International Research Journal of Engineering and Technology (IRJET) e-ISSN: 2395-0056 Volume: 09 Issue: 06 | June 2022 www.irjet.net p-ISSN: 2395-0072 © 2018, IRJET | Impact Factor value: 7.211 | ISO 9001:2008 Certified Journal | Page 3178 Investigation of a Double Tube Heat Exchanger with Dimples Wael I. A. Aly, Nermin Mahmoud*, A.G. Gomaa, Ashraf Mimi Elsaid Department of Refrigeration and Air Conditioning Technology, Faculty of Technology and Education, Helwan University, 11282, Cairo, Egypt ---------------------------------------------------------------------***--------------------------------------------------------------------- Abstract - The experimental investigation of the double concentric-tube heat exchanger is presented with inner tube dimples. The internal tube side heat transport enhancement characteristics of double tube heat exchangers composed of a dimpled tube with an elliptic shape that are investigated experimentally in the range of Reynolds number from 2570 to 14405 for inner tube fluid while for outer tube fluid is 1477 to 8103. The purpose is to present a clear view of the thermofluid characteristics of this type of heat exchanger with different key design parameters leading to design optimization. The effects of dimple angles on the inner tube side thermo fluid characteristics are also explored. The effectiveness, number of transfer units, and Nusselt number of the double tube heat exchanger with dimples are higher than that of the double tube heat exchanger without dimples by approximately 87%, 107.8% and 53%, respectively. Key Words: Double tube, Dimple tube, Cooling performance, Heat exchangers. 1. INTRODUCTION The need to achieve high thermo-hydraulic performance and compact heat exchangers have pushed a Lot of industries to find new ways to promote heat transfer of double tube heat exchanger [1-7]. Applying dimpled surface is one of the passive techniques for heat transfer enhancement of double tube heat exchanger. In this respect, the dimpled surface has emerged as one way to increase thermo-hydraulic performance. The improved characters create rotating and secondary flows that increase the effectiveness of the heat transfer area of a double tube heat exchanger. It interrupts thermal and velocity boundary layer development with increasing degrees of turbulence close to the rough tube wall. These accompanying increases the convective heat transfer constant (h) with a resultant increase in the pressure drop on the tube. Sarmadian et al. [8] investigated condensation heat transfer in a helically dimpled tube using refrigerant R- 600a as a working fluid. Their results showed that the heat transfer rate was increased up to two times greater than that of a smooth straight tube. Ming Li et al. [9] examined the thermo-hydraulic performance of a dimpled tube in steady-state single-phase (liquid-to-liquid) fluid flow for Reynolds numbers ranging from 500 to 8000 and for a water/glycol. Shuai Xie et al. [10] studied the heat transfer and thermal performance of enhanced tube with cross ellipsoidal dimples. They observed that the cross ellipsoidal dimples induced the transverse and longitudinal dimples, which caused the reattachment and periodic impingement flows that helped improve thermal performance. Aroonrat and Wong wises [11] studied the effect of dimpled depth on heat transfer enhancement, pressure loss and overall performance of dimpled tubes. Their results showed that the dimpled tube with the largest dimpled depth gave the highest heat transfer enhancement and pressure loss up to 83 and 892% over those of the smooth tube. Recently, nanofluids were applied in dimpled tubes for further heat transfer enhancement. In the present study, a thermo-hydraulic performance of a dimpled tube heat exchanger is investigated. The effects of a dimpled tube with dimple angles of 45˚, 60˚ and plain tube are studied. 2. EXPERIMENTAL SETUP AND PROCEDURES The experimental setup includes a double dimpled tube heat exchanger and three circuits, namely, closed loop chilled water cycle, R-22 vapor compression refrigeration cycle, and closed loop hot water cycle. The double dimpled tube heat exchanger made of copper with a thickness of 1 mm, and a length of 1100 mm associated with the necessary measuring instruments. The inner tube diameter is 18 mm, and the outer tube diameter is 20 mm. The geometrical characteristics data of the double dimpled tube heat exchanger are given in Table [1]. Two fluids are being considered which are chilled water in the outer tube and hot water in the inner tube of the heat exchanger. The chilled water system includes a 0.5 m 3 thermally insulated tank, and a cooling system that is controlled by adjusting a temperature controller, the chilled water is generated by the cooling system. The closed loop hot water cycle consists of a 0.56 m 3 thermally insulated tank supplied with 5 electrical heaters with total power of 7.5 kW and an adjustable temperature controller. Figure [1] shows a photograph of the experimental setup and the double tube heat exchanger. The setup contains two