Metal Foam Heat Research Articles

Experimental and artificial neural network investigation on the effect of inclination angle on the interface temperature of CPU/metal foam heat sink

Purpose This study aims to clarify the relationship between inclination angle of hot surface of CPU and its temperature in absence and presence of aluminum foam as a cooling system. It proposes application of the artificial neural [multi-layer perceptron (MLP) and radial basis function] networks and adaptive neuron-fuzzy inference system (ANFIS) to predict interface temperature of central processing unit (CPU)/metal foam heat sink. Design/methodology/approach To provide a consistent set of data, the surface of an aluminum cone with and without installing Duocel aluminum foam was heated in a natural convection using an electrical resistor. The hot surface temperature was measured using five K-type thermocouples (±0.1°C). To develop the predictive models, ambient temperature, input power and inclination angle are taken as input which varied from 23°C to 32°C, 4 to 20 W and 0° to 90°, respectively. The hot surface temperature is taken as the output. Findings The results show that in the presence of foam, the hot surface temperature was less sensitive to the variations of angle, and the maximum enhancement of the heat transfer coefficient was 23 per cent at the vertical position. Both MLP network and ANFIS are comparable, but the values predicted by MLP network are in more conformity with the measured values. Originality/value The effect of metal foam on the inclination angle/hot surface temperature dependence is identified. The optimum angle is clarified. The applicability of the MLP networks to predict interface temperature of CPU/heat sink is approved.

Kelvin-cell-based metal foam heat exchanger with elliptical struts for low energy consumption

Conventional Kelvin-cell-based metal foam (KMF) has triangular or circular struts. This study proposes a KMF with elliptical struts instead of such struts. Two schemes of transforming a circular strut into an elliptical strut are proposed: (1) maintaining the same circumference as the circular strut cross-section and (2) maintaining the same cross-section area as the circular strut cross-section. Five KMFs with different struts were designed. The fluid flow, heat transfer, and performance characteristics of the KMFs were numerically investigated. Two dimensionless diameters for the strut section are introduced to analyze the hydro-thermal characteristics of KMFs: λa, the major diameter ratio of the elliptical strut to the circular strut, and λb, the minor diameter ratio of the elliptical strut to the circular strut. The major axis of all struts was set parallel to the main flow direction. The pressure drop and heat transfer characteristics of KMFs are explained by the pressure drag and friction drag. As λb decreases from 1.000 to 0.556, the pressure drop of the KMF decreases by 44% because the pressure drag and blockage ratio decrease with λb. However, the decrease of λb results in decreases in the free stream velocity around the strut, which has a penalty on the heat transfer performance. This can be partially compensated by increasing λa by increasing the flow tortuosity, surface area, and separation angle. Therefore, using the scheme with the same cross-sectional area provides better performance than using the scheme with the same circumference. A KMF using the scheme with the same cross-section area scheme required 32% less pumping power than a KMF with circular struts and the same thermal resistance.

Experimental investigation of heat transfer and pressure drop of alumina–water nano-fluid in a porous miniature heat sink

ABSTRACTThe thermal management of electronic devices has become of great concern in the last decade. Recent research has focused on heat transfer enhancement methods for heat removal from such devices. In this study, we investigated the convective heat transfer and pressure drop of alumina–water nano-fluids in a miniature heat sink occupied with a copper porous medium under a 62.5 heat flux. Nano-fluid volume fractions of 0.1, 0.3, and 0.5% were synthesized by a two-step method, and the thermal conductivity and kinematic viscosity of the nano-fluids were measured experimentally. Two copper metal foam heat sinks, with different pore densities of 15 pores per inch (PPI) and 30 PPI, were manufactured and inserted into a channel. The results indicated an enhancement in the convective heat transfer coefficients and Nusselt numbers with increase in the Reynolds number. The enhancement in the heat transfer coefficients using different nano-fluid volume fractions in higher- and lower-porosity media varied from 1 to 22% and 2.3 to 26.4%, respectively. An increment in the nano-fluid volume fraction, and pore density, led to a higher heat transfer rate. However, the higher pore density medium caused larger pressure drops. The thermal resistance of the samples was also studied. The results indicated an increase in Reynolds number, and volume fraction decreased the thermal resistance. According to the efficiency index, a lower pore density medium as well as higher nano-fluid volume fractions provided better performance in this porous heat sink.

