With advanced gas turbine combustor and internal combustion engine designs, autoignition can happen alongside flame propagation. Laminar flame speeds are required to model flame propagation. Determining laminar flame speeds using simulation assumes flames are freely propagating, an assumption that is not valid when autoignition does occur. From a CFD modeling viewpoint however, it is useful to have extrapolated laminar flame-speed values over a wide range of conditions, to allow CFD to operate smoothly and avoid discontinuities while calculating flame-propagation properties. In this work we focus on developing an approach for generating laminar flame-speed libraries under both nonigniting and autoigniting conditions. Following a test of whether autoignition occurs, laminar flame speeds are either modeled or extrapolated. The details of the approach implemented and its validation are explained. We assess the accuracy of the extrapolation employed by calculating relevant coefficients based on flame speeds from nearby operating points. Recommendations are made for the time scales to be used in determining autoignition occurrence. Fuel effects are also explored in this context.

Flamelet Generated Manifold (FGM) has proven to be an efficient approach to model turbulent combustion across different regimes of combustion. The manifolds are generally created by solving laminar premixed or opposed flow configurations. Gas turbine combustors often involve many strong non-adiabatic events such as multiple temperature boundaries, quenching from cooling and effusion holes, conjugate heat transfer, soot radiation interaction, phase change from spray and the modulation of inlet conditions. The adiabatic assumption of the underlying flamelet generation in the FGM is, therefore, prone to errors in the prediction of flame speed, liner temperatures, and pollutant formation. In this work, a novel approach to generate fully non-adiabatic manifold is proposed and validated. The FGM manifold is created using a series of non-adiabatic flamelets, each flamelet is solved in one-dimensional physical space. The non-adiabatic flamelets are generated with an optimal combination of freely propagating and burner stabilized flames. This hybrid method of the flamelet configuration allows modeling large heat gain and loss without encountering any unrealistic temperature in the flamelet solution. Such fully non-adiabatic flamelets are then convoluted to generate a five-dimensional Non-adiabatic Flamelet Generated Manifold (NFGM) Probability Density Function (PDF.). The average properties such as temperature, mixture density, species concentration, rate of reaction, etc. from PDF are then coupled with the CFD solution. The non-adiabatic flamelets and corresponding NFGM is implemented into ANSYS Fluent software version 2020R1. This approach is validated first for canonical cases, followed by gas turbine like conditions of swirl stabilized methane fueled turbulent flame, developed at DLR Stuttgart as the PRECCINSTA combustor. The experimental data for this combustor is available for multiple operating conditions. A stable operating point (φ = 0.83, P = 30 kW) is chosen. The proposed nonadiabatic NFGM is used with Stress blended eddy simulation (SBES). The current NFGM-SBES results are compared with experimental data as well as the previously published works. The impact of modeling heat release in flamelet is used to analyze the M-shape versus V-shape flame transition and the peaks of the carbon monoxide in mixing shear layers. The findings from the current work, in terms of accuracy, validity and best practices while modeling NFGM-SBES are discussed and summarized. The improved results of NFGM compared to adiabatic FGM are encouraging and provides a potential high-fidelity tool for accurate, yet efficient modeling of turbulent combustion inside gas turbines.

