Numerical Simulation Of Turbulent Combustion Research Articles

Implications of Using Scalar Forcing to Sustain Reactant Mixture Stratification in Direct Numerical Simulations of Turbulent Combustion

A recently proposed scalar forcing scheme that maintains the mixture fraction mean, root-mean-square and probability density function in the unburned gas can lead to a statistically quasi-stationary state in direct numerical simulations of turbulent stratified combustion when combined with velocity forcing. Scalar forcing alongside turbulence forcing leads to greater values of turbulent burning velocity and flame surface area in comparison to unforced simulations for globally fuel-lean mixtures. The sustained unburned gas mixture inhomogeneity changes the percentage shares of back- and front-supported flame elements in comparison to unforced simulations, and this effect is particularly apparent for high turbulence intensities. Scalar forcing does not significantly affect the heat release rates due to different modes of combustion and the micro-mixing rate within the flame characterised by scalar dissipation rate of the reaction progress variable. Thus, scalar forcing has a significant potential for enabling detailed parametric studies as well as providing well-converged time-averaged statistics for stratified-mixture combustion using Direct Numerical Simulations in canonical configurations.

Turbulent Flame Propagation in Hydrogen-Air and Methane-Air Mixtures in the Field of Synthetic Turbulence: Direct Numerical Simulation

A technique alternative to the direct numerical simulation of turbulent combustion of gas mixtures is proposed. It is based on the solution of the three-dimensional transport equations for species concentrations and the energy conservation equation in the “synthetic” field of constant-pressure homogeneous, isotropic and statistically stationary (forced) turbulence using the detailed reaction mechanism. The synthetic turbulence with given spatial and temporal correlation functions is generated using the Monte Carlo method, assuming that the components of the vector of fluctuation velocity obey the normal Gaussian distribution. The technique is applied to the problem of turbulent combustion of fuel-lean and stoichiometric mixtures of hydrogen and methane with air at a turbulence intensity up to 10 m/s. The calculated turbulent flame propagation velocities agree satisfactorily with the values measured in the fan-stirred bomb. The predicted volume fractions of active reaction centers H, O, and OH in a turbulent flame are shown to be less than in a laminar flame up to an order of magnitude, which also agrees with the experiment. In general, calculations indicate that the “wrinkled flame” model is applicable to fuel-lean and stoichiometric mixtures of hydrogen and methane with air at turbulence intensities up to 10 m/s

CFD Analysis and NOx Prediction in H2 and Ch4 Turbulent Non-premixed Flame Compared with Swirling Flame

This work is devoted to the comparative study for the formation and dissociation of nitrogen oxides by the numerical simulation of turbulent combustion without premix in a combustion chamber having a cylindrical shape with two coaxial jets, two flames using the ANSYS fluent software16.0. The study focuses on the influence of the type of fuel on the composition of discharges in content with NOx, that is to say two cases are treated and compared. Turbulence is modeled by the k-ε model and the chemical aspect of combustion is treated by the PDF model for each flame. The calculation results relate to the characteristics of dynamic fields, temperature, the mass fractions of different species involved in the combustion process and the NOx prediction. The effect of the swirl is also tested in this study with a CFD prediction of non premixed swirling g flame. These results are compared with measurements and confrontations is satisfactory.

Numerical simulation of turbulent combustion of methaneair mixture in moderate Reynolds number conditions

Numerical simulation of turbulent combustion of methaneair mixture in moderate Reynolds number conditions

Three-Dimensional Direct Numerical Simulation of Turbulent Combustion of Hydrogen-Air Mixtures in a Synthetic Turbulent Field

In this paper, we propose a method of three-dimensional direct numerical simulation of turbulent flame propagation in gas-reacting mixtures under stationary, homogeneous, and isotropic turbulence conditions. This method is based on the detailed kinetic mechanism of combustion of a multicomponent mixture, does not contain any adjustable parameters, and is applied to the calculation of turbulent combustion of fuel-lean hydrogen–air mixtures. The calculated propagation speeds of the turbulent flame are in satisfactory agreement with the measured values. The calculated concentrations of the reaction active centers, such as OH, H, and O, in a turbulent flame are lower than those in a laminar flame, which also agrees with the experimental results.

