Local effectiveness and Nusselt number distributions for a rectangular jet impinging on a cylindrical convex surface at different angles

Abstract

Local effectiveness and Nusselt number distribution measurements for heated rectangular jets impinging perpendicularly and obliquely on a cylindrical convex surface are reported here. The jets exit from a rectangular nozzle of height ‘H’ and width ‘W’ with aspect ratios, H/W, equal to 10 and 5 and the corresponding the curvature ratios (ratio of hydraulic diameter of nozzle to diameter of the impinging convex surface), b/D, were equal to 0.083 and 0.076. Studies were conducted for non dimensional jet-to-target distances, L/b, equal to 2, 3, 4 and 5, and for inclination of jet axis with the convex target surface, θ, equal to 0°, 30° and 45°. The Reynolds numbers, Re (based on average exit velocity of jet and hydraulic diameter of nozzle) was kept constant at 17000.The entrainment of the fluid from the surroundings into the walljet from the top surface and edge regions, which is symmetric on the either side of the impingement zone for perpendicular impingement, gives a ‘rhombus like’ shape for the effectiveness contours. However, the entrainment is asymmetric for oblique impingement, with the drop in effectiveness rapid in the uphill side and gradual in the downhill side, giving the effectiveness contours a ‘kite like’ shape. The fluid entrainment from the top surface and edge regions influence the Nusselt number distribution also. The relatively higher mixing of the jet fluid with the ambient at the edge regions increases the turbulence and thereby increases the Nusselt number at these locations, resulting in contour shapes different from those observed for the effectiveness.