Large eddy simulation of turbulent combustion systems

This paper reviews recent and ongoing work on numerical models for turbulent combustion systems based on a classical LES approach. The work is confined to single-phase reacting flows. First, important physico-chemical features of combustion-LES are discussed along with several aspects of overall LES models. Subsequently, some numerical issues, in particular questions associated with the reliability of LES results, are outlined. The details of chemistry, its reduction, and tabulation are not addressed here. Second, two illustrative applications dealing with non-premixed and premixed flame configurations are presented. The results show that combustion-LES is able to provide predictions very close to measured data for configurations where the flow is governed by large turbulent structures. To meet the future demands, new key challenges in specific modelling areas are suggested, and opportunities for advancements in combustion-LES techniques are highlighted. From a predictive point of view, the main target must be to provide a reliable method to aid combustion safety studies and the design of combustion systems of practical importance.     

Numerical simulation of a turbulent magnetic dynamo

We present numerical simulations of a turbulent magnetic dynamo mimicking closely the Riga-dynamo experiment at Re approximately 3.5x10(6) and 15< or =Rem< or =20. The Reynolds-averaged Navier-Stokes equations for the fluid flow and turbulence field are solved simultaneously with the direct numerical solution of the magnetic field equations. The fully integrated two-way-coupled simulations reproduced all features of the magnetic self-excitation detected by the Riga experiment, with frequencies and amplitudes of the self-generated magnetic field in good agreement with the experimental records, and provided full insight into the unsteady magnetic and velocity fields and the mechanisms of the dynamo action.     

Small scales,many species and the manifold challenges of turbulent combustion

A major goal of combustion research is to develop accurate, tractable, predictive models for the phenomena occurring in combustion devices, which predominantly involve turbulent flows. With the focus on gas-phase, non-premixed flames, recent progress is reviewed, and the significant remaining challenges facing models of turbulent combustion are examined. The principal challenges are posed by the small scales, the many chemical species involved in hydrocarbon combustion, and the coupled processes of reaction and molecular diffusion in a turbulent flow field. These challenges, and how different modeling approaches face them, are examined from the viewpoint of low-dimensional manifolds in the high-dimensional space of chemical species. Most current approaches to modeling turbulent combustion can be categorized as flamelet-like or PDF-like. The former assume or imply that the compositions occurring in turbulent combustion lie on very-low-dimensional manifolds, and that the coupling between turbulent mixing and reaction can be parameterized by at most one or two variables. PDF-like models do not restrict compositions in this way, and they have proved successful in describing more challenging combustion regimes in which there is significant local extinction, or in which the turbulence significantly disrupts flamelet structures. Advances in diagnostics, the design of experiments, computational resources, and direct numerical simulations are all contributing to the continuing development of more accurate and general models of turbulent combustion.     

Numerical simulation of turbulent mixing in plane laser targets

Results of 1D (with allowance for thek−∈ model) and 2D numerical simulations of laser-driven acceleration of corrugated foils are presented. They are made on the basis of two Lagrangian-Eulerian codes, the ATLANT code (Lebedev Physical Institute and Institute of Mathematical Modeling, Moscow) and the EGAK-LAZ code (All-Russian Scientific-Research Institute of Experimental Physics, Sarov). The simulations are made for the experimental conditions of the MISHEN’ laser installation (Nd laser with pulse energy of 50–100 J, TRINITI, Troitsk). The simulations based on the ATLANT and EGAK codes are in good agreement. The foil velocity obtained in the calculations agrees well with the experimental data. It is shown that turbulent mixing leads to a lower average density of nonablative foils. This enables one to determine empirical parameters of thek−∈ model from a comparison of experimental and calculation results. Translated from Preprint of the P. N. Lebedev Physical Institute No. 19 (1997).     

Numerical simulation of super-continuum laser propagation in turbulent atmosphere

Considering the atmospheric extinction and turbulence effects,we investigate the propagation performances of supercontinuum laser sources in atmospheric turbulence statistically by using the numerical simulation method,and the differences in propagation properties between the super-continuum(SC)laser and its pump laser are also analyzed.It is found that the propagation characteristics of super-continuum laser are almost similar to those of the pump laser.The degradation of source coherence degree may cause the relative beam spreading and scintillation indexes to decrease at different propagation distances or different turbulence strengths.The root-mean-square value of beam wandering is insensitive to the variation of source correlation length,and less aperture averaging occurs when the laser source becomes less coherent.Additionally,from the point of view of beam wandering,the SC laser has no advantage over the pump laser.Although the pump laser can bring about a bigger aperture average,the SC laser has a lower scintillation which may be due to the multiple wavelength homogenization effects on intensity fluctuations.This would be the most important virtue of the SC laser that can be utilized to improve the performance of laser engineering.     

A fractal flame-wrinkling large eddy simulation model for premixed turbulent combustion

Combustion plays an important role in a wide variety of industrial applications, such as gas-turbines, furnaces, spark-ignition engines, and various air-breathing engines. The ability to predict and understand the behavior of reacting flows in practical devices is fundamental to improved combustors with higher efficiency and reduced levels of emissions. At present, large eddy simulation is considered the most promising approach for premixed combustion modeling since the large-scale energy containing flow structures are resolved on the grid. However, the typically thin reaction zone cannot be resolved. To overcome this difficulty flamelet models, in which the reaction is assumed to take place in thin layers, wrinkled by the turbulence can sometimes be used. In these models, the turbulent flame speed can be represented as the product of the laminar flame speed, S_u, corrected for the effects of stretch (strain and curvature) and the flame-wrinkling, Ξ. In this study, we propose to model Ξ using fractal theory. This model requires sub-models for the fractal dimension, and the inner and outer cut-offs—the latter being set by the grid. A model is proposed for the inner cut-off, whereas an empirical parameterization is used to provide the fractal dimension. The proposed model is applied to flame kernel growth in homogeneous isotropic turbulence in a fan-stirred bomb and to a lean premixed flame in a plane symmetric dump combustor. Good qualitative and quantitative agreement with experimental data were obtained for the proposed model in both cases. Comparison with other well-known turbulent flame speed closure models shows that the proposed model behaves at least as good, or even better, than the reference models.     

