الفهرس 
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Abstract
Augmentation of heat transfer in the annulus side of tube in tube heat exchanger was investigated experimentally and numerically. The heat transfer as well as the follow characteristics was investigated. The test section used in present study is horizontal annular passage formed by two concentric tubes. The outer tube is made from PVC material and the inner tube is a circular brass. The inner tube is heated under uniform heat flux condition to simulate the hot stream side and the outer PVC tube is well insulated. Copper wires of three different diameters are used a artificial surface roughness enhancement elements. The copper wires are wrapped helically on the outer surface of the inner tube at four different pitches . 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, from 7,950 to 28,900.
Also, the numerical solutions for the heat transfer and flow characteristics are obtained using Computational Fluid Dynamic commercial code Fluent 6.2. The flow and thermal development in the annular side of tube-in tube heat exchanger using helical wires as roughness elements is predicted. The numerical results of the average Nusselt number and the friction factor values in the annulus were compared with the experimental data obtained in the present study. The CFD simulations were in a good agreement with the present experimental data measured in the present study.
The result obtained from the present study illustrated that the heat transfer rates is increased with increasing the wire diameter and with decreasing the wire pitch. The highest enhancement tario based on constsnt mass flow rate reached to value of 2.04 compared with the smooth annulus case (without helical wire ) at Re=7950 with helical wire having the pitch of 10 mm and helical wire diameter 1.5 mm for outer tube diameter 47 mm.
The increasing heat transfer rate due to the helical wires is accompanied with a considerable increase in pressure DROP and 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 10 mm 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. Als, Empirical correlations expressing the average Nusselt number, friction factor, enhancement ratio and friction factor ratio with the different operating and design conditions were obtained.