EXPERIMENTAL INVESTIGATION OF THE EFFECTS OF POROUS MEDIUM ON SUBCOOLED FLOW BOILING HEAT TRANSFER IN A VERTICAL ANNULUS TUBE

Flow boiling heat transfer enhancement has been considered an interesting topic in the industry. Insertion of high porosity metal foams in heat exchangers is one technique to increase the rate of heat transfer because there is a high surface area in a small volume in these materials. In this study, an experimental apparatus is constructed to study a porous medium's effects on subcooled flow boiling of water in a vertical annulus tube with an inner diameter of 50.7 mm and an outer diameter of 70.6 mm at near atmospheric pressure. The tube is filled with 10 PPI open-cell nickel metal foam. The effects of different parameters such as heat flux, mass flow rate (ranging from 0.012 to 0.0286 kg/s), and the inlet subcooling temperature (i.e., 35-50 degrees) on heat transfer coefficients are explored. The results revealed that increasing the imposed heat flux leads to an enhancement in the subcooled boiling heat transfer coefficient. Also, the subcooled heat transfer coefficient increases with a decrease in inlet subcooling temperature. Using 10 PPI nickel porous inside the annular tube, the subcooled boiling heat transfer coefficient can be increased up to 30% compared to an empty tube. The surface area enhancement due to using metal foam leads to an increment in the active nucleation sites, which are visualized in this paper. The theoretical relations also validate the heat fluxes in the subcooled boiling regime.