Effect of porosity on heat transfer and pressure drop characteristics of wet air in hydrophobic metal foam under dehumidifying conditions

In order to evaluate the performance of the hydrophobic metal foam heat exchangers under dehumidifying conditions, the effect of porosity on the heat transfer and pressure drop characteristics of wet air in hydrophobic metal foam was experimentally investigated and compared with that in uncoated ones. The test results show that, as the porosity decreases from 95% to 85%, the heat transfer coefficient and pressure drop in hydrophobic metal foams are increased by 6%–18% and 56%–162%, respectively. The heat transfer coefficient of wet air in metal foams is increased by the hydrophobic coating at most experimental conditions except for the porosity of 85% and relative humidity of 70%–90%, and the maximum increment is 33%. The pressure drop of wet air in hydrophobic metal foams is 3%–139% larger than that in uncoated ones. The comprehensive performance of hydrophobic metal foam is better than that of uncoated foams at porosity of 95% and relative humidity of 30%–50%, and it becomes worse than that of uncoated ones with the decreasing porosity and increasing relative humidity.

A comparative study of PCM melting process in a heat pipe-assisted LHTES unit enhanced with nanoparticles and metal foams by immersed boundary-lattice Boltzmann method at pore-scale

Latent heat thermal energy storage (LHTES) has attracted lots of attention due to its nearly constant working temperature and large thermal energy storage density. However, the thermal conductivity of phase change materials (PCMs) is usually low which impedes the heat transfer efficiency in the LHTES system. Adding high thermal conductivity nanoparticles or metal foams are the two common approaches to enhance the thermal performance of the PCMs. In the current work, the PCM melting performance in a heat pipe-assisted LHTES unit enhanced by nanoparticle-metal foam combination is numerically investigated by immersed boundary-lattice Boltzmann method (IB-LBM) at pore scale. The microstructure of metal foam is reconstructed using the quartet structure generation set (QSGS). The PCM melting performance in LHTES is studied in terms of porosity and pore size of metal foams, volume fraction of nanoparticles, and radius of heat pipe. A comparative study is carried out to illustrate the effectiveness of enhancing PCM melting performance with different combinations of nanoparticles and metal foams. The results indicate that there exist the optimum metal foam porosity and heat pipe radius for the energy storage efficiency in the LHTES unit. Besides, it is found that using metal foams is more effective than adding nanoparticles for the improvement of PCM heat transfer capability.

Numerical investigation on aluminum foam application in a tubular heat exchanger

A numerical study has been conducted to examine the thermal and fluiddynamic behaviors of a tubular heat exchanger in aluminum foam. A plate in metal foam with a single array of five circular tubes is the geometrical domain under examination. Darcy–Forchheimer flow model and the thermal non-equilibrium energy model are used to execute two-dimensional simulations on metal foam heat exchanger. The foam is characterized by porosity and (number) pores per inch respectively equal to 0.935 and 20. Different air flow rates are imposed to the entrance of the heat exchanger with an assigned surface tube temperature. The results are provided in terms of local heat transfer coefficient and Nusselt number evaluated on the external surface of the tubes. Furthermore, local air temperature and velocity profiles in the smaller cross section, between two consecutive tubes are given. Finally, the Energy Performance Ratio (EPR) is evaluated in order to demonstrate the effectiveness of the metal foam.

