On the behaviour of impinging sweeping jets: effects of the mixing chamber length, the feedback channel minimum cross-sectional area and the nozzle-to-plate distance

Abstract

An experimental investigation on the effects of the mixing chamber length, the feedback channel minimum cross-sectional area and the nozzle-to-plate distance on the external flow field of impinging sweeping jets is performed by employing the Particle Image Velocimetry (PIV) technique. Three sweeping jet devices, characterized by different mixing chamber lengths (Lf/w=4.5,3.5,2.5, being w the width of the exit nozzle throat section), are investigated. Furthermore, in order to analyse different minimum cross-sectional area values, the geometry of the feedback channels of the device characterized by the longest mixing chamber length is varied. In particular, five different values of g/w (1, 0.67, 0.50, 0.33, and 0.17) are taken into account, being g the minimum passage width of the feedback channels. Time-averaged and phase-averaged measurements are carried out for non-dimensional nozzle-to-plate distances H/w ranging between 2 and 10, with a spacing of 2. As for the effects of the mixing chamber length, the devices characterized by Lf/w<4.5 do not generate an oscillating jet, thus the time-averaged flow field exhibits a single jet structure, while a twin-jet pattern is found for the device having Lf/w=4.5, due to the oscillating motion of the jet. Furthermore, due to the absence of the jet oscillation, the values assumed by the phase-correlated kinetic energy (PKE) are significantly lower for the devices having Lf/w<4.5. Moreover, for the latter devices, the highest values of the turbulent kinetic energy (TKE) are reached in the jet shear layers, which are aligned to the streamwise direction, instead, for the device having Lf/w=4.5, the TKE assumes higher values in proximity of the jet centreline. As for the effects of the feedback channel minimum cross-sectional area, the decrease of g/w determines an increase of the oscillation frequency and a reduction of the jet oscillation amplitude, with a subsequent strong decrease of the PKE and an increase of the TKE values in proximity of the jet centreline. As regards the effects of the nozzle-to-plate distance, by increasing this parameter for the device having Lf/w=4.5 the jet no longer sweeps in a rigid-like way between the two most deflected positions in the external flow field and the PKE values in the stagnation regions decrease.