EAGER: Experimental investigation of physical-space scalar structure and unresolved mixing to improve large-eddy simulation of turbulent combustion

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This project investigates a key process, turbulent mixing, in combustion. Turbulent combustion occurs and plays an important role in many engineering applications, such as jet engines, gas turbine generators, internal combustion engines, and chemical processing plants. The modern design process of such devices needs to employ numerical modeling to evaluate design choices and to optimize design parameters. However, the current state-of-the art modeling approach, large-eddy simulation (LES), despite decades of research efforts, still does not have the accuracy needed for advanced engineering design, primarily because the unresolved mixing process is not well predicted. Previous efforts to understand mixing had been partially hampered by the predominant LES approach. This research will investigate mixing using the novel LES approach developed by S. B. Pope as the framework, which overcomes the limitations of the traditional approach. The new approach, however, also poses challenges in experimental techniques and data analysis methods. The researchers will employ a new laser diagnostic technique and new statistical analysis method. The research will significantly advance the understanding of the fundamental physics in turbulent combustion, which will potentially enable development of combustion models with sufficient accuracy for designing high-performance and low emission engineering devices, which will have a positive impact on society and the environment. The understanding of the physics gained will also benefit other research areas involving interactions between turbulence and strongly nonlinear phenomena, such as chemical engineering processes, atmospheric physics, atmospheric chemistry, and environment systems.This project investigates the physical-space structure and the unresolved mixing as well as their interaction with chemistry using the recently developed self-conditioned fields LES approach (Pope 2010) as the framework, which overcomes the limitations of the filtering LES approach. The researchers will conduct experiments in turbulent non-reactive coaxial jets and in jet flames. Two-dimensional images will be obtained in coaxial jets. A new photo-dissociation-based diagnostic technique and new statistical analysis method will be used. Direct numerical simulation of a Sydney flame (Thorsten et al. 2019), which contains 3D images, will complement the experiments. Researchers will analyze the physical-space scalar structure and its representation in the self-conditioned joint PDF and the unclosed mixing terms in its transport equation. The results provide a basis for investigating the mixing of mixture fraction and temperature in piloted turbulent partially premixed (Sandia) flames. The researchers will analyze the effects of the physical-space scalar structure on the self-conditioned scalar JMDF and the unclosed mixing terms. The project is the first to use the novel self-conditioned LES approach as the framework. It is also the first time true two-dimensional images of mixture fraction and temperature with resolved dissipation scales are obtained in turbulent hydrocarbon flames without major compromising assumptions. The results are expected to significantly advance the understanding of the unresolved physics essential for developing improved mixing models capable of accurately predicting multiscalar mixing and turbulence-chemistry interaction.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.