Todd M. Bandhauer Modine Manufacturing Company, Racine, WI 53403 Akhil Agarwal Srinivas Garimella e-mail: srinivas.garimella@me.gatech.edu GWW School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, GA 30332-0405 Measurement and Modeling of Condensation Heat Transfer Coefficients in Circular Microchannels A model for predicting heat transfer during condensation of refrigerant R134a in hori- zontal microchannels is presented. The thermal amplification technique is used to mea- sure condensation heat transfer coefficients accurately over small increments of refrig- erant quality across the vapor-liquid dome 0  x  1. A combination of a high flow rate closed loop primary coolant and a low flow rate open loop secondary coolant ensures the accurate measurement of the small heat duties in these microchannels and the deduction of condensation heat transfer coefficients from measured UA values. Measurements were conducted for three circular microchannels 0.506  D h  1.524 mm over the mass flux range 150  G  750 kg/m 2 s. Results from previous work by the authors on condensa- tion flow mechanisms in microchannel geometries were used to interpret the results based on the applicable flow regimes. The heat transfer model is based on the approach origi- nally developed by Traviss, D. P., Rohsenow, W. M., and Baron, A. B., 1973, “Forced- Convection Condensation Inside Tubes: A Heat Transfer Equation For Condenser De- sign,” ASHRAE Trans., 79(1), pp. 157–165 and Moser, K. W., Webb, R. L., and Na, B., 1998, “A New Equivalent Reynolds Number Model for Condensation in Smooth Tubes,” ASME, J. Heat Transfer, 120(2), pp. 410–417. The multiple-flow-regime model of Ga- rimella, S., Agarwal, A., and Killion, J. D., 2005, “Condensation Pressure Drop in Circular Microchannels,” Heat Transfer Eng., 26(3), pp. 1–8 for predicting condensation pressure drops in microchannels is used to predict the pertinent interfacial shear stresses required in this heat transfer model. The resulting heat transfer model predicts 86% of the data within ±20%. DOI: 10.1115/1.2345427 Keywords: condensation, microchannel, phase change, heat transfer coefficient Introduction Microchannels are increasingly being used in industry to yield compact geometries for heat transfer in a wide variety of applica- tions. Considerable literature exists on single-phase flow, pressure drop, and heat transfer in microchannels, as can be seen in some recent reviews of the literature 1–4. Similarly, boiling and evaporation pool boiling and convective boiling in microchan- nels have also been studied due to the interest in heat removal at high heat fluxes in the electronics cooling industry. But limited research has been conducted on flow regimes, and the measure- ment of pressure drop and heat transfer coefficients during con- densation in microchannel geometries, i.e., in the submillimeter range of hydraulic diameters. The prominence of studies on boil- ing and evaporation to date can be attributed to the electronics cooling industry’s need to remove high heat fluxes through vapor- ization from compact devices that must be maintained at relatively low temperatures while being not readily accessible or conducive to the installation of large and complex cooling systems. How- ever, the ultimate rejection of these large heat duties through com- pact condensers has not been addressed adequately. It should be noted that such compact condensers have been designed and used for some years by the automotive industry, whose air-conditioning condensers consist of rectangular tubes with multiple parallel mi- crochannels cooled by air flowing across multilouver fins. The microchannels used in these condensers often have hydraulic di- ameters in the 0.4– 0.7 mm range, although an understanding of the fundamental condensation phenomena in these heat exchang- ers is just beginning to emerge. A fundamental understanding of condensation at the microscales will yield far reaching benefits not only for these industries, but also for other as-yet untapped applications such as portable personal cooling devices, hazardous duty and high ambient air conditioning, and medical/surgical de- vices, to name a few. The essential issue and research challenge in microscale con- densation is that two-phase flow mechanisms and flow regime transitions in these small channels are considerably different from those found in the more conventional larger diameter tubes. This is because of significant differences between large round tubes and the smaller noncircular tubes in the relative magnitudes of gravity, shear, and surface tension forces, which determine the flow regime established at a given combination of liquid and vapor-phase velocities. Coleman and Garimella experimentally demonstrated and interpreted these differences in flow mecha- nisms and transitions for air-water flows 5 as well as for the condensation of refrigerants 6,7. Because heat transfer coeffi- cients and two-phase pressure drops depend on the corresponding flow patterns, it is reasonable to expect that heat transfer coeffi- cients in microchannels may not be predicted adequately by the existing correlations for larger diameter tubes. Having established the pertinent flow mechanisms and transition criteria for the con- densation of refrigerants in microchannels, Garimella et al. devel- oped experimentally validated models for pressure drops during intermittent condensing flows in circular 8 and noncircular 9 microchannels. In addition, they developed a model for conden- Contributed by the Heat Transfer Division of ASME for publication in the JOUR- NAL OF HEAT TRANSFER. Manuscript received June 17, 2005; final manuscript received March 7, 2006. Review conducted by Satish G. Kandlikar. Paper presented at the 3rd International Conference on Microchannels and Minichannels ICMM2005, June 13–15, 2005, Toronto, Ontario, Canada. 1050 / Vol. 128, OCTOBER 2006 Copyright © 2006 by ASME Transactions of the ASME Downloaded From: http://heattransfer.asmedigitalcollection.asme.org/ on 01/28/2016 Terms of Use: http://www.asme.org/about-asme/terms-of-use