#### 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://tigerprints.clemson.edu/all_dissertations/1717