Experimental Study On The Influence Of Multilayer Gradient Copper Foam Of Pool Boiling Heat Transfer Performance

With the continuous development of science and technology,the integration of electronic components is becoming more and more advanced,and conventional heat dissipation methods such as air cooling and liquid single-phase forced convection are difficult to meet the demand for efficient heat dissipation.Pool boiling heat transfer is a heat transfer method with high heat flow density,which does not require high superheat to achieve very high heat flow density and has an important role in high efficiency heat transfer.Foam metal,as a new porous medium,can effectively strengthen the pool boiling heat transfer performance and alleviate the current phenomenon of increased heat transfer demand for electronic equipment.In this paper,experimental studies on the pool boiling heat transfer performance of multilayer foam metals were carried out by building a visualization test bench.Firstly,the enhanced pool boiling heat transfer performance of single-layer copper foam with nine pore densities（10～90 PPI）were divided into low pore density（10～40 PPI）and high pore density（50～90 PPI）,and the changes of bubble dynamic behavior and liquid replenishment position were analyzed.The results show that the boiling heat transfer performance of the copper foam reinforced cell increases and then decreases with increasing pore density,with the weakest reinforced heat transfer effect for 10 PPI copper foam and the best for 70 PPI copper foam,and the maximum surface heat transfer coefficient increases by 1.27 times compared with that of smooth surface.By observing the dynamic behavior of escaping bubbles on the surface of copper foam,a variety of boiling bubble detachment forms were analyzed,and the theoretical analysis of the direction of liquid replenishment on the macroscopic surface of copper foam with different pore densities revealed that the increase of boiling bubble escape resistance and the decrease of external liquid replenishment position were the reasons limiting the enhanced boiling heat transfer performance of copper foam.For gradient foam metal,as a new type of enhanced boiling heat exchange material,it can effectively weaken the resistance of boiling bubbles in the detachment process,clarify the escape path and enhance the boiling heat exchange performance significantly.In this paper,the effect of copper foam with different gradient structures on the pool boiling heat transfer performance is studied.When the gradient copper foam is double-layer structure,the copper foam with larger gradient difference has stronger boiling heat transfer performance in the pre-boiling stage,and in the middle and late boiling stage,the copper foam with 20 PPI gradient difference has the best heat transfer performance.The enhanced boiling heat transfer performance of copper foam with positive gradient increases with the increase of pore density.The direction of gas-liquid flow of copper foam with inverse gradient changes,and the liquid replenishment rate and bubble detachment resistance are the decisive factors affecting the heat transfer performance of copper foam with inverse gradient.When the copper foam pore density is low,an increase in the number of gradient foam metal gradient layers can improve the heat transfer effect.The change in the arrangement order of the multilayer gradient copper has a significant effect on the dynamic behavior of the boiling bubbles.When the bubbles detach upward,the temperature at the upper surface of the foam metal is lower and the temperature gradient is larger.When the bubbles detach to the side,the escape distance of boiling bubbles is shortened,the bubble escape time is shorter,and the frequency and size of bubble detachment increase.In this paper,gas-liquid behavior and heat transfer models are established for different structures of gradient foam metals,and three new multilayer gradient foam metal structures are proposed.Based on the abundant experimental data,the empirical exponents in the Rohsenow boiling correlation were derived to make the Rohsenow correlation consistent with the pool boiling heat exchange performed on the surface of foam metal.Combining the nature of pool boiling heat exchange and the Rohsenow correlation,a new correlation is derived for pool boiling heat exchange on foam metal,which is more consistent with the pool boiling heat exchange on foam metal surface than the Rohsenow correlation by combining the porous nature and three-dimensional structure of foam metal itself.