Characterization of diabatic two-phase flows in microchannels: Flow parameter results for R-134a in a 0.5 mm channel

Abstract An optical measurement method for two-phase flow pattern characterization in microtubes has been utilized to determine the frequency of bubbles generated in a microevaporator, the coalescence rates of these bubbles and their length distribution as well as their mean velocity. The tests were run in a 0.5 mm glass channel using saturated R-134a at 30 °C (7.7 bar). The optical technique uses two laser diodes and photodiodes to measure these parameters and to also identify the flow regimes and their transitions. Four flow patterns (bubbly flow, slug flow, semi-annular flow and annular flow) with their transitions were detected and observed also by high speed video. It was also possible to characterize bubble coalescence rates, which were observed here to be an important phenomena controlling the flow pattern transition in microchannels. Two types of coalescence occurred depending on the presence of small bubbles or not. The two-phase flow pattern transitions observed did not compare well to a leading macroscale flow map for refrigerants nor to a microscale map for air–water flows. Time averaged cross-sectional void fractions were also calculated indirectly from the mean two-phase vapor velocities and compared reasonably well to homogeneous values.

[1]  Xiuhan Li,et al.  Microscale boiling heat transfer in a micro-timescale at high heat fluxes , 2005 .

[2]  Ziping Feng,et al.  Two-phase flow in microchannels , 2002 .

[3]  Akimaro Kawahara,et al.  Effects of Channel Diameter and Liquid Properties on Void Fraction in Adiabatic Two-Phase Flow Through Microchannels , 2005 .

[4]  J. Thome,et al.  Flow Boiling in Horizontal Tubes. Part 1; Development of a Diabatic Two–Phase Flow Pattern Map , 1998 .

[5]  W. Riebold,et al.  Measurement of the Velocity of Gas Bubbles in Water by a Correlation Method , 1970 .

[6]  A. Kawahara,et al.  Investigation of two-phase flow pattern, void fraction and pressure drop in a microchannel , 2002 .

[7]  Keith Cornwell,et al.  Two-Phase Flow Regimes and Heat Transfer in Small Tubes and Channels , 1998 .

[8]  Chien-Yuh Yang,et al.  Flow pattern of air–water and two-phase R-134a in small circular tubes , 2001 .

[9]  Mark E. Steinke,et al.  Flow Boiling and Pressure Drop in Parallel Flow Microchannels , 2003 .

[10]  M. W. Wambsganss,et al.  Microscale flow visualization of nucleate boiling in small channels: Mechanisms influencing heat transfer , 1997 .

[11]  John R. Thome,et al.  Heat Transfer Model for Evaporation in Microchannels, Part I: Presentation of the Model , 2004 .

[12]  Albert Mosyak,et al.  Two-phase flow patterns in parallel micro-channels , 2003 .

[13]  Chih-Ming Ho,et al.  Transport of bubbles in square microchannels , 2002 .

[14]  P. Kew,et al.  Correlations for the prediction of boiling heat transfer in small-diameter channels , 1997 .

[15]  M. W. Wambsganss,et al.  Boiling Heat Transfer in a Horizontal Small-Diameter Tube , 1993 .

[16]  Said I. Abdel-Khalik,et al.  Gas–liquid two-phase flow in microchannels Part I: two-phase flow patterns , 1999 .

[17]  John R. Thome,et al.  Heat Transfer Model for Evaporation in Microchannels, Part II: Comparison with the Database , 2004 .

[18]  K. S. Rezkallah,et al.  Flow regime identification in microgravity two-phase flows using void fraction signals , 1999 .

[19]  Mikio Suo,et al.  Two phase flow in capillary tubes , 1964 .