Effects of choking on flow and heat transfer in micro-channels

The gas flows through micro-channels are encountered in many engineering applications such as the cooling devices of electronic chips, semiconductors, micro-electro-mechanical systems (MEMS), etc. Many works have been performed to investigate the flow and heat transfer characteristics generally occurring in the micro channels. According to these investigations, the majority of heat was transferred in the entrance region of the channel, due to high strain rate of the developing flow. These findings are valid only for unchoked micro channel flows. Once the gas flow is choked, the major flow features may be changed but no detailed works have been carried out to date. In these regards, the choked flow characteristics should be known to investigate the heat transfer phenomena in the micro channel flows. In the present study, numerical simulations have been used to provide detailed flow and heat transfer characteristics of micro-channel gas flows. The main objectives of the present effort are to understand the evolution of choking inside micro-channels with isothermally-heated-walls and to elucidate the regions of high heat transfer. The results obtained show that for choked flow conditions, high heat transfer is generated at both the entrance and the exit of the micro-channel. The exit effects like increased strain rate, high temperature gradient and the thinning of the boundary layer cause a rapid increase in heat transfer at the exit of the micro-channel. The location where the flow is choked is practically important in determining the heat transfer phenomena at the vicinity of the channel exit.