Saturated pool boiling heat transfer from highly conductive graphite foams

This paper presents an investigation of heat transfer enhancement using highly conductive graphite foam as porous insert in a pool boiling evaporator. Graphite foams of different bulk thermal conductivities and pore sizes were investigated with phase change coolants FC-72 and HFE-7000 and compared with a copper block in a designed thermosyphon. The heater wall temperature, superheat and thermal resistances were obtained to evaluate the boiling performance. Associated with the analysis of various boiling regimes, nondimensional parameters including the Capillary, Grashof and Bond numbers were used to analyze the bubble formation from the porous insert. The experimental results show that the boiling thermal resistances of the system with a graphite foam insert are about 2 and 3 times lower than those of the copper block immersed in FC-72 and HFE-7000, respectively. The nondimensional analysis showed that the bubbles from the small pore diameter graphite foam have to overcome a large surface tension force before departure, although the foam possesses high thermal conductivity. This implies that a balanced relation between thermal conductivity and pore diameter could maximize the enhancement on pool boiling heat transfer. The current thermosyphon with a porous insert can allow a heat flux of 112 W/cm2 to be removed with a maximum heater temperature of 90 °C.