Experimental study of the three-stream scalar mixing in a turbulent coaxial jet.

摘要

In the present study we investigate three-stream scalar mixing in a turbulent coaxial jet. In this flow the center jet and the annulus, consisting of acetone-doped air and ethylene respectively, are mixed with the co-flow air. A unique aspect of this study compared to previous studies of three-scalar mixing is that two of the scalars (the center jet and air) are separated from the third (annulus); therefore, this flow better approximates the mixing process in a nonpremixed turbulent reactive flow. Planar laser-indiced fluorescence and Rayleigh scattering are employed to measure the mass fractions of the acetone-doped air and ethylene, respectively. The results show that the most unique development of the three-scalar mixing occurs in the near field of the flow. The mixing process in this part of the flow are analyzed in detail using the scalar means, variances, correlation coefficient, joint probability density functions (JPDF), conditional diffusion, and conditional dissipation rate. The conditional scalar diffusion velocity streamlines in scalar space generally converge quickly to a manifold along which they continue at lower velocities. Current mixing models do not exhibit such a trend. The approach to the manifold is generally in the direction of the annulus scalar. The different magnitudes of the diffusion velocity components for the two scalars cannot be accounted for by their different dissipation time scales. The mixing processes during the approach to the manifold, therefore, cannot be modeled by using different dissipation time scales alone. While the three scalars in this flow have the same distance in scalar space, mixing between two of the scalars can occur only through the third, forcing a detour of the manifold (mixing path) in scalar space. This mixing path provides a challenging test for mixing models as most mixing models use only scalar-space variables and do not take into account the spatial (physical-space) scalar structure. The scalar JPDF and the conditional dissipation rates obtained in the present study have similarities to these of mixture fraction and temperature in turbulent flames. The present study, therefore, is an important step towards understanding and modeling multiscalar mixing in reactive flows.