A comparative study of passive control on flow structure evolution and convective heat transfer enhancement for impinging jet

The present study experimentally and numerically investigated the passive control on flow structure evolution and convective heat transfer enhancement for impinging jet. Four different impinging jets, including a baseline circular jet (CJ) and three passive controlled jets, i.e., an elliptic jet (EJ), a circular-chevron jet (CCJ) and an elliptic-chevron jet (ECJ), were comparatively analyzed by utilizing the Particle Image Velocimetry (PIV) technique, infrared (IR) thermography and large eddy simulation (LES) over a wide range of jet-to-wall distances (H/D) at the jet Reynolds number (Re) of 20,000. The results showed that, unlike CJ which presented a general shedding of axisymmetric toroidal vortices, EJ showed highly deformed toroidal structures accompanied with the axis switching effect, both CCJ and ECJ exhibited the well-organized counterrotating streamwise vortex pairs developing from the chevron notches. All the three passive controlled strategies were found to induce a stronger mixing and fluctuating activity near around the stagnation region, especially for ECJ (i.e., the passive-passive controlled device) which showed the highest turbulence level approaching the target wall due to the double-passive enhancement. Moreover, compared with the baseline jet CJ, all the passive controlled jets achieved a significant heat transfer improvement in the vicinity of the stagnation point, particularly for ECJ which presented the highest heat transfer enhancement of about 41% at H/D = 5. Whereas both CCJ and ECJ were found to exhibit a less-than-ideal heat transfer performance at a small H when the heat transfer uniformity was specifically considered, due to the anisotropic thermal imprint distributions. (C) 2018 Published by Elsevier Ltd.