The subgrid-scale scalar variance under supercritical pressure conditions

To model the subgrid-scale (SGS) scalar variance under supercritical-pressure conditions, an equation is first derived for it. This equation is considerably more complex than its equivalent for atmospheric-pressure conditions. Using a previously created direct numerical simulation (DNS) database of transitional states obtained for binary-species systems in the context of temporal mixing layers, the activity of terms in this equation is evaluated, and it is found that some of these new terms have magnitude comparable to that of governing terms in the classical equation. Most prominent among these new terms are those expressing the variation of diffusivity with thermodynamic variables and Soret terms having dissipative effects. Since models are not available for these new terms that would enable solving the SGS scalar variance equation, the adopted strategy is to directly model the SGS scalar variance. Two models are investigated for this quantity, both developed in the context of compressible flows. The first one is based on an approximate deconvolution approach and the second one is a gradient-like model which relies on a dynamic procedure using the Leonard term expansion. Both models are successful in reproducing the SGS scalar variance extracted from the filtered DNS database, and moreover, when used in the framework of a probability density function (PDF) approach in conjunction with the β-PDF, they excellently reproduce a filtered quantity which is a function of the scalar. For the dynamic model, the proportionality coefficient spans a small range of values through the layer cross-stream coordinate, boding well for the stability of large eddy simulations using this model

Large Eddy Simulations (LES) and Direct Numerical Simulations (DNS) for the computational analyses of high speed reacting flows

The principal objective is to extend the boundaries within which large eddy simulations (LES) and direct numerical simulations (DNS) can be applied in computational analyses of high speed reacting flows. A summary of work accomplished during the last six months is presented

Large eddy simulations and direct numerical simulations of high speed turbulent reacting flows

The main objective is to extend the boundaries within which large eddy simulations (LES) and direct numerical simulations (DNS) can be applied in computational analyses of high speed reacting flows. In the efforts related to LES, we were concerned with developing reliable subgrid closures for modeling of the fluctuation correlations of scalar quantities in reacting turbulent flows. In the work on DNS, we focused our attention to further investigation of the effects of exothermicity in compressible turbulent flows. In our previous work, in the first year of this research, we have considered only 'simple' flows. Currently, we are in the process of extending our analyses for the purpose of modeling more practical flows of current interest at LaRC. A summary of our accomplishments during the third six months of the research is presented

Assessment of LES sub-grid models for turbulent reacting flows

Imperial Users onl

Mixing and non-premixed combustion at supercritical pressures

This thesis is devoted to the numerical investigation of mixing and non- premixed combustion of cryogenic propellants at supercritical pressures. These severe conditions are commonly encountered in high pressure combustion chambers, such as those of liquid-fueled rocket engines (LRE), and lead to significant deviations from the ideal gas thermodynamic behavior of the reacting mixtures. The non-premixed laminar flame structure of liquid oxygen (LOx) and methane or liquid natural gas (LNG) mixtures, a recently proposed LRE propellants com- bination, is investigated by means of a general fluid unsteady flamelet solver. Real gas effects are analyzed on prototypical unsteady flame phenomena such as autoignition and re-ignition/quenching caused by strain perturbations. Such effects influence different flame regions depending on pressure, as well as the critical strain values that a laminar flame can sustain before quenching occurs. Moreover the flame structure is also influenced by the composition of the LNG, in particular the early stage soot precursors production and oxidation. In order to shed light on real gas mixing, a low-Mach approximation for real gas reacting mixtures is presented. A single species non-reacting real gas model is implemented in a highly scalable spectral element computational fluid dynamic (CFD) code with state of the art thermodynamic and transport properties. Transcritical and supercritical planar temporal jets, are chosen as representative test cases for investigating high-pressure mixing by means of direct numerical simulations. The pseudo-boiling phenomenon, occurring in transcritical flows, significantly influences the jet development, mitigating the development of shear layer instabilities and leading to a liquid-like jet break-up. Moreover pseudo-boiling is confined in a narrow spatial region suggesting particular care in the turbulent combustion modeling of non-premixed flames when transcritical thermodynamic conditions are encountered. The results of the present thesis, its physical insights as well as the modeling considerations involved, can be of support in the development of future CFD tools capable of simulating real engine operative conditions and configurations

Doctor of Philosophy

dissertationThis dissertation presents the development and validation of a variant of the One Dimensional Turbulence model (ODT) in an Eulerian reference frame. The ODT model solves unfiltered governing equations in one spatial dimension with a stochastic model for turbulence. The stand-alone ODT model implemented for this work resolves the full range of length and time scales associated with the flow, in 1D, with detailed chemistry, thermodynamics and transport in the gas phase. The model is first applied to a planar nonpremixed turbulent jet flame and results from the model prediction are compared with DNS data. Results indicate that the model accurately reproduces the DNS data set. Turbulence-chemistry interactions, including trends for extinction and reignition, are captured by the model. Differences observed between model prediction and data are the result of early excess extinction observed in the model. The reasons for the early extinction are discussed within the model context. A parameter sensitivity is also done for the current model. Simulations are performed over a range of jet Reynolds numbers for reacting and nonreacting configurations. Results from the simulations are compared with DNS and experimental data for reacting and nonreacting cases, respectively. Based on the identified sensitivity an empirical correlation is proposed and conclusions are drawn about the parameter estimation. The model is also applied to a planar premixed turbulent jet flame and results from the ODT simulations are compared with DNS data. Two different Da cases are considered in the study and comparisons between the model and DNS data in physical space are shown. Results indicate that the model qualitatively reproduces the DNS data set. Mixing is well captured by the model and the quantitative differences observed between model and data for thermochemistry are due to the curvature effects in the data. The reasons for the differences observed are discussed within the model context

An evaluation of the one-dimensional turbulence model: comparison with direct numerical simulation of CO/H2 jets with extinction and reignition

Journal ArticleAbstract A variant of the One-Dimensional Turbulence (ODT) model formulated in an Eulerian reference frame is applied to a planar nonpre mixed turbulent jet flame and results from the model prediction are compared with DNS data. The model employed herein solves the full set of conservation equations for mass, momentum, energy, and species on a one-dimensional domain corresponding to the transverse jet direction. The effects of turbulent mixing are modeled via a stochastic process, while the full range of diffusive-reactive length and time scales are resolved directly on the one-dimensional domain. A detailed chemical mechanism consisting of 11 species and 21 reactions and mixture averaged transport is used in this study (consistent with DNS simulations). Comparisons between the model and DNS data in physical and state space are shown, including conditional statistics. Results indicate that the model accurately reproduces the DNS data set. Turbulence-chemistry interactions, including trends for extinction and re-ignition, are captured by the model. Differences observed between model prediction and data are the result of early excess extinction observed in the model. The reasons for the early extinction are discussed within the model context

LES, DNS and RANS for the analysis of high-speed turbulent reacting flows

The purpose of this research is to continue our efforts in advancing the state of knowledge in large eddy simulation (LES), direct numerical simulation (DNS), and Reynolds averaged Navier Stokes (RANS) methods for the computational analysis of high-speed reacting turbulent flows. In the second phase of this work, covering the period 1 Sep. 1993 - 1 Sep. 1994, we have focused our efforts on two research problems: (1) developments of 'algebraic' moment closures for statistical descriptions of nonpremixed reacting systems, and (2) assessments of the Dirichlet frequency in presumed scalar probability density function (PDF) methods in stochastic description of turbulent reacting flows. This report provides a complete description of our efforts during this past year as supported by the NASA Langley Research Center under Grant NAG1-1122