Coupled CFD-DEM simulation of oscillatory particle-laden fluid flow through a porous metal foam heat exchanger: Mitigation of particulate fouling

A coupled finite volume-discrete element (FVM-DEM) numerical method is developed to investigate oscillating multiphase foulant-laden air (solid-gas) flow and particulate fouling in a porous heat exchanger channel comprising an array of circular cylinders. Oscillatory fluids serve as a steppingstone to advance existing conventional anti-fouling techniques and to yield optimum energy consumption. A performance evaluation criteria based on various amplitudes of pulsation and foulant injection rates is established. The fouling characteristics and pressure drop is compared against a non-oscillating steady flow. For the solid foulant particles considered in this study, the results show that the time-averaged deposition fraction is sensibly invariant with increasing foulant injection rate; however, the same observation is not realized for steady-flow cases. The 1 Hz case showed no significant disparity in terms of deposition fraction at high injection rates compared with the non-oscillating steady flow case. In addition, the 1 Hz case showed miniscule reduction of low-density foulants. The higher-frequency spectrum has superior fouling mitigation capabilities although comparatively higher pressure drop is generated. The 6-pore configuration outperforms the 3-pore under certain oscillatory flow conditions. An optimum frequency of 5 Hz exhibits superior fouling mitigation performance while achieving a low time-averaged pressure drop.

Experimental Investigation of the Effect of the Metal Foam Material on CPU Cooling

One of the most important component of computers is processors (CPU). Because of the reasons like running too many programs simultaneously and playing games may cause the processor to overheat. This situation may cause unwanted computer malfunction. The CPU capacity and heat quantities increase with increasing the CPU speed and its performance. In order to run the system effectively, this heat must be removed and the critical surface temperature of the CPU must be kept below approximately 85 °C. In this present study, aluminum open cell metal foam (Al-6101) heat sink having pore density of 10 and 40 PPI (pore per inch) was used instead of the conventional plate type aluminum heat sink. 10 and 40 PPI aluminum metal foam materials are compared with each other and the results are presented in graphics at different fan speeds (1140, 1742, 2439, 2944, 3308 and 4187 rpm). According to the results, in %0-10 CPU usage (standby), lower surface temperatures were reached while using 10 PPI aluminum metal foam material. However, in %100 CPU usage, lower surface temperatures were reached while using 40 PPI aluminum metal foam material.

Influence of pore density and porosity on the wet air flow in metal foam under different operation conditions

In order to optimize the parameters of metal foam heat exchanger operating under dehumidifying conditions, the influence of pore density and porosity on the heat transfer and pressure drop characteristics of wet air in metal foam under different conditions were investigated experimentally. The experimental conditions cover PPI (number of pores per inch) of metal foam from 5 to 40 and porosity from 85% to 95%. The results show that, as PPI increases, the global heat transfer coefficient always increases under low relative humidity conditions, while it initially increases then decreases under high relative humidity conditions, presenting a maximum increment of 12–21% at 20 PPI. As the porosity increases from 85% to 95%, the global heat transfer coefficient of wet air in metal foam decreases maximally by 32%, and the pressure drop of wet air in metal foam decreases by 47%.

Two- and four-way coupling of cohesive poly-disperse particulate foulants on a metal foam fibre immersed in quiescent fluid

The ubiquity and complexity of the unsteadiness of fouling and multiphase flows in various engineering systems signify the need to develop advanced numerical methods to study the underlying phenomena of two-phase particle-laden fluid flows in heat exchanger systems such as, compact electronics cooling (i.e. heat sinks) and HVAC&R systems. Fouling is omnipresent in many industries such as power generation, chemical, petroleum, among others. The mechanisms governing fouling coupled with multiphase foulant-laden fluid flow in porous heat exchangers, such as metal foams, are very complex and poorly understood. This investigation forms the basis for addressing the implications of fouling for a myriad of industrial processes. This study will discuss the development of a coupled finite volume method and discrete element method (FVM-DEM) numerical framework to investigate the mechanisms governing particulate fouling in an idealized metal foam heat exchanger. This study resolves four-way and two-way coupled interactions based on poly-disperse cohesive foulants in fluid-saturated foam. The significance stems from the inclusion of cohesiveness between particle-particle and particle-wall contacts which play a decisive role in the foulant aggregation process prevalent in particles with a diameter smaller than 50μm. The present results show that the cohesive foulants exhibit strong tendency to aggregate with time and form chain-like projections. A rigid aggregate stack is formed which alters the fluid velocity of the fluid-filled foam. Quantitative analysis of the foulant count and time-averaged aggregate count is discussed. The presented results and the numerical framework could potentially be used to optimize heat exchanger designs by considering operating conditions and foam morphology (i.e. pore diameter, ligament thickness, porosity) that is most susceptible to particulate fouling.

