Heat Transfer Rate, Heat Exchanger, Rotating Insert, Twisted Tapes, Heat Transfer Coefficient.
Enhancement of heat transfer using various techniques has received strong attention over the years. Many techniques have been developed for enhancing heat transfer rate in heat exchanger like active methods and passive methods. Any techniques for enhancing the heat transfer should be optimized between higher heat transfer coefficient and lower pumping cost. This paper reviews the work undertaken by researchers on the passive techniques such as ribs, conical ring inserts, twisted tapes etc. to enhance the thermal performance in heat exchangers. This paper shows that a tube employed with passive techniques has better heat transfer rate than that of plain tube with minimum pumping power requirement. Heat transfer can also be enhanced by using rotating inserts in a round tube. These rotating insert acts as a swirl generator. The use of the swirl generator is expected to create the tangential velocity or swirling flow to prolong residence time of the flow and to enhance the tangential and radial fluctuation, therefore leading to increase in heat transfer inside the test tube. The higher heat transfer occurs with lowest pitch ratio than that of other pitch ratios. For the lowest pitch ratio stronger swirl flow will create. So turbulence intensity will be higher for lowest pitch ratio. This stronger swirl flow results in higher pressure drop because of more blockage of fluid. So a tube with lowest pitch ratio has higher friction factor than other pitch ratios.