A critical review on various factors affecting the thermohydraulic performance in transition-flow regime

Heat transfer and pressure drop in laminar and turbulent flow have been studied for a long time. However, the thermohydraulic of fluid flow in transition flow is still in the embryonic stage and needs further exploration. Primarily, this article complied all the fragmented research works linked to thermal and flow performance in transition-flow regimes. Several detailed research works pertaining to developing and fully developed transition flow were reported in the past three decades which significantly contribute to the field. It was also found that the transition-flow regime shifted with the change in the operating conditions such as free, forced and mixed convection, developing and fully developed flow, inlet geometries, type of working fluid, channel geometries, roughness, the orientation of test section, etc. Hence, by altering the shape of the entrance, the amount of heat passing through, or the surface of the tube, one could manipulate the range of Reynolds numbers where the transition took place. The temperature of fluids affects their densities, so when heat is applied to the tube wall, it creates temperature differences within the thermal boundary layer. These differences in temperature then cause changes in density and buoyancy due to the force of gravity. Several novel correlations have been developed based on these findings to determine the heat transfer and pressure drop in different flow regimes. In order to develop accurate correlations for heat transfer and pressure drop in the transitional-flow regime, it is necessary to comprehend the factors that impact the beginning and end of this regime. This understanding is crucial for selecting or creating suitable correlations.