A numerical study of condensation heat transfer and pressure drop in horizontal round and flattened minichannels

Significance Statement

Previous numerical research studying condensing flow in microchannels focused mainly on heat transfer with little efforts made on pressure gradients. A better understanding of pressure drop characteristics coupled with heat transfer would provide better information on condensation process and design of efficient heat exchangers.

Research led by Professor Wei Li and Dr. Jingzhi Zhang from Zhejiang University and in collaboration with Professor S.A. Sherif from University of Florida carried out numerical investigations with effects of mass flux, vapor quality and aspect ratio on heat transfer and pressure drop characteristics for R410A and R134a refrigerants inside a horizontal flattened tubes deformed from round tubes with an inner diameter of 3.78mm. This research was implemented to improve understanding on condensation process in flattened tubes by using numerical approaches. The work appeared in the peer-reviewed journal, International Journal of Thermal Sciences.

Heat transfer coefficient for R134a was higher in range of 10-32% than R410A at similar mass flux and vapor quality due to difference in properties of the two refrigerants. The flattened tube was found to enhance heat transfer coefficients especially at higher mass flux and vapor quality values when compared with round tubes but decreases as vapor quality decreases.

Researchers’ numerical results on heat transfer coefficients of flattened tubes when compared with previous empirical heat transfer correlations for round tubes by using hydraulic diameter showed reasonable agreement for both refrigerants.

Liquid film thickness of the two refrigerants decreased with an increase in aspect ratio, mass flux and vapor quality aiding heat transfer for flattened tubes. A thinner film thickness for R134a was observed due to higher shear and surface tension force. The average thin film thickness is 10% lower for flattened tube than round tubes at a vapor quality of 0.8.

When the authors looked at stream traces for round and flattened tubes for refrigerants R410A and R134a a more significant gravity effect was observed in round tubes with the same cross-sectional perimeter compared with flattened tubes. The stream trace started from the vapor phase leading to the liquid-vapor interface where vapor condenses to liquid. The movement of vapor phase from the core region to wall region also enhanced heat transfer. The liquid phase at the bottom of liquid film region thickened the liquid film at tube corners.

Pressure gradients of the two tubes increase with the value of vapor quality less than 0.8 but decreases at a higher vapor quality. It was also found to increase with mass flux. Pressure gradients for flattened tubes increased with an increase in aspect ratio, and gets larger with increase in mass flux and vapor quality for R410A. Frictional pressure drop for R134a was much higher than R410A at the same mass flux due to higher working fluid velocity.

The numerical data results of pressure gradients when compared with previous empirical correlations obtained a best prediction of mean absolute deviation and mean relative deviation of 16.34% and approximately 3.83% respectively when using equivalent diameter.

The numerical approach used in this study gives a better understanding for condensation process in flattened tubes thereby pointing out its advantages.

About the author

Wei Li is ASME Fellow and Professor of Heat and Mass Transfer at the Zhejiang University (ZJU). His 200 papers in journals and conference proceedings in English and Chinese are about experimental and computational studies on two phase heat transfer, falling film evaporation, fouling, chemical reaction flow, compact heat exchangers, supercritical fuels, air conditioning, chemico-physical characteristics of heat-transfer surfaces, and etc.

His eight labs provide research space 2,500 m2 for fundamental and applied studies.  His empirical correlations on heat transfer coefficient, CHF, and friction factor were adopted in Chapter 5, Two-Phase flow of “2012-2016 ASHRAE Handbook – Fundamentals”.

Wei Li received BS in ME in the Xi’an Jiao Tong University and obtained his MS and PhD degrees in heat transfer in the Pennsylvania State University in 1998. After eight years of engineering practice serving as a director of engineering in the United States, he joined ZJU as a full professor in 2006. ZJU has been ranked No. 4 as one of the Best Engineering Universities for graduate study in the World in the U.S. NEWS and Report since 2014.

He was a conference plenary speaker twice at the 2007 and 2009 Chinese National Heat Transfer Conference, respectively. His Ph.D. student was one of the two receipts of Outstanding Student Award of WU Zhong-Hua Fund, the highest award for graduate students in heat transfer in China; his Master student continued Ph.D. study in ME in MIT; his international graduate students are all supported under China National Scholarship (Application deadline of CNS is around middle of April for the 2017 Fall semester).

He is serving as associate editors for ASME Journal of Thermal Sciences and Engineering Applications and Journal of Enhanced Heat Transfer. He is one of the thirties members of executive committee of Chinese Society of Heat Transfer. 

About the author

Jingzhi Zhang received his B.Sc. and M.Sc. in thermal engineering in Shandong University, Jinan, China. Now, he is a Ph. D. student in Zhejiang University, Hangzhou, China, supervised by Prof. Wei Li who is an ASME Fellow.

His main research interests include numerical simulation, multiphase flow, phase-change heat transfer enhancement techniques, supercritical fluids, and compact heat exchangers (Plate Heat Exchangers, Shell and Tube Heat Exchangers, and Plate-Fin Heat Exchangers). He has co-authored about 20 papers in international journals and conferences. 

About the author

Dr. S.A. Sherif is Professor of Mechanical and Aerospace Engineering and is the Founding Director of the Wayne K. and Lyla L. Masur HVAC Laboratory and Director of both the UF Industrial Assessment Center and the UF Mobile Energy Laboratory.

He served as Co-Director of the Southeastern Center for Industrial Energy Intensity Reduction (2009-2013). He is a Fellow of ASME, a Fellow of ASHRAE, an Associate Fellow of AIAA, a Member of Commission B-1 on Thermodynamics and Transfer Processes of the International Institute of Refrigeration, and a Member of the Advisory Board of Directors of the International Association for Hydrogen Energy. He is a past chair for the ASME Advanced Energy Systems Division, K-19 Committee on Environmental Heat Transfer, the Coordinating Group on Fluid Measurements and the Fluid Applications and Systems Technical Committee all of ASME, the Steering Committee of the Intersociety Energy Conversion Engineering Conference, ASHRAE’s Standards Project Committee 41.6 on Measurement of Moist Air Properties, and ASHRAE’s TC1.1 Committee on Thermodynamics and Psychrometrics.

He was the Head of the Refrigeration Section of ASHRAE, the Technical Conference Chair of the 2008 ASME Summer Heat Transfer Conference and the General Conference Chair of the 2013 ASME Summer Heat Transfer Conference.

He is the 2013-2014 Chair of the ASME Heat Transfer Division Executive Committee (2009-2016) and a member of the ASME’s Basic Engineering Group Operating Board (2010-2014). Dr. Sherif is a Founding Member of the Board of Directors of the American Society of Thermal and Fluids Engineers (ASTFE).

He is Technical Editor of the ASME Journal of Thermal Science and Engineering Applications and is on the editorial boards of 22 thermal science journals. He has over 400 publications and two US patents. 

Journal Reference

Jingzhi Zhang1, 2, Wei Li1, S.A. Sherif3. A numerical study of condensation heat transfer and pressure drop in horizontal round and flattened minichannels, International Journal of Thermal Sciences 106 (2016) 80-93.

[expand title=”Show Affiliations”]
  1. Department of Energy Engineering, Zhejiang University, 38 Zheda Road, Hangzhou, 30027 Zhejiang, China
  2. Department of Energy Engineering, Co-Innovation Center for Advanced Aero-Engine, Zhejiang University, 38 Zheda Road, Hangzhou, 30027 Zhejiang, China.
  3. Department of Mechanical and Aerospace Engineering, University of Florida, 232 MAE Bldg. B, P.O. Box 116300, Gainesville, FL 32611, USA.
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