The effect of jet pulsation on the flow field of a round impinging jet and the radially expanding wall jet

A pulsating round jet having a constant average mass flow rate may increase heat transfer in comparison to a steady jet impinging on a surface. However, while several studies have focused on the heat transfer, flow field characteristics remain poorly understood. Here, the mean and turbulent flow characteristics of a pulsating, round jet impinging on a flat plate were measured using planar, phase-locked particle image velocimetry (PIV) and compared to those of a steady jet. The surface to jet nozzle stand-off distance equaled two nozzle diameters and Reynolds numbers based on the average steady jet exit velocity were Re = 4606, 8024 and 13,513. Jet exit velocities varied nearly sinusoidal and Strouhal numbers ranged between 2 x 10(-3) <= Sr <= 15.6 x 10(-3). The results indicate that jet pulsation did not affect the normalized, phase-averaged radial and axial velocity distributions in the impingement region and these could be predicted based on a quasi-steady assumption. However, at the center of the jet, rms values of radial and axial velocity fluctuations were enhanced at pulsation phases for which the jet exit velocity was lowest. In contrast, profiles of radial and axial velocity fluctuations and the Reynolds shear stress in the wall jet were significantly affected by jet pulsation. For the here investigated range of parameters, our results indicate that outer layer self-similarity in the wall jet is not attained when the jet is pulsating, and the wall jet cannot be treated as quasi-steady. (C) 2019 Elsevier Ltd. All rights reserved.