FORCED CONVECTION AND FLOW BOILING IN MONO- AND BI- DISPERSED POROUS CHANNELS

Forced convection and flow boiling in channels packed with sintered copper bi-dispersed porous media (with micro-pores diameter of 80 μm and macro-pore diameters of 200/400/800 μm) were investigated in this paper. For comparison, the same channels were packed with sintered copper mono-dispersed porous media with pore diameters of 80/800 μm. The experiments were conducted using distilled water as the working fluid. The absolute permeabilities of the porous channels packed by the sintered copper mono- and bi-dispersed media were first determined by measuring pressure drops at different inlet velocities of the isothermal flow of subcooled water. Experiments were then performed for forced convection in the porous channels heated from below with constant heat flux. It was found that the convection heat transfer coefficients of fluid flow through bi-dispersed porous media are smaller than those of the same fluid flow through mono-dispersed porous media (with the same pore diameter as the micro-pore of the bi-dispersed media) at the same inlet fluid velocity. As the heat flux is increased further and boiling begins to occur, the pressure drop of the two-phase flow and characteristic curve of boiling were determined. It is found that the boiling phenomena inside the mono- and bi-dispersed porous media begins at very low wall superheats because of the increase of the specific heat transfer surface area and nucleation sites. The results indicated that the porous material is a highly effective two-phase heat sink, especially for bi-dispersed porous materials which have a lower flow resistance than that of the mono-dispersed porous materials having the same pore diameter as the micro-pore diameter of the bi-dispersed porous material.