Effect of compressible volume on flow boiling instability of water in the pin-fin microchannel

Abstract

Flow boiling instability induced by compressible volume is significant for the premature occurrence of the limiting critical heat flux (CHF) condition in microchannles. To suppress the flow boiling instability, some methods have been studied, including increasing inlet restrictors on the microchannel inlet, forming reentrant cavities on the microchannel inner surface, and et al. These methods focused on microchannels themselves, while little attention was given to compressible volume in upper stream of the microchannels. In this paper, flow boiling instability of water in the silicon-based pin-fin microchannel was studied when there existed compressible volume in the upper stream of the microchannel. This pin-fin microchannel had circular/staggered pin-fin arrays arranged on the undersurface of the microchannel with a hydraulic diameter of 210.4 μ m. During the experiments, heating power was unchanged, and working conditions were adjusted by decreasing mass flux. The experimental results showed that the mass fluxes of onset of flow instability (OFI) for the compressible volumes of V =0, 0.5, 1.0 and 2.0 cm3 were always the same. This indicated that the compressible volume has no effect on OFI. After OFI, flow boiling instability occurred in the microchannel with the periodic oscillations of temperature and pressure when there existed compressible volume in the upper stream of the microchannel. The amplitudes and periods of these periodic oscillations increased with the increase of the compressible volume. In this unstable boiling region, slight decrease of mass flux could result in significant increase in pressure drop through the microchannel, and the amplification increased with the increase of compressible volume. However, when there was no compressible volume in the upper stream of the microchannel, the periodic oscillations of temperature and pressure did not occur. The compressible volume has several forms, such as driving the water flow in the experimental system by a pressure tank and connecting the pipes in the upper stream of the microchannel with PVC soft-pipes, and they all introduced the compressible volume and induced the flow boiling instability.