Direct Numerical Simulation of Turbulent Combustion of Hydrogen—Air Mixtures of Various Compositions in a Two-Dimensional Approximation

A technique of two-dimensional direct numerical simulation of turbulent flame propagation in reacting gas mixtures under stationary homogeneous isotropic turbulence conditions is proposed. This technique is based on a detailed kinetic mechanism of combustion of a multicomponent mixture and uses no fitting parameters. It is applied to the calculation of turbulent combustion of a hydrogen-air mixture. A condition is proposed to compare the results of two-dimensional calculations (dependences of flame propagation velocity on turbulence intensity) with the data of actual three-dimensional experiments. The obtained agreement between the calculated and measured dependences confirmed the validity of the proposed condition. The effects of pressure on the flame propagation velocity were considered. The calculated concentrations of the active reaction centers-hydroxyl (OH) and H and O atoms-in turbulent flame are lower than those in laminar flame, which also agrees with experimental results.

Numerical studies of turbulent swirling reacting flows using LES and URANS

Numerical studies of turbulent swirling reacting flows are performed using unsteady Reynolds-averaged Navier-Stokes (URANS) and large-eddy simulation (LES) approaches. Aiming at the reduction of computational costs, the present research assesses the feasibility of URANS for the computations of turbulent reacting flows. The conserved scalar-mixture fraction-based thermo-chemical variables are described using the steady laminar flamelet model. The Favre-filtered scalars are obtained from the presumed beta probability density function (β–PDF) approach. The flow field exhibits upstream recirculation zone due to the bluff body and a downstream vortex breakdown region induced by the swirling flow. The study reveals that the strength of the vortex decays with the increase of distance from the injection region. The temperature exhibits high values in the region surrounding the flame. The present research shows that in spite of its low computational cost, the URANS approach is quite dissipative when compared with LES. Therefore, LES is a more appropriate approach for the numerical simulation of turbulent combustion, when the solution accuracy is of concern.

Trigger Detection for Adaptive Scientific Workflows Using Percentile Sampling

The increasing complexity of both scientific simulations and high-performance computing system architectures are driving the need for adaptive workflows, in which the composition and execution of computational and data manipulation steps dynamically depend on the evolutionary state of the simulation itself. Consider, for example, the frequency of data storage. Critical phases of the simulation should be captured with high frequency and with high fidelity for postanalysis; however, we cannot afford to retain the same frequency for the full simulation due to the high cost of data movement. We can instead look for triggers, indicators that the simulation will be entering a critical phase, and adapt the workflow accordingly. In this paper, we present a methodology for detecting triggers and demonstrate its use in the context of direct numerical simulations of turbulent combustion using S3D. We show that chemical explosive mode analysis (CEMA) can be used to devise a noise-tolerant indicator for rapid increase in heat release. However, exhaustive computation of CEMA values dominates the total simulation, and thus is prohibitively expensive. To overcome this computational bottleneck, we propose a quantile sampling approach. Our sampling-based algorithm comes with provable error/confidence bounds, as a function of the number of samples. Most importantly, the number of samples is independent of the problem size, and thus our proposed sampling algorithm offers perfect scalability. Our experiments on homogeneous charge compression ignition and reactivity controlled compression ignition simulations show that the proposed method can detect rapid increases in heat release, and its computational overhead is negligible. Our results will be used to make dynamic workflow decisions regarding data storage and mesh resolution in future combustion simulations.

Implications of laminar flame finite thickness on the structure of turbulent premixed flames

A layered description of the structure of turbulent flame brushes is provided for situations featuring large but finite values of the Damköhler number, which correspond to the wrinkled flame regime of turbulent premixed combustion. One special focus of this study is placed on the description of the leading edge of the turbulent flame brush, the role of which is known to be essential with respect to propagation, transport and stabilization issues. On the basis of rather simple and well-identified working hypotheses, the influence of slight increases in the Karlovitz number values is revealed. The phenomenology and associated statistics are also investigated analytically, which leads to a mathematical description of the leading edge internal structure. With respect to the progress variable statistics, i.e. probability density function, this leading edge can indeed be thought of as the inner part of a boundary layer where the influence of the finite thickness of laminar flamelets can no longer be neglected. From the proposed description, standard fast-chemistry closures, which are currently used to perform the numerical simulation of turbulent combustion, may easily be generalized to account for the finite-rate chemistry effects occurring in this sublayer, thus emphasizing the interest of the present analysis for turbulent combustion theory and modelling.