Heat Transfer During High Temperature Gas Flow Through Metal Foam Heat Exchangers

An experimental study of heat transfer in metal foam heat exchangers fabricated from 10 and 40 pores per inch (PPI) was conducted. Heat exchangers were made by either brazing Inconel sheets to foam or plasma spraying Inconel skins on the foam. A burner test rig was built to produce high temperature combustion gases at either 550 °C or 750 °C that were passed over the exposed surface of heat exchangers that were cooled by passing air through them at rates of up to 200 SLPM. Both pressure drop and temperature rise of the air were measured. Friction factors and volumetric heat transfer coefficients were calculated for air velocities varying from 0.1 to 5 m/s and dimensionless correlations to predict these derived. The heat exchangers with 40 PPI foam were measured to have higher heat transfer rates and larger pressure drop than those with 10 PPI foam. Thermal sprayed heat exchangers were found to perform better than those that were brazed since they had lower thermal contact resistance between the external shell and foam struts. An analytical model was developed assuming local thermal nonequilibrium (LTNE) and predictions from model were found to be in good agreement with experimental results.

Natural convection in metal foam heat sinks with open slots

Natural convection in metal foam heat sinks with open slots was investigated experimentally. The test sample was made in-house by attaching several foam strips having fixed width of 10mm and length of 100mm onto a substrate of 100×100×4mm3 in size. A total of 29 test samples with different slot widths (s=0/2.86/5/8/12.5/20mm) and foam heights (H=10/20/40/60/80mm) were tested under both horizontal and vertical orientations for each sample. The zero width of open slots corresponds to a heat sink with a single foam block as considered in open literature, while the non-zero open slot width 2.86/5/8/12.5/20mm corresponds to 8/7/6/5/4 foam strips in a heat sink, respectively. Compared to a single foam block, the foam with open slots are more permeable for air penetrating through the heat sink. More foam strips means more heat transfer area available but less permeation of the heat sink. Experimental results revealed that there exists an optimum open slot width (5–8mm) for maximum heat transfer coefficient at given heat sink volume. With optimum open slot width, the heat transfer coefficient is enhanced by 14.9%, 21.3%, 37.6%, and 38.2% relative to single foam block (s=0mm) at the foam height of 10mm, 20mm, 40mm, and 80mm, respectively.

A coupled finite volume & discrete element method to examine particulate foulant transport in metal foam heat exchangers

The exorbitant costs associated with particulate fouling necessitates the need to formulate advanced methods to comprehend mass transport and fouling in heat exchangers. A coupled finite volume and discrete element method is developed to investigate the mechanisms that govern particle-laden gas flows and particulate fouling in idealized porous metal foam heat exchangers. This meticulous examination will take great precedence in addressing the negative impact particulate fouling has in the industry. The numerical method will permit engineers to better optimize porous metal foams for applications such as air-cooled heat aluminium heat exchangers. The robustness of this numerical method is validated against the original and modified Darcy-Forchheimer analytical equations through a novel modified porosity theory. Good agreement is obtained between the numerical and analytical results. It is shown that both 2D and 3D heat exchanger configurations of identical porosities with different geometric profiles have shown similar deposition fraction and pressure drop magnitudes albeit having a slightly different fouling layer distribution. This is attributable to the particle properties and the variation between the 2D and 3D inlet injection plane surface area. It is found that the commencement of sandstone and sawdust deposition in a 6-pore configuration differs by 0.57s, whereas a three pore configuration completely nullifies particulate fouling irrespective of foulant type. A staggered row configuration has shown significant reduction in pressure drop as compared to the 6-pore heat exchanger configuration. For the case of sandstone particles, the optimum heat exchanger geometry exhibits 78% less pressure drop and 100% less deposition fraction compared with the original 6-pore configuration.

