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% – 90% while the variation of the duty-cycle is performed between DC = 35% – 65%. During all investigations, the dimensionless distance between adjacent nozzles is fixed at x/D = y/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 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 cooling ratio.

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