Experimental Investigation of Two-Phase Pressure Drop in a Microchannel

Abstract

Experimental results of two-phase pressure drop in a horizontal circular microchannel are reported in this paper. A test tube was made of fused silica having an internal diameter of 781 μm with a total length of 261 mm and a heated length of 191 mm. The outer surface of the test tube was coated with an electrically conductive thin layer of ITO (indium tin oxide) for direct heating of the test section. Refrigerants R134a and R245fa were used as the working fluids, and mass flux during the experiments was varied between 100 and 650 kg/m²-s. Experiments were performed at two different system pressures corresponding to saturation temperatures of 25°C and 30°C for R134a and at three different system pressures corresponding to saturation temperatures of 30°C, 35°C, and 40°C for R245fa. Two-phase frictional pressure drop characteristics with variation of mass flux, vapor fraction, saturation temperature, and heat flux were explored in detail. Finally, the prediction capability of some well-known correlations available in the literature, some developed for macrochannels and others especially developed for microchannels, was assessed.

Acknowledgments

The authors acknowledge the valuable discussions with Professor Tim Ameel from the University of Utah and Claudi Martin-Callizo from the Sapa heat transfer AB Sweden. Rashid Ali and Mohammad H. Maqbool were financially supported by the Higher Education Commission of Pakistan for a Ph.D. scholarship to carry out this research work.

Rashid Ali is a Ph.D. student at the Department of Energy Technology, Royal Institute of Technology (KTH), Stockholm, Sweden. He obtained a bachelor's degree in mechanical engineering from the Mehran University of Engineering and Technology, Jamshoro, Pakistan, in 2001 and a master's degree in power engineering (with specialization in nuclear power) from the NED University of Engineering and Technology, Pakistan, in 2003. He joined the Department of Energy Technology at KTH, Stockholm, in 2006 as a Ph.D. student. Since then, he has been involved in heat transfer research at the Division of the Applied Thermodynamics and Refrigeration of the department. He has carried out research in single-phase flow and heat transfer in microchannels. Currently, he is engaged in experimental flow boiling and heat transfer in microchannels. He has carried out research in the field of two-phase flow and heat transfer focusing on different aspects such as heat transfer, pressure drop, dryout, and flow visualization.

Björn Palm is head of the Division of Applied Thermodynamics and Refrigeration, Department of Energy Technology, at the Royal Institute of Technology, in Stockholm, Sweden. His main research interests are related to heat transfer connected to refrigeration systems, i.e., evaporation and condensation in compact heat exchangers, and enhancement of boiling heat transfer and boiling in minichannels. He is also active in the area of application of natural refrigerants, such as hydrocarbons, ammonia, and carbon dioxide, in different types of refrigeration systems. He has also been working on cooling of electronics by two-phase thermosyphon loops. He has authored or coauthored about 100 papers presented at international conferences and in scientific journals.

Mohammad H. Maqbool received his B.Sc. degree in mechanical engineering from the University of Engineering and Technology, Lahore, Pakistan, in 2000. He completed his master's degree in nuclear power engineering from NED University, Karachi, Pakistan, in 2002. In 2007 he was selected for Ph.D. scholarship abroad. Currently, he is a Ph.D. student in the department of Energy Technology at the Royal Institute of Technology, Stockholm, Sweden. He is involved in two-phase flow and heat transfer research in microchannels, especially using ammonia as a working fluid.