Influence of Excitation Frequency, Phase Shift, and Duty Cycle on Cooling Ratio in a Dynamically Forced Impingement Jet Array

Abstract

The cooling ratio on a dynamically forced 7 × 7 impingement jet array is studied experimentally. The current study is focused on determining the influence of a phase shift between every row of nozzles as well as the impact of a duty cycle variation on the cooling ratio. Both parameters are studied in dependency of the impingement distance (H/D = 2, 3, 5), the (nozzle-) Reynolds-number (ReD = 3200, 5200, 7200), and the excitation frequency (f = 0 Hz − 1000 Hz). For every set of parameters, the phase shift between every row of nozzles is varied between Φ=0% and 90%, while the variation of the duty cycle is performed between duty cycle (DC) = 35% and 65%. During all investigations, the dimensionless distance between adjacent nozzles is fixed at Sx/D = Sy/D = 5, and liquid crystal thermography is used to acquire the wall temperatures, which are further processed to calculate the local Nusselt numbers. Generally, the implementation of an excitation frequency allows a case-depending increase in the cooling ratio of up to 52%. Further implementation of a phase shift yields an additional frequency-depending improvement of the cooling ratio. In case of duty cycle variation, the best case revealed an additional 19% improvement in the cooling ratio.