Heat transfer and friction factor of discrete V-down rib roughened solar air heater ducts

Abstract

In this paper, results of experimental investigation on heat transfer and friction in rectangular ducts roughened with a new configuration of “discrete V-down ribs” on one broad wall are presented. The discretization of V-down rib is done by providing small symmetrical gap equal to rib height in both legs of V-shape. The rectangular duct has width-to-height ratio of 12 and the Reynolds number ranged from 3000 to 15 000. The roughened wall is uniformly heated while the remaining three walls are insulated. These boundary conditions correspond closely to those found in solar air heaters. The rib roughness has height-to-hydraulic diameter ratio of 0.043, angle of attack of 60°, and pitch-to-height ratio of 10. The relative gap position was varied from 0.20 to 0.80. The effects of gap position on Nusselt number and friction factor have been discussed and results compared with continuous V-down rib roughened duct and smooth duct under similar flow and boundary conditions to investigate the enhancement in Nusselt number and friction factor. The maximum enhancement in Nusselt number and friction factor has been obtained at relative gap position of 0.65 and is of order of 2.88 and 3.03 times of that of the smooth duct, respectively. The thermohydraulic performance of solar air heater provided with such rib roughness is considerably enhanced.