Three-dimensional vortex dynamics and convective heat transfer in circular and chevron impinging jets

This paper describes an experimental investigation at Reynolds number equal to 5000 on circular and chevron impinging jets by means of time-resolved tomographic particle image velocimetry (TR-TOMO PIV) and infrared (IR) thermography. TR-TOMO PIV experiments are performed at kilo-hertz repetition rate in a tailored water jet facility where a plate is placed at a distance of 4 diameters from the nozzle exit. Using air as working fluid, time-averaged convective heat transfer is measured on the impinged plate by means of IR thermography with the heated-thin-foil heat transfer sensor for nozzle-to-plate distances ranging from 2 to 10 diameters. The circular impingement shows the shedding and pairing of axisymmetric toroidal vortices with the later growth of azimuthal instabilities and counter-rotating streamwise vortices. In the chevron case, instead, the azimuthal coherence is replaced by counter-rotating pairs of streamwise vortices that develop from the chevron notches. The heat transfer performances of the chevron impingement are compared with those of the circular one, analyzing the influence of the nozzle-to-plate distance on the distribution of Nusselt number. The chevron configuration leads to enhanced heat transfer performances for all the nozzle-to-plate distances hereby investigated with improvements up to 44% at the center of the impinged area for nozzle-to-plate distance of 4. Such enhancements are discussed in relation to the streamwise structures that, compared with the toroidal vortices, are associated with an earlier penetration of turbulence towards the jet axis and a higher arrival speed. (C) 2012 Elsevier Inc. All rights reserved.