ENHANCED HEAT TRANSFER IN THE ENTRANCE REGION OF MICROCHANNELS

摘要

A detailed heat transfer analysis has been performed on the data from previously-reported experiments (Rahman and Gui 1993) to investigate the characteristics of high-heat-flux (10~6 W/m~2) microchannel cooling in electronic chips. The use of microchannel directly etched into a silicon wafer has shortened the cooling path and improved the performance by significantly minimizing the thickness of the heat transfer layer. Experimental convective heat transfer coefficients (up to 45 kW/m~2 K) for low-temperature single-phase flow was an order of magnitude higher than conventional heat transfer coefficients; and reached the level of two-phase boiling heat transfer.The flow and heat transfer modes and their transitions in the experiments were investigated. The influence of the microchannel passage geometry, fluid property variation, and the fluid flow mode on the local Nusselt number in the entrance region of the microchannel has been analyzed. The analysis indicates that the significant enhancement obtained in microchannel cooling results from four key aspects: a thinner thermal boundary layer, entry effects, the roughness of the channel, and strong pre-existing turbulence at the inlet. The preexisting turbulence delayed the formation of the thermal boundaries, thereby increasing the entry effects on heat transfer. The critical Re shifted from 2,300 to 1,400 in microchannels, possibly due to the wall roughness which docs not affect the Re_c otherwise in normal size tubes.