Flow Boiling Heat Transfer in Horizontal Metal-Foam Tubes

The two-phase flow and boiling heat transfer in horizontal metal-foam filled tubes are experimentally investigated. The results show that the heat transfer is almost doubled by reducing the cell size from 20 ppi to 40 ppi for a given porosity, thanks to more surface area and strong flow mixing for the smaller cell size. The boiling heat transfer coefficient keeps steady rising, albeit slowly, by increasing the vapor quality for high mass flow rates, while the same story does not hold for the cases of low mass flow rates. The flow pattern can be indirectly judged through monitoring the cross-sectional wall surface temperature fluctuations and wall-refrigerant temperature difference. As the operating pressure increases, the boiling heat transfer at low vapor quality $(x<0.1)$ exhibits similar behavior with pool boiling heat transfer, namely, the heat transfer is enhanced by improving the pressure. However the flow boiling heat transfer is suppressed to some extent as the pressure increases. The heat transfer coefficient of copper foam tubes is approximately three times higher than that of plain tubes.