Numerical framework for transcritical real-fluid reacting flow simulations using the flamelet progress variable approach

An extension to the classical FPV model is developed for transcritical real-fluid combustion simulations in the context of finite volume, fully compressible, explicit solvers. A double-flux model is developed for transcritical flows to eliminate the spurious pressure oscillations. A hybrid scheme with entropy-stable flux correction is formulated to robustly represent large density ratios. The thermodynamics for ideal-gas values is modeled by a linearized specific heat ratio model. Parameters needed for the cubic EoS are pre-tabulated for the evaluation of departure functions and a quadratic expression is used to recover the attraction parameter. The novelty of the proposed approach lies in the ability to account for pressure and temperature variations from the baseline table. Cryogenic LOX/GH2 mixing and reacting cases are performed to demonstrate the capability of the proposed approach in multidimensional simulations. The proposed combustion model and numerical schemes are directly applicable for LES simulations of real applications under transcritical conditions.Comment: 55th AIAA Aerospace Sciences Meeting, Dallas, T

Raman imaging of counterflow diffusion flames

Ph. D. University of Missouri--Columbia, 1995."December 1995."Hydrogen versus air counterflow diffusion flames are studied with linewise Raman imaging system. Raman measurements are gathered over 20 second duration with high precision (2%) and high spatial resolution (160 $\mu$m). Axial strain rate values are measured by laser Doppler velocimetry. Three different fuel jet concentrations are studied: mole fractions of 21% hydrogen diluted with 79% nitrogen, 50% hydrogen diluted with 50% nitrogen and undiluted hydrogen. Axial strain rate is varied for each of the fuel dilution cases. Raman images of these flames are converted into temperature and major species mole fraction profiles. Mixture fraction and scalar dissipation rate profiles are generated from the mole fraction information. Strain rate and differential diffusion effects are clearly illustrated by the results. Numerically predicted strain rate and differential diffusion effects are experimentally matched. Peak temperatures values agree well with numerical estimates for all of the fuel dilutions except for the undiluted fuel jet cases. At least 50 K higher temperatures are observed for undiluted fuel jet flames when compared to numerically predicted temperatures of these flames. A library of mixture fraction and scalar dissipation rate is generated for hydrogen-air combustion

On the dynamics of the collapse of a diffusion-flame hole

The collapse dynamics of a diffusion-flame hole in the presence of a counterflow are studied. We construct unsteady solutions of the one-dimensional edge-flame model of Buckmaster (1996), in which heat and mass transverse losses are algebraic. The flame structure is determined in the classical limit of large activation energy. Solutions for both planar and axisymmetric strain geometry are considered for the particular case of unity Lewis number. It is shown that the final stage of the edge-flame collapse is determined by a dominant balance between the time rate of change of the mass fractions (and temperature) and diffusion, giving a self-similar structure in which the size of the edge-flame hole approaches zero, to leading (zeroth) order, as a 1/2-power of time. This solution suggests an expansion of the full model equations in 1/2-powers of time that allows detailed analysis of the effects of side losses and flow distribution in the edge-flame collapse process. It is found that side loss effects are apparent at the first order, whereas convection by the counterflow is first felt during collapse at the second order in the fractional-time expansion. Numerical integrations of the governing equations are found to verify the analytic results

