LES of Reacting Mixing Layers: Species Concentration Boundedness and Inflow Conditions

The present work carries out large-eddy simulations of the low-speed, high-Reynolds number, chemically-reacting mixing layer experiments by Slessor et. al. In particular, we study the low-heat release case with prescribed turbulent inflow conditions. The objective of the present work is to gain insight into the physics of the reacting shear layer and to address some associated computational challenges. This set of experiments are at subsonic conditions and use hydrogen and fluorine as the fuel and oxidizer, respectively. The hypergolic reaction between H_2 and F_2, as it was run in the Slessor et al. experiments, is characterized by a large Damköhler number, making the chemistry fast compared to the flow time scales: the product formation and temperature-rise in the flow is mixing-limited. In this work, we attempt to address the issue of overshoots and undershoots of species mass-frictions, often observed in LES of high-Reynolds number flows, by modifying the convective fluxes. We observe that the modified fluxes eliminate the global excursions of species mass-fraction concentration. A three dimensional simulation is performed by imposing synthetic turbulence at the inflow, generated using the digital filter approach of Klein et al., to mimic the experimental flow conditions. The velocity profiles, growth rate, and product thickness obtained from the simulations show a good match with the experimental data, but the peak value of temperature-rise is slightly over predicted

Large-Eddy Simulation of Autoignition-Dominated Supersonic Combustion

The simulation of low-speed combustion flows is well established. However, at high-speed conditions where radical formation and ignition delay are important, there is much less experience with turbulent combustion modeling. In the present work, a novel evolution variable manifold (EVM) approach of Cymbalist and Dimotakis is implemented in a production CFO code and preliminary RANS and large-eddy simulations are computed for a hydrogen combustion test case. The EVM approach solves a scalar conservation equation for the induction time to represent ignition delay. The state or the combustion products is tabulated as a function of density, energy, mixture fraction, and the evolution variable. A thermodynamically-consistent numerical flux function is developed and the approach for coupling the EVM table to CFD is discussed. Initial simulations show that the EVM approach produces good agreement with full chemical kinetics and model simulations. Work remains to be done to improve the numerical stability, extend the grid, and increase the order or accuracy of the simulations

LES of Reacting Mixing Layers: Species Concentration Boundedness and Inflow Conditions

The present work carries out large-eddy simulations of the low-speed, high-Reynolds number, chemically-reacting mixing layer experiments by Slessor et. al. In particular, we study the low-heat release case with prescribed turbulent inflow conditions. The objective of the present work is to gain insight into the physics of the reacting shear layer and to address some associated computational challenges. This set of experiments are at subsonic conditions and use hydrogen and fluorine as the fuel and oxidizer, respectively. The hypergolic reaction between H_2 and F_2, as it was run in the Slessor et al. experiments, is characterized by a large Damköhler number, making the chemistry fast compared to the flow time scales: the product formation and temperature-rise in the flow is mixing-limited. In this work, we attempt to address the issue of overshoots and undershoots of species mass-frictions, often observed in LES of high-Reynolds number flows, by modifying the convective fluxes. We observe that the modified fluxes eliminate the global excursions of species mass-fraction concentration. A three dimensional simulation is performed by imposing synthetic turbulence at the inflow, generated using the digital filter approach of Klein et al., to mimic the experimental flow conditions. The velocity profiles, growth rate, and product thickness obtained from the simulations show a good match with the experimental data, but the peak value of temperature-rise is slightly over predicted

Large-Eddy Simulation of Autoignition-Dominated Supersonic Combustion

The simulation of low-speed combustion flows is well established. However, at high-speed conditions where radical formation and ignition delay are important, there is much less experience with turbulent combustion modeling. In the present work, a novel evolution variable manifold (EVM) approach of Cymbalist and Dimotakis is implemented in a production CFO code and preliminary RANS and large-eddy simulations are computed for a hydrogen combustion test case. The EVM approach solves a scalar conservation equation for the induction time to represent ignition delay. The state or the combustion products is tabulated as a function of density, energy, mixture fraction, and the evolution variable. A thermodynamically-consistent numerical flux function is developed and the approach for coupling the EVM table to CFD is discussed. Initial simulations show that the EVM approach produces good agreement with full chemical kinetics and model simulations. Work remains to be done to improve the numerical stability, extend the grid, and increase the order or accuracy of the simulations

Development of the US3D Code for Advanced Compressible and Reacting Flow Simulations

Aerothermodynamics and hypersonic flows involve complex multi-disciplinary physics, including finite-rate gas-phase kinetics, finite-rate internal energy relaxation, gas-surface interactions with finite-rate oxidation and sublimation, transition to turbulence, large-scale unsteadiness, shock-boundary layer interactions, fluid-structure interactions, and thermal protection system ablation and thermal response. Many of the flows have a large range of length and time scales, requiring large computational grids, implicit time integration, and large solution run times. The University of Minnesota NASA US3D code was designed for the simulation of these complex, highly-coupled flows. It has many of the features of the well-established DPLR code, but uses unstructured grids and has many advanced numerical capabilities and physical models for multi-physics problems. The main capabilities of the code are described, the physical modeling approaches are discussed, the different types of numerical flux functions and time integration approaches are outlined, and the parallelization strategy is overviewed. Comparisons between US3D and the NASA DPLR code are presented, and several advanced simulations are presented to illustrate some of novel features of the code

