Date of Award

12-2014

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Legacy Department

Mechanical Engineering

Advisor

Tong, Chenning

Committee Member

Miller , Richard

Committee Member

Xuan , Xiangchun

Committee Member

Ochterbeck , Jay

Abstract

The subgrid-scale (SGS) mixing of mixture fraction, temperature, and species mass fraction in turbulent partially premixed (Sandia) flames is studied. We focus on the effects of the SGS mixing regimes and the spatial relationships among the scalars on the SGS mixing of the species mass fractions. High resolution lines images are used to obtain the scalar filtered mass density function and the conditionally filtered diffusion. The results show that for small SGS variance the scalar fields have a relatively simple structure. For large SGS variance the structure is more complex. The spatial relationships among the scalars result in different SGS scalar structures and SGS mixing characteristics, with the mass fraction of CO2 being the simplest and that of CO being the most complex. The results in this study present a challenging test for SGS mixing models. Recent studies of subgrid-scale (SGS) mixing and turbulence-chemistry interaction have shown that turbulent flames contain different structures. In flamelets diffusion of reactive scalars and chemical reaction are tightly coupled. Most mixing models used in probability density and filtered density methods, however, are based on non-reactive scalars. To investigate the effects of the coupling on the diffusion we decompose a reactive scalar into a steady flamelet part and perturbations from it. The diffusion of the former can be obtained from a flamelet solution while the latter is unclosed. The conditionally filtered diffusion and dissipation of the reactive scalar perturbations are analyzed using high-resolution line images obtained in turbulent partially premixed (Sandia) flames. For SGS scalar containing flamelets, the perturbation diffusion has characteristics similar to that of a non-reactive scalar, in contrast with the flamelet part. The functional form of the conditionally filtered diffusion is well described by the Interaction by Exchange with the Mean (IEM) model. Our perturbation analysis of the flamelet equation shows that for perturbations having length scales smaller than the reaction zone width, the reactive scalar diffusion are largely controlled by the mixture fraction field, thus have the characteristics of non-reactive scalar mixing. For perturbations with length scales larger than the reaction width, the conditionally filtered diffusion has the same form as non-reactive scalar mixing, with the mixing time scale given by the flamelet. The IEM model predictions based on this mixing time scale are in good agreement with the experimental results for a range of SGS conditions, suggesting that the perturbations for the conditions studied are consistent with unsteady flamelets. Thus, mixing models based on non-reactive scalars can potentially model the SGS mixing accurately. The results in the present study can be useful for developing a unified mixing model that can predict all combustion regimes accurately. There already have extensive studies about turbulent mixing based on two-scalars. However, in many applications such as reactive flows, at least three scalar (two reactants and one product) are involved. The mixing process is multiscalar and more complex. In the present study, we investigate three-scalar mixing in temporally developing plane jets using direct numerical simulation (DNS). The flow configuration consists of five streams: a central jet, two off-central jets surround the central jet, and two coflows surround the off-central jets. The mixing process is analyzed in detail using joint probability density functions (JPDF), conditional diffusion, and conditional dissipation rate. The results show that the mixing process is greatly influenced by the upstream conditions, with high quality of mixing favored by high initial velocity gradients and high ratios between the central and off-central jet widths. The scalar JPDF and the conditional dissipation rates obtained in current simulations show some similarities to these of mixture fraction and temperature in turbulent flames. The present study, therefore, provide a basis for understanding the multiscalar mixing in reactive flows.

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