Modeling and Simulation of Turbulent Reacting Flows


Modeling and Simulation of Turbulent Reacting Flows
Turbulent combustion is a spatially three-dimensional, highly transient, complex hydrochemical phenomena. It has been the subject of intense research over the past fifty years and continues to be of high priority in view of worldwide concerns about energy and pollution control. The inherent unsteadiness and limited spatial resolution has rendered experimental measurement of turbulent flames infeasible. And though experimental analysis of laminar flames have yielded valuable information, rarely are laminar flames realized in engineering combustion processes. In addition to new governing equations, the extension of computational fluid dynamics to combustion introduces new molecular properties, chemical reactions with different time scales, and multiphase dynamics amongst other complexities.
Figure 1: Instantaneous temperature contours in a turbulent diffusion-flame.	Recently, a new computational technique has been developed for simulating turbulent combustion. This approach separates the solution process into two parts-solution of the hydrodynamic field and solution of the scalar (chemical) field. The hydrodynamic field is obtained using conventional methodologies (i.e., finite differences/elements/volumes, spectral methods, etc.) to solve the conservation laws. The scalar field is obtained by using a stochastic method to solve the evolution of the joint probability density function of the scalars. (In the case of combustion, the scalars are the chemical species CH₄, O₂, OH, H₂O, etc.) This novel methodology utilizes Monte Carlo elements that represent realizations of the entire chemical state of the flow. This method has an advantage over existing approaches in that regardless of the complexity of the chemical reaction mechanism there are no associated modeling issues.
The resulting stochastic scheme is a robust, computationally affordable method for use in the numerical simulation of combustion. This method has proven extremely effective for a variety of turbulent reacting flows and has enormous potential for performing accurate simulations of turbulent combustion (see Figure 1). Currently, this methodology is being used by Supercomputing Institute Principal Investigator Professor Sean Garrick and his research group in the Mechanical Engineering Department at the University of Minnesota to perform simulations of non-premixed flames to study the evolution of the flame structure in turbulent reacting flows.

They are performing further simulations on confined flows. These simulations could yield valuable information that can help to elucidate the nature of flame-wall interactions. These are manifest in devices such as pulse detonation engines, where the nature of the governing equations is such that the behavior at the physical boundaries is not necessarily confined to that region. Disturbances originating at the boundaries may quickly saturate the entire flow domain. The resulting interactions are of great consequence when studying phenomenon such as pollutant emissions and combustion stability. Future work includes modeling the formation and growth of particles (from gas phase reactants to nanoscale particulates) in combustion environments and the control of soot production.	Figure 2: Vorticity flame-surface interaction in a round jet.

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