Experimental investigation on pool boiling mechanism of two-level gradient metal foams in deionized water, aqueous surfactant solutions and polymeric additive solutions

In the present study, metal skeleton temperature, bubble growth, pool boiling curves, and the solid-liquid static contact angle of two-level gradient foams have been experimentally investigated in deionized water, aqueous surfactant solutions and polymeric additive solutions under atmospheric pressure. The surfactant is Triton X-100 and the polymeric additive is polyvinyl pyrrolidone. A two-level gradient foam sample is made by a uniform copper foam layer, a uniform nickel foam layer and a copper plate with the thickness of 2 mm. The pore densities of uniform foams are 5 PPI, 20 PPI and 40 PPI. The foam porosity is fixed as 0.98. The experimental results show that, foam layer position reversal has a great effect on pool boiling heat transfer of the gradient foam. The pool boiling heat transfer rate of the gradient nickel-copper foam increases with increasing bottom foam pore density. The temperature of corner skeleton on the upper surface of the gradient foam is higher than that of inner skeleton because of the heat flow from escaping bubbles by collision, cutting, stagnation, etc. The metal skeleton temperature is heavily affected by the additives of Triton X-100 and polyvinyl pyrrolidone due to the variations of the solid-liquid contact angles.