可压缩容积对内肋阵列微通道流动沸腾不稳定性影响

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

实验研究了通道入口上游存在不同的可压缩容积条件下,水力直径为210.4mm的硅基内肋阵列微通道内去离子水流动沸腾不稳定性,实验采用的内肋结构为圆形叉排内肋阵列.实验过程中,实验段加热功率保持恒定,采用逐渐减小去离子水质量流量的方法改变工况.定量分析了不同可压缩容积条件下,沸腾不稳定起始点(OFI)、沸腾不稳定现象产生时,温度和压力振荡的振幅和周期变化规律.结果表明,几种可压缩容积(V=0,0.5,1.0和2.0cm^3)条件下,OFI点对应的质量流量几乎相同,这表明可压缩容积对OFI没有影响.当通道上游存在可压缩容积时,OFI点之后通道内产生了沸腾不稳定性现象,伴随着温度和压力的周期性振荡.而且随着可压缩容积增加,温度和压力振荡的振幅逐渐增大,周期线性增加.在非稳态沸腾区,流量稍有减小便会引起平均压降突然增大,且增幅随着可压缩容积增加而增大.而通道上游不存在可压缩容积时,通道内未产生沸腾不稳定性现象.微通道入口上游可压缩容积有多种形式,采用压力罐驱动和上游管道采用软管连接都会引入可压缩容积,从而引起可压缩容积不稳定性振荡.

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 mm. 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 cm^3 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.