Porous-wall microchannels generate high frequency “eye-blinking” interface oscillation, yielding ultra-stable wall temperatures

Abstract

The proposed gradient–porous-wall microchannels consist of bare channels and pin–fin array regions, fabricated by MEMS (microelectricalmechanicalsystem) technique. Boiling experiments were performed with acetone as the working fluid. Ultra-stable wall temperatures are achieved with oscillation amplitudes in the range of 0.02–0.18°C. Bubble nucleation is found to happen in the porous wall. The generated vapor flows towards bare channels due to surface tension driving flow. The vapor ejection direction is periodically switched between neighboring channels, called the “bubble emission switch”. The bubble confinement ratio is newly defined. Bubbles become fat and slim in bare channels to generate high frequency “eye-blinking oscillation”. Bubble confinement ratios display sine function, and out-of-phase characteristic between neighboring channels. We confirm the “eye-blinking” oscillation as a density wave oscillation, propagating in the channel width direction. Because the porous-wall width is much smaller than the channel length, the “eye-blinking” frequencies are 10–100 times higher than that of the axially propagated density wave oscillation. The “integration parameter model” establishes the connection between “eye-blinking” oscillation and wall temperatures. The convective heat transfer intensity in bare channels is assumed to follow the bubble confinement ratio variation. The wall temperature oscillation amplitude is inversely proportional to the “eye-blinking” frequency. The phase angle between bubble confinement ratios and wall temperatures are 3π/2, being the negative feedback mechanism to inhibit wall temperature oscillations. The porous-wall microchannels open a new way to eliminate flow instabilities for heat exchangers and thermal energy systems.