MAE PhD Defense – Joseph Scroggins

Title: On Modeling Lifted Jet Flames with the RIF-ist Framework

Advisor: Kevin Lyons

Date and Time: Friday, March 15, 2019 at 3:30 PM

Location: EB3 – 3115

Abstract: Turbulent non-premixed combustion powers many practical engineering devices, but predictive combustion modeling remains challenging and computationally intensive. Tabulated chemistry models have significantly reduced computational cost for CFD simulations and have been used extensively for modeling both premixed and non-premixed combustion with both RANS and LES turbulence models. Despite wide-spread adoption in industry, tabulated models are limited by simplifying assumptions, such as being tabulated at steady scalar dissipation rates and thus neglecting history effects that can be important in modeling pollutant formation. Further, tabulated approaches are constrained by memory when many manifolds are required. The multiple Representative Interactive Flamelet approach, where the unsteady flamelet equations are solved online and integrated with a presumed PDF at each time step at each computational cell, captures history effects and is not constrained by dimensionality. The traditional RIF approach however is computationally costly and becomes increasingly more so for large LES grids. A recently developed formulation of the RIF model, the RIF in situ tabulation (RIF-ist) model, combines the advantages of online flamelet calculations and the speed of tabulated models by replacing the costly PDF integration at each computational cell with a 2D interpolation of a mixture fraction and mixture fraction variance table generated at each time step for each flamelet. The RIF-ist model has been demonstrated to perform similarly to the traditional RIF method for predicting flame liftoff length and ignition time for ECN Spray A at significantly reduced computational cost. However, quantitative validation, such as prediction of temperature and species, was not assessed. The focus of the present work is to extend the RIF-ist model to gaseous jet flames and validate the framework using comprehensive species data sets. The model is first used to simulate the Cabra vitiated coflow burner, which has been used to examine two autoigniting flames: a nitrogen-diluted hydrogen flame; and an air-diluted methane flame. The methane-air flame species and temperature results are well-predicted along the centerline as well at the downstream radial measurements, with predictions comparing favorably with other flamelet models. The hydrogen-nitrogen flame is found to be subject to large oscillations due to oscillating heat release in the domain. Heat release oscillations are common in RIF simulations, with a faster flamelet injection rate leading to reduced oscillations. Limitations of the current implementation are discussed. The RIF-ist framework is then extended to transitional/MILD n-heptane flames. Using both RANS and LES, the framework is shown to capture flame structure and liftoff trends. The present work thus validates the model for a range of conditions. Potential model updates to extend the applicability of the model are also discussed.

Biography: Joe attended Georgia Tech and graduated with a BS in Mechanical Engineering. Following graduation, he attended North Carolina State University, where he received an MS in Aerospace Engineering and continued on toward a PhD. After the departure of his original advisor, Joe took a position in the propulsion industry, where he has worked on thermal/fluid problems with gas turbines, solid rockets, scramjets, launch vehicles, and re-entry vehicles. He has continued to pursue his PhD part-time. He is currently the Lead for the Thermal Environments group at Northrop Grumman Innovation Systems Launch Vehicles business unit.