Effects of Freestream Turbulence on an Axisymmetric CO2 Turbulent Jet Diffusion Field

Abstract

Effects of freestream turbulence on an axisymmetric CO2 turbulent jet diffusion field is experimentally investigated. The composite probe consisting of two concentration-sensitive I-type hot-wires is used for the simultaneous measurement of instantaneous axial velocity and concentration. To measure grid turbulence without the jet, I- and X-type hot wire probes are used. The fractal square grid is used to generate freestream turbulence. The Taylor Reynolds number is Reλ = 80 - 160. The results are compared with previous measurements in which weak freestream turbulence at Reλ-35 is generated by the classical biplane grid (regular grid). For both flows, the mesh Reynolds number based on the grid mesh size M and freestream speed U0 is 6,000 and the jet Reynolds number based on the diameter d (= 3 mm) of jet exit and relative velocity between freestream and exit velocity, UJ-U0, is 5,000. The results show that profiles of normalized mean velocity and concentration in the CO2 jet do not change at the present Reλ in the freestream flow, while growth of half widths of mean velocity and mean concentration, turbulent intensity, turbulent mass flux and eddy diffusivity in the CO2 jet are significantly increased under the effect of strong freestream turbulence. The eddy diffusivity in the CO2 jet becomes approximately three times of that in the regular grid turbulence.