NUMERICAL STUDY OF THE HEAT TRANSFER ENHANCEMENT INSIDE AN INTERNALLY HELICAL GROOVED TUBES CONTAINING TWISTED TAPE INSERTS

Abstract

Heat exchangers have a major role in energy conservation. It is necessary to improve the performance of heat exchangers in order to save energy, cost, and material and to reduce environmental degradation. Therefore, a Computational Fluid Dynamics (CFD) investigation has been carried out to study the heat transfer enhancement in a smooth tube and enhanced tubes with twisted tape inserts. Grooved pitch sizes (19, 57 and 95 (mm)) and twisted tape ratios (3, 9 and 14) were investigated. The numerical simulations were performed in the Reynolds number range of 12000-22000 using water as the working fluid. The CFD simulations carried out in this study present the effects of groove size, twisted tape ratio and inlet mass flow rate on increasing the heat transfer in internally grooved tubes fitted with twisted tape inserts. This study revealed an increment in Nusselt number and friction factor by decreasing both the twist ratio from 14 to 3 and the groove pitch size from 95 to 19 mm indicating that pitch size and twist ratio plays an important role in the device heat transfer enhancement. It was also found that by increasing the Reynolds number, the Nusselt number is increased in all cases due to more turbulence which led to mixing the flow. The Nusselt number increased about 2 to 34% in grooved tubes without twisted tape; 46 to 112% for plain tube containing twisted tapes; and 46 to 159% for grooved tube containing twisted tapes in comparison to smooth tubes. Also, the friction factor increased about 16 to 73% in grooved tubes without twisted tape; 73 to 175% for plain tube containing twisted tapes; and 75 to 193% for grooved tube containing twisted tapes in comparison to smooth tubes. Through the thermal performance factor evaluation, the condition under which the choice heat transfer enhancement technique is favourable was characterized in this study. The thermal enhancement factor for internally helical grooved tubes containing 9 twisted tape inserts tended to increase by increasing the Reynolds number and the overall highest thermal enhancement factor of about 1.37 was observed for grooved tubes (pitch size of 19 mm) containing twisted tape of twist ratio 3 at Reynolds number of 14000. For all investigated enhanced tubes an enhancement up to 37% was obtained. The heat transfer performance of grooved tubes containing twisted tapes investigated revealed that the CFD simulations can be used to improve future design considerations of heat exchanger devices, which would lead to improvement of thermal system efficiency