射流抛光多相紊流流场的数值模拟

 理论分析了射流抛光的紊动冲击射流特点,构建了射流抛光的垂直冲击射流模型和斜冲击射流模型。根据射流抛光冲击射流的特点,比较各种流体模型后,采用RNG k-e 模型应用于射流抛光模型的计算。利用计算流体力学理论的二阶迎风格式对抛光模型方程离散,用SIMPLEC数值计算方法对射流抛光过程的紊动冲击射流和离散相磨粒分布进行数值模拟,得到了射流抛光过程的连续流场和离散相磨粒与水溶液的耦合流场,同时计算出了抛光液射流在工件壁面上的压力、速度、紊动强度、剪切力分布和磨粒体积质量分布,分析了垂直射流抛光模型和斜冲击射流抛光模型紊流流场的特点。     

射流抛光多相紊流流场的数值模拟

理论分析了射流抛光的紊动冲击射流特点,构建了射流抛光的垂直冲击射流模型和斜冲击射流模型。根据射流抛光冲击射流的特点,比较各种流体模型后,采用RNG k-e 模型应用于射流抛光模型的计算。利用计算流体力学理论的二阶迎风格式对抛光模型方程离散,用SIMPLEC数值计算方法对射流抛光过程的紊动冲击射流和离散相磨粒分布进行数值模拟,得到了射流抛光过程的连续流场和离散相磨粒与水溶液的耦合流场,同时计算出了抛光液射流在工件壁面上的压力、速度、紊动强度、剪切力分布和磨粒体积质量分布,分析了垂直射流抛光模型和斜冲击射流抛光模型紊流流场的特点。     

Numerical simulation of turbulent wakes with variable total excess momentum

Numerical simulation of the dynamics of plane and axisymmetric turbulent wakes in a homogeneous fluid is performed using the modified e ～ ? model. The calculation results are in good agreement with the known experimental data. Asymptotic decay of axisymmetric turbulent wakes with small nonzero total excess momentum is numerically simulated.     

Challenges for turbulent combustion

Turbulent combustion will remain central to the next generation of combustion devices that are likely to employ blends of renewable and fossil fuels, transitioning eventually to electrofuels (also referred to as e-fuels, powerfuels, power-to-x, or synthetics). This paper starts by projecting that the decarbonization process is likely to be very slow as guided by history and by the sheer extent of the current network for fossil fuels, and the cost of its replacement. This transition to renewables will be moderated by the advent of cleaner engines that operate on increasingly cleaner fuel blends. A brief outline of recent developments in combustion modes, such as gasoline compression ignition for reciprocating engines and sequential combustion for gas turbines, is presented. The next two sections of the paper identify two essential areas of development for advancing knowledge of turbulent combustion, namely multi-mode or mixed-mode combustion and soot formation. Multi-mode combustion is common in practical devices and spans the entire range of processes from transient ignition to stable combustion and the formation of pollutants. A range of burners developed to study highly turbulent premixed flames and mixed-mode flames, is presented along with samples of data and an outline of outstanding research issues. Soot formation relevant to electrofuels, such as blends of diesel-oxymethylene ethers, hydrogen-methane or ethylene-ammonia, is also discussed. Mechanisms of soot formation, while significantly improved, remain lacking particularly for heavy fuels and their blends. Other important areas of research, such as spray atomization, turbulent dense spray flames, turbulent fires, and the effects of high pressure, are briefly mentioned. The paper concludes by highlighting the continued need for research in these areas of turbulent combustion to bring predictive capabilities to a level of comprehensive fidelity that enables them to become standard reliable tools for the design and monitoring of future combustors.     

Fractal modelling of turbulent combustion

In a previous paper we proposed a new model for turbulent flows, called the fractal model (FM), which is applicable both to RANS and LES formulations. Here, the model is extended to the reactive case with the goal of simulating turbulent flames, both premixed and non-premixed.

FM is a subgrid model that describes the physics of the small scales of turbulence building on the phenomenological concept of vortex cascade and on fractal theory. The physics of the small scales is summarized by a turbulent ‘viscosity’ μ_t, to be added to the molecular one. μ_t is zero where the flow is laminar and, in particular, goes to zero at solid walls.

The fundamental assumption in treating combustion in this work is that chemical reactions take place only at the dissipative scales of turbulence, i.e. near the so-called ‘fine structures’ (the eddy dissipation concept). FM predicts the growth of dissipative scales due to heat release; therefore, it enables a local DNS in the hot regions of the flow where the dissipative scale may grow up to the cell dimension. FM can also estimate the volume fraction γ* occupied by the ‘fine structures’; this quantity is critical for modelling the reaction rate, and therefore the source terms in the energy and species equations. FM can also estimate the local surface of the reactive ‘fine structures’, that is, the local flame front area. It also takes into account, although in approximate manner, