Effect of dimple pitch on thermal-hydraulic performance of tubes enhanced with ellipsoidal and teardrop dimples

This paper aims to numerically investigate the thermal-hydraulic performance of dimpled tubes having various geometric pitches under a constant external heat flux of 10 kW/m2 for a wide range of Reynolds numbers (Re). The performance of enhanced tubes consisting of ellipsoidal 0°, teardrop, and ellipsoidal 45° dimples of equivalent volumes are compared using steady-state Reynolds-Averaged Navier-Stokes simulations. It is observed that the performance of dimpled tubes vary significantly with dimple pitch (P) and Re variation. Therefore, optimum pitches and working ranges of Re are identified for all dimpled tube configurations by varying 3.2mm≤P≤13.2mm and 9000≤Re≤40000. The ellipsoidal 45° and teardrop dimpled tubes have been found to have an optimum pitch of 3.2 mm, and suitable working ranges of 9000≤Re≤30000 and 14000≤Re≤40000. The ellipsoidal 45° and teardrop dimpled tubes with optimum pitch enhanced thermal-hydraulic performance up to 45.7% and 31.2%, respectively. However, ellipsoidal 0° tubes have relatively short working ranges with limited thermal-hydraulic performance improvement. An in-depth analysis of local drag and heat transfer characteristics of individual dimples along with their effects on the overall performance of the tubes are also carried out. Lastly, new correlations of Nusselt number and friction factor are proposed for optimum dimpled tubes.