Experimental investigation on the heat transfer augmentation and friction factor inside tube enhanced with deep dimples

This study aims to investigate experimentally heat transfer improvement in the longitudinal and circumferential direction and pressure drop of distilled water when flowing inside deep dimpled tubes. Also, the effect of three mechanisms that cause enhancement in the thermal performance of this new tube geometry is observed. Several tests were conducted on plain and deep dimpled tubes in a laminar regime with Reynolds numbers ranging from 500 to 2250 and constant heat flux using an experimental setup. The results indicate that implementing deep dimples on the tube improves the convective heat transfer coefficient up to 3.25 times better than a smooth tube; however, the friction factor ratio increases by just 1.69 in Re = 2250. The circumferential investigation illustrated the influence of exerting dimple on the flow field and revealed how generating vortexes enhanced heat transfer. Besides, results show that modifying tube geometry causes altering the zone of different flow regimes, which can be specified by Nusselt number and friction factor figures. Since the deep dimple tube has a low improvement penalty and appropriate thermal enhancement, increased the performance evaluation criteria (PEC) in the studying region up to 2.71. Also, it has been revealed that a deep dimpled tube cannot be applicable for Reynolds numbers below 1000. However, in higher velocities, replacement of them with simple tubes in conventional heat exchangers is highly suggested.