Flamelet Library Approach Research Articles

Validation Assessment of Turbulent Reacting Flow Model Using the Area-Validation Metric on Medium-Scale Methanol Pool Fire Results

Abstract Accident analysis and ensuring power plant safety are pivotal in the nuclear energy sector. Significant strides have been achieved over the past few decades regarding fire protection and safety, primarily centered on design and regulatory compliance. Yet, after the Fukushima accident a decade ago, the imperative to enhance measures against fire, internal flooding, and power loss has intensified. Hence, a comprehensive, multilayered protection strategy against severe accidents is needed. Consequently, gaining a deeper insight into pool fires and their behavior through extensive validated data can greatly aid in improving these measures using advanced validation techniques. A model validation study was performed at Sandia National Laboratories (SNL) in which a 30-cm diameter methanol pool fire was modeled using the SIERRA/Fuego turbulent reacting flow code. This validation study used a standard validation experiment to compare model results against, and conclusions have been published. The fire was modeled with a large eddy simulation (LES) turbulence model with subgrid turbulent kinetic energy closure. Combustion was modeled using a strained laminar flamelet library approach. Radiative heat transfer was accounted for with a model utilizing the gray-gas approximation. In this study, additional validation analysis is performed using the area validation metric (AVM). These activities are done on multiple datasets involving different variables and temporal/spatial ranges and intervals. The results provide insight into the use of the area validation metric on such temporally varying datasets and the importance of physics-aware use of the metric for proper analysis.

Study of radiative heat transfer and flow physics from medium-scale methanol pool fire simulations

A medium-scale (30 cm diameter) methanol pool fire was simulated using Sandia National Laboratories’ Sierra/Fuego low-Mach number multi-physics turbulent reacting flow code. Large Eddy Simulation (LES) with subgrid turbulent kinetic energy closure was used as the turbulence model. Combustion was modeled using a strained laminar flamelet library approach. Radiative heat transfer was modeled using the gray-gas approximation. This paper details analysis done to support a validation study for the fire model. In this analysis, integral quantities were primarily examined. The radiant fraction was computed and used as a model calibration parameter. Integrated buoyancy flux was calculated and compared to an engineering correlation. Entrainment rate was computed with and without a mixture fraction threshold filter and compared to engineering correlations. Turbulent kinetic energy was computed and the effect of mesh size on the subgrid and total turbulent kinetic energy was examined. Flame height was calculated using an intermittency definition with two input parameters. A sensitivity study was then conducted to determine the sensitivity of the estimated flame height to the input parameters. This analysis aided in achieving the primary validation study objectives by providing model calibration and expanding the scope of the validation effort. In addition, the range of physics examined was increased, enhancing the understanding of the model's overall performance and of the relationship between phenomena.

Near-field flame dynamics of liquid oxygen/kerosene bi-swirl injectors at supercritical conditions

The flame dynamics of liquid bi-swirl injectors are numerically investigated using the large eddy simulation technique. Liquid oxygen (LOX) and kerosene at subcritical temperatures are injected into a supercritical pressure environment. The theoretical framework is based on the full conservation laws and accommodates real-fluid thermodynamics and transport theories over the entire range of fluid states. Turbulence/chemistry interaction is modeled with a laminar flamelet library approach, the validity of which is demonstrated in the present work. The near-field flow and flame characteristics are carefully studied. The flame is anchored in the wake of the inner injector post by two counter-rotating vortices, and further stabilized by center and corner recirculation zones in the downstream region. Differences in the flow patterns between the cold-flow and combustion cases are recognized. Various geometric parameters, including recess region, post thickness, and kerosene annulus width, are examined in depth to explore their influence on flame characteristics. A recess region is found to be necessary to achieve efficient mixing and combustion. The absence of a recess region increases the penetration depth of the kerosene stream in the downstream region and reduces the thermal protection provided to the injector faceplate. On the other hand, a thicker LOX post or a wider kerosene annulus protects the faceplate more efficiently, and introduces larger recirculation zones near the LOX post surface and thus higher flow residence time to better anchor the flame. However, the flame attachment for thicker post and wider annulus induces a stronger heat flux to the post surface, and thus increases the risk of thermal failure of the injector device. The dynamic characteristics of the flame field are also discussed. The flow oscillations within the injector are found to be dominated by a quarter acoustic wave, while the oscillatory field near the injector exit is characterized by vortex shedding. The characteristic frequency of the vortex shedding is similar for different LOX post thicknesses and annulus widths, and is determined by the exit velocity profiles.

