Fully developed forced convection heat transfer and pressure drop in a smooth tube in the transitional flow regime

Abstract

Extensive work has been done on characterising convective heat transfer and pressure drop in smooth tubes in the laminar and turbulent flow regimes. However, little work was completed in the transitional flow regime. In all previous transitional studies, experiments that were conducted between the laminar and turbulent flow regimes were with mixed convection in the laminar flow regime and not in the forced convection flow regime. The secondary flow that occurs during mixed convection should most probably influence the characteristics in the transitional flow regime. It can therefore be expected that the transitional flow characteristics of forced convection and mixed convection will be different. However, the transitional characteristics of forced convection flow have not yet been determined. The purpose of this study was therefore to determine the heat transfer and pressure drop transitional characteristics specifically in the forced convection flow regime. Furthermore, to focus on determining these factors for a circular, horizontal smooth tube for fully developed flow. The characteristics were determined in an experimental set-up through which flow occurred through a test section consisting of a horizontal and circular smooth tube. The test-section inside diameter was 4.04 mm, and the tube length was 8.4 m. Water was used as the test fluid and was circulated through the test section which was heated at a constant heat flux. A calming section with a square edge inlet was upstream of the test section. Temperatures at the tube inlet, outlet and outer surface of the test section were measured with a total of 58 thermocouples. Two pressure taps was also installed on the test section and was connected to a pressure transducer for pressure drop measurements. Experiments were conducted mainly on the last part of the test section where fully developed flow occurred. Experiments were conducted between Reynolds numbers of 1 000 to 10 000, Prandtl numbers of 3 to 8, and Rayleigh numbers of 330 and 11 000 (heat fluxes of 0.89 kW/m2 to 3.26 kW/m2). It was found that the heat transfer transitional range coincided with the friction factor transition range with a Reynolds number range of 2 484 to 2 849. Forced convection results in the laminar regime was achieved and compared well to literature. The results were mapped on published flow regime maps. This was inconclusive as the published flow regime maps have been specifically developed for fixed parameters that did not match the parameters of this study.