Effect of aluminum-foam heat sink on inclined hot surface temperature in the case of free convection heat transfer

The need to more powerful electronic chips cause more heat generation as a consequence. So it leads to apply metal foam heat sink as a powerful cooling methods. The objective of this paper is to experimentally investigate the effect of inclination angle on the temperature of a heated surface with and without metal foam in the case of free convection heat transfer conditions. Experimental modeling has been conducted using an electrical resistor on an aluminum plate with and without aluminum metal foam in different angles with respect to the horizontal direction. The results show that the temperature variations have a linear dependency to the sine of inclination angle up to 45° and 60° for plate with and without metal foam, respectively. The slope of the lines are positive in low power input and negative in high power input. The effect of applying metal foam depends on the input power and inclination angle. In the presence of foam, the surface temperature has a decrease of 16°C with respect to a flat plate. The maximum cooling efficiency using foam is about %17 at 90° inclination angle.

Experimental investigation on water drainage characteristics of open-cell metal foams with different wettabilities

For optimizing the metal foam heat exchangers used in dehumidification apparatus, the influence of surface wettability on drainage characteristics should be explicit. In the present study, the water drainage characteristics of metal foams with different wettabilities were experimentally investigated. The experimental results show that the stabilization time of the water drainage process for the hydrophobic sample is the shortest among the tested metal foam samples, and the water retention mass in the hydrophobic metal foam is much smaller than that in the other samples; the wettability impact factors for water retention are within 0.78–1.1 and 0.40–0.77 for the hydrophilic and hydrophobic samples, respectively. The water retention mass in the copper foam is 9.5%–52.6% smaller than that in the aluminum foam. For the hydrophobic copper foam, the water retention mass per unit volume in metal foams with PPI of 5–20 is 45%–66% smaller than that in fin–tube heat exchangers.

Geometric Mean of Fin Efficiency and Effectiveness: A Parameter to Determine Optimum Length of Open-Cell Metal Foam Used as Extended Heat Transfer Surface

High porosity open-cell metal foams have captured the interest of thermal industry due to their high surface area density, low weight, and ability to create tortuous mixing of fluid. In this work, application of metal foams as heat sinks has been explored. The foam has been represented as a simple cubic structure and heat transfer from a heated base has been treated analogous to that of solid fins. Based on this model, three performance parameters namely, foam efficiency, overall foam efficiency, and foam effectiveness have been evaluated for metal foam heat sinks. Parametric studies with varying foam length, porosity, pore density, material, and fluid velocity have been conducted. It has been observed that geometric mean of foam efficiency and foam effectiveness can be a useful parameter to exactly determine the optimum foam length. Additionally, the variation in temperature profile of different foams heated from one end has been determined experimentally by cooling these with atmospheric air. The experimental results have been presented for open-cell metal foams (10 and 30 PPI) made of copper/aluminium/Fe–Ni–Cr alloy with porosity in the range of 0.908–0.964.

