Effect of Injection Parameters on Jet Array Impingement Heat Transfer

The purpose of the present study is to clarify the heat transfer characteristics with multiple jet impingement aiming at the highly efficient cooling performance. In the study, we investigated the effect of injection parameters on circular jet array impingement heat transfer. As we focus on interference among the adjacent impinging jets, tests are mainly conducted at the minimum crossflow condition. The experiments are also conducted at injection distance from 2 to 8 jet hole diameters and jet-to-jet spacing from 4 to 8 jet hole diameters. Jet hole diameter Reynolds number is 4,680. Thermochromic liquid crystal is used to obtain heat transfer coefficient. Wall pressure measurement and oil flow visualization on the target surface are performed to understand the flow pattern of impinging jet and wall jet. The effect of injection parameters, such as injection distance, jet-to-jet spacing and number of jets, on jet array impingement heat transfer is clarified. NOMENCLATURE Af = jet exit area ratio to target surface area, Af = πD/4S Cp = wall pressure coefficient, Cp = 2(Pw Pref)/ρUref D = jet hole diameter, m (D = 3 mm) h = local heat transfer coefficient, W/m K k = thermal conductivity of air, W/m K (k = 0.0272 in this study) L = injection distance from nozzle to target surface, m N = number of jet holes Nu = local Nusselt number, Nu = hD/k Nu = area averaged Nusselt number Nu4D = 4D-square area averaged Nusselt number (origin of area is center of central jet.) Pref = reference pressure (= ambient pressure), Pa Pw = local pressure on target surface, Pa qw = heat flux supplied to heat transfer surface, W Re = Reynolds number based on jet hole diameter S = jet-to-jet spacing, m Tj = jet temperature, K Tw = target surface wall temperature, K Uref = jet velocity, m/s (Uref = 25 m/s) X = streamwise distance, normal to target surface, m Y = horizontal spanwise distance, along target surface, m Z = vertical spanwise distance, along target surface, m ρ = density of air, kg/m INTRODUCTION In order to improve the performance of the jet engine, it is necessary to increase turbine inlet temperature and to improve the efficiency of its components. Therefore, it is desirable to develop the more efficient cooling mechanism by the minimum mass flow rate of coolant. The impingement cooling system is applied to the inside of turbine blade and combustion liner even now. One of the main reasons is that the impinging jet has the most efficient cooling performance based on the high kinetic momentum. It also has an advantage of a simplified structure. So, an array of the impinging jets has been widely used to provide an effective cooling performance for the hot part of industrial product. However, the array of impinging jets produces the crossflow due to the existence of the spent flow passing through a confined channel wall and fountain introduced by the impingement of wall jets. It is well known that the presence of the crossflow tends to disturb the impinging jet flow pattern, thicken wall boundary layers and degrade heat transfer rates. And the presence of the fountain also tends to affect the impinging jet behavior, weaken kinetic momentum and cause a distortion of the impinging jet. Since these behaviors cause a degradation of heat transfer on the impingement surface, various remedies are proposed. Uysal et al. [1] made experiment on an in-line array varying the jet hole-size in a systematic manner. They show the influence of the flow rate varied by the jet hole-size on the crossflow. Esposito et al. [2] performed experiment on four types of injection plate with in-line arrays. They show the heat transfer improvement by changing the configuration of the injection plate. Rhee et al. [3] conducted experiment at square arrays with extraction holes on the injection plate. They show that effusion holes play an important role in inhibiting a generation of the crossflow and give a high heat transfer at the narrow injection distance. The purpose of our study is to investigate the heat transfer characteristics for the highly efficient cooling performance with multiple jet impingement. In the current study, we investigate the effect of injection parameters on the heat transfer of target surface under interference among the adjacent impinging jets. To clear the effect of the interference, we select a minimum crossflow configuration, which is insensitive to the crossflow, for multiple impinging jet arrays. Steady state thermochromic liquid crystal method is employed to obtain heat transfer coefficient. Wall pressure measurement and oil flow visualization on the target surface are conducted to understand the flow pattern of impinging jet and wall jet. We clarify the effect of injection distance, jet-to-jet spacing and number of jets on jet array impingement heat transfer. International Journal of Gas Turbine, Propulsion and Power Systems February 2012, Volume 4, Number 1 Copyright © 2012 Gas Turbine Society of Japan Presented at International Gas Turbine Congress 2011 Osaka, November 13-18, Osaka, Japan, IGTC2011-0228 Review completed February 15, 2012