Flow boiling instability characteristics in microchannels with porous-wall

Abstract

In this work, a porous-wall microchannel heat sink was designed and fabricated, and the wall regions were etched with the micro pin fin arrangement instead of solid walls. The porous-wall can be divided into three regions: the densely/intermediate densely/sparely pin fin region with the fin gap of 5 μm/7.5 μm/10 μm, respectively. Flow boiling tests in porous-wall microchannels were carried out at mass fluxes of 250–510 kg/(m2·s) and heat fluxes of 120–720 kW/m2. The effects of the porous-wall on suppressing the flow instabilities and manipulating the boiling flow were studied, and different boiling flow behaviors of pressure drops and temperatures as well as corresponding flow patterns were measured and observed. The experimental results showed that: (1) With the porous-wall inter-connect effect, the premature ONB (Onset of Nucleate Boiling) can be occurred, the nucleate boiling flow in whole channel can be triggered in less than 2 ms, the boiling flow instability can be suppressed, and the duration of two-phase flow can be prolonged; (2) The boiling instabilities in porous-wall microchannels were classified into four flow modes, and the temporal behaviors of temperatures and pressure drops corresponding to each flow instability mode were observed and analyzed. (3) By PSD (Power Spectral Density) combined with WT (Wavelet Transform) analysis method, the coupled wall temperature oscillations in each instability type were decoupled into different instability sub-types at different amplitudes and timescales. Moreover, the analysis of ms-timescale flow instability induced by the flow pattern transition helps to better understanding the dominant influence of the porous-wall on the resulting temperature oscillations.