Large-Eddy Simulation of Supersonic Reacting Mixing Layers

We study a class of chemically reacting, spatially evolving, supersonic mixing layers via large eddy simulation. Specifically, the goal is to reproduce the experimental results on molecular mixing and heat release performed at Caltech by Bonanos et al. Here, the mixing layer is formed as a result of the interaction of supersonic and subsonic streams: the supersonic stream expands over a 30° perforated ramp and interacts with a subsonic stream of fluid injected into the combustor through the ramp. The primary (top, supersonic) stream contains a small amount of H_2 as the fuel. The secondary stream (injected through the ramp) contains a fractional amount of F_2 which acts as the oxidizer. The hypergolic reaction between hydrogen and fluorine is characterized by a large Damköhler number, making the chemistry fast compared with the flow time scales. Hence, the product formation and temperature-rise in the flow is mixing limited. Both reacting and non-reacting simulations are performed with two turbulence models (Smagorinsky and Vreman) and the comparisons are made with the available experimental data. The reconstructed species concentrations, used in the flux evaluation, are limited using ideas from a recent paper by Zhang and Shu in order to ensure boundedness for these quantities. The simulations show close agreement of the velocity profiles and the temperature-rise profiles to those measured in the experiment

Large-Eddy Simulation of Supersonic Reacting Mixing Layers

We study a class of chemically reacting, spatially evolving, supersonic mixing layers via large eddy simulation. Specifically, the goal is to reproduce the experimental results on molecular mixing and heat release performed at Caltech by Bonanos et al. Here, the mixing layer is formed as a result of the interaction of supersonic and subsonic streams: the supersonic stream expands over a 30° perforated ramp and interacts with a subsonic stream of fluid injected into the combustor through the ramp. The primary (top, supersonic) stream contains a small amount of H_2 as the fuel. The secondary stream (injected through the ramp) contains a fractional amount of F_2 which acts as the oxidizer. The hypergolic reaction between hydrogen and fluorine is characterized by a large Damköhler number, making the chemistry fast compared with the flow time scales. Hence, the product formation and temperature-rise in the flow is mixing limited. Both reacting and non-reacting simulations are performed with two turbulence models (Smagorinsky and Vreman) and the comparisons are made with the available experimental data. The reconstructed species concentrations, used in the flux evaluation, are limited using ideas from a recent paper by Zhang and Shu in order to ensure boundedness for these quantities. The simulations show close agreement of the velocity profiles and the temperature-rise profiles to those measured in the experiment

LES of a high-Reynolds number, chemically reacting mixing layer

In this work we perform large eddy simulations of high-Reynolds number, chemically reacting, spatially developing mixing layers with the goal of reproducing the experimental results obtained by Slessor at al. The chemical mechanism is the reaction of hydrogen and fluorine to produce HF. This is an exothermic, kinetically-fast reaction (Da » 1) in which the heat release (and the consequent temperature rise) is a direct measure of the product formation. The upper stream has a velocity of U_1 = 100 m/s and it is composed of a mixture of H_2 and inert gases, while the bottom stream has a lower velocity, U_2 = 40 m/s, and carries F_2 diluted in inert gases. Both streams have the same density. The mixing layer develops from a splitter plate and is characterized by a fairly large Reynolds number (Re_(δT) = 2·10^5 ). Although we do not explicitly model the boundary layers developing on the splitter plate, we impose laminar boundary-layer profiles at the inflow consistent with those reported in Slessor et al. The three-dimensional simulations show an excellent agreement with the experiments for the mean velocity, although some discrepancies are found in the temperature/product formation profiles. LES results tend to overestimate the molecular mixing in the flow: In the very high Damköhler regime this results In an overprediction of product formation and temperature rise. We study these issues by conducting some two-dimensional simulations using the Filtered Mass Density Function methodology which alleviates this problem. We compute the probability density functions of the mixture fraction as a function of the transverse coordinate and we confirm that the most probable mixture fraction in the layer is the one predicted by the asymmetric entrainment ratio model. In particular, about ~30% more mass is entrained into the layer from the high-speed stream as compared to the lower stream

LES of a high-Reynolds number, chemically reacting mixing layer

In this work we perform large eddy simulations of high-Reynolds number, chemically reacting, spatially developing mixing layers with the goal of reproducing the experimental results obtained by Slessor at al. The chemical mechanism is the reaction of hydrogen and fluorine to produce HF. This is an exothermic, kinetically-fast reaction (Da » 1) in which the heat release (and the consequent temperature rise) is a direct measure of the product formation. The upper stream has a velocity of U_1 = 100 m/s and it is composed of a mixture of H_2 and inert gases, while the bottom stream has a lower velocity, U_2 = 40 m/s, and carries F_2 diluted in inert gases. Both streams have the same density. The mixing layer develops from a splitter plate and is characterized by a fairly large Reynolds number (Re_(δT) = 2·10^5 ). Although we do not explicitly model the boundary layers developing on the splitter plate, we impose laminar boundary-layer profiles at the inflow consistent with those reported in Slessor et al. The three-dimensional simulations show an excellent agreement with the experiments for the mean velocity, although some discrepancies are found in the temperature/product formation profiles. LES results tend to overestimate the molecular mixing in the flow: In the very high Damköhler regime this results In an overprediction of product formation and temperature rise. We study these issues by conducting some two-dimensional simulations using the Filtered Mass Density Function methodology which alleviates this problem. We compute the probability density functions of the mixture fraction as a function of the transverse coordinate and we confirm that the most probable mixture fraction in the layer is the one predicted by the asymmetric entrainment ratio model. In particular, about ~30% more mass is entrained into the layer from the high-speed stream as compared to the lower stream