Impact of chemistry models on flame–vortex interaction

In numerical simulations of turbulent combustion, accurate modeling of the fuel chemistry is a critical component of capturing the relevant physics. Various chemical models are currently being used including detailed chemistry, tabulated chemistry, one-step chemistry, and rate-controlled constrained-equilibrium. However, the differences between these models and their impacts on the fluid dynamics are not well understood. Towards that end, the interaction between a laminar premixed hydrogen-air flame and a two-dimensional monopole vortex is investigated through simulations using the four previously-mentioned chemistry models. In these simulations, the flame speed, viscosity, diffusivity, conductivity, heat capacity, density ratio, and initial vortex characteristics are virtually identical providing a comparison of the effects of each chemical model alone. The results with each model are compared by considering the evolution of the flame structure and the characteristics of the vortex. All four models predict very similar results for the interaction with a small, fast vortex. In the interaction with a large, slow vortex, results from one-step chemistry are different due to a different flame thickness. The large, fast vortex case showcases differences in the flame structure and larger discrepancies between the modeling approaches.

Simulation of a high Reynolds number reactive transverse jet and the formation of a triple flame

This paper briefly describes a hybrid Eulerian–Lagrangian approach for the numerical simulation of turbulent combustion and its application to the study of transverse reactive jets. Because of their interesting mixing properties, transverse jets are important to a variety of industrial applications such as film cooling, primary or dilution jets in gas turbines, and flame stabilization in high speed combustion. To capture the jet complex structure and the associated reaction dynamics, we developed a fast, multiscale and parallel 3D code using a Lagrangian particle method to solve the vorticity transport equation and an Eulerian adaptive grid-based method to solve the reactive transport equations.Flame anchoring computations were done for a pure methane jet in a crossflow of air at a Reynolds number Re=1000 and at a velocity ratio of 5 between the jet and the crossflow velocity. The results show that the reactive jet structure strongly resembles that of the non-reactive case, although the reactive jet is slower to bend into the cross flow and it is wider in the spanwise direction. The flame is anchored downstream of the nozzle in a relatively low velocity region and shows a triple flame structure. This triple flame seats in a region of relatively low scalar dissipation rate. We describe the three dimensional topology of the triple flame, as well as the impact of the jet counter-rotating vortex pair (CVP) on the flame front.

Turbulent combustion of biomass syngas

Biomass syngas, a low-calorific gas continuously produced in the process of biomass pyrolysis and gasification, a relatively little-known fuel, can be used for heat production in an industrial plant. However, stringent emission norms have to be fulfilled at all times. In this paper, we present numerical simulations of turbulent combustion of the biomass syngas. They show that strict norms can be kept and that the process does not need any additional oil-burners to sustain the mandatory temperature. We have created a 3D model of the real industrial syngas combustion chamber of complex geometry involving an axial co-flow jet and six additional air inlets to enhance the swirl and to promote the generation of vortices. We have run a series of non-premixed combustion simulations for the syngases produced from wood chips and from turkey feathers.

Numerical simulation of turbulent combustion of subsonic gas jet flows

A physical and mathematical model of turbulent combustion of subsonic gas fuel jet flows flowing into an air space is proposed. The processes are described by averaged equations of the boundary layer with a turbulent viscosity model and a combustion diffusion model. As turbulent viscosity models, the well-known two-parameter k-ɛ standard and k-ω models are taken. The results of the averaged and pulsating flow characteristics’ comparison of numerical calculations with the experimental data are presented.

Numerical investigation of the application of high temperature air combustion in an industrial furnace

Numerical simulation of turbulent combustion in the radiation section of an industrial steam cracking furnace is carried out in classical premixed and newly developed high temperature air combustion (HiTAC) regimes. The eddy dissipation concept coupled with the detailed KEE-58 mechanism is employed to resolve turbulence–chemistry interaction and combustion chemistry. Three simulations with highly preheated atmospheric air (19%wt oxygen) and diluted fuel are performed to study the attractive features of the so-called HiTAC regime. It is shown that when the inlet air temperature increases to 1100 K, fuel dilution can provide favorable characteristics of the high temperature air combustion. In this temperature, combustion continues even under extremely fuel-lean conditions such as 1.5%wt of methane in the reactants. Calculation of the Damköhler number determined that a finite-rate reaction zone is volumetrically extended throughout the furnace. Computation of NO x emission from different chemical routes revealed that 80% reduction of NO x formation in fuel-diluted high temperature air combustion regimes could be achieved compared to the normal firing condition. Moreover, the results implied that the emission of NO x in normal firing and high temperature air combustion conditions was mainly attributed to thermal/prompt and prompt/NNH mechanisms, respectively.