Supercritical Mixing and Combustion of Liquid-Oxygen/ Kerosene Bi-Swirl Injectors

The mixing and combustion characteristics of liquid-oxygen/kerosene bi-swirl injectors are investigated under the supercritical conditions typical of contemporary rocket engines. The basis of the study is a large-eddy simulation technique combined with a unified treatment of real-fluid thermodynamics. The turbulence/chemistry interaction is treated using a laminar flamelet library approach. Emphasis is placed on the near-field flow and flame development downstream of the inner swirler. The flame is found to be stabilized by two counter-rotating vortices in the wake region of the liquid-oxygen post, which is covered by the kerosene-rich mixture. The width of the kerosene annulus is found to significantly affect the injector behavior. A wider annulus induces a larger spreading angle of the liquid-oxygen stream, which intercepts the kerosene stream in a more efficient way. Increasing the annulus width, however, imposes a wake region in a broader zone. The resultant flame becomes relatively unstable if the flame thickness is larger than the liquid-oxygen post thickness. Variation of the kerosene annulus width has a negligible effect on the dominant frequency of the pressure fluctuation, but it changes the amplitude of fluctuation.

Aspects of 0D and 3D Modeling of Soot Formation for Diesel Engines

In this work, zero-dimensional (0D) and three-dimensional (3D) models were applied to the same engine experiment, investigating aspects of 0D and 3D modeling of combustion and soot formation for diesel engines. The 0D simulations were carried out using a direct injection stochastic reactor model (DI-SRM), which is built on a probability density function (PDF) approach. The 0D model allows for the use of detailed chemistry for calculation of combustion, emission formation, and interaction between chemistry and turbulent flow. The 3D computational fluid dynamics (CFD) simulations were performed using a PDF-time scale combustion model and a flamelet library soot source term model. The DI-SRM results demonstrate the applicability of the flamelet model for the combustion process and also elucidate the limitations of the interactive flamelet model when calculating emission formation. The emission results, if plotted in mixture fraction space, show a dispersion for species such as NO and CO, but a flamelet structure for species such as C2H2 and OH, which makes the latter ones applicable for calculation of the source terms of soot formation in mixture fraction space. The CFD calculations were used to verify assumptions made in the DI-SRM and the DI-SRM results were used to verify the assumptions inferred by using tabulated chemistry. It is demonstrated that the DI-SRM can be used for soot modeling under diesel engine conditions and that the flamelet library approach for modeling of soot formation in CFD is sound.

A tabulated closure for turbulent non-premixed combustion based on the linear eddy model

A tabulated closure based on conditional statistics from fully-resolved one-dimensional scalar profiles has been formulated for application using the large eddy simulation technique. These scalar profiles are obtained from both experiments (i.e., line imaging of Raman/Rayleigh/CO-LIF) and from the simulations of linear eddy model. Such profiles provide a local description of species and temperature evolution due to the full range of turbulent scales in the predominant direction of scalar transport. Instantaneous realizations of these scalar profiles are processed as a function of pre-determined filter widths in space to obtain the conditional statistics of filtered scalars. Key statistical quantities needed for LES, are then parameterized as a function of a unique filtered state vector. Here, we use the filtered mixture fraction Z∼, filtered scalar dissipation rate χ∼, and the subgrid Reynolds number ReΔ as the filtered state vector. Conditional PDF’s of temperature P(T|Z∼,χ∼,ReΔ), species mass fractions P(Yi|Z∼,χ∼,ReΔ), and relevant thermodynamic and transport properties are tabulated as a function of Z∼, χ∼, and ReΔ. A thermo-chemical state library is then constructed in a manner directly analogous to the conventional flamelet-library approach. The potentially novel feature of this approach is that the relation between filtered and instantaneous quantities is known directly. This eliminates the need for a posteriori models that link these quantities. An a priori analysis of the proposed model was carried out for non-premixed combustion using measurements obtained from CH4/H2/N2 jet flames at a Reynolds numbers of 15,200. This flame exhibits an interesting range of Damköhler and Schmidt numbers through local extinction, reignition, and differential diffusion. Results show good quantitative agreement with measurements.

