Pressure drop and flow boiling instabilities in silicon microchannel heat sinks

Abstract

A simultaneous visualization and measurement experiment has been performed to study pressure drop and flow instabilities at various mass fluxes and heat fluxes during flow boiling of deionized water in a silicon microchannel heat sink. The silicon micro heat sink consisted of eight parallel microchannels 60 mm long, having a trapezoidal cross section with a hydraulic diameter of 72.7 µm. It is found that (1) the onset of nucleate boiling (ONB) depends on both the amount of heat flux and mass flux. The mass flux, at which the onset of nucleate boiling occurs, increases as the heat flux is increased; (2) the difference in mass fluxes between the ONB and the onset of flow instability (OFI) decreases as the heat flux is increased; (3) both oscillation amplitude and oscillation period of various measurements near the OFI are small; (4) as the mass flux is decreased further from the OFI while keeping the heat flux constant, the following boiling instability flow modes occurred sequentially: the liquid/two-phase alternating flow (LTAF) and the liquid/two-phase/vapor alternating flow (LTVAF); (5) the occurrence of LTAF and LTVAF, as well as their induced large-amplitude/long-period oscillations of various measurements, are owing to the reversed flow of vapor core because of the limited expansion space in the microchannels; (6) the appearance of the reversed flow of the vapor core in parallel microchannels is not in phase due to the flow or nucleation nonuniformity. The results presented in this paper help us to better understand the two-phase flow instabilities occurring in the silicon-based micro heat sinks.