance at high fluid velocities or for the case with a large height.     

Numerical and experimental study of forced convection in graphite foams of different configurations

Forced convection heat transfer in a channel with different configurations of graphite foams is experimentally and numerically studied in this paper. The physical properties of graphite foams such as the porosity, pore diameter, density, permeability and Forchheimer coefficient are determined experimentally. The local temperatures at the surface of the heat source and the pressure drops across different configurations of graphite foams are measured. In the numerical simulations, the Navier–Stokes and Brinkman–Forchheimer equations are used to model the fluid flow in the open and porous regions, respectively. The local thermal non-equilibrium model is adopted in the energy equations to evaluate the solid and fluid temperatures. Comparisons are made between the experimental and simulation results. The results showed that the solid block foam has the best heat transfer performance at the expense of high pressure drop. However, the proposed configurations can achieve relatively good enhancement of heat transfer at moderate pressure drop.     

Investigation of functionally graded metal foam thermal management system for solar cell

Concentrated photovoltaic cell (CPV) is a solar energy harvesting device that converts solar energy into electrical energy. However, the performance and efficiency of the CPV are heavily dependent on the temperature. Besides, nonuniformity of temperature distribution on the CPV will lead to thermal aging and affects the cycle life. Hence, an effective cooling system is required to remove excess heat generated to ensure that the CPV operates at optimum operating temperature with minimum variation of temperature. Metal foam is a new class of material that possesses huge potential for thermal management. In this study, a functionally graded metal foam is proposed for the CPV thermal management system. Computational thermal fluid dynamic analysis is conducted to investigate the effect of porosity and pore density on the flow field and thermal performance of the aluminum foam heat sink. The investigation results revealed that 10 PPI functionally graded aluminum foam heat sink with two stages of porosity gradient 0.794 and 0.682 produced the lowest pressure drop and highest thermal performance. Temperature difference of 3.9°C was achieved for a solar cell with total heat generation of 900 W under water mass flow rate of 20 gs⁻¹.     

Application of CFD to closed-wet cooling towers

Computational fluid dynamics (CFD) is applied to predicting the performance of closed-wet cooling towers (CWCTs) for chilled ceilings according to the cooling capacity and pressure loss. The prediction involves the two-phase flow of gas and water droplets. The predicted thermal performance is compared with experimental measurement for a large industrial CWCT and a small prototype cooling tower. CFD is then applied to the design of a new cooling tower for field testing. The accuracy of CFD modelling of the pressure loss for fluid flow over the heat exchanger is assessed for a range of flow velocities applied in CWCTs. The predicted pressure loss for single-phase flow of air over the heat exchanger is in good agreement with the empirical equation for tube bundles. CFD can be used to assess the effect of flow interference on the fluid distribution and pressure loss of single- and multi-phase flow over the heat exchanger.     

Numerical Investigation of PCM Based Heat Sinks with Embedded Metal Foam/Crossed Plate Fins

Three-dimensional numerical models for phase change material based heat sinks equipped with thermal conductivity enhancers like aluminum metal foam and crossed plate fins are validated with the experimental data found in literature. For the aluminum metal foam embedded in the heat sink filled with phase change material, the porosity and the pores per inch of the metal foam were varied and natural convection currents were studied. Maintaining the volume fraction of the phase change material as a constant, the thermal performance enhancement as a result of the introduction of thermal conductivity enhancer into the heat sinks is determined.     

A synthesis of fluid and thermal transport models for metal foam heat exchangers

Metal foam heat exchangers have considerable advantages in thermal management and heat recovery over several commercially available heat exchangers. In this work, the effects of micro structural metal foam properties, such as porosity, pore and fiber diameters, tortuosity, pore density, and relative density, on the heat exchanger performance are discussed. The pertinent correlations in the literature for flow and thermal transport in metal foam heat exchangers are categorized and investigated. Three main categories are synthesized. In the first category, the correlations for pressure drop and heat transfer coefficient based on the microstructural properties of the metal foam are given. In the second category, the correlations are specialized for metal foam tube heat exchangers. In the third category, correlations are specialized for metal foam channel heat exchangers. To investigate the performance of the foam filled heat exchangers in comparison with the plain ones, the required pumping power to overcome the pressure drop and heat transfer rate of foam filled and plain heat exchangers are studied and compared. A performance factor is introduced which includes the effects of both heat transfer rate and pressure drop after inclusion of the metal foam. The results indicate that the performance will be improved substantially when a metal foam is inserted in the tube/channel.     

