Experimental Research On Steam Condensation Flow And Heat Transfer In Silicon Microchannels

Nowadays most of the investigations on convective condensation heat transfer are focused on the channels with the hydraulic diameter larger than 200μm. However, there is little research on the condensation in the microchannels with the hydraulic diameter less than 200μm, which greatly hinders the application of micro-chip heat exchanger and the in-depth grasp of the microscale phase change phenomena. So in this paper an experimental investigation has been performed on the flow pattern transitions, heat transfer and flow friction characteristics during the steam condensation in the four kinds of silicon microchannels (Dh=53.0μm , 77.5μm, 93.0μm, 128.5μm). The findings and conclusions are as follows. The condensation flow in the microchannels was categorized into following different flow regimes alternatively from the inlet to outlet: the mist flow, annular flow, injection flow, slug/bubbly flow and bubbly flow.The common existence of injection flow in microchannels was confirmed experimentally. The formation mechanism of the injection flow was analyzed. The effect of flow velocity, heat flux and hydraulic diameter on the injection location was documented. Based on the experimental data, dimensionless correlations of injection location were obtained for the microchannels. Meanwhile the injection flow frequency was explored with the aids of high speed camera.Simultaneous investigations of the wall temperature distribution and corresponding flow patterns in microchannels show that the wall temperature decreased sharply nearby the injection flow location. The upstream of injection flow (mist flow and annular flow) have a relatively high wall temperature, while the downstream of injection flow (slug/bubbly flow and bubbly flow) have a low and smoothly decreasing wall temperature. So the injection flow location can be determined according to the wall temperature distributions.The heat transfer experimental results show that the condensation Nusselt number in microchannels was dependant greatly on the Reynolds number, Jakob number and hydraulic diameter. The dimensionless correlation of Nusselt number and its application range were obtained for microchannels.The investigations also show that the two-phase friction factor had a close relationship with the Reynolds number, Jakob number and hydraulic diameter of microchannels. And the dimensionless correlation of the two-phase friction factorΦ2Lo during the flow condensation in microchannels was obtained.It is also found that when there was liquid droplet trapped into the inlet of microchannels, there appeared the out-of-phase fluctuations of wall temperatures at the upstream and downstream of the injection flow. This is because the flow patterns at the upstream and downstream of the injection flow were disturbed periodically by the trapped liquid droplet, which had a lower temperature than the annular flow but a higher temperature than the slug/bubbly flow. So it is necessary to insulate the steam piping before the testing section in order to obtain the stable experimental data.The findings and conclusions obtained in this paper help to better understand the condensation flow and heat transfer phenomena in the micriochannels with the hydraulic diameter less than 200μm.