Assessment of implementation variants of conditional scalar dissipation rate in LES-CMC simulation of auto-ignition of hydrogen jet (original) (raw)
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Numerical modeling of hydrogen diffusion jet flame: the role of the combustion model
The prediction of turbulent reactive flows largely depends on the proper choice of a combustion model. The closure of the highly non linear production term in the species conservation equation is one of the most challenging aspects when modeling turbulent combustion with RANS/FANS approaches. The main value of the species reaction rates cannot be obtained by simply substituting the mean variables into the Arrhenius equations, and the effects of turbulence on temperature and species concentration fields must be taken into account. Several approaches have been proposed in the literature to provide a proper modelling of mean reaction rates. The Eddy Dissipation Model is based on the infinitely fast chemistry hypothesis and assumes that the reaction rate is controlled by turbulent mixing . A generalized formulation of the Eddy Dissipation Model, i.e. EDM/FR, has been proposed in order to take into account finite rate chemistry effects, however, this model is not suited to treat detailed combustion mechanisms and fails with a number of reactions larger than three or four. The Eddy Dissipation Concept (EDC) is an extension of EDM which allows detailed kinetics to be included in the calculations . The model assumes that chemical reactions occur within the smallest turbulent structures, called fine structures. These are treated as perfectly stirred reactors (PSR) which exchange mass with the surrounding fluid. The overall reaction rate in each PSR is controlled by chemical kinetics. The properties of the fine structures are derived from a step-wise energy cascade model and expressed with quantities related to the main flow, such as the turbulent kinetic energy, k, and the turbulent dissipation rate, _. Two formulations of EDC have been developed, for fast and finite rate chemistry respectively [2]. Both combustion models described above are based on the reaction rate approach. A different approach is based on the primitive variable and originates a group of combustion models. In its simplest formulation, this approach allows the decoupling of chemistry and flow field calculation. The chemical reaction rates are computed off-line and stored in lookup tables, accessible by the flow solver. The instantaneous variables are expressed in terms of mixture fraction and enthalpy and, then, integrated over an assumed _-PDF probability density function to get the mean scalar variables. In the Flamelet approach, the flame is modelled as an ensemble of thin and steady laminar flames, strained by the turbulent motion. According to this model, two scalar equations, for the mixture fraction and its variance, have to be solved independently of the number of species involved in the chemical mechanism. The influence of the outer flow field on the inner reaction zone is described by the scalar dissipation rate. This represents a reciprocal residence time within the flame sheets which is increased by stretch effects of turbulence motion and reduced by diffusion. At a critical value of the dissipation rate the flame shows a threshold behaviour and extinguishes. The mean values of scalar properties are computed from the instantaneous values which are integrated over a probability density function with a presumed _-PDF shape. The integration is not carried out during the CFD calculation but is part of the flamelet library generation, in order to reduce the computational cost. The flamelet libraries are created with a mixture fraction ranging between zero and unity and for discrete
Modeling of Scalar Dissipation Rates in Flamelet Models for HCCI Engine Simulation
50th AIAA Aerospace Sciences Meeting including the New Horizons Forum and Aerospace Exposition, 2012
T he flamelet approach offers a viable framework for combustion modeling of homogeneous charge compression ignition (HCCI) engines under stratified mixture conditions. Scalar dissipation rate acts as a key parameter in flamelet-based combustion models which connects the physical mixing space to the reactive space. T he aim of this paper is to gain fundamental insights into turbulent mixing in low temperature combustion (LT C) engines and investigate the modeling of scalar dissipation rate. T hree direct numerical simulation (DNS) test cases of two-dimensional turbulent auto-ignition of a hydrogen-air mixture with different correlations of temperature and mixture fraction are considered, which are representative of different ignition regimes. T he existing models of mean and conditional scalar dissipation rates, and probability density functions (PDFs) of mixture fraction and total enthalpy are a priori validated against the DNS data. T he underlying reasons behind the performance of the existing models are explored by looking into the local and global dynamics of mixing. In the context of Reynolds-averaged Navier-Stokes (RANS) framework, the current models are not able to capture the local rise in scalar dissipation rate due to the effects of differential diffusion of hydrogen. However, the beta PDF gives reasonably accurate results when differential diffusion effects are discarded. It is also observed that model constants for the mean scalar dissipation rates should be taken as 3.0 instead of 2.0 for more accurate results. In the context of large eddy simulation (LES), it is found that mixing is completely uncorrelated with turbulence and that the mean scalar dissipation rates can be accurately modeled as being proportional to the scalar variances. Based on this understanding, novel simplistic models for the mean and cross scalar dissipation rates are proposed and are shown to perform very well under the wide range of auto-ignition regimes considered in this study.
