Effect of aspect ratio on heat transfer enhancement in alternating oval double pipe heat exchangers

In this paper, a numerical study for laminar, incompressible flow for alternating oval double pipes is carried out in order to analyze the suggested configuration under different conditions. In addition to Reynolds number, the effect of pipe aspect ratio, as one of the significant non-dimensional variables on thermal performance is analyzed quantitatively. The results show that in all aspect ratios, heat transfer rate accepts higher values comparing to circular counterpart. However, it is not considered as oval double pipes are always worth taking the place of circular types, since the pressure loss effects may be dominant over the heat transfer improvement. For this purpose, Enhancement ratio (ER) is investigated as a function of aspect ratio (gamma) and Reynolds number (Re). It is revealed that, at a specific Reynolds number, ER increases with aspect ratio until it reaches a max value, then starts declining. Therefore, to determine whether using this configuration is economically justified or not, a map for ER in terms of mentioned parameters is given. It is revealed that in particular pairs of (gamma,Re), the ER > 1 condition can be achieved. It is also found that the region of ER > 1 becomes more limited at extreme values of aspect ratio resulting in an optimum value for each Reynolds ratio. In addition, for Reynolds values less than 600, ER is always less than one regardless of aspect ratio value. In the end, mathematical expressions for the extreme values of gamma that result in higher than one values of ER is suggested. (C) 2017 Elsevier Ltd. All rights reserved.