Thermal performance of finned aluminum heat sink filled with ERG aluminum foam: Experimental and numerical approach

The rapid improvements in electronic devices have led to a high demand for effective cooling techniques. The purpose of this study was to investigate the heat transfer characteristics and performance of different aluminum heat sinks filled with aluminum foam for an Intel core i7 processor. The aluminum foam heat sinks were subjected to water flow covering the non-Darcy flow regime (300-600 Reynolds numbers). The bottom side of the heat sinks was heated with a heat flux between 8.5 and 13.8 W/cm². Three different heat sinks were examined in this study. Models A, B, and C contained two, three and four channels, respectively. Each channel gap was filled with ERG aluminum foam. The distributions of the local surface temperature and the local Nusselt number were measured for each heat sink design. The experimental data were compared with the numerical results. The average Nusselt number was obtained for the range of Reynolds numbers, and an empirical correlation of the average Nusselt number as a function of the Reynolds number was derived for each heat sink. The pressure drop across the characteristics of each heat sink design was measured. The thermal performance of each aluminum foam heat sink was evaluated based on the average Nusselt number and the required pumping power. The experimental results revealed that model B achieved the highest average Nusselt number compared with models A and C. However, model C had the highest surface to volume ratio; the thermal boundary layers, which are formed on adjacent fin surfaces inside the aluminum foam, interface with each other causing a reduction in the overall heat transfer. The numerical results were in good agreement with experimental data of local Nusselt number and local temperature with maximum relative errors of 2% and 1%, respectively.     

Experimental study of thermal performance improvement in a cross-flow heat exchanger by using copper foam

In this study, copper foam was used as a porous medium in place of traditional aluminum fins. A comparison between the two heat exchangers—one with fins and the other with copper foam—was conducted under various conditions. The air inlet velocity ranged from 0.9 to 9.3 m/s, and the water inlet temperature ranged from 10°C to 18°C. Different water flow rates were tested. A comparison was made between the performance of copper foam and aluminum fins by calculating several parameters, including thermal resistance, heat exchanger effectiveness, Colburn factor, Nusselt number, friction factor, and area goodness factor (AG). The experimental results showed that at low air velocities, the heat transfer coefficient for both types of heat exchangers was almost equal. However, at high air velocities, the copper foam exhibited a higher heat transfer coefficient. The Colburn factor was higher for the heat exchanger with copper foam than in the conventional fins, where it was equal to 0.1959 for the copper foam and 0.1186 for the fins. On the other hand, the AG was higher in the case of fins than in the heat exchanger with copper foam.     

Ultralightweight Compact Heat Sinks With Metal Foams Under Axial Fan Flow Impingement

Experimental results of ultralightweight compact heat sinks with open-celled copper (Cu) foams under the impingement of axial fan flows are presented. The thermal resistance of the system and the pressure coefficient on heat sink base plate were measured, focusing on the influences of foam height (H_f ) and impinging distance between fan exit and base plate (H). With the impinging distance fixed at H/D = 0.5 (D being the fan diameter), it is demonstrated that an optimum foam height exists at H_f /D = 0.22, providing the lowest thermal resistance. Furthermore, with the foam height fixed at H_f /D = 0.22, reducing the impinging distance to the foam height level reduces the thermal resistance further. In comparison with conventional aluminum (Al) plate-fin heat sinks under identical flow conditions, the Cu foam heat sinks require only 30% of the weight and 50% of the volume to achieve a similar level of heat dissipation performance.     

Experimental Investigation of the Thermal Performance of Graphite Foam for Evaporator Enhancement in Both Pool Boiling and an FC-72 Thermosyphon

High-conductivity graphite foam is investigated for use as a surface enhancement for improved thermal performance in both pool boiling and an FC-72 thermosyphon. The influences of heat load and fluid level on the overall system thermal performance including surface superheat, effective heat transfer coefficient, and thermal resistance are examined. The thermal resistance of the foam heat sink is found to be extremely low at a minimum of 0.024 K/W, well below that of many other methods. The featured low thermal resistance is the primary benefit of this system. The thermal resistance is found to rise with increasing heat flux, but still remains advantageously low and exhibits excellent potential for high heat flux dissipation with low surface superheat, making it suitable for thermal management of advanced electronics.     

Hydrogen storage systems based on hydride materials with enhanced thermal conductivity

