Investigation On Heat Transfer And Bubble Behavior Of Pool Boiling In Gradient Metal Foams

To save energy and ensure the safety of heating equipment,improving heat transfer performance effectively is urgently needed.Pool boiling is considered to be an effective method for cooling high heat-generating electronic components with less noise and equipment.The gradient metal foams not only remain high surface area and other advantages of metal foams,but also the enlarging pore structures make bubble escape easily,which significantly enhance pool boiling heat transfer.In this paper,a pool boiling experiment system is designed and built.A high-speed camera is used to capture the bubbles of pool boiling.The heat transfer performance and bubble behavior in gradient metal foams under pool boiling conditions are investigated.The metal foams materials are copper and nickel.The porosity remains at the fixed value of0.98,while the pore density and thickness are from 5 PPI to 100 PPI,2 mm to 14 mm respectively.The working liquid is pure deionized water,SDS,TX-100,n-heptanol and alumina nanoparticle solutions.The heat transfer investigation results show that gradient metal foams significantly enhance pool boiling heat transfer,which is heavily dependent on metal foams thickness,pore density,material and surfactant,etc.The gradient nickel foams with middle-low pore density combination have better heat transfer performance than the foams with high-low and high-middle pore density combination.The effect of material depends on the value of heat flux,and when heat flux is low,the copper-nickel foams have higher boiling heat transfer coefficient.Adding SDS into pure water can improve pool boiling heat transfer,but the enhancement decreases with increasing concentrations.Triton X-100 enhancement is weak,which even deteriorates boiling heat transfer.Alumina nanoparticle adversely affects the boiling heat transfer due to nanoparticle-deposited on the metal skeletons.Based on the investigation of bubble behavior in gradient metal foams under pool boiling conditions,it is found that surfactant reduces bubble diameter and improves bubble generating rate,and the activation of TX-100 is better than SDS.N-heptanol increases the wettability of the copper surface and adversely affects the boiling heat transfer.By observing the bubbles behavior of pool boiling in n-heptanol solutions,it is found that small departure bubbles attach onto the metal skeleton and then decrease in size at low heat flux.As heat flux increases,two common phenomena occur in the gradient foams:bubbles escape from the metal skeletons without cracking and bubbles separate into two smaller bubbles.When heat flux increases to 1×10⁵ W·m^-2,two neighboring bubbles move upwards and are then sucked into the metal foam due to coalescence.Finally,two-dimension simplified metal foams are built,and bubble escape in the metal foams is researched by numerical simulation.The simulation results show that the upper enlarging pores of gradient metal foams make bubble escape more easily,which also result in more violent velocity volatility of the departing bubble.However,the skeletons arrangement at the junction of two-layer metal foams has effect on the movement of bubbles.If skeletons at the junction are staggered,the departing bubble will be split,which adversely affects bubble departure.