Matching Items (3)
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Description
Next generation gas turbines will be required to produce low concentrations of pollutants such as oxides of nitrogen (NOx), carbon monoxide (CO), and soot. In order to design gas turbines which produce lower emissions it is essential to have computational tools to help designers. Over the past few decades, computational

Next generation gas turbines will be required to produce low concentrations of pollutants such as oxides of nitrogen (NOx), carbon monoxide (CO), and soot. In order to design gas turbines which produce lower emissions it is essential to have computational tools to help designers. Over the past few decades, computational fluid dynamics (CFD) has played a key role in the design of turbomachinary and will be heavily relied upon for the design of future components. In order to design components with the least amount of experimental rig testing, the ensemble of submodels used in simulations must be known to accurately predict the component's performance. The present work aims to validate a CFD model used for a reverse flow, rich-burn, quick quench, lean-burn combustor being developed at Honeywell. Initially, simulations are performed to establish a baseline which will help to assess impact to combustor performance made by changing CFD models. Rig test data from Honeywell is compared to these baseline simulation results. Reynolds averaged Navier-Stokes (RANS) and Large Eddy Simulation (LES) turbulence models are both used with the presumption that the LES turbulence model will better predict combustor performance. One specific model, the fuel spray model, is evaluated next. Experimental data of the fuel spray in an isolated environment is used to evaluate models for the fuel spray and a new, simpler approach for inputting the spray boundary conditions (BC) in the combustor is developed. The combustor is simulated once more to evaluate changes from the new fuel spray boundary conditions. This CFD model is then used in a predictive simulation of eight other combustor configurations. All computer simulations in this work were preformed with the commercial CFD software ANSYS FLUENT. NOx pollutant emissions are predicted reasonably well across the range of configurations tested using the RANS turbulence model. However, in LES, significant under predictions are seen. Causes of the under prediction in NOx concentrations are investigated. Temperature metrics at the exit of the combustor, however, are seen to be better predicted with LES.
ContributorsSpencer, A. Jeffrey (Author) / Herrmann, Marcus (Thesis advisor) / Chen, Kangping (Committee member) / Adrian, Ronald (Committee member) / Arizona State University (Publisher)
Created2012
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Description
This dissertation develops and demonstrates a new physics-based approach that provides computational stability of subgrid stress models in large eddy simulations while producing far smaller changes in the original subgrid stress and subgrid production fields than do current \textit{ad hoc} stabilization methods. A pseudo-spectral code that is shown here

This dissertation develops and demonstrates a new physics-based approach that provides computational stability of subgrid stress models in large eddy simulations while producing far smaller changes in the original subgrid stress and subgrid production fields than do current \textit{ad hoc} stabilization methods. A pseudo-spectral code that is shown here to be almost entirely non-dissipative yet inherently stable without any subgrid model is used to conduct simulations with stable and unstable subgrid stress models. Results show that initial instability, subsequent exponential growth, and eventual machine overflow occur via a highly localized dynamical process that results from interactions among terms in the kinetic energy and enstrophy transport equations. This process begins first at one material point and then occurs at increasingly more material points, with local exponential growth rates of kinetic energy and enstrophy being the same for all points, until machine overflow eventually occurs at the material point where the process began first. A Lagrangian backtracking scheme is developed and applied to this material point, allowing backward-in-time tracking of all terms in the kinetic energy and enstrophy transport equations. This gives insights into the dynamics that produce this local instability and its subsequent exponential growth, with the initial instability shown to result from interactions between the subgrid production and subgrid redistribution terms. Elementary backscatter limiting based on locally reducing individual subgrid stress components that contribute to local kinetic energy backscatter is shown to stabilize any stress model, but still produces substantial changes in the stress and production fields. The rational Boolean stabilization method developed here instead uses the local subgrid production and subgrid redistribution rates to determine where and how individual subgrid stress components must be rescaled to provide local backscatter limiting and/or forward scatter amplification. This stabilizes all subgrid stress models while producing only small changes in the subgrid stress and production fields. Rational Boolean stabilization is computationally fast, and can be generalized to stabilize models for other subgrid terms in large eddy simulations while producing only small changes in their resulting fields. This solves a key problem that has previously limited the accuracy of large eddy simulations.
ContributorsTorres, Emilio Elijah (Author) / Dahm, Werner JA (Thesis advisor) / Calhoun, Ronald J (Committee member) / Herrmann, Marcus A (Committee member) / Kasbaoui, Mohamed H (Committee member) / Mikellides, Pavlos G (Committee member) / Arizona State University (Publisher)
Created2022
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Description
Advancements to a dual scale Large Eddy Simulation (LES) modeling approach for immiscible turbulent phase interfaces are presented. In the dual scale LES approach, a high resolution auxiliary grid, used to capture a fully resolved interface geometry realization, is linked to an LES grid that solves the filtered Navier-Stokes equations.

Advancements to a dual scale Large Eddy Simulation (LES) modeling approach for immiscible turbulent phase interfaces are presented. In the dual scale LES approach, a high resolution auxiliary grid, used to capture a fully resolved interface geometry realization, is linked to an LES grid that solves the filtered Navier-Stokes equations. Exact closure of the sub-filter interface terms is provided by explicitly filtering the fully resolved quantities from the auxiliary grid. Reconstructing a fully resolved velocity field to advance the phase interface requires modeling several sub-filter effects, including shear and accelerational instabilities and phase change. Two sub-filter models were developed to generate these sub-filter hydrodynamic instabilities: an Orr-Sommerfeld model and a Volume-of-Fluid (VoF) vortex sheet method. The Orr-Sommerfeld sub-filter model was found to be incompatible with the dual scale approach, since it is unable to generate interface rollup and a process to separate filtered and sub-filter scales could not be established. A novel VoF vortex sheet method was therefore proposed, since prior vortex methods have demonstrated interface rollup and following the LES methodology, the vortex sheet strength could be decomposed into its filtered and sub-filter components. In the development of the VoF vortex sheet method, it was tested with a variety of classical hydrodynamic instability problems, compared against prior work and linear theory, and verified using Direct Numerical Simulations (DNS). An LES consistent approach to coupling the VoF vortex sheet with the LES filtered equations is presented and compared against DNS. Finally, a sub-filter phase change model is proposed and assessed in the dual scale LES framework with an evaporating interface subjected to decaying homogeneous isotropic turbulence. Results are compared against DNS and the interplay between surface tension forces and evaporation are discussed.
ContributorsGoodrich, Austin Chase (Author) / Herrmann, Marcus (Thesis advisor) / Dahm, Werner (Committee member) / Kim, Jeonglae (Committee member) / Huang, Huei-Ping (Committee member) / Kostelich, Eric (Committee member) / Arizona State University (Publisher)
Created2023