T he flamelet approach offers a viable framework for combustion modeling of homogeneous charge compression ignition (HCCI) engines under stratified mixture conditions. Scalar dissipation rate acts as a key parameter in flamelet-based combustion models which connects the physical mixing space to the reactive space. T he aim of this paper is to gain fundamental insights into turbulent mixing in low temperature combustion (LT C) engines and investigate the modeling of scalar dissipation rate. T hree direct numerical simulation (DNS) test cases of two-dimensional turbulent auto-ignition of a hydrogen-air mixture with different correlations of temperature and mixture fraction are considered, which are representative of different ignition regimes. T he existing models of mean and conditional scalar dissipation rates, and probability density functions (PDFs) of mixture fraction and total enthalpy are a priori validated against the DNS data. T he underlying reasons behind the performance of the existing models are explored by looking into the local and global dynamics of mixing. In the context of Reynolds-averaged Navier-Stokes (RANS) framework, the current models are not able to capture the local rise in scalar dissipation rate due to the effects of differential diffusion of hydrogen. However, the beta PDF gives reasonably accurate results when differential diffusion effects are discarded. It is also observed that model constants for the mean scalar dissipation rates should be taken as 3.0 instead of 2.0 for more accurate results. In the context of large eddy simulation (LES), it is found that mixing is completely uncorrelated with turbulence and that the mean scalar dissipation rates can be accurately modeled as being proportional to the scalar variances. Based on this understanding, novel simplistic models for the mean and cross scalar dissipation rates are proposed and are shown to perform very well under the wide range of auto-ignition regimes considered in this study.
The potential of a progress variable formulation for predicting autoignition and subsequent kernel development in a nonpremixed jet flame is explored in the LES (Large Eddy Simulation) context. The chemistry is tabulated as a function of mixture fraction and a composite progress variable, which is defined as a combination of an intermediate and a product species. Transport equations are solved for mixture fraction and progress variable. The filtered mean source term for the progress variable is closed using a probability density function of presumed shape for the mixture fraction. Subgrid fluctuations of the progress variable conditioned on the mixture fraction are neglected. A diluted hydrogen jet issuing into a turbulent coflow of preheated air is chosen as a test case. The model predicts ignition lengths and subsequent kernel growth in good agreement with experiment without any adjustment of model parameters. The autoignition length predicted by the model depends noticeably on the chemical mechanism which the tabulated chemistry is based on. Compared to models using detailed chemistry, significant reduction in computational costs can be realized with the progress variable formulation.
Ignition Source Effect on Modelling Hydrogen Premixed Combustion
Athens Journal of Sciences, 2018
This paper presents numerical study of premixed combustion of hydrogen-air mixtures inside a laboratory scale combustion chamber. The study is carried out on a premixed lean mixture of hydrogen/air flames with an equivalence ratio of 0.7. The main focus of the current work is to examine the effects of the way the source of ignition would have on the overall characteristics of combustion. An in-house Large Eddy Simulation (LES) modelling technique has been used to carry out the numerical simulations. The model predictions have been validated against available published experimental data where ignition has been introduced through the use of a laser beam. Successful numerical representation of the experimental initial and boundary conditions has resulted in good agreement between the experimental and numerical results for the generated combustion flame and pressure-time history. It was concluded that the combustion characteristics are sensitive to the size of the ignition source in terms of the timing occurrence of the peak pressure but not its magnitude. This finding has practical importance in analyzing explosion hazards, internal combustion engines and gas turbine combustors.
