Effectiveness and heat transfer characteristics for a single heated rectangular jet with different aspect ratios impinging perpendicularly on a flat surface

Abstract

Results from an experimental investigation to study the distribution of local effectiveness and Nusselt number for a heated rectangular jet with different aspect ratios impinging perpendicularly on a flat surface are reported here. The jet exits from a rectangular nozzle of height ‘H’ and width ‘W’ with a developing length equal to 50 times the hydraulic diameter of the nozzle geometry, b. Nozzle aspect ratios, H/W, equal to 10, 5 and 1, Reynolds numbers (based on average exit velocity of jet and hydraulic diameter of nozzle) equal to 11,000 and 17,000 and non dimensional jet-to-plate distances, L/b, equal to 2, 3, 4 and 5 were studied. The contours of effectiveness and Nusselt number are initially similar to the exit shape of the nozzle but very quickly take the shape similar to that of a rhombus with rounded corners with diagonals along the major and minor axes of the rectangular jet. The effectiveness variations show that after impingement, the jet fluid entrains the surrounding fluid from the edges as well as the top surface. The edge entrainments progress towards the center, giving the contours the characteristic ‘rhombus like’ shape. The Nusselt number is also influenced by the fluid entrainment, as indicated by the contours. Regions of high heat transfer coefficients are observed due to high turbulence generated by mixing with the stagnant ambient, especially at the low L/b ratios.