Date of Award

5-2009

Document Type

Thesis

Degree Name

Master of Science (MS)

Legacy Department

Mechanical Engineering

Advisor

Tong, Chenning

Committee Member

Figliola , Richard

Committee Member

Miller , Richard

Abstract

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.

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