Internal Groove Influenced Thermohydraulic Performance on an Air-Channel

Internal fins, protrusions, and porous foams enhance the local convective heat transfer at the channel walls in the heat exchangers, electronic chips for cooling, and solar panels for cooling and heating by promoting the local turbulence in flow. However, the fluid pumping power in the channel suffers due to the blockage and high pressure drop caused by the fins, protrusions, and foams. The present article reports the experimental friction factors and Nusselt numbers in a rectangular channel with an array of internal grooves in one wall. The grooves provide the minimum flow blockage, but still promote the local turbulence to enhance the surface heat transfer. The cylindrical grooves are machined at 45° to the mean air-flow direction in one of the wide walls of the channel of aspect ratio of 40.6:1. The ratio of groove print-diameter to depth is 2.83:1. The objectives are to investigate the effects of groove pitch on the thermal performance in the channel as the flow Reynolds number (Re) varies between 600 and 15000. Two ratios of groove-pitch to print-diameter are employed — 3.2:1 and 4.5:1. The measurements include the distributions of the wall staticpressure and heat transfer coefficient along the channel wall having the grooves. The pressure distributions are obtained at the adiabatic condition. The heat transfer coefficients are obtained with the constant heat flux boundary condition from the grooved surface. The measurements are also obtained in the smooth channel when the grooved wall is replaced by a smooth wall for comparisons with the grooved channel. The results indicate the ratio of Darcy friction factor (f/f_o) for the grooved channel (f) to that for the smooth channel (f_o) increases with the Re by 27% and 41% at the maximum for the two grooves. The f/f_o ratios are slightly higher for the grooves with the smaller pitch. The fully developed Nusselt number ratio (Nu/Nu_o) for the grooved channel (Nu) to that for the smooth channel (Nu_o) increases with the Re by 37% at the maximum for the two grooves. However, the thermal performance quantified by the ratio (Nu/Nu_o)/(f/f_o)^1/3 is higher for the smaller pitch grooves for most of the Reynolds numbers > 2000. The results thus contribute to the design of heat exchangers and cooling channels for high thermal performance with the ratio (Nu/Nu_o)/(f/f_o)^1/3 > 1.0 based on the smaller surface area, pumping power, and heat duty.