Direct Numerical Simulations of Turbulent Combustion: Fundamental Insights Towards Predictive Models

Evatt R. Hawkes, Ramanan Sankaran, James C. Sutherland, Joseph C. Oefelein, and Jacqueline H. Chen
Sandia National Laboratories

The advancement of our basic understanding of turbulent combustion processes and the development of physics-based predictive tools for design and optimization of the next generation of combustion devices are strategic areas of research for the development of a secure, environmentally sound energy infrastructure. In recent years, the rapid advance of computational capabilities has presented significant opportunities for high-fidelity simulations of turbulent combustion flows. Realistic simulations that address complex multi-physics interactions have become accessible through the growth of processor speed, computer memory and storage, and significant improvements in computational algorithms and chemical models. In direct numerical simulation (DNS) approaches, all scales of the reacting flow problem are resolved. However, because of the magnitude of this task, DNS of practical high Reynolds number turbulent hydrocarbon flames is out of reach of even terascale computing. For the foreseeable future, the approach to this complex multi-scale problem is to employ distinct but synergistic approaches to tackle smaller sub-ranges of the complete multi-scale problem. In Reynolds-Averaged-Navier-Stokes (RANS) and Large Eddy Simulation (LES) approaches, models are required for unresolved scales. With full access to the spatially and temporally resolved fields, DNS can play a major role in the development of these models and in the development of fundamental understanding of the micro-physics of turbulence-chemistry interactions. Two examples, performed at terascale Office of Science computing facilities, are presented to illustrate the role of DNS in delivering new insights to advance the predictive capability of models. In the first example, simulations of stratified auto-ignition are used to provide new fundamental understanding of the combustion modes and modeling of a new clean and efficient engine concept called homogeneous charge compression ignition (HCCI). In the second example, results are presented from new three-dimensional DNS with detailed chemistry of turbulent non-premixed jet flames revealing the differences between mixing of passive and reacting scalars.