Stationary edge flames in a wedge with hydrodynamic variable-density interaction

Edge flames are a canonical two-dimensional flame structure appearing in more complicated combustion problems, such as lifted jet flames and in the dynamics of the growth and repair of flame holes in nonpremixed turbulent combustion. Typical theoretical configurations to study edge flames are unable to evaluate retreating edge flames with strong hydrodynamic-coupling. A new computational configuration is introduced which places the edge flame in a wedge-shaped counterflow with a mass sink, providing control over the position of the edge flame, and allowing access to stationary, hydrodynamically-coupled retreating flames (at high strain). This framework is first used to evaluate edge flames using a simple global one-step chemistry model and Fickian transport. This simple model is used to characterize the behavior of the resulting edge flames, including the relationship between flame speed and transverse strain rate and response to Lewis number variations. The details of the computational method will be discussed, including the underlying finite element method, the generation of boundary data, and the continuation of the flame through regions of varying transverse strain. This configuration is then applied to detailed ethylene-air combustion using a skeletal reduction of the USC Mech II combustion reaction model and a detailed transport model. The details of the ethylene-air edge flame are discussed, and comparisons are made between stoichiometric, fuel-lean, and fuel-rich compositions. Novel results characterizing the dilatation and vorticity near the flame front are provided, data which are necessary for the construction of potential flow approximations of hydrodynamically-coupled edge flames

Development of flamelet generated manifolds for partially-premixed flame simulations

Accuracy of simulations of combustion processes does not only depend on a meticulous description of the turbulent flow field, its accuracy and detail depends on the representation of combustion chemistry and its interaction with turbulence as well. Simulations with a high level of exactness are very time consuming due the large range in length- and timescales in turbulence and chemical kinetics. The contribution of solving the flow field to the computational cost can be reduced by only solving the major turbulent motions containing the largest part of the turbulent kinetic energy. In this method, the Large Eddy Simulation (LES) method, the influence of small, non-resolved eddies has to be modeled, but good models for this unresolved transport of momentum are readily available. Reducing the chemical kinetics is typically a more difficult job due to the large number of elementary (chemical) reactions to be taken into account and the associated stiffness of the system of equations due to the (very) large range in time scales. The Flamelet Generated Manifold (FGM) reduction method resolves this issue by the creation of a chemical manifold out of one-dimensional flame structures. These one-dimensional flame structures, called flamelets, are computed using detailed chemistry and are subsequently tabulated in composition space as a function of a small number of control variables. During a numerical simulation of a flame, only transport equations for the control variables have to be solved and these variables are typically chosen in a way that the stiffness of the resulting system of equations remains low. The FGM method can thus be interpreted as a combination of flamelet and manifold methods: the low-dimensional chemical manifold which is used in simulations of multi-dimensional flames is based of flame structures containing all transport and chemical phenomena as observed in real-life. A small number of archetypical flamelet types exist of which the premixed and counterflow diffusion are used most commonly. They represent premixed and non-premixed flames, two limiting combustion modes. Partially-premixed combustion is considered to occur somewhere between these two types and no archetypical flamelet type exists for stratified combustion. This doctoral dissertation focusses on the following main research question: Can partially-premixed combustion be adequately modeled by FGM tables based on premixed flamelets, counterflow diffusion flamelets or a combination of both types? The use of the FGM reduction method already can reduce computational costs with a few orders of magnitude [?], but the requirement that the reaction layer should be resolved still claims a large number of grid points in the simulation of turbulent flames. The Flame Surface Density (FSD) model allows much coarser grids to be used, but this method has been specifically defined for premixed flames. In chapter 3 it is shown that the FSD approach can also be used in partially-premixed flames having stratification levels as typically observed in gas turbines. Direct Numerical Simulations (DNS) of turbulent planar Bunsen flames was enabled by the use of the FGM method. A priori analysis of the DNS results indicate that simple Presumed PDF subfilter models for the filtered mass burning rate yield fairly accurate predictions when filter widths of up to eight flame thicknesses are used. The implementation of the FSD model using mass burning rate data from flamelets can therefore be considered to be a feasible possibility for the simulation of turbulent stratified flames in an industrial environment. In chapter 4 the focus shifts from flames with only a moderate stratification towards flames with a larger range of equivalence ratios: the well-documented Sandia Flames. For CO, CO2, H2, H2O and OH mass fractions, a priori comparisons are made with experimentally obtained data from flames with Reynolds numbers ranging from 13.400 for the moderately turbulent Flame C to 44.800 for Flame F in which local quenching plays an important role. It is concluded that counterflow diffusion flamelet-based FGM’s prove to be significantly more accurate than premixed-based ones for H2, CO2 and CO mass fraction predictions. For fuel-rich conditions, premixed flamelet-based FGM’s tend to severely overestimate H2 and CO mass fractions while underestimating CO2 mass fractions. Preferential diffusion effects were only visible at low Reynolds numbers and close to the burner nozzle: in general the unit Lewis number assumption is an appropriate one for these flames. Where all numerical and modeling errors in the Computational Fluid Dynamics (CFD) simulations have been excluded in the a priori analysis in chapter 4, in chapter 5 these are taken into account as well in LES of Sandia Flame D and F. Simulation results appear to be very sensitive to boundary conditions for turbulent velocity fluctuations. However, when outcomes for the reaction progress variable, H2 and CO are viewed in composition space, a fairly good resemblance with experimental results is obtained. Using additional transport equations for H2 and CO instead of interpolating them directly from the FGM table does not improve results. A priori predictions of H2 and CO do show a significant improvement in accuracy compared to LES results. It can be concluded that tabulated FGM chemistry can yield accurate predictions even for difficult species like H2 and CO, provided that the CFD solver predicts control variable fields with high accuracy. For all flames considered in chapters 3 and 5 the type of flamelets to use for the generation of the FGM table was clear, either from literature or from a priori analysis. Chapter 6 tackles the question whether FGM tables based on either premixed or non-premixed flamelets can be combined for the simulation of partially-premixed flames. In many industrially-relevant flames, it is simply not known beforehand which flamelet type is most applicable and therefore an adaptive method is highly desirable. For the two species this dissertation focusses on, H2 and CO, the combination of two FGM tables by means of the proposed smooth transition function does not yield the desired results. Predictions for H2 and CO do not consistently improve when the combined FGM approach is used, although often the results are better then those obtained with the least applicable FGM table. Like in chapter 5, additional transport equations for H2 and CO in which the chemical source term comes from the combination of FGM tables does not improve results either. It can be concluded that the FGM method is a powerful tool which renders DNS and LES of complex flames possible. Without this reduction method, simulations discussed in chapter 3 and 5 would have taken amounts of computational resources which are simply not available. The FGM method obtains accurate predictions for species mass fractions, of which H2 and CO are mainly treated in this dissertation, provided that control variables are accurately predicted by the CFD solver and appropriate assumptions are made for the flamelets from which the FGM table is created. This aspect however, requires a certain insight in combustion physics implying that using the FGM method in partially-premixed flames is not yet "plug-and-play"

