Direct Quadrature Conditional Moment Closure for Turbulent Non-Premixed Combustion
Abstract
The accurate description of the turbulence chemistry interactions that can determine chemical conversion rates and flame stability in turbulent combustion modelling is a challenging research
area. This thesis presents the development and implementation of a model for the treatment of fluctuations around the conditional mean (i.e., the auto-ignition and extinction phenomenon) of
realistic turbulence-chemistry interactions in computational fluid dynamics (CFD) software. The
wider objective is to apply the model to advanced combustion modelling and extend the present analysis to larger hydrocarbon fuels and particularly focus on the ability of the model to capture
the effects of particulate formation such as soot.
A comprehensive approach for modelling of turbulent combustion is developed in this work. A direct quadrature conditional moment closure (DQCMC) method for the treatment of realistic turbulence-chemistry interactions in computational fluid dynamics (CFD) software is described. The method which is based on the direct quadrature method of moments (DQMOM) coupled with the Conditional Moment Closure (CMC) equations is in simplified form and easily
implementable in existing CMC formulation for CFD code. The observed fluctuations of scalar dissipation around the conditional mean values are captured by the treatment of a set of mixing environments, each with its pre-defined weight. In the DQCMC method the resulting equations
are similar to that of the first-order CMC, and the “diffusion in the mixture fraction space” term
is strictly positive and no correction factors are used. Results have been presented for two mixing environments, where the resulting matrices of the DQCMC can be inverted analytically.
Initially the DQCMC is tested for a simple hydrogen flame using a multi species chemical scheme containing nine species. The effects of the fluctuations around the conditional means are
captured qualitatively and the predicted results are in very good agreement with observed trends from direct numerical simulations (DNS). To extend the analysis further and validate the model
for larger hydrocarbon fuel, the simulations have been performed for n-heptane flame using detailed multi species chemical scheme containing 67 species. The hydrocarbon fuel showed improved results in comparison to the simple hydrogen flame. It suggests that higher
hydrocarbons are more sensitive to local scalar dissipation rate and the fluctuations around the
conditional means than the hydrogen. Finally, the DQCMC is coupled with a semi-empirical soot
model to study the effects of particulate formation such as soot. The modelling results show to
predict qualitatively the trends from DNS and are in very good agreement with available
experimental data from a shock tube concerning ignition delays time. Furthermore, the findings
suggest that the DQCMC approach is a promising framework for soot modelling.
Authors
Ali, ShaukatCollections
- Theses [3321]