Experimental Studies of Turbulent Mixing in Impinging Jets

Abstract: This thesis concerns the experimental study of mixing and flow characteristic in turbulent circular impinging jets without and with jet orifice manipulators (i.e. passive-fixed perturbations atthe nozzle exit), without and with swirl. The goal of the studies is to understand the basic mechanisms that control the mixing processes in turbulent impinging jets. The applications of such mixing processes are vast, including cooling of Gas Turbine engine components to mass-transfer in bio-reactors. The thesis consisted of three main parts: Each of these is characterized by the flow conditions at the nozzle exit: unperturbed and perturbed circular non-swirling flow and swirling circular jets. Several stand-alone and combined, laser techniques have been employed in these studies: standard and time-resolved Particle Image Velocimetry (PIV), Laser-induced Fluorescence (LIF), Laser Doppler Velocimetry (LDV) and simultaneous PIV/LIF. A comprehensive measurement of flow characteristics and the mixing of a passive tracer, in circular impinging jets has been made for several jet-Reynolds numbers and several impingement-distance configurations. First, a parametric study of velocity and turbulence fields in the circular impinging jets has been carried out using PIV. The effect of impingement-distance and Reynolds number on the development of jets are presented and discussed. Thereafter, we consider the effects of the mixing boundary conditions at the nozzle exit on the mixing in a circular impinging jet using LIF. Additionally, the mixing of a locally injected passive tracer has been quantified. Emphasis is made on the mixing in the deflection region of the jet. The results are given in terms of statistics and probability density functions of concentration at key positions, The influence of shear-layer instabilities and secondary vortices that are formed on the wall at 1.5-2 nozzle diameter from the stagnation point on the mixing are pointed out and quantified. The combination of PIV and LIF is used to simultaneously measure the instantaneous concentration and velocity fields in the impinging jets, which provides the estimates of the turbulent mass transport term in the plane. These information enables one to compare directly the validity of different models for the turbulent fluxes. Such data is very scarce in the literature. In the case of the perturbed nozzle inlet of the jet, the turbulent mixing is measured using simultaneous PIV/PLIF. The unperturbed nozzle (N-0) is compared to nozzles with two (N-2) and three (N-3) needle shaped rods, respectively. The results show that the low turbulent region is shorter in the N-2 and N-3 cases. This is caused by an earlier transition to turbulence in the shear layer, which also leads to enhanced mixing indicated by the evolution of the dye tracer and the turbulent fluxes. The perturbation effect is also evident in other statistical properties, such as turbulent kinetic energy. In the case of the swirling impinging jet, we investigate the effects of swirl number in two different cases and compare the results to the non-swirling case (S-0, S-1 and S-2). The effects of Reynolds number and impingement distance (H/D=0.5-2) on the turbulent mixing and flow field has also been considered. Time-Resolved PIV and LIF are employed to extract the mean data as well as the additional spectral information. LDV measurement is used for assessing measurement errors and further understanding the high frequency modes of the flow. The results shows that even at the relatively low level of swirl, vortex breakdown is found if the impingement wall is close enough. This vortex break-down leads to a recirculation zones, with en increase in the spreading rate of he jet. Furthermore, at H/D=1 and 2, the mixing is enhanced by the introduction of the swirl. When further decreasing the distance to the impingement wall to H/D=0.5, the effect of the wall becomes more important and the large scale structure dominates the transport of the passive scalar. Therefore, the non-swirling jet gives the best mixing performance. Simultaneous PIV/PLIF measurements provide the whole (2-D) field of turbulent transport of the passive scalar. This thesis also aims at providing a database of experimental data of good quality to enable comparisons and validations of computational tools within the Reynolds Averaged Navier-Stokes (RANS) and Large Eddy Simulation (LES) frameworks.