Development and implementation of turbulence models in the combustion code Ares (original) (raw)
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RANS Modelling of Turbulence in Combustors
Turbulence Modelling Approaches - Current State, Development Prospects, Applications, 2017
Turbulence modelling is a major issue, affecting the precision of current numerical simulations, particularly for reacting flows. The RANS (Reynolds-averaged Navier-Stokes) modelling of turbulence is necessary in the development of advanced combustion systems in the foreseeable future. Therefore, it is important to understand advantages and limitations of these models. In this chapter, six widely used RANS turbulence models are discussed and validated against a comprehensive experimental database from a model combustor. The results indicate that all six models can catch the flow features; however, various degrees of agreement with the experimental data are found. The Reynolds stress model (RSM) gives the best performance, and the Rk-ε model can provide similar predictions as those from the RSM. The Reynolds analogy used in almost all turbulent reacting flow simulations is also assessed in this chapter and validated against the experimental data. It is found that the turbulent Prandtl/Schmidt number has a significant effect on the temperature field in the combustor. In contrast, its effect on the velocity field is insignificant in the range considered (0.2-0.85). For the present configuration and operating conditions, the optimal turbulent Prandtl/Schmidt number is 0.5, lower than the traditionally used value of 0.6-0.85.
2021
The present study investigates a model for the large-scale simulation of turbulent non-premixed flames. In these flames, fuel and oxidizer arrive separately in the reaction zone.Easier to design than premixed flames (no prior mixing of the reagents in proportions compatible with the flammability limits), these flames are also safer since there is no risk of a flashback, which motivates their use in a certain number of situations (industrial furnaces, rocket motors, etc.).On the other hand, they are generally less efficient, and the inability to control their maximum temperature favors the formation of nitrogen oxides.Numerical simulation has now become essential to help design efficient burners. Despite the continued growth in the power of computing resources, direct numerical simulations (DNS), without modeling the flame/turbulence interaction, remain impossible for combustion chambers of practical interest.Large-scale simulation (LES) represents a good compromise in terms of infor...
Modeling and Simulation of Turbulent Combustion
Energy, Environment, and Sustainability, 2018
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Numerical simulation of turbulent combustion: Scientific challenges
Science China Physics, Mechanics & Astronomy, 2014
Computational Fluid Dynamics (CFD) is increasing its importance in the design of systems involving reacting turbulent flows like industrial combustion devices. The associated complex physics demands for accurate models and advanced numerical methods. Nowadays supercomputing makes feasible using powerful CFD approaches like LES (Large Eddy Simulations), which is able to meet such demands. CRESCO platform, recently installed in Portici, has absolutely increased computing capabilities of the ENEA research team in combustion. The aim of this work is to show some results of turbulent combustion related problems obtained by means of the ENEA HeaRT code run on CRESCO. HeaRT (Heat Release and Turbulence) is a fully compressible, reactive, unsteady flow solver that implements an original LES subgrid scale model, named Fractal Model. In particular, the main issues of two test cases are discussed. The first one is the Sandia/ETH-Zurich CO/H2/N2 non-premixed unconned turbulent jet ame. This flame is simulated in a three-dimensional axisymmetric formulation. The most important scientific issue related to this flame is its anchoring, analyzed in detail to evidence the controlling physical mechanisms. Besides this, comparison with experimental data is also provided.
Investigation of Modeling for Non-Premixed Turbulent Combustion
Flow Turbulence and Combustion, 1998
A method for predicting filtered chemical species concentrations and filtered reaction rates in Large-Eddy Simulations of non-premixed, non-isothermal, turbulent reacting flows has been demonstrated to be quite accurate for higher Damköhler numbers. This subgrid-scale model is based on flamelet theory and uses presumed forms for both the dissipation rate and subgrid-scale probability density function of a conserved scalar. Inputs to
LES of explosions in venting chamber: A test case for premixed turbulent combustion models
Combustion and Flame, 2017
This paper presents a new experimental and Large Eddy Simulation (LES) database to study upscaling effects in vented gas explosions. The propagation of premixed flames in three setups of increasing size is investigated experimentally and numerically. The baseline model is the well-known laboratory-scale combustion chamber from Sydney (Kent et al., 2005; Masri et al., 2012); two exact replicas at scales 6 and 24.4 were set up by GexCon (Bergen, Norway). The volume ratio of the three setups varies from 1 to more than 10,0 0 0, a variation unseen in previous experiments, allowing the exploration of a large range of Reynolds and Damköhler numbers. LES of gaseous fully premixed flames have been performed on the three configurations, under different operating conditions, varying the number of obstacles in the chamber, their position and the type of fuel (hydrogen, propane and methane). Particular attention is paid to the influence of the turbulent combustion model on the results (overpressure, flame front speed) comparing two different algebraic sub-grid scale models, the closures of Colin et al. (20 0 0) and Charlette et al. (2002), used in conjunction with a thickened flame approach. Mesh dependency is checked by performing a highly resolved LES on the small-scale case. For a given scale and with a fixed model constant, LES results agree with experimental results, for all geometric arrangement of the obstacles and all fuels. However, when switching from small-scale cases to medium-scale or large-scale cases this conclusion does not hold, illustrating one of the main deficiencies of these algebraic models, namely the need for an a priori fitting of the model parameters. Although this database was initially designed for safety studies, it is also a difficult test for turbulent combustion models.
