An Experimental Investigation on Heat Transfer Characteristics of Air and Co2 in Microtubes

Several researches dealing with the single-phase forced convection heat transfer inside micro channels have been published in the past decades. The performance of liquid flow has been proved that agree with the conventional correlations very well (Yang and Lin [2007]). However, owing to the low heat transfer coefficient of gaseous flow, it is more difficult to eliminate the effects of thermal shunt and heat loss than water flow while measuring its heat transfer performance. This study provides an experimental investigation on forced convective heat transfer performance of air and gaseous carbon dioxide flowing through two microtube with inner diameter of 920 μm. A non-contacted liquid crystal thermography (LCT) temperature measurement method that proposed by Lin and Yang [2007] was used in this study to measure the surface temperature of microtube. The test results show that the conventional heat transfer correlations for laminar and turbulent flow can be well applied for predicting the fully developed heat transfer performance in microtubes while taking account of the compressibility effect of high pressure gaseous flow in micro tubes. There is no significant difference between CO2 and air in both heat transfer and friction. INTRODUCTION Owing to the fabrication technology development during the past decades, the so-called micro tubes with internal diameters smaller than 1 mm can be easily made and used for increasing the compactness of heat exchangers. These kinds of heat exchangers are able to attain extremely high heat transfer surface area per unit volume, high heat transfer coefficient and low thermal resistance. The study on heat transfer performance in micro tubes has become more important due to the rapid growth of the application for high heat flux electronic devices cooling. However, the conventional forced convection heat transfer correlations were derived from tubes with diameter much larger than those used in micro-channels. They have not been verified to work well for predicting the heat transfer coefficient inside small diameter tubes. Several researches dealing with the single-phase forced convection heat transfer in micro tubes have been published in the past decades. Yu et al. [1995] studied the fluid flow and heat transfer characteristics of nitrogen gas and water in circular tubes with diameters of 19, 52 and 102 μm and Reynolds numbers ranging from 250 to near 20,000. The measured friction factors were slightly lower than the Moody chart values for both laminar and turbulent regimes. However, the Nusselt numbers for cooling of water in the turbulent regime were considerably higher than those would be predicted for larger tubes, suggesting that the Reynolds analogy does not hold for micro-channel flow. Adams et al. [1998] investigated turbulent single-phase forced convection of water in circular micro-channels with diameters of 0.76 and 1.09 mm. Their data suggested that the extent of enhancement increases as the channel diameter decreases and Reynolds number increases. Based on the data they obtained, along with earlier data for small circular channels by Yu et al. [1995], they developed a correlation for the Nusselt number for turbulent, single-phase, forced convection in circular micro-channels with diameters range from 0.102 mm to 1.09 mm. Mala and Li [1999] investigated water flow through micro tubes with diameters ranging from 50 to 254 μm. The experimental results indicate that at high Reynolds number laminar flow condition, the friction factor is higher than that given by the conventional Poiseuille flow theory. Celata et al. [2002] reported the results of refrigerant R114 flowing in capillary tubes with a diameter of 130 μm. They 1 Copyright © 2011 by ASME Proceedings of the ASME 2011 9th International Conference on Nanochannels, Microchannels, and Minichannels ICNMM2011 June 19-22, 2011, Edmonton, Alberta, CANADA