A new solution-adaptive mesh refinement capability has been implemented in ANSYS Forte CFD. In this paper, we discuss its implementation and application to engine simulations. This feature is built upon the automatic meshing framework in Forte. The automatic and on-the-fly mesh generation capability in Forte generates a Cartesian cell mesh and the solver uses an immersed boundary method. The mesh generation employs an octree data structure to represent the computational cells inside the computational domain, and all computational cells are perfectly orthogonal. In many scenarios of engine application, high mesh resolution is required to resolve fine geometrical structures or sharp gradients in a physical model. It is impractical and often unnecessary to apply tiny cells everywhere inside the whole domain; thus adaptive mesh refinement and coarsening can be important for obtaining high local mesh resolution while keeping the computational cost low. There are three types of refinement controls implemented in Forte: 1) Fixed Mesh Refinement – Cells along certain boundary surfaces or in predefined volumes are refined to a userspecified level during specified time or crank-angle intervals. 2) Geometry-Adaptive Mesh Refinement – This control is mainly concerned with moving walls. For example, the cell size within valve gaps is dynamically controlled based on the valve lift profiles and user-specified minimum lift thresholds and minimum number of cells in the gap. 3) Solution-Adaptive Mesh Refinement – In this control, cells are adaptively refined or coarsened based on userspecified solution parameters (or their gradients) following certain refinement criteria. This control can help apply high resolution at locations where the high resolution is most needed. We refer to this control as SAM in this paper. In typical engine cases, the first two types of refinement are always used. In this work we focus on the implementation and application of solution-adaptive mesh refinement (SAM). Our first objective is to demonstrate the usage of SAM in different modeling components involved in engine simulations, including basic flow solution, spark ignition and flame propagation, and sprays. We explore different SAM control parameters, demonstrating that refinement is applied at expected locations, and then derive best practices for SAM application. A second objective is to take advantage of SAM’s flexibility to study the mesh sensitivity and mesh convergence of several key sub-models in Forte. A natural question about mesh refinement is: how much should the mesh be refined? The answer will certainly vary for different CFD implementations, mesh types, and sub-models. In this paper, we use SAM to explore the mesh sensitivity of the flame propagation model and spray model. From this we provide guidance for users to select cost effective refinement levels in their engine applications.

A detailed chemistry model is necessary to simulate the effects of variations in fuel composition on soot emissions. In this work, we have developed a detailed chemistry model for the soot formation and oxidation chemistry, with a focus on the surface kinetics of the soot-particle. The model has been compared to a unique set of soot particle-size data measured in flames for several single-component fuels. Fuel components used in the experiments represent the chemical classes found in jet, gasoline, and diesel fuels, including n-heptane (representative of n-alkanes) and toluene (aromatic). Measurements were taken in burner-stabilized stagnation-flame (BSSF) experiments, which can be simulated well using the 1-dimensional BSSF flame model in CHEMKIN-PRO. Soot volume fraction and particle size distributions are modeled using the sectional method option for Particle Tracking, within CHEMKIN-PRO software. The well-characterized flow of the BSSF experiments allows the modeling to focus on the kinetics. Validated detailed reaction mechanisms for fuel combustion and PAH production, combined with the new soot surface-kinetics mechanism, were used in the simulations. Simulation results were compared to measurements for both particle size distributions and total soot volume fraction. Observed effects of fuel, temperature, pressure, equivalence ratio and residence time on the soot size distribution shape and soot quantity were reproduced by the model. The chemistry in the soot surface model includes particle nucleation, growth through the HACA (hydrogen-abstraction/carbon-addition) and PAH-condensation (polycyclic aromatic hydrocarbons) pathways, as well as soot-oxidation pathways. In addition to soot chemistry, the physics of particle coagulation and aggregation were included in the model. The results demonstrate the ability of well-validated chemistry to predict both dramatic and subtle effects related to soot mass and soot particle size.

A sectional method for determining particle size distributions has been implemented within the particle tracking module included with CHEMKIN-PRO. The module is available for use with many types of reactor models, ranging from 0-D batch reactors to laminar flame simulations. Coupled with the Burner-stabilized Stagnation Flame (BSSF) Model, the sectional model offers a high-fidelity, robust, and efficient computational framework for simulating flame synthesis of particles in a laminar, premixed stagnation flame environment. The CHEMKIN-PRO coupling allows inclusion of detailed gas-phase chemistry that determines key particle-formation precursors, along with physical processes such as nucleation and coagulation of particles. These capabilities are demonstrated for two flame-particle systems of practical importance, viz. nanocrystalline titania synthesis and soot formation. The results are compared with experimental data obtained at the University of Southern California (USC) flame facility. Computed particle size distributions show good agreement with experimental data. Simulations have led to exploration of the parameter space for particle production and particle-size influences.

