Heat transfer in the laminar and transitional flow regimes of smooth vertical tube for upflow direction

Abstract

Due to design limitations or system upgrade, heat exchangers are required to operate in the laminar-to-turbulent transition region in order to achieve a high heat transfer with low pressure drop. The present research investigates the effect of vertical upflow with heating on the single-phase heat transfer in the transitional flow regime of a vertical tube. The experimental setup consists of a swinging test bench which allows for horizontal flow and vertical upflow direction under constant heat flux boundary condition. A smooth circular tube with inner diameter of 5.1 mm and a heated length of 4.52 m was used as the test section with water at Prandtl number ranging between 5 to 7 as working fluid. The experiment covers Reynolds number range of 1 000 to 10 000 at horizontal and vertical orientations of the test section using a squared-edged inlet geometry. For fully developed vertical upflow direction, transition is delayed when compared to horizontal flow direction where the effect of secondary flow increases the laminar flow heat transfer and causes transition to occur much earlier. The width of transition region for vertical tubes is significantly smaller than that of the horizontal tubes.