Thermal Performance And Structure Optimization Of Metal Foam Heat Sink

With the rapid development of micro-electro-mechanical systems（MEMS）,large-scale integrated circuits and high-power light-emitting diodes,the miniaturization and integration of advanced electronic components and systems are increasing,resulting in the transient heat flux and working temperature continuously rising,which would seriously affects the stability and reliability of electronic components.Metal foam is a novel kind of porous medium with the advantages of good structural rigidity and multi-functionality.Metal foams as compact and efficient heat sinks provide a feasible cooling solution for high powered electronics.Compared to the traditional heat sink,the flow and heat transfer characteristics of metal foam heat sink would be different.Therefore,a detailed investigation on fluid flow and heat transfer within metal foam heat sink is of significance to illustrate the forced convection heat transfer characteristics.The relevant research results could provide theoretical guidelines for designing high efficiency and low resistance heat sink.In this paper,a combined numerical and experimental study was adopted to deeply investigate the forced convection heat transfer within metal foam heat sink for the potential application of high powered electronics cooling.Numerical simulations were conducted to analyze the effects of metal foam morphological parameters,thermophysical properties and channel geometrical parameters on local thermal non-equilibrium heat transfer phenomena within metal foam heat sink.On the basis of obtaining the flow and heat transfer mechanism within metal foam,a macroscopic numerical model involving viscous resistance,inertia resistance and the conjugated heat transfer between the solid matrix and fluid was established.The effects of different metal foam configurations and gradient metal foam designs on flow drag reduction and heat transfer enhancement are analyzed in detail.To design more compact metal foam heat sink with high heat dissipation performance,a novel scheme by combining metal foam with high thermal conductivity solid material was proposed.The metal foam/solid compound fin heat sink,and metal foam and pin fin hybrid heat sink are developed to enhance the heat transfer performance.The fluid flow and heat transfer characteristics of aluminum foam heat sinks were experimentally investigated to measure the pressure drop and temperature distributions,and the enhanced heat transfer performance of aluminum foam and pin fin heat sink was experimentally validated.In light of the superior heat transfer performance of metal foam/pin fin heat sink,a multi-objective optimization method was adopted to optimize its geometrical parameters.The main contents and conclusions are summarized as follows:（1）Both pore-scale and macroscopic scale numerical simulations were conducted to investigate the fluid flow and heat transfer characteristics within metal foam.For pore scale simulations,the simplified tetrakaidecahedron metal foam model was employed to investigate the flow and heat transfer characteristics.Macroscopic modeling of heat transfer in metal foams including local thermal equilibrium model and local thermal non-equilibrium model.The difference between them are demonstrated,and the heat transfer performance predicted by the two models are compared in terms of temperature profile,temperature field distribution and average Nusselt number.The results show that metal foam morphological parameters,the solid-to-fluid thermal conductivity ratio,velocity and channel aspect ratio have considerably effect on the local thermal non-equilibrium heat transfer.All the measures that can lower the local convective thermal resistance between foam ligament and fluid,which can also reduce the local thermal non-equilibrium effect.（2）The volume-average technique was used for modeling the fluid flow and heat transfer in high porosity metal foam heat sink.In this part,the authors mainly focused on investigating the effects of the homogenous or heterogeneous metal foam with various geometrical configurations on velocity distribution,temperature distribution,flow resistance and heat transfer performance.The mechanism of heat transfer enhancement with metal foam was also analyzed by using the field synergy principle.The results demonstrated that metal foam can significantly enhance the heat transfer performance,for the studied cases,the thermal resistance of metal foam fully filled heat sink was reduced by 55%～75%.The enhanced heat transfer with metal foam is attributed to that the synergy between velocity filed and temperature gradient filed is effectively improved.The average field synergy angle can be reduced by 12.6^o～26.2^o.Gradient metal foam with pore diameter decreasing vertical to the flow direction provides a new concept for achieving the improved thermo-hydraulic performance simultaneously.（3）The novel metal foam/solid compound fin heat sink,and metal foam and pin fin hybrid heat sink（MFPFH）are proposed.Thermal performance of these novel heat sinks are numerically investigated in terms of velocity distribution,temperature distribution,flow resistance and heat transfer performance.The effects of thermal contact thermal resistance（TCR）between pin fin and metal foam on the thermal performance of MFPFH are illustrated.The results show that heat transfer enhancement and pressure drop reduction can be achieved simultaneously with proper design of the porous fin dimensionless thickness,and the optimal dimensionless porous fin thickness for the studied cases is approximately 0.2.MFPFH heat sink can effectively suppress the flow separation,and the vortices are disappeared behind pin fins in MFPFH heat sink.MFPFH provides a much larger effective thermal conductivity than metal foam heat sink.The Num of the MFPFH heat sink is approximately 266.6%and 36.3%higher than the traditional pin fin heat sink and metal foam heat sink.When the value of thermal contact resistance is 10^-3 m²·K/W,the Nu_mis reduced by 36%.（4）The commercially available ERG aluminum foam samples with porosity of 0.88 and pore densities of 10 PPI and 20 PPI are used to manufacture the aluminum foam heat sinks and aluminum/pin fin heat sinks.All the foam samples have the same overall dimension,which is cut by using the WEDM（wire electronic discharge machine）method.Experimental tests were conducted to measure the pressure drop and bottom wall temperature distribution.Thermo-hydraulic performance of aluminum foam heat sink is also evaluated in comparison with aluminum-pin fin heat sink.Compared with the aluminum foam heat sink,the implementation of the aluminum/pin fin heat sinks significantly improves the heat transfer performance.The enhancement in Num of aluminum/pin fin heat sinks is over 65%.Under the same pumping power consumption,the aluminum/pin fin heat sink shows a better convection heat transfer performance,with the Num approximately 1.5 times higher than the aluminum foam heat sink.It is also demonstrated that,compared with empty channel,aluminum foam heat sink significantly enhances the heat transfer performance,with the heated wall temperature reduced by 30 ^oC.（5）The influences of structure parameters（porosity,pore density and fin diameter）and operating conditions（inlet velocity）on pressure drop and maximum temperature of MFPFH heat sink are numerically analyzed.The quadratic polynomial between thermal performance（pressure drop and maximum temperature）and design variables is fitted by using response surface method.Based on these two quadratic polynomials,the multi-objective optimization of MFPFH heat sink were conducted by using genetic algorithm（NSGA-II）.A series of Pareto optimal sets containing the optimal design parameters,and the related pressure drop and maximum temperature are obtained.Several typical Pareto optimization solutions are selected in comparison with the numerical results,the predicted results fits well with the Pareto sets,which verifies the accuracy of the optimization results.