Experimental Investigation of Turbulent Heat Transfer in the Entrance Region of Microchannels

Abstract

Within the entrance region of a closed channel, the effects on flowfield and heat transfer mechanisms are significant. Where microchannels and microdevices are concerned, there are limited heat transfer studies into this region, and those available are for laminar flows. In the turbulent regime, where the hydraulic resistance is conventionally increased, higher pumping powers are required. However, future microscale applications may have a need for turbulent flow data in microchannels. There is currently no heat transfer data available on the turbulent entrance region of microchannels. An experimental investigation has been carried out to explore turbulent convection heat transfer in the entrance region of uniformly heated microtubes. The measurement of local wall temperatures is achieved through the use of unencapsulated thermochromic liquid crystals, a state-of-the-art, nonintrusive thermal-measurement technique. Heat transfer data was obtained for two stainless steel microtubes, with nominal inner diameters of 1.067 and 0.508 mm, over a Reynolds number range of 4000 to 9000. The working fluid is FC-72, and adequate tube entry length is provided for hydrodynamic flow development before heating. Local temperature data and Nusselt values are obtained in the thermal entrance region of the microchannels. The thermal turbulent entrance length is found to remain relatively constant for the Reynolds number range considered for both microtube diameters. This is in good agreement with conventional thermal turbulent entrance studies for pipes.