Subcooled flow boiling on hydrophilic and super-hydrophilic surfaces in microchannel under different orientations

Abstract

An experimental investigation of subcooled flow boiling in the rectangular microchannel with hydraulic diameter of 0.94 mm (5.01 mm × 0.52 mm) for different orientations on hydrophilic and super-hydrophilic surfaces is conducted utilizing deionized water as the working fluid. The contact angles of the bare silicon surface and the nano-silica surface prepared through the plasma enhanced chemical vapor deposition method are 65° ± 3° and less than 5° respectively. With an inlet subcooling temperature of 10 °C, the mass fluxes and heat fluxes are tested in the range of 200–500 kg/(m2 s) and 4–25 W/cm2 respectively, while the orientation angles are 0° (bottom-heated horizontal flow), 90° (vertical upflow), 180° (top-heated horizontal flow) and 270° (vertical downflow). Significant appearances at the onset of nucleate boiling (ONB) accompanied with bubble nucleation are observed in the boiling curves. An earlier occurrence of critical heat flux (CHF) is presented for vertical downflow. With the direction of buoyancy opposite to the fluid inflow, the buoyancy force impedes the departure and movement of bubbles against the inertia, causing easier bubble coalescence and elongation as well as prolonged dry-out, which deteriorates heat transfer. At low mass fluxes the pressure drop for the case of vertical downflow is the highest among others, whereas at high mass fluxes the bottom-heated horizontal flow exhibits the maximum pressure drop. Most intense pressure drop fluctuation is obtained for vertical downflow, indicating the most severe flow instability under this orientation. The effect of orientation is weakened with the increment of mass flux due to the dominance of inertia force. At the mass flux of 200 kg/(m2 s), the super-hydrophilic surface delays CHF without increased pressure drop penalty in the vertical orientation, while in the horizontal orientation the effect of surface wettability seems more significant.