H Najafi - School of Mechanical Engineering, College of Engineering, University of Tehran, Iran;
M.A. Akhavan-Behabadi - School of Mechanical Engineering, College of Engineering, University of Tehran, Iran;

The present study investigated the effect of coiled wire insert on heat transfer nhancement and pressure drop increasing in an evaporator. For this purpose, Wire coils having different coil pitches and wire diameters were inserted into a horizontal plain copper tube of 7.5 mm inside diameter. The coil pitches were 5, 8, 10 and 13 mm, and the wire diameters were 0.5, 0.7, 1.0 and 1.5 mm. Refrigerant HFC-134a flowing inside the tube was heated by a flexible electrically coiled heater wrapped around it. The ranges of refrigerant mass fluxes were between 54 and 136 kg/m2s.Analysis of the collected data indicated that, the coiled wire inserts enhance the heat transfer coefficient but with a higher penalty due to the increasing of pressure drop, in comparison to that for the plain flow. By considering the heat transfer and pressure drop, It has been found that the plain tube has the best performance. Therefore, the coiled wire inserts are recommended only under special conditions by considering the appropriate consistence between heat transfer performance and the rate of pumping power increasing.Using the present collected data, a new correlation was developed for estimation of pressure drop during evaporation inside a horizontal coiled wire inserted tube. This correlation predicts the experimental data of this study within an error band of ± 20%.

Coiled wire, Heat transfer, Pressure drop, Evaporator