Flow boiling characteristics in open microchannel heat sinks with flexible membrane-based cover plates

The confinement of expanding vapor slugs during flow boiling in a microchannel heat sink (MHS) is one of the primary causes that triggers flow instability, resulting in substantial fluctuations in pressure and wall temperatures. Although several geometric modifications of microchannels have been explored, the pivotal role of the top cover plate in mitigating the flow boiling instabilities remains largely unexplored. The present study investigates the impact of flexible, porous, and non-porous membrane-based cover plates on mitigating flow boiling instabilities in open microchannel configurations. Flow boiling experiments are conducted in a flow boiling loop using deionized (DI) water as the working fluid, with heat fluxes ranging from 20 to 240 W/cm² and coolant mass flows from 256 to 536 kg/m²s. The test module comprises an array of 11 rectangular microchannels with a hydraulic diameter of 0.5 mm (500 microns). The open microchannel features a 300 micron gap between the microchannel face and the cover plate. The influence of a flexible-porous PTFE (Polytetrafluoroethylene) membrane-based cover plate (OVV) and a flexible-non-porous Nitrile membrane-based cover plate (ONV-1& ONV-2) on flow boiling is analyzed in comparison to an open microchannel with a rigid cover plate (OMC). The porous PTFE membrane-based cover plate has considerably reduced the pressure drop and has effectively alleviated flow boiling instabilities. Continuous vapor venting through the pores enables periodic extraction of excess vapor slugs from the MHS, thereby reducing pressure and temperature fluctuations in the channels