The Effect of Tube Flattening on Flow Boiling Heat Transfer Enhancement

Abstract

The present study investigated the effect of smooth tube flattening on heat transfer enhancement in an evaporator. The tubes with internal diameter of 8.7 mm were flattened into an oblong shape with different inside heights. The test setup was basically a vapor compression refrigeration system equipped with all necessary measuring instruments. Refrigerant R-134a flowing inside the tube was heated by an electrical coil heater wrapped around it. The ranges of mass velocities were from 74 to 106 kg/m²-s and vapor quality varied from 25% to 95%. Analysis of the collected data indicated that the heat transfer coefficient elevates by increasing the mass velocity and vapor quality in flattened tubes just like the round tube. The flow boiling heat transfer coefficient increases when the flattened tube is used instead of the round tube. The highest heat transfer coefficient enhancement of 172% was achieved for the tube with the lowest inside height at mass velocity of 106 kg/m²-s and vapor quality of 85%. Finally, based on the present experimental results, a correlation was developed to predict the heat transfer coefficient in flattened tubes.

Acknowledgments

The authors express their thanks to the School of Mechanical Engineering, College of Engineering, University of Tehran, for financial support of the present experimental work.

M. Nasr recently received his M.Sc. degree in energy conversion field under the supervision of Prof. M. A. Akhavan-Behabadi from the School of Mechanical Engineering, College of Engineering, University of Tehran, Tehran, Iran. He received his B.Sc. degree in mechanical engineering from Power and Water University of Technology (PWUT), Tehran, Iran, in 2006. He is currently working on augmentation of heat transfer by different passive techniques in single-phase and two-phase flow systems.

M. A. Akhavan-Behabadi is an associate professor of mechanical engineering at the University of Tehran, Iran. He received his Ph.D. from the Indian Institute of Technology at Roorkee, India, in 1993. He is the head of the School of Mechanical Engineering, College of Engineering, University of Tehran, Tehran, Iran. He has co-authored more than 70 journal and conference publications. His research interests include experimental two-phase and single-phase convective heat transfer. He is currently working on augmentation of heat transfer by different passive techniques in two-phase flow and also nanofluid single-phase flow.

S. E. Marashi has recently received his M.Sc. degree in energy conversion under the supervision of Prof. M. A. Akhavan-Behabadi from the School of Mechanical Engineering, College of Engineering, University of Tehran, Tehran, Iran. He received his B.Sc. degree in mechanical engineering from Sharif University of Technology, Tehran, Iran, in 2006. He is currently working on augmentation of heat transfer by different passive techniques in single-phase and two-phase flow systems.