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Title LES/PDF approach for turbulent reacting flows
Publication Date
Date Accessioned
Degree PhD
Discipline/Department Aerospace Engineering
Degree Level doctoral
University/Publisher University of Texas – Austin
Abstract The probability density function (PDF) approach is a powerful technique for large eddy simulation (LES) based modeling of turbulent reacting flows. In this approach, the joint-PDF of all reacting scalars is estimated by solving a PDF transport equation, thus providing detailed information about small-scale correlations between these quantities. The objective of this work is to further develop the LES/PDF approach for studying flame stabilization in supersonic combustors, and for soot modeling in turbulent flames. Supersonic combustors are characterized by strong shock-turbulence interactions which preclude the application of conventional Lagrangian stochastic methods for solving the PDF transport equation. A viable alternative is provided by quadrature based methods which are deterministic and Eulerian. In this work, it is first demonstrated that the numerical errors associated with LES require special care in the development of PDF solution algorithms. The direct quadrature method of moments (DQMOM) is one quadrature-based approach developed for supersonic combustion modeling. This approach is shown to generate inconsistent evolution of the scalar moments. Further, gradient-based source terms that appear in the DQMOM transport equations are severely underpredicted in LES leading to artificial mixing of fuel and oxidizer. To overcome these numerical issues, a new approach called semi-discrete quadrature method of moments (SeQMOM) is formulated. The performance of the new technique is compared with the DQMOM approach in canonical flow configurations as well as a three-dimensional supersonic cavity stabilized flame configuration. The SeQMOM approach is shown to predict subfilter statistics accurately compared to the DQMOM approach. For soot modeling in turbulent flows, an LES/PDF approach is integrated with detailed models for soot formation and growth. The PDF approach directly evolves the joint statistics of the gas-phase scalars and a set of moments of the soot number density function. This LES/PDF approach is then used to simulate a turbulent natural gas flame. A Lagrangian method formulated in cylindrical coordinates solves the high dimensional PDF transport equation and is coupled to an Eulerian LES solver. The LES/PDF simulations show that soot formation is highly intermittent and is always restricted to the fuel-rich region of the flow. The PDF of soot moments has a wide spread leading to a large subfilter variance. Further, the conditional statistics of soot moments conditioned on mixture fraction and reaction progress variable show strong correlation between the gas phase composition and soot moments.
Subjects/Keywords Probability density function approach; Large eddy simulation; Supersonic combustion modeling; Soot modeling; Turbulent reacting flows; Direct quadrature method of moments; Semi-discrete quadrature method of moments; Quadrature based methods; Lagrangian Monte Carlo methods; Supersonic combustors; Flame stabilization; Polycyclic aromatic hydrocarbons; Soot-turbulence-chemistry interactions; Shock-turbulence-chemistry interactions
Contributors Raman, Venkat (advisor); Clemens, Noel (committee member); Ezekoye, Ofodike (committee member); Goldstein, David (committee member); Moser, Robert (committee member)
Language en
Country of Publication us
Record ID handle:2152/19481
Repository texas
Date Indexed 2020-10-15
Grantor The University of Texas at Austin
Note [] text; [department] Aerospace Engineering;

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…for the LES/PDF approach. The PDF approach for supersonic combustion modeling is reviewed in Chapter 3, and errors introduced by existing methods are discussed. A new approach called semi-discrete quadrature method of moments (SeQMOM) is then…

…obtain the solution to the PDF transport equation. The stochastic approach can be further divided into Lagrangian [42, 44, 45, 55] and Eulerian [52, 56] techniques. These methods are coupled to a deterministic LES solver in order to…

…1 2 4 6 8 8 . . . . . 10 10 11 14 14 17 Chapter 3. PDF approach for supersonic combustion modeling 3.1 Quadrature based moment methods . . . . . . . . . . . . . . . 3.1.1 Quadrature method of moments (QMOM) . . . . . . . . 3.1.2 Direct…

…quadrature method of moments (DQMOM) . . . 3.2 Numerical errors in DQMOM . . . . . . . . . . . . . . . . . . 3.3 Semi-discrete quadrature method of moments (SeQMOM) . . . 3.4 Numerical characteristics of the SeQMOM approach…

…Evolution of the second moment of a conserved scalar obtained using various methods after 0.1 flow through time. Note that the solution obtained from SeQMOM is very similar to that obtained from MOM. . . . . . . . . . . . . . . . . . . . . . . . Time…

…averaged contours of mixture fraction (top). Variance of mixture fraction obtained using various methods (bottom) is compared at (A) x/h=15, (B) x/h=30, and (C) x/h=43. Note that the variance prediction…

…chemistry, or in high Mach number flows, the joint-PDF of the thermochemical composition vector needs to be estimated in each computational cell by solving a PDF transport equation. The corresponding approach leads to LES/PDF 7 methods for turbulent…

…turbulence-chemistry interactions. While the LES/PDF approach is well suited for describing these subfilter correlations, existing methods for estimating the PDF were found to be inaccurate. Consequently, a novel approach for solving the PDF transport…