Thermographic PIV for Turbulent Flux Measurement

To comprehend the role of turbulence in heat and mass transport of the flows applied to modern engineering systems, a measurement technique for turbulence is needed. Thermographic particle image velocimetry (PIV), one of laser diagnostics for simultaneous measurement of velocity and temperature, has been rose as promising for this purpose, recently. The technique uses luminous phosphor particles as PIV tracer that serve for temperature measurements as well. In this study, the technique has been further developed and validated. First, to extend the applicable temperature range, eight prospective phosphors as thermographic PIV tracer have been investigated in an optically accessible oven. A theoretical model, which considers the propagation of camera errors, has been used to predict the temperature precision of each phosphor based on the measured spectra in the oven and the signal measured from aerosolized particles in air of a reference phosphor. Furthermore, by using the theoretical model, an optimized filter set for a phosphor could be selected. Through this investigation, SRMG and BAM:EuMn have been recognized as useful tracers in the intermediate temperature range of 300 – 600 K. Although YAG:Dy showed weak intensity in the low temperature range, its delayed thermal quenching and the temperature sensitivity at high temperature make the phosphor most promising one for the high temperature application. BAM has been selected as the tracer of this study for its high performance in the wide temperature range upto 800 K. Three potential sources of bias of the thermometry have been pointed out: the effect of laser fluence, the multiply scattered phosphorescence originating from the cold flow on the high temperature flow, and possible preferential absorption of phosphorescence. A power scan has been performed in the measurement of spectra from packed powder in the oven and also in the measurements of intensity ratio from individualized particles in air to investigate the effect of laser fluence. Increasing laser fluence resulted in a shift of spectra to the UV regime and the increasing intensity ratio. Varying crystal field according to the increased number of excited activators has been suggested as the possible reason for the shift of the BAM spectra. To avoid a bias by this effect, a homogenized beam by using only the center part of the beam has been applied in this study. For the multiply scattered phosphorescence from the cold flow, the images with seeding density ratio of the hot and cold flow larger than two have been selected for further processing. Here, a particle locating algorithm has been applied to count the number of particles. By seeding the non-absorbing MgO particles instead of BAM in the cold flow, the potential preferential absorption has been suggested, but could not be confirmed for the remaining multiple scattering effect. The application of the thermographic PIV to near- and transitional fields of heated round jet (x=1~15 d) enabled the verification of the measurement technique by comparing with previous results. The axial and radial profiles of mean, rms of velocity and temperature, the integral length scales, the Reynolds shear stress and scalar flux demonstrated good agreements. Based on the eddy viscosity and gradient diffusion model, the turbulent viscosity, turbulent heat diffusivity and the turbulent Prandtl number have been extracted and compared with the previous works