The reaction of hydrogen gas with a metal to form a metal hydride is exothermic. If the heat released is not removed from the system, the resulting temperature rise of the hydride will reduce the hydrogen absorption rate. Hence, hydrogen storage systems based on hydride materials must include a way to remove the heat generated during the absorption process. The heat removal rate can be increased by (i) increasing the effective thermal conductivity of the metal hydride by mixing it with high-conductivity materials such as aluminum foam or graphite, (ii) optimizing the shape of the tank, and (iii) introducing an active cooling environment instead of relying on natural convection. This paper presents a parametric study of hydrogen storage efficiency that explores quantitatively the influence of these parameters. An axisymmetric mathematical model was formulated in Ansys Fluent 12.1 to evaluate the transient heat and mass transfer in a cylindrical metal hydride tank, and to predict the transient temperatures and mass of hydrogen stored as a function of the thermal conductivity of the enhanced hydride material, aspect ratio of the cylindrical tank, and thermal boundary conditions. The model was validated by comparing the transient temperature at selected locations within the storage tank with concurrent experiments conducted with LaNi₅ material. The parametric study revealed that the aspect ratio of the tank has a stronger influence when the effective thermal conductivity of the metal hydride bed is low or when the heat removal rate from the tank surface is high (active cooling). It was also found that for a hydrogen filling time of 3 min, adding 30% aluminum foam to the metal hydride maximizes hydrogen absorption under natural convection, whereas the addition of only 10% aluminum foam maximizes the hydrogen content under active cooling. For filling times beyond 3 min, the amount of aluminum foam required to maximize hydrogen content can be reduced for both natural convection and active cooling. This study should prove useful in the design of practical metal hydride-based hydrogen storage systems.     

Effect of oscillatory frequency on heat transfer in metal foam heat sinks of various pore densities

In this paper, an experimental investigation was performed to study the heat transfer performance of metal foam heat sinks of different pore densities subjected to oscillating flow under various oscillatory frequencies. The variations of pressure drop and flow velocity along the kinetic Reynolds number of oscillating flow through aluminum foams were compared. The measured pressure drops, velocities and surface temperatures of oscillating flow through aluminum 10, 20 and 40 PPI foams were presented in detail. The calculated cycle-averaged local temperature and Nusselt number for different kinetic Reynolds numbers were analyzed and compared with finned heat sinks. The results of length-averaged Nusselt number for both oscillating and steady flows indicate that higher heat transfer rates can be obtained in metal foams subjected to oscillating flow. For the purpose of designing a novel heat sink using metal foam, the characteristics of the pumping power of the cooling system for aluminum foam with different pore densities were also analyzed.     

Performance of Forced Convection Heat Transfer in Porous Media Based on Gibson–Ashby Constitutive Model

A novel simulation model is developed for predicting the performance of forced convection heat transfer in the porous metal foam. Based on the physical geometry of the Gibson-Ashby constitutive model, the theoretical model proposed is able to predict the mechanical behaviors and thermal physical properties of porous materials simultaneously. The theoretical predictions of the overall heat transfer coefficient and pressure drop were compared with available experimental data for two different porous foam tubes. The first tube has a porous diameter of 0.6mm and porosity of 0.402, and the other tube has a diameter of 1.6mm and porosity of 0.462. The results show that the relative deviation of the flow pressure drop between the prediction and the experimental data are in the range from 5% to10% while the relative deviation of the overall heat transfer coefficient is about 20%. These deviations are acceptable for applications in engineering. So the feasibility of the Gibson-Ashby constitutive model to be used to predict the performance of flow resistance and convective heat transfer in porous foam ducts is satisfactorily validated.     

泡沫金属-PCM-液冷复合方式下动力电池散热分析

电池是电动汽车的核心动力元件,而电池的热管理系统是动力电池发挥最佳工作性能的重要保障,在保证最佳工作性能的同时提升汽车安全性能、电池寿命及能源利用效率。基于21700NCA圆柱型三元锂离子电池,建立以泡沫铝为支撑骨架的电池组系统,在骨架和电池之间的孔隙注入相变材料（PCM）以提高结构内部温度均匀性,在电池底部添加液冷板来强化冷却效果,利用计算流体力学（CFD）仿真技术分析单体电池的耦合散热效果。结果表明,与单一冷却模式相比,使用泡沫金属与相变材料、液体冷却的耦合散热系统,可以达到更加良好的散热效果;对于相变材料,在一定密度范围内,密度越大,对电池系统的冷却效果越好,混合比主要影响相变材料的凝固融化速率。     

Pressure loss and forced convective heat transfer in an annulus filled with aluminum foam

This experimental study investigates non-Darcy flow and heat transfer in an annulus with high porosity aluminum foams to attain the miniaturization of thermal systems. The local wall temperature distribution, inlet and outlet pressures, and temperatures and heat transfer coefficient were measured for heat flux of 13.6–31.4 kW/m². The results show that aluminum foam enhances heat transfer from a surface compared with that of laminar flow in a clear annulus. Correlations for the friction factor and the Nusselt number are proposed and used for design of thermal applications.     