LAMINAR AND TURBULENT STUDY OF COMBUSTION IN STRATIFIED ENVIRONMENTS USING LASER BASED MEASUREMENTS

Practical gas turbine engine combustors create extremely non-uniform flowfields, which are highly stratified making it imperative that similar environments are well understood. Laser diagnostics were utilized in a variety of stratified environments, which led to temperature or chemical composition gradients, to better understand autoignition, extinction, and flame stability behavior. This work ranged from laminar and steady flames to turbulent flame studies in which time resolved measurements were used. Edge flames, formed in the presence of species stratification, were studied by first developing a simple measurement technique which is capable of estimating an important quantity for edge flames, the advective heat flux, using only velocity measurements. Both hydroxyl planar laser induced fluorescence (OH PLIF) and particle image velocimetry (PIV) were used along with numerical simulations in the development of this technique. Interacting triple flames were also created in a laboratory scale burner producing a laminar and steady flowfield with symmetric equivalence ratio gradients. Studies were conducted in order to characterize and model the propagation speed as a function of the flame base curvature and separation distance between the neighboring flames. OH PLIF, PIV and Rayleigh scattering measurements were used in order to characterize the propagation speed. A model was developed which is capable of accurately representing the propagation speed for three different fuels. Negative edge flames were first studied by developing a one-dimensional model capable of reproducing the energy equation along the stoichiometric line, which was dependent on different boundary conditions. Unsteady and laminar negative edge flames were also simulated with periodic boundary conditions in order to assess the difference between the steady and unsteady cases. The diffusive heat loss was unbalanced with the chemical heat release and advective heat flux energy gain terms which led to the flame proceeding and receding. The temporal derivative balanced the energy equation, but also aided in the understanding of negative edge flame speeds. Turbulent negative edge flame velocities were measured for extinguishing flames in a separate experiment as a function of the bulk advective heat flux through the edge and turbulence level. A burner was designed and built for this study which created statistically stationary negative edge flames. The edge velocity was dependent on both the bulk advective heat flux and turbulence levels. The negative edge flame velocities were obtained with high speed stereo-view chemiluminescence and two dimensional PIV measurements. Autoignition stabilization was studied in the presence of both temperature and species stratification, using a simple laminar flowfield. OH and CH2O PLIF measurements showed autoignition characteristics ahead of the flame base. Numerical chemical and flow simulations also revealed lower temperature chemistry characteristics ahead of the flame base leading to the conclusion of lower temperature chemistry dominating the stabilization behavior. An energy budget analysis was conducted which described the stabilization behavior