Numerical Simulation of Self-Ignition of Hydrogen-Hydrocarbons Mixtures in a Hot Supersonic Air Flow
42nd AIAA/ASME/SAE/ASEE Joint Propulsion Conference & Exhibit, 2006
The present paper, carried out in the framework of the French Promethée Program, deals with the comparative study of the self-ignition of hydrogen and hydrocarbons/hydrogen mixtures jets in co-stream vitiated Mach 2 airflow. Here, ethylene and methane are used as surrogate combustible for hydrocarbons fuels. Experimental studies have been performed to obtain a large database of hydrocarbons/hydrogen mixtures auto-inflammation. Due to heavy setup , hostile environment of the combustor, it is essential to develop CFD codes capable of numerically simulating flight conditions faced in the combustion chamber. Experimental results, simple 0D kinetic analysis using Perfectly Stirred Reactor (PSR) and Partially Stirred Reactor (PaSR) models have shown the importance of micromixing in the supersonic mixing layer. Therefore, a micromixing model, that takes into account the physics of the flow and that is low cost for the CPU, has been implemented to the ONERA's CFD code CEDRE. Results obtained using this model with 2D Large Eddy Simulations (LES) shows that both wall pressure distribution in the combustion chamber and ignition delay are well estimated.
1 Modeling of Scalar Dissipation Rates in Flamelet Models for HCCI Engine Simulation
2016
The flamelet approach is considered a viable framework to the modeling of homogeneous charge compression ignition (HCCI) engines under stratified mixture conditions. However, there are several issues that need further improvement. In particular, accurate representation of the scalar dissipation rate, which is the key parameter to connect the physical mixing space to the reactive space, requires further investigation. This involves a number of aspects: (i) probability density functions, (ii) mean scalar dissipation rates, and (iii) conditional scalar dissipation rates, for mixture fraction (Z) and total enthalpy (H). The present study aims to assess the validity of existing models in HCCI environments both in the RANS and LES contexts, and thereby suggest alternative models to improve on the above three aspects. Nomenclature Z = mixture fraction H = total enthalpy Z = Z scalar dissipation rate H = H scalar dissipation rate Z = Mean Z scalar dissipation rate H = Mean H scalar dissipat...
Peculiarities of mathematical modeling of combustion of hydrogen-air mixtures
Journal of Physics: Conference Series, 2019
Paper presents a comprehensive study of contemporary opportunities in the numerical analysis of transient combustion regimes on the example of the topical problem of hydrogen combustion. The elementary processes determining the development of combustion under different conditions are considered. In view of this analysis, it is concluded that the most appropriate for numerical modeling of classic deflagration inside confined space is the low-dissipation numerical schemes of low (second) order of approximation. For near-critical conditions where distinct mechanisms determine the combustion development one is able to utilize the simplifications such as low Mach number approximation.
ICFD12-EG-5052 Numerical Study of Hydrogen Premixed Combustion-Effects of Ignition Source
This paper presents numerical study of premixed combustion of hydrogen-air mixtures inside a laboratory scale combustion chamber. The study is carried out on a premixed lean mixture of hydrogen/air flames with an equivalence ratio of 0.7. The main focus of the current work is to examine the effects of the way the source of ignition would have on the overall characteristics of combustion. An in-house Large Eddy Simulation (LES) modelling technique has been used to carry out the numerical simulations. The model predictions have been validated against available published experimental data where ignition has been introduced through the use of a laser beam. Successful numerical representation of the experimental initial and boundary conditions has resulted in good agreement between the experimental and numerical results for the generated combustion flame and pressure-time history. It was concluded that the combustion charachtertics are sensitive to the size of the ignition source in terms of the timing occurrence of the peak pressure but not its magnitude. This finding has practical importance in analyzing explosion hazards, internal combustion engines and gas turbine combustors.