泡沫铝通道内瞬态流动的平均换热系数

针对泡沫铝金属填充矩形通道内的对流换热开展了瞬态实验研究,分析了泡沫铝孔径(孔隙率)、流体流量(流速)等关键参数的影响。为了有效地处理实验数据,重新定义并推导了平均换热系数的计算公式,得到了泡沫铝通道内流动的平均换热系数,并引入了基于渗透率的雷诺数和达西数,确定了相关换热、流动准则数关系。实验研究表明,流速的增大有利于对流换热的强化:而平均换热系数对泡沫金属孔径较敏感;对于低孔隙率泡沫金属,渗透率成为影响换热强度的主要因素,相同或接近的孔隙率下,孔径越大,渗透率和达西数越大,越有利于换热,且压损减小。     

Analysis of an Ultrathin Graphite-Based Compact Heat Exchanger

The emerging production of ultrathin graphite material is applied to thermal management in a numerical comparison of aluminum and graphite-based plate-fin heat exchangers. Considering anisotropic thermal conductivity in which out-of-plane transport is about two orders of magnitude lower than in-plane values, the ultrathin graphite-based solution outperforms aluminum by rejecting up to 20% more heat on a volumetric basis. Thermal and hydraulic performance is characterized for both solutions over a range of airflow rates in a notional water/air device. Laminar through fully turbulent regimes are considered. Steady and unsteady three-dimensional (3-D) conjugate simulations reveal a faster equilibration rate for the ultrathin graphite-based solution, minimizing thermal lag that must be accounted for in on-demand electronics cooling. Fin optimization studies predict equivalent conductance with graphite at one-tenth the thickness of aluminum. The combination of improved heat rejection, rapid response rate, and low material density make an ultrathin graphite-based solution uniquely suited to aerospace thermal management.     

Metallic foam with cross flow heat exchanger: A review of parameters,performance, and challenges

The cross-flow heat exchanger involves a tractor moving between two fluids that flow in a direction perpendicular to each other, and one of the fluids is often a liquid and the other is a gas. This type of heat exchanger has been studied in many previous studies for its importance in air conditioning applications and many industrial applications. In this type of heat exchanger, the surface area for heat transfer is very large. Therefore, many techniques have been used to improve the thermal and dynamic performance of this type of heat exchanger. In this study, previous studies that used metallic foam as one of the ways to improve the performance of heat exchangers were reviewed. The most important techniques that were used in previous studies during the process of evaluating the thermal performance of a cross-flow heat exchanger in the presence of different types of metal foam were clarified. The use of metal foam depends on important factors, including (1) the type of material, where copper and aluminum were used in most of the previous studies, due to their availability and ease of foam formation using these materials, in addition to the good thermal properties, (2) porosity, where the porosity value of metal foam ranged between 0.85 and 0.98 in the previous studies, (3) the density of pores, most of the studies ranged between 10 and 40 PPI.     

Combustion and heat transfer at meso-scale with thermal recuperation

Results are presented from successfully designed and fabricated meso-scale ceramic combustors that incorporate internal thermal energy recirculation. The combustor provided sustained operation using propane and air as the reactants. Flames could be obtained well below the normal quenching distance. The development required examination of several different combustor designs and materials. Flammability limits of these combustors have been determined experimentally. Experimental investigations have been performed on the effects of flame holder geometry, material conductivity, equivalence ratio, and inlet Reynolds number on the combustor performance. Measurement of the reactant preheating and product exhaust temperatures was performed using K-type thermocouples which were installed with minimal intrusion to the flow. The reactant preheating temperatures were observed to be in the range 700 K–1000 K. However, the combustor suffered significant overall heat loss (50–85%) which was implied by the low exhaust temperatures (500 K–750 K). For a constant fuel flow rate, the exhaust temperature increased monotonously with decrease in equivalence ratio until the blow-off condition implying that the combustor’s maximum thermal efficiency occurs at its lean blow-off limit. Thermal imaging of the combustor walls was performed using infrared camera to obtain the temperature distribution within the combustor. Numerical simulations were performed with the aid of CFD software using a heat loss coefficient chosen so as to give best correlation with experimental results. These CFD simulations helped to obtain better insight of the dependence of combustor performance on thermal conductivity of the material and heat load.     

CFD modeling and performance evaluation of multipass solar air heaters

This article investigates the impacts of flow configurations on the thermal performance of a solar heater system. Recycled aluminum cans (RACs) have been utilized as turbulators with a double pass single duct solar air collector. The CFD software of COMSOL Multiphysics V5.3a is used to model three designs: Cocurrent (model A), countercurrent (model B), and U-shape (model C). The numerical results reveal that the U-shape design offers a greater thermal performance of 5.4% and 6.5%, respectively, compared with the cocurrent and countercurrent flow models. Furthermore, an outdoor experiment is performed based on the numerical modeling of flow configurations. The experimental setup is examined for three configurations of model C, namely, solar air heater (SAH) without RAC model C-I (plain model), SAH with in-line RAC layout (model C-II), and SAH with staggered RAC layout (model C-III). We found the double pass single duct solar air collector (model C) design is in a good agreement with the experimental data, and model C-III has a better thermal efficiency of 60.2%, compared to those of model C-II, 53.1%, and model C-I, 49.4%.