Browsing by Author "Siwaborworn, Papakorn"

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    Conservative LES-CMC modelling for turbulent jet flames
    (2013) Siwaborworn, Papakorn; Kronenburg, Andreas (Prof. Dr.)
    The objective of the present study is to analyze turbulent non-premixed flames by utilizing a combined large eddy simulation - conditional moment closure (LES-CMC) method. LES is based on spatial filtering, and it resolves large scales of turbulent motion while modelling the small turbulent structures using a subgrid model, here the Smagorinsky model. CMC is a conserved scalar method where fluctuations of the reactive scalar variables can be associated with fluctuations of the mixture fraction. Therefore, CMC is applied to turbulent combustion modelling in this work using mixture fraction as the conditioning variable. In the last decade, computations using a non-conservative LES-CMC formulation have provided good predictions of major and minor species for different flames. However, inaccurate predictions occur in CMC cells which have large temporal variations of the mixture fraction field. A lack of weighting the convective term by a filtered density function (FDF) ratio in non-conservative CMC is believed to be a major reason for these inaccurate predictions. In contrast to non-conservative LES-CMC, the present conservative formulation is inherently mass conserving. It considers weighting the convective term by an FDF ratio so that improved predictions of local conditional scalars can be obtained. Investigations of turbulent jet flames (Sandia Flames D, E and F) are performed using the conservative LES-CMC approach. Flame D is used as the first test case to validate the numerical results in comparison with well-established experimental data. A study of the flow and mixing parameters is carried out first to establish the parameters for Flames E and F. Results from these studies show that the optimal values of Schmidt number, Sc, and turbulent Schmidt number, Sct are 0.7 and 0.4, respectively. The appropriate value of the subgrid-scale variance modelling constant is 0.2. A sensitivity analysis of the results demonstrates that inflow velocity variance levels corresponding to 2/3 , 1/3 and 2/9 of the measured variances at z/D = 0.14 are suitable inflow conditions for Flames D, E and F, respectively. Subsequently, parametric studies of the combustion model are performed for all test cases. The statistical predictions of scalars compared with measurements show that the LES with the conservative CMC formulation is better than the one based on non-conservative equations. However, similar predictions are obtained from two different flux approximation methods (computing the CMC convective fluxes based on the LES cells located at the CMC faces or based on the CMC cell centres). A parametric study of the CMC grid resolution shows that a resolution of 8*8*80 cells in x-, y- and z-directions yields appropriate predictions within a reasonable computational time. Simulation results of Flames E and F show that the CMC simulations presented here cannot capture local extinction and re-ignition accurately. This is partially due to the averaging effect of the large CMC cells on the modelled conditional dissipation. Much finer CMC cells of the order of the LES cell size will capture more of the fluctuation of scalar dissipation rates and may lead to a more accurate prediction of the local extinction events. The parametric study of CMC grid resolution for Flames E and F shows that a finer CMC grid (16*16*80) predicts slightly better results than the reference grid (8*8*80), but predictions could still be improved. It is possible that the problem is associated with the accuracy of the chemical source term. Hence, some possible solutions, such as second-order closure and doubly conditional reaction source terms, are suggested for future works.