Effect of pin-fins on the onset of flow instability of water in silicon-based microgap

This paper is focused on the effect of inner pin-fins on the onset of flow instability (OFI) in microgaps. An experiment is performed on the OFI of water through four types of microgaps with different shaped pin-fins and a plain microgap without pin-fins. The microgaps have a hydraulic diameter of 210 μm. It is found that the OFI is delayed in the pin-fin microgap with larger friction factor as compared to that with smaller one. A theoretical analysis for the effect of friction factor on OFI is further developed. It is shown that OFI moves to the direction of decreasing mass flux with the increasing friction factor, which well explains the experimental results. However unexpectedly, when the temperature and pressure oscillations occur due to the flow boiling instability after OFI, the oscillation amplitudes for the pin-fin microgap with the larger friction factor are larger than those with the smaller one. To account for this unexpected result, the flow patterns during the flow boiling instability just after OFI are presented. The effects of pin-fins and inlet restrictors on OFI are also compared. It is found that the pin-fin configurations delay the occurrence of OFI as compared to the plain microgap. However, when the flow boiling instability occurs after OFI, the former increase the oscillation amplitudes of temperature and pressure measurements, and the latter decrease them. The mechanism analysis of the different effects between pin-fins and inlet restrictors is also presented.