An experimental study on the characteristics of heat transfer on the turbulent round impingement jet according to the inclined angle of convex surface using the liquid crystal transient method

Abstract

Measurements of the local heat transfer coefficients on a hemispherically convex surface with a round oblique impinging jet were made. The liquid crystal transient method was used for these measurements. This method, which is a variation of the transient method, suddenly exposes a preheated wall to an impinging jet while video recording the response of liquid crystal for the surface temperature measurements. The Reynolds number used was 23,000 and the nozzle- to-surface distance was L/ d = 2, 4, 6, 8, and 10 and the jet angle was α = 0°, 15°, 30° and 40°. In the experiment, the Nusselt number at the stagnation point decreases as the jet angle increases and has the maximum value for L/ d = 6. The X-axis Nusselt number distributions exhibit secondary maxima at 0° ⩽ α ⩽ 15° L/ d ⩽ 4 for X/ d < 0 (upstream) and at 0° ⩽ α ⩽ 40°, L/ d ⩽ 4 and at 30° ⩽ α ⩽ 40°, 4 < L/ d ⩽ 6 for X/ d > 0 (downstream). The secondary maximum occurs at long distance from the stagnation point as the jet angle increases or the nozzle-to-surface distance decreases. The Y-axis Nusselt number distributions exhibit secondary maxima at Y/ d = ±2 for 0° ⩽ α ⩽ 30° and L/ d ⩽ 4, and for α = 40° and L/ d = 2. The displacement of the maximum Nusselt number from the stagnation point increases as the jet angle increases or the nozzle-to-surface distance decreases and the maximum distance is about 0.67 times of the nozzle diameter. The ratio of the maximum Nusselt number to the stagnation Nusselt number increases as the jet angle increases.