Thermal Performance Analysis of Biporous Metal Foam Heat Sink

The present study presents a concept of biporous metal foam heat sink applicable to electronic cooling. This heat sink has two metal foam layers arranged in parallel along the primary flow direction, with different metal foam thickness, porosity, and pore density for each layer. The forced convective heat transfer in biporous metal foam heat sink is numerically investigated by employing the Forchheimer–Brinkman extended Darcy momentum equation and local thermal nonequilibrium energy equation. The effects of geometrical and morphological parameters on thermal and hydraulic performance are discussed in detail, and the heat transfer enhancement mechanism of biporous metal foam is analyzed. The thermal performance of biporous metal foam heat sink is compared with that of uniform metal foam heat sink. The results show that the thermal resistance of the biporous metal foam heat sink decreases with decrease of top layer metal foam porosity at a fixed bottom metal foam porosity of 0.9. It is seen that the biporous metal foam heat sink can outperform the uniform metal foam heat sink with a proper selection of foam geometrical and morphological parameters, which is attributed to the presence of high velocity gradient at the boundary layer that can enhance the convective heat transfer. The best observed thermal performance of biporous metal foam heat sink is achieved by employing 30 pores per inch (PPI) metal foam at the bottom layer, with a fixed 50 PPI metal foam at the top layer for the porosities of both layers equal to 0.9, and the optimal thickness of the bottom foam layer is about 1 mm.

Thermal-hydraulic performance of metal foam heat exchangers under dry operating conditions

High porosity metal foams with novel thermal, mechanical, electrical, and acoustic properties are being more widely adopted for application. Due to their large surface-area-to-volume ratio and complex structure which induces better fluid mixing, boundary layer restarting and wake destruction, they hold promise for heat transfer applications. In this paper, the thermal-hydraulic performance of open-cell aluminum metal foam heat exchanger has been evaluated. The impact of flow conditions and metal foam geometry on the heat transfer coefficient and gradient have been investigated. Metal foam heat exchanger with same geometry (face area, flow depth and fin dimensions) consisting of four different type of metal foams have been built for the study. Experiments are conducted in a closed-loop wind tunnel at different flow rate under dry operating condition. Metal foams with a smaller pore size (40PPI) have a larger heat transfer coefficient compared to foams with a larger pore size (5PPI). However, foams with larger pores result in relatively smaller pressure gradients. Current thermal-hydraulic modeling practices have been reviewed and potential issues have been identified. Permeability and inertia coefficients are determined and compared to data reported in open literature. On the basis of the new experimental results, correlations are developed relating the foam characteristics and flow conditions through the friction factor f and the Colburn j factor.

3D modeling and analysis of the thermo-mechanical behavior of metal foam heat sinks

Metal foams are metallic materials that contain a high percentage of porosity. They can be manufactured as open-cell or closed-cell foams. Open-cell metal foams are ideal for thermal applications due to the high ratio of surface area to volume, which provide more surface contact between the material and the fluid. In this work, a detailed approach to utilize micro-computed tomography scan to create a detailed and accurate 3D metal foam model is discussed. The model is implemented into a finite element analysis software and used in an application of heat sinks. Experiments are performed to record the temperature profile of the heat sink, and then the heat transfer coefficient h for the numerical model is calibrated using the temperature data. The 3D metal foam heat sink model is validated against experiments as well. Finally, the complete validated model is used to study the thermo-mechanical behavior of 1, 2, and 3 fins metal foam heat sinks. The developed model and approach provides more insight into the coupled behavior of metal foam heat sinks by being able to calculate the developed stresses due to the thermal loading of the heat sinks.

Numerical study of metal foam heat sinks under uniform impinging flow

The ever-increasing demand for performance improvement and miniaturization of electronics has led to a significant generation of waste heat that must be dissipated to ensure a reliable device operation. The miniaturization of the components complicates this task. In fact, reducing the heat transfer area, at the same required heat rate, it is necessary to increase the heat flux, so that the materials operate in a temperature range suitable to its proper functioning. Traditional heat sinks are no longer capable of dissipating the generated heat and innovative approaches are needed to address the emerging thermal management challenges. Recently, heat transfer in open-cell metal foams under an impinging jet has received attention due to the considerable heat transfer potential of combining two cooling technologies: impinging jet and porous medium. This paper presents a numerical study on Finned Metal Foam (FMF) and Metal Foam (MF) heat sinks under impinging air jet cooling. The analysis is carried out by means of the commercial software COMSOL Multiphysics®. The purpose is to analyze the thermal performance of the metal foam heat sink, finned or not, varying its geometric parameters. Results are presented in terms of predicted dissipated heat rate, convective heat transfer coefficient and pressure losses.