The objectives of this project have been to develop a comprehensive set of fundamental data regarding the combustion behavior of jet fuels and appropriately associated model fuels. Based on the fundamental study results, an auxiliary objective was to identify differentiating characteristics of molecular fuel components that can be used to explain different fuel behavior and that may ultimately be used in the planning and design of optimal fuel-production processes. The fuels studied in this project were Fischer-Tropsch (F-T) fuels and biomass-derived jet fuels that meet certain specifications of currently used jet propulsion applications. Prior to this project, there were no systematic experimental flame data available for such fuels. One of the key goals has been to generate such data, and to use this data in developing and verifying effective kinetic models. The models have then been reduced through automated means to enable multidimensional simulation of the combustion characteristics of such fuels in real combustors. Such reliable kinetic models, validated against fundamental data derived from laminar flames using idealized flow models, are key to the development and design of optimal combustors and fuels. The models provide direct information about the relative contribution of different molecular constituents to the fuel performance and can be used to assess both combustion and emissions characteristics.

Validated surrogate models have been developed for two Fisher-Tropsch (F-T) fuels. The models started with a systematic approach to determine an appropriate surrogate fuel composition specifically tailored for the two alternative jet-fuel samples. A detailed chemical kinetic mechanism has been assembled for these model surrogates starting from literature sources, and then improved to ensure self-consistency of the kinetics and thermodynamic data. This mechanism has been tested against fundamental laboratory data on auto-ignition times, laminar flame-speeds, extinction strain rates, and NOx emissions. Literature data used to validate the mechanism include both the individual surrogate-fuel components and actual F-T fuel samples where available. As part of the validation, simulations were performed for a wide variety of experimental configurations, as well as a wide range of temperatures and equivalence ratios for fuel/air mixtures. Comparison of predicted surrogate-fuel behavior against data on real F-T fuel behavior also show the effectiveness of the surrogate-matching approach and the accuracy of the detailed-kinetics mechanisms. The resulting validated mechanism has been also reduced through application of automated mechanism reduction techniques to provide progressively smaller mechanisms, with different degrees of accuracy, that are reasonable for use in CFD simulations employing detailed kinetics.

Blends of n- and iso-alkane components are employed as surrogates for Fischer–Tropsch (F–T) and biomass-derived jet fuels. The composition of the blends has been determined based on data available for two F–T fuel samples obtained from different sources, using a systematic optimization approach. A detailed chemical kinetic mechanism for combustion of the surrogate blends has been assembled. The mechanism has been validated against fundamental experimental data. While drawing initially from other studies in the literature, the mechanism has been improved by enforcing self-consistency of the kinetic and thermodynamic data for the various surrogate-fuel components represented by the mechanism. These improvements have led to more accurate predictions of flame propagation, flame extinction, and NOx emissions. As part of the validation process, simulations were performed for a wide variety of experimental configurations, as well as for a wide range of temperatures and equivalence ratios for fuel/air mixtures. Comparison of the model predictions to the available literature data confirms the accuracy of the mechanism as well as of the approach for selecting the surrogate blends.

Numerical simulations of the interaction of a polydispersed water mist and a flame, with the specific goal of determining fire suppression characteristics, are very computation intensive. Even in geometrically simple configurations, such as co-flowing flames, the computational cost of a full Navier-Stokes solution of the reacting flow alone, without droplets, is very high. A boundary-layer approach is developed here which provides the needed efficiency; a typical nonpremixed flame including chemical kinetics with hundreds of reactions and multicomponent transport is solved in a few minutes on a personal computer. This is compared to many hours or even days that are required to simulate the equivalent problems with full Navier-Stokes equations. This paper discusses the range of circumstances for which the boundary-layer approximations are valid. Preliminary results characterizing the one-way coupled interaction of a water mist with a laminar non-premixed flame are also presented.