Effect of swirl on combustion dynamics in a lean-premixed swirl-stabilized combustor

The effect of inlet swirl on the flow development and combustion dynamics in a lean-premixed swirl-stabilized combustor has been numerically investigated using a large-eddy-simulation (LES) technique along with a level-set flamelet library approach. Results indicate that when the inlet swirl number exceeds a critical value, a vortex-breakdown-induced central toroidal recirculation zone is established in the downstream region. As the swirl number increases further, the recirculation zone moves upstream and merges with the wake recirculation zone behind the centerbody. Excessive swirl may cause the central recirculating flow to penetrate into the inlet annulus and lead to the occurrence of flame flashback. A higher swirl number tends to increase the turbulence intensity, and consequently the flame speed. As a result, the flame surface area is reduced. The net heat release, however, remains almost unchanged because of the enhanced flame speed. Transverse acoustic oscillations often prevail under the effects of strong swirling flows, whereas longitudinal modes dominate the wave motions in cases with weak swirl. The ensuing effect on the flow/flame interactions in the chamber is substantial.

Investigation of Turbulence Models Applied to Premixed Combustion Using a Level-Set Flamelet Library Approach

Most of the common modeling approaches to premixed combustion in engineering applications are either based on the assumption of infinitely fast chemistry or the flamelet assumption with simple chemistry. The level-set flamelet library approach (FLA) has shown great potential in predicting major species and heat release, as well as intermediate and minor species, where more simple models often fail. In this approach, the mean flame surface is tracked by a level-set equation. The flamelet libraries are generated by an external code, which employs a detailed chemical mechanism. However, a model for the turbulent flame speed is required, which, among other considerations, depends on the turbulence intensity, i.e., these models may show sensitivity to turbulence modeling. In this paper, the FLA model was implemented in the commercial CFD program Star-Cd, and applied to a lean premixed flame stabilized by a triangular prism (bluff body). The objective of this paper has been to investigate the impact on the mean flame position, and hence on the temperature and species distribution, using three different turbulent flame speed models in combination with four different turbulence models. The turbulence models investigated are: the standard k-ε model, a cubic nonlinear k-ε model, the standard k-ω model and the shear stress transport (SST) k-ω model. In general, the computed results agree well with experimental data for all computed cases, although the turbulence intensity is strongly underestimated at the downstream position. The use of the nonlinear k-ε model offers no advantage over the standard model, regardless of flame speed model. The k-ω based turbulence models predict the highest turbulence intensity with the shortest flame lengths as a consequence. The Mu¨ller flame speed model shows the least sensitivity to the choice of turbulence model.

Effects of flame stretch and wrinkling on co formation in turbulent premixed combustion

This paper presents an investigation of CO formation in a lean premixed propane/air turbulent flamein an afterburner configuration. A previous experiment showed that a high amount of CO was formed in the mean turbulent flame brush. To explain the high CO concentration in the flame zone, the effects of flame stretch and flame wrinkling are studied, based on an ensemble averaged laminar flamelet library approach. It is shown that the flame stretch decreases the laminar burning velocity by 20% under the studied flame conditions, and the stretched flamelet model predicts the non-equilibrium CO concentration in the postflame zone. However, the high CO concentration in the mean flame brush cannot be predicted by a stretched flamelet library model alone. A flamelet model accounting for wrinkled flamelets in the mean turbulent flame brush, and the effect of flame stretch (mainly strain rate), is tested. The model is based on a level-set G-equation for the mean position of the turbulent flame brush and an esemble average of strained laminar flamelet libraries. A comparison of the numerical results with the experimental data and a previous translating flamelet model clearly shows that the wrinkled flamelet model predicts the intermediate species, such as CO, more accurately. The major species such as O2 and CO2, as well as temperature, are found to be not sensitive to the flame wrinkling.

