Manifold-Based Modeling of Turbulent Reacting Flows: Cool Flames and Multi-Modal Combustion

Abstract

Simulation tools are becoming progressively more important in industrial settings because of their ability to facilitate the design of increasingly efficient combustion systems. In particular, Large Eddy Simulation (LES) stands out as both an accurate and computationally affordable method to simulate quantities of interest in turbulent reacting flows. However, it requires models for unclosed combustion terms that are simultaneously physically accurate and computationally affordable. One traditional closure approach is that of a manifold-based model that restricts the thermochemical state to some lower-dimensional manifold based on physical assumptions. Because of the low dimensionality of the manifold space, such models are very computationally efficient. However, their generality suffers due to restrictive physical assumptions about the flame's structure. This dissertation seeks to generalize manifold-based turbulent combustion models.

First, the capability of traditional manifold-based models in describing turbulent cool flames is explored. A combined experimental and computational study of a turbulent nonpremixed jet cool flame lays the foundation for the study. Experimental measurements agree well with Direct Numerical Simulation (DNS) results, which in turn agree well with solutions to nonpremixed manifold equations, indicating that nonpremixed manifold-based models can describe turbulent cool flames. Additional DNS are performed in order to study the effect of the Damkoehler number on manifold-based model validity, which suggest that multi-modal combustion is relevant at low Damkoehler numbers. The states locally identified as nonpremixed are still found to be well described by a traditional manifold-based model, but describing all states requires a more general multi-modal model.

Such a multi-modal manifold-based combustion model is then developed using two-dimensional manifold equations. The need for this model is examined through multiple laminar flame configurations, and closure for the dissipation rates is achieved. A new In-Situ Adaptive Manifolds approach is developed to overcome computational tractability issues, and the model is applied to LES of a multi-modal turbulent lifted jet flame. The cost per timestep is comparable to traditional manifold-based models for a single asymptotic mode, indicating that this model generality comes at no additional computational overhead. Additionally, the model allows for simple combustion mode analysis through examination of the local dissipation rates and source terms.