A hydrophobic porous substrate-based vapor venting technique for mitigating flow boiling instabilities in microchannel heat sink

Abstract

• Porous hydrophobic exit is used to mitigate flow instabilities in microchannels. • It facilitates the removal of elongated bubbles during intermittent flow regime. • The vapor venting technique suppresses wall temperature and pressure oscillations. • A 32% enhancement in HTC is observed compared to the conventional configuration. • A 63% reduction in pressure drop is observed compared to the conventional design. With the onset of nucleation, flow boiling in microchannels is susceptible to undesirable flow instabilities resulting in fluctuating wall temperature and large pressure drops. It is imperative to develop a passive technique to mitigate flow boiling instabilities and enhance the thermo-hydraulic performance of the heat sink without additional pressure drops. The present work explores a hydrophobic porous polydimethylsiloxane (PDMS) substrate-based vapor venting technique to mitigate flow boiling instabilities in microchannels. The hydrophobic porous substrate fitted at the channel exit manifold attracts the vapor plug, assists in vapor venting, and thus facilitates the easy evacuation of the bubbles from the channel. Flow boiling experiments are performed with deionized water in rectangular cross-section microchannels for a heat flux range of 10–260 W/cm 2 and the coolant mass flux range of 206–335.49 kg/m 2 s. Heat transfer and pressure drop characteristics of the proposed configuration are compared with those of the conventional configuration. The proposed design suppresses the wall temperature oscillation and pressure oscillations. It produces a 32 % enhancement in heat transfer coefficient and a 63 % reduction in pressure drop compared to the conventional outlet configuration. The new configuration significantly reduces the bubble ebullition cycle, resulting in an efficient bubble evacuation from the channels. Further, frequent flushing of the vapor slug in the new design reduces the temperature fluctuations and enhances the heat transfer coefficient. The heat transfer augmentation is more pronounced at higher heat flux with more vapor generation rates.