Computational simulations are used to predict and understand the influence of fine water mists on the suppression of laminar, freely propagating methane–, propane–, and hydrogen–air atmospheric-pressure premixed flames. The model solves a coupled, chemically reacting, two-phase-flow problem. Flame suppression is measured in terms of a reduction in burning velocity. The effects of droplet diameter, net water loading, and fuel–air stoichiometry are reported. The results show similar qualitative features for all the flames. Generally speaking, smaller droplets are more effective than the larger droplets. Sufficiently small droplets (approximately 10 μm diameter for methane–air flames) are in a small-droplet limit, where even smaller droplets have the same suppression characteristics for the same net mass loading. Droplets above a certain diameter (approximately 30 μm for methane–air flames) lead to a turning-point extinction, where the burning velocity at the turning point is approximately half of the unperturbed burning velocity without any water-mist loading.

A Kinetic Monte Carlo (KMC) model is presented which simulates the open-circuit voltage and electrical double layer of a doped electrolyte. The computational grid approximates the defect spacing in an electrolyte, so the length scale is atomistic. Numerical results for steady state, open-circuit voltage match analytical predictions over a wide range of oxygen pressure differentials. A general analytical solution is then presented for the distribution of ions in a doped electrolyte subjected to an external voltage. Using available data for Yttria-Stabilized Zirconia (YSZ), it is found that the KMC simulator computes ion concentration profiles and electrical double layers in close agreement with the predictions of the analytical model. A localized or differential updating scheme is used for the electric field that significantly reduces the computation time.

The burning behavior and  ame structure of magnesium in pure carbon dioxide and pure carbon monoxide atmospheres in low-gravity conditions are investigated. Cylindrical specimens are suspended by a thermocouple wire and are radiatively ignited. Spherical  ames are obtained during steady-state burning of the metal sample with increasing metal-oxide accumulation in an outer shell. Burning times twice as long as in normal gravity are observed, revealing a diffusion-controlled reaction. The burning time is proportional to the square of the metal sample diameter. Combustion of magnesium with carbon monoxide is not possible without constant heating of the sample. A one-dimensional, quasi-steady numerical model of the spherically symmetric diffusion  ame using elementary gas-phase reactions and detailed transport property calculations shows qualitative agreement with the observed structure of the  ames. It predicts a maximumtemperature close to the vaporization–decomposition point of themetal oxide, as well as the coexistence of the gaseous and condensed phases of the oxideproduct. It also predicts a diffusion-controlledreaction formagnesiumburning in oxygen, air, and carbon dioxide and provides an accurate comparison of the burning rates of these systems. The discrepancies between the numerical simulation and the experimental observations may be attributed to the absence of accurate condensation, radiation, and surface-reaction models.

Ongoing exploration and future mission2001110444 s to Mars have given impetus to research on the use of natural resources of the planet. Since carbon dioxide (CO2) constitutes approximately 95% of the Mars atmosphere and since it reacts directly and vigorously with several metals, this investigation focuses on metal-CO2 reactions as a possible combination for rocket-propellant production and energy generation. Magnesium (Mg) has been initially selected as the metal fuel owing to its low ignition temperature and high specific impulse and burning rate in CO2. Our studies in this field started with low gravity (g) combustion tests of Mg in O2, CO2, and CO. Reduced gravity provided a clear picture of the burning phenomena by eliminating the intrusive buoyant flows in high-temperature metal reactions and by removing the destructive effect of gravity on the shape of molten metal samples. Suspended cylindrical metal samples of 2, 3, and 4-mm in diameter and length were radiatively ignited in low-g to generate free-floating samples exhibiting a spherically symmetric flame with increasing metal-oxide accumulation in an outer shell. For the Mg-CO2 combination, burning times twice as long as in normal-g and five times longer than in Mg-O2 flames were observed, revealing a diffusion-controlled reaction. The burning time is proportional to the square of the sample diameter. In tests conducted with pure CO, combustion was not possible without constant heating of the sample due to the formation of a thick carbon-containing coating around the Mg sample generated by surface reactions. The following work presents two new studies that attempt to explain some of the low-g experimental observations. First, a simplified one-dimensional, quasi-steady numerical model is developed to obtain temperature, species concentrations, and burning rates of the spherically symmetric diffusion flame around the Mg sample burning in O2 and CO2. Second, a Planar Laser Induced Fluorescence (PLIF) technique is implemented to provide spatially resolved measurements of magnesium oxide (MgO) in the reaction zone of Mg samples burning in O2 and CO2. These experiments reveal fundamental differences between the two combustion systems.