Prospects of Numerical Simulations of Turbulent Combustion Using the K Computer

Prospects of Numerical Simulations of Turbulent Combustion Using the K Computer

Three-dimensional hydrodynamic simulations of the combustion of a neutron star into a quark star

We present three-dimensional numerical simulations of turbulent combustion converting a neutron star into a quark star. Hadronic matter, described by a micro-physical finite-temperature equation of state, is converted into strange quark matter. We assume this phase, represented by a bag-model equation of state, to be absolutely stable. Following the example of thermonuclear burning in white dwarfs leading to Type Ia supernovae, we treat the conversion process as a potentially turbulent deflagration. Solving the non-relativistic Euler equations using established numerical methods we conduct large eddy simulations including an elaborate subgrid scale model, while the propagation of the conversion front is modeled with a level-set method. Our results show that for large parts of the parameter space the conversion becomes turbulent and therefore significantly faster than in the laminar case. Despite assuming absolutely stable strange quark matter, in our hydrodynamic approximation an outer layer remains in the hadronic phase, because the conversion front stops when it reaches conditions under which the combustion is no longer exothermic.

A high-order public domain code for direct numerical simulations of turbulent combustion

A high-order scheme for direct numerical simulations of turbulent combustion is discussed. Its implementation in the massively parallel and publicly available Pencil Code is validated with the focus on hydrogen combustion. This is the first open source DNS code with detailed chemistry available. An attempt has been made to present, for the first time, the full set of evolution and auxiliary equations required for a complete description of single phase non-isothermal fluid dynamics with detailed chemical reactions. Ignition delay times (0D) and laminar flame velocities (1D) are calculated and compared with results from the commercially available Chemkin code. The scheme is verified to be fifth order in space. Upon doubling the resolution, a 32-fold increase in the accuracy of the flame front is demonstrated. Finally, also turbulent and spherical flame front velocities are calculated and the implementation of the non-reflecting so-called Navier–Stokes Characteristic Boundary Condition is validated in all three directions.

Numerical simulation of turbulent combustion in porous materials

This paper presents one-dimensional simulations of combustion of an air/methane mixture in porous materials using a model that explicitly considers the intra-pore levels of turbulent kinetic energy. Transport equations are written in their time-and-volume-averaged form and a volume-based statistical turbulence model is applied to simulate turbulence generation due to the porous matrix. Four different thermo-mechanical models are compared, namely Laminar, Laminar with Radiation Transport, Turbulent, Turbulent with Radiation Transport. Combustion is modeled via a unique simple closure. Preliminary testing results indicate that a substantially different temperature distribution is obtained depending on the model used. In addition, for high excess air peak gas temperature is reduced and the flame front moves towards the exit of the burner. Also, increasing the inlet flow rate for stoichiometric mixture pushes the flame out of the porous material.

Monte Carlo method of radiative transfer applied to a turbulent flame modeling with LES

Radiative transfer plays an important role in the numerical simulation of turbulent combustion. However, for the reason that combustion and radiation are characterized by different time scales and different spatial and chemical treatments, the radiation effect is often neglected or roughly modelled. The coupling of a large eddy simulation combustion solver and a radiation solver through a dedicated language, CORBA, is investigated. Two formulations of Monte Carlo method (Forward Method and Emission Reciprocity Method) employed to resolve RTE have been compared in a one-dimensional flame test case using three-dimensional calculation grids with absorbing and emitting media in order to validate the Monte Carlo radiative solver and to choose the most efficient model for coupling. Then the results obtained using two different RTE solvers (Reciprocity Monte Carlo method and Discrete Ordinate Method) applied on a three-dimensional flame holder set-up with a correlated-k distribution model describing the real gas medium spectral radiative properties are compared not only in terms of the physical behavior of the flame, but also in computational performance (storage requirement, CPU time and parallelization efficiency). To cite this article: J. Zhang et al., C. R. Mecanique 337 (2009).

Terascale direct numerical simulations of turbulent combustion — fundamental understanding towards predictive models

Advances in high-performance computational capabilities enable scientific simulations with increasingly realistic physical representations. This situation is especially true of turbulent combustion involving multiscale interactions between turbulent flow, complex chemical reaction, and scalar transport. A fundamental understanding of combustion processes is crucial to the development and optimization of next-generation combustion technologies operating with alternative fuels, at higher pressures, and under less stable operating conditions, such as highly dilute, stratified mixtures. Direct numerical simulations (DNS) of turbulent combustion resolving all flow and chemical features in canonical configurations are used to improve fundamental understanding of complex flow processes and to provide a database for the development and validation of combustion models. A description of the DNS solver and its optimization for use in massively parallel simulations is presented. Recent DNS results from a series of three combustion configurations are presented: soot formation and transport in a nonpremixed ethylene jet flame, the effect of fuel stratification in methane Bunsen flames, and extinction and reignition processes in nonpremixed ethylene jet flames.