Experimental Study of Gradient Copper Foam Effect on Pool Boiling Heat Transfer Performance and Gas-Liquid Behavior Characteristics

Gradient metal foam (GMFs), as a new type of enhanced boiling heat transfer material, can effectively weaken the resistance of bubbles in the detachment process, clarify the escape path and enhance the boiling heat transfer performance significantly. In this paper, an experimental visualization setup was built, the effect of different gradient structures copper foam on the pool boiling heat transfer performance and bubble behavior characteristics were investigated. The results showed that in the early boiling stage, the copper foam with a higher gradient difference showed a stronger boiling heat transfer performance. However, in the middle and late boiling stages, the copper foam with 20 PPI gradient difference showed the best heat transfer performance. The boiling heat transfer performance of uniform metal foam (UMF), positive gradient metal foam (PGMFs) and the inverse gradient metal foam (IGMFs) were compared and the liquid complementary analysis was performed for the three structures. Under the same condition, the IGMFs showed the best heat transfer performance. The effect of pore density on the PGMFs and the IGMFs was significantly different. It was found that the liquid replenishment rate and bubble detachment resistance are the decisive factors affecting the heat transfer performance of IGMFs. Three forms of bubble detachment were obtained, and corresponding force analysis was performed