Pressure effect on soot formation in turbulent diffusion flames

Soot formation in a methane–air turbulent jet diffusion flame is investigated numerically using a semi-empirical model. The temperature, density and species (the soot precursor C 2H 2) fields are calculated using detailed chemical kinetic mechanism based on the flamelet library approach. The influence of pressure on the soot formation and the behavior of the semi-empirical model in different flame situations are investigated. It is found that the flame shape and the flame temperature can be well predicted by the flamelet library approach. The calculated soot yield is mostly sensitive to the soot surface growth rate and the increase of pressure. The increase of pressure leads to the increase of soot surface growth rate and therefore to the increase of soot volume fraction. By adjusting a model constant in the soot surface growth rate, the soot emissions in both pressure p=1 atm and p=3 atm are properly simulated by the current semi-empirical soot model.

Level-set flamelet library approach for premixed turbulent combustion

Abstract Modeling of a lean premixed propane/air turbulent flame stabilized by a bluff body is presented. The laminar flamelet library approach, which has been successfully used for modeling of nonpremixed turbulent flames, is extended and applied to premixed combustion. In this approach the mean flame location and mean flame thickness are modeled by a level-set G-equation and a G-variance equation, along with empirical expressions. The detailed species concentrations, temperature and density are calculated using a presumed probability density function (PDF) together with a laminar flamelet library, which is generated using a detailed chemical kinetic mechanism. The sensitivity of the results to the model components is investigated. It is shown that the temperature and major species concentrations are not very sensitive to the model components such as the turbulent flame speed and the shape of the presumed PDF. Species which only exist in the inner layer of the laminar flamelet are found to be more sensitive to the model components. Comparisons of calculations with experimental data indicate that the flamelet library approach is able to predict the major properties of the mean turbulent flame. In particular the method exhibits potential in simulation of minor pollutant species such as NOx, with fairly detailed chemistry at a low cost.

Modeling of NOx and Soot Formation in Diesel Combustion

N Modeling of NO x and Soot Formation in Diesel Combustion N An approach to model the formation and oxidation or reduction of soot and NO in turbulent diffusion flames is presented. The model is based on the flamelet library approach and extended to account for radiative heat losses in the flame. Due to the rather slow processes leading to soot and NO a modified flamelet library approach is used. Instead of taking the mass fractions directly from flamelet libraries the different source terms for soot and NO formation are calculated and a transport equation for the mean mass fractions is solved in the CFD calculation. The source terms are obtained from laminar counterflow-flame calculations using a detailed chemistry model for the gas phase species and the formation and oxidation of soot. Transport equations for the mean mixture fraction and the mixture fraction variance are solved and the chemical source term is closed by presuming a beta-function like distribution of mixture fraction and a log-normal distribution of the scalar dissipation rate. The model was first tested in laminar and turbulent jet flames. By applying a reduction strategy for the flamelet libraries of the source terms it was made applicable to the simulation of soot formation in a Diesel spray taking different oxidizer temperatures and pressures into account. Additionally, different formulations of the flamelet equations have been tested and their accuracy has been evaluated by comparing them to turbulent flame experiments.

Detailed soot modeling in turbulent jet diffusion flames

An approach for modeling the interaction between the formation and oxidation of soot, the radiative heat loss, and the flowfield in turbulent jet diffusion flames is presented. These interactions are modeled by the flamelet library approach in the framework of prescribed probability density functions (PDFs). The formation and oxidation of soot is calculated from a detailed chemical soot model. The laminar flamelet concept is applied to model the rates of soot particle inception, soot volume dependent surface growth, and oxidation, as well as species and temperature fields, Radiative heat transfer from the soot particles and the gas-phase species, CO2 and H2O, decreases the peak flame temperature, which in turn influences the flamelet structures. Experiments from Young et al. on a turbulent ethylene diffusion flame are used to validate the modeling approach. The calculated fields of mean mixture fraction, temperature, and soot volume fraction are found to be in agreement with the experimental data. The spatially resolved rates of soot inception, surface growth, and oxidation are presented. The maximum rates of the surface independent production occur in the fuel-rich region at a radial position of about 10 mm. In contrast, the maximum rates of surface growth and oxidation are found on the centerline with the oxidation occurring at a higher location in the flame. The total rate has its maximum on the centerline, whereas soot formation and destruction balances on the slightly rich side near the stoichiometric contour. This shows the strong interaction of soot formation and oxidation in the flame. A sensitivity analysis of the calculated soot volume fraction on different model parameters is presented. The rate coefficient of the heterogeneous surface growth reaction is the most sensitive parameter. A 20% increase of this rate leads to a 65% increase of the calculated maximum soot volume fraction.