Effects of H2O, CO2, and N2 Air Contaminants on Critical Airside Strain Rates for Extinction of Hydrogen-Air Counterflow Diffusion Flames

Coaxial tubular opposed jet burners (OJB) were used to form dish shaped counterflow diffusion flames (CFDF), centered by opposing laminar jets of H2, N2 and both clean and contaminated air (O2/N2 mixtures) in an argon bath at 1 atm. Jet velocities for flame extinction and restoration limits are shown versus wide ranges of contaminant and O2 concentrations in the air jet, and also input H2 concentration. Blowoff, a sudden breaking of CFDF to a stable ring shape, occurs in highly stretched stagnation flows and is generally believed to measure kinetically limited flame reactivity. Restore, a sudden restoration of central flame, is a relatively new phenomenon which exhibits a H2 dependent hysteresis from Blowoff. For 25 percent O2 air mixtures, mole for mole replacement of 25 percent N2 contaminant by steam increased U(air) or flame strength at Blowoff by about 5 percent. This result is consistent with laminar burning velocity results from analogous substitution of steam for N2 in a premixed stoichiometric H2-O2-N2 (or steam) flame, shown by Koroll and Mulpuru to promote a 10 percent increase in experimental and calculated laminar burning velocity, due to enhanced third body efficiency of water in: H + O2 + M yields HO2 + M. When the OJB results were compared with Liu and MacFarlane's experimental laminar burning velocity of premixed stoichiometric H2 + air + steam, a crossover occurred, i.e., steam enhanced OJB flame strength at extinction relative to laminar burning velocity

Assessment of tabulated chemistry models for the les of a model aero-engine combustor

Tabulated chemistry methods present a compromise between computational cost and the ability to capture complex combustion physics in high-fidelity numerical simulations. The application of such models entails a number of modeling decisions, that may affect the simulation results significantly, especially in partially premixed combustion, where the assumption of the existence of underlying premixed or non-premixed flamelet structures is arguable. In this work, different classical tabulation strategies are assessed in terms of their ability to predict the lift-off induced by localized extinction in a model aero-engine combustion chamber: the Cambridge swirl spray flame. The lift-off dynamics of the stable n-heptane spray flame are compared using: i) premixed flamelets, ii) stable and unstable counterflow diffusion flamelets, iii) stable and unsteady extinguishing counterflow diffusion flamelets, iv) unsteady extinguishing and reigniting counterflow diffusion flamelets at a given strain rate. The extinction and reignition events associated to the lift-off are validated against OH-PLIF measurements, and the temporal evolution of the lift-off and reattachment is analyzed.The research leading to these results has received funding from the European Union’s Horizon 2020 Programme under the CoEC project, grant agreement No. 952181 and the Clean Sky 2 Joint Undertaking ESTiMatE project under grant agreement No 821418.Peer ReviewedPostprint (published version