الفهرس 
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Abstract
Enhancing heat transfer is used in many engineering applications such as heat exchangers, air conditioning, chemical reactors and refrigeration systems. Hence many techniques have been investigated for enhancement of heat transfer rate and decreasing the size and cost of the equipment. One of the most important techniques used are passive heat transfer technique by using swirl flow devices. This work presents an experimental study of heat transfer enhancement and friction factor characteristics in decaying turbulent swirl flow generated by a short helical tape placed at the entrance of the inner tube of double pipe heat exchanger. Experiments were conducted for cold water flow rates in inner tube in the range of 3432≤ Re ≤5108. Experiments were carried out for three helical tapes with different lengths LH of 130 mm, 270 mm, and 370 mm. Also, experiments were carried out for two successive helical tapes of length 135 mm at different free space Ls. The local and average heat transfer coefficients and pressure DROP were measured and compared with the base case of plain tube. The local heat transfer coefficients were found to increase to very high values in the region of the helical tape, and then decrease downstream of the helical tape. The thermal performance factor increases with increasing helical tape length at each Reynolds number examined and decrease with increasing Reynolds number in the range of 3432≤ Re ≤5108. The maximum performance factor of ƞ=1.45 was obtained for the length of 370 mm at Re=3432. The helical tape of length LH=370 mm provides the highest increase in the average heat transfer rate by about 27% over the plain tube but increase the pressure DROP by 2.6 %. To overcome the increasing of pressure drop, different free-spacing ratio S=1.8, 3.6, and 5.4 between two successive helical tape of length LH=135mm at Re=4083 were examined. It was found that the thermal performance factor of two successive helical iv tapes of length LH=135 mm with free space ratio S=1.8 is better than the continuous helical tape of length LH=270 mm at Re=4083. A comparison between the results of present work and published results of full length helical and twisted tapes inserts has shown superior heat transfer enhancement using short length helical tapes inserted in cold water tube as compared to full length twisted tapes in the Reynolds number range of Re=3432–5108 and same performance of the full length helical tapes without a centered rod in the Reynolds number range of Re=4083–5108.