Augmentation of heat transfer in the annulus side of tube in tube heat exchanger was investigated experimentally. The heat transfer as well as the flow characteristics was investigated. The test section used in the present study is a horizontal annular passage formed by two concentric tubes. The outer tube is made from PVC material and two different sizes of 47 and 72 mm inner diameters are used. The inner tube is a circular brass tube of 22 mm outer diameter and 6 mm thickness. A cylindrical heating element of 10 mm diameter is tightly inserted in the inner tube to simulate the hot stream side. The outer PVC tube is well insulated. Copper wires having diameters of 0.7, 1, and 1.5 mm are used as an artificial surface roughness enhancement elements. The copper wires are wrapped helically on the outer surface of the inner tube at four different pitches of 10, 20, 40 and 60 mm respectively. Several experimental runs were performed to show the effect of the helical wire diameter and pitch as well as the annulus diameter ratio. Also, all the experiments were carried out in the range of Reynolds of number, based on hydraulic diameter of the flow passage, from 7950 to 28900.
The results obtained from the present study showed that the heat transfer rates is increased with increasing the wire diameter and with decreasing the wire pitch. The highest enhancement ratio based on constant mass flow rate reached to a value of 2.04 compared with the smooth annulus case (without helical wire) at Re= 7950 with outer tube diameter of 47 mm. The increasing heat transfer rate due to the helical wires is accompanied with a considerable increase in the pressure drop and the friction factor. The highest average increase in friction factor is about 5.787 times the value of the smooth annulus and this took place at Re=21900 with helical wire having the pitch of 10mm and a diameter of 1.5 mm for outer tube diameter 72 mm. Also, the maximum net energy gain at the expense of increased pressure drop caused by helical wire insertions is about 30% and took place at wire diameter of 1 mm, wire pitch of 40 mm, Re=7950 and outer tube diameter of 47 mm. Also, some empirical correlations expressing the different heat transfer and friction parameters with the operating and design conditions were obtained. |