W/BN multilayers are theoretically efficient x-ray mirrors at the nitrogen and boron K-(alpha) lines (31.3 angstroms and 67.6 angstroms, respectively). Their most attractive potential application is detection of light elements by x-ray fluorescence spectrometry. The performances of W/BN mirrors depend not only on the structural quality of the multilayers but also on the stoichiometry of the boron nitride layers, especially in the water window (20 - 40 angstroms). In order to get stoichiometric BN layers with low surface roughness, the deposition of thick boron nitride films has been studied in detail. In-situ kinetic ellipsometry, x-ray photoemission, grazing x-ray reflection and scanning electron microscopy show that quasi-stoichiometric BN films with low surface roughness are obtained only with a low total deposition pressure and an additional nitrogen partial pressure. This result is related to the chemical and structural properties of the BN films. W/BN multilayers with medium period value (2d approximately equals 120 angstroms) show about 80 of the maximum reflectivity at the W M4-5 line. When the period is reduced, the performances are reduced, but good quality W/BN multilayers with very low period values (2d approximately equals 50 angstroms) and a great number of periods ( 100) have been fabricated. The best structural quality is obtained when a low nitrogen partial pressure is introduced during the deposition of the BN layers. The optical indice contrast is improved and the tungsten-boron interdiffusion is reduced.

Using a diode rf-sputtering technique, different magnesium silicide based multilayer systems have been deposited in very thin films for optical applications in the soft x-ray range. A detailed structural analysis of the different multilayers has been made using in-situ kinetic ellipsometry, ex-situ grazing x-ray reflection at the copper K-(alpha) line and transmission electron microscopy. The multilayer performances have been measured by synchrotron radiation at the magnesium K-(alpha) and L-(alpha) lines and related to the structural characteristics. For short wavelength, the W/Mg2Si system shows characteristics very similar to those of the more common W/Si system. Non-negligible interdiffusion and limited interface roughness allow the layer thicknesses to be reduced to very low values. Well-defined Bragg peaks are observed even when the double period is as low as 44 angstrom. First Bragg peak reflectivity as high as 31 has been measured at 9.89 angstrom for a multilayer with a double period equal to 84 angstrom and a limited number of periods. This preliminary result is very promising for future applications in the field of x-ray fluorescence analysis. W/Mg2Si and Si3N4/Mg2Si multilayers have also been fabricated for use at higher wavelengths around the Mg L-(alpha) line (286 angstrom). In the case of the W/Mg2Si multilayers have also been fabricated for use at higher wavelengths around the Mg L-(alpha) line (286 angstrom). In the case of the W/Mg2Si system, the tungsten layers are crystallized due to their higher thickness and consequently the interface roughness is slightly higher. In spite of this, more than 20 reflectivity at the first Bragg peak has been measured at normal incidence on different W/Mg2Si samples, with a selectivity two times better than conventional Mo/Si mirrors ((lambda) /(delta) (lambda) approximately equals 20). When we replace tungsten by a thin silicon nitride layer deposited by reactive sputtering, we increase the selectivity up to (lambda) /(delta) (lambda) approximately equals 30, and the thermal stability is drastically improved ( 800 degree(s)C).