Numerical Implementation and validation of turbulent premixed combustion model for lean mixtures
MATEC Web of Conferences, 2018
The present paper discusses the numerical investigation of turbulent premixed flames under lean conditions. Lean premixed combustion, a low NOx emission technique but are prone to instabilities, extinction and blow out. Such flames are influenced by preferential diffusion due to different mass diffusivities of reactants and difference between heat and mass diffusivities in the reaction zone. In this numerical study, we estimate non-reacting flow characteristics with implementation of an Algebraic Flame Surface Wrinkling Model (AFSW) in the open source CFD code OpenFOAM. In these flows, the mean velocity fields and recirculation zones were captured reasonably well by the RANS standard k-epsilon turbulence model. The simulated turbulent velocity is in good agreement with experiments in the shear-generated turbulence layer. The reacting flow study was done at three equivalence ratios of 0.43, 0.5 and 0.56 to gauge the ability of numerical model to predict combustion quantities. At equi...
Simulation of flow development in high-speed combustor in 2D and 3D formulations
2018
Model high-speed combustor on gaseous hydrocarbon fuel, prepared for experiments in T-131 wind tunnel of TsAGI, is presented. Experiments are projected to create an experimental database for validation of calculations and physical models of turbulence and combustion. Geometry of combustor and prepared measurements are described. The main subject of paper is preliminary calculations of this combustor. Numerical methods for 2D and 3D URANS calculations are described. Special attention is given to numerical techniques allowing fast calculations of 3D unsteady flow development in the combustor. Approach to parallel realization of Fractional Time Stepping (FTS) technology is described. One way of In Situ Adaptive Tabulation (ISAT) of kinetic equations solution during the calculation is presented. Possible gasdynamic structure of flow in the combustor with the flame stabilization both in subsonic and in supersonic regime is described. Asymmetrical stationary solution (for the symmetrically-expanding duct) and symmetrical solution with flame oscillations are found and analyzed. PREPARATION OF THE EXPERIMENTAL MODEL Today it is impossible to imagine the creation of perspective aircraft without supplementation of the experiments with numerical simulation. However, modern possibilities to calculate practical flows with combustion in aircraft engines are essentially limited by the huge computer cost for calculation of 3D viscid turbulent flows with finite-rate reactions [1,2] and by the low accuracy of the available models of turbulence, of chemical kinetics, and of turbulence/combustion interaction [3-5]. In 2017, the scientific laboratory "Studies and development of physical models and numerical technologies for description of different combustion regimes in aircraft engines" has been created in Propulsion department of TsAGI under the support of Russian Ministry of education and science. Goals of the laboratory are the development and validation of physically-grounded models for various combustion regimes in air-breathing engines, as well as the creation of special software for use in the cycle of aerodynamic design for new aircraft engines. The laboratory develops and improves physical and mathematical models of turbulent combustion, oriented to calculations in the framework of RANS (Reynolds-Averaged Navier-Stokes) and LES (Large Eddy Simulation). These models are implemented into computer codes, specially adjusted for concrete class of flows to get the best prediction of flow characteristics. Such adjustment is based on experimental data for flows of the considered class. To create the basis for such activities, the new "fire" aerodynamic experiments are prepared in TsAGI. Experiments will be performed on the unique high-speed wind tunnel T-131 (see detailed information about wind
Computational modeling of combustion instabilities in lean premixed turbulent combustors
In this work the reaction-rate response of different species to inlet flow variations have been studied using an unsteady perfectly stirred reactor model. Transient simulations of variations in mass flow rate, temperature and mixture equivalence ratio at the reactor inlet have been conducted. Combustion of methane and propane, with both global single-step and detailed chemical kinetic mechanisms, has been simulated. The detailed mechanisms predict similar general trends. The global and detailed mechanism for methane predict almost the same reaction rates, whereas the predicted reaction rates from the global and detailed mechanism for propane are very different at high equivalence ratios, near stoichiometry. The reaction-rate oscillations were not very sensitive to imposed small oscillations on the inlet temperature. An imposed small oscillation on the inlet mass flow rate gave reaction-rate oscillations that were almost constant at both rich and lean mixtures. The largest variations in reaction rate oscillations between rich and lean mixtures were found when imposing a small oscillation on the equivalence ratio of the mixture at the inlet. The present study indicates that variations in the inlet mixture equivalence ratio may lead to combustion instabilities in lean premixed combustion.
Simulation of lean premixed turbulent combustion
Journal of Physics: Conference Series, 2006
There is considerable technological interest in developing new fuel-flexible combustion systems that can burn fuels such as hydrogen or syngas. Lean premixed systems have the potential to burn these types of fuels with high efficiency and low NOx emissions due to reduced burnt gas temperatures. Although traditional scientific approaches based on theory and laboratory experiment have played essential roles in developing our current understanding of premixed combustion, they are unable to meet the challenges of designing fuel-flexible lean premixed combustion devices. Computation, with its ability to deal with complexity and its unlimited access to data, has the potential for addressing these challenges. Realizing this potential requires the ability to perform high fidelity simulations of turbulent lean premixed flames under realistic conditions. In this paper, we examine the specialized mathematical structure of these combustion problems and discuss simulation approaches that exploit this structure. Using these ideas we can dramatically reduce computational cost, making it possible to perform high-fidelity simulations of realistic flames. We illustrate this methodology by considering ultra-lean hydrogen flames and discuss how this type of simulation is changing the way researchers study combustion.