Date of Award
8-2016
Document Type
Dissertation
Degree Name
Doctor of Philosophy (PhD)
Legacy Department
Mechanical Engineering
Committee Member
Dr. Chenning Tong, Committee Chair
Committee Member
Dr. Richard Miller
Committee Member
Dr. Jay Ochterbeck
Committee Member
Dr. Xiangchun Xuan
Abstract
The effects of the velocity and length scale ratios of the annular flow to the center jet on three-scalar mixing in turbulent coaxial jets are investigated. In this flow a center jet and an annular flow, consisting of acetone-doped air and ethylene respec-tively, are mixed with the co-flow air. Simultaneous planar laser-induced fluorescence and Rayleigh scattering are employed to measure the mass fractions of the acetone-doped air and ethylene. The velocity ratio alters the relative mean shear rates in the mixing layers between the center jet and the annular flow and between the annular flow and the co-flow, modifying the scalar fields through mean-flow advection, turbu-lent transport, and small-scale mixing. The length scale ratio determines the degree of separation between the center jet and the co-flow. The results show that while varying the velocity ratio can alter the mixing characteristics qualitatively, varying the annulus width only has quantitative effects. Increasing the velocity ratio and the annulus width always delays the evolution of the scalar fields. The evolution of the mean scalar profiles are dominated by the mean-flow advection, while the shape of the joint probability density function (JPDF) is largely determined by the turbulent transport and molecular diffusion. The JPDF for the higher velocity ratio cases is bimodal at some locations while it is unimodal for the lower velocity ratio cases. The diffusion velocity streamlines in scalar space representing the conditional diffusion generally converge quickly to a manifold along which they continue at a lower rate. The curvature of the manifold is significantly larger for the higher velocity ratio cases. Predicting the mixing path along the manifold as well as its dependence on the velocity and length scale ratios presents a challenging test for mixing models. The three-scalar subgrid-scale (SGS) mixing in the context of large eddy simu-lation and its dependence on the velocity and length scale ratios are also investigated. The analysis reveals two SGS mixing regimes depending on the SGS variance value of the center jet scalar. For small SGS variance the scalars are well mixed with uni-modal filtered joint density function (FJDF) and the three-scalar mixing configuration is lost. For large SGS variance, the scalars are highly segregated with bimodal FJDFs at radial locations near the peak of the mean SGS variance of the center jet scalar. Two competing factors, the SGS variance and the scalar length scale, are important for the bimodal FJDF. For the higher velocity ratio cases, the peak value of the SGS variance is higher, thereby resulting in stronger bimodality. For the lower velocity ratio cases, the wider mean SGS variance profiles and the smaller scalar length scale cause bimodal FJDFs over a wider range of physical locations. The diffusion stream-lines first converge to a manifold and continue on it toward a stagnation point. The curvature of the diffusion manifold is larger for the larger velocity ratio cases. The manifold provides a SGS mixing path for the center jet scalar and the co-flow air, and thus the three-scalar mixing configuration characteristics is maintained for the large SGS variance. The SGS mixing characteristics observed present a challenging test for SGS mixing models. The scalar dissipation rate structures have similarities to those of mixture fraction and temperature in turbulent nonpremixed/partially pre-mixed flames. The results in the present work, therefore, also provide a basis for investigating multiscalar SGS mixing in turbulent reactive flows.
Recommended Citation
Li, Wei, "Effects of Mean Shear and Scalar Initial Length Scale on Three-Scalar Mixing in Turbulent Coaxial Jets" (2016). All Dissertations. 1717.
https://open.clemson.edu/all_dissertations/1717