Abstract
The enhancement of heat exchanger performance was investigated using dimpled tubes tested at different Reynolds numbers, in the present work four types of dimpled tubes with a specified configuration manufactured, tested and then compared performance with the smooth tube and other passive techniques performance. Two dimpled arrangements along the tube were investigated, these are inline and staggered at constant pitch ratio X/d=4, the test results showed that Nusselts number (heat transfer) of the staggered array is higher than the inline array by 13%. The effect of different depths of the dimple (14.5 mm and 18.5 mm) has been also investigated; a tube with large dimple diameter enhanced the Nusselts number by about 25% for the range of Reynolds number between 4000- 20000. The overall enhancement ratio was used to differentiate the passive technique and comparison of different configurations for the technique itself, the result depicted that the 10 mm dimple diameter with a pitch ratio of X/d= 4 gives better performance than the static mixer and twisted tape techniques. The overall enhancement ratio of the 10 mm dimple diameter is varied from (1.21 to 1.65) that means good performance for a range of Reynolds numbers (4000-20000). This indicates that 21- 65 % of heat transfer area can be saved at the same pumping power compared with the smooth tube heat exchanger.
Highlights
The techniques used in enhance the performance of heat exchangers can be classified into two main categories, passive and active techniques, in addition to a hybrid technique which includes two or more from each of passive and active technique [1]
The overall enhancement ratio is defined as the ratio of heat transfer enhancement ratio to the friction factor ratio; this parameter was derived according to using same pumping power for different configurations. [14]
From the test results presented in Figure (6) the effect of different depths of the dimple (14.5 mm and 18.5 mm), tube with 18.5 mm dimple diameter enhanced the Nusselts number by 25% compared with the tube of 14.5 mm dimple diameter for the range of Reynolds number between 400020000,this due to first, large dimple diameter makes the kinetic energy of the core flow dominant and much higher than the shear force induced by the dimpled surface
Summary
The techniques used in enhance the performance of heat exchangers can be classified into two main categories, passive and active techniques, in addition to a hybrid technique which includes two or more from each of passive and active technique [1]. Kumar & Murugesan [2] considered and evaluated the heat transfer and pressure drop investigations of more than thirty published work from (2001 to 2012) of various twisted tapes placed in heat exchangers. Udaya Kumar et al [3] reviewed the heat transfer enhancement techniques in square ducts of more than ten published works from years (2004 to 2013) They found that the heat transfer of square ducts was found considerably higher than the circular tube. There is a shortage of the available data in the literature for the effect for dimpled tube geometrical parameters on the heat transfer and pressure drop in turbulent flow. The relationship between the thermal and hydraulic performance must be considered, by means of the overall enhancement ratio concept
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