Multi-dimensional and transient effects on flamelet modeling for turbulent pulverized coal combustion
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Fuel (2019)

The transport along mixture fraction iso-surfaces is generally denoted as multi-dimensional effect, which is not considered in the classical one-dimensional (1D) flamelet model. This work investigates multi-dimensional and transient effects on flamelet modeling for turbulent pulverized coal combustion. To this end, instantaneous flamelets are extracted from a fully resolved 3D DNS database generated for a turbulent pulverized coal flame with a 33 array of particles. At first, the importance of transport along the iso-mixture fraction surface is analyzed based on the flamelet regime diagram. By this means, the flamelet regime of the local combustion is clarified. Then, the relevance of transient effects is investigated using budget analyses for transient terms in both physical space and composition space. Particularly, the significance of the Eulerian and Lagrangian transient terms is quantified. It is found that compared to the other transient processes, the transient change in the inner flame structure, i.e., the Lagrangian transient process, is dominant. Interestingly, the budget analyses show that the Lagrangian transient term can be partially recovered by the steady flamelet model. The findings obtained from the flamelet regime diagram and the budget analyses are consistent with the flamelet solutions where the multi-dimensional and transient terms are included in/excluded from the flamelet equations. The generalized flamelet equations including both multi-dimensional and transient terms can effectively reproduce the instantaneous flamelets in the DNS. Finally, a priori analyses are conducted to evaluate the multi-dimensional and transient effects on the final performance of the flamelet model. Overall, the a priori results are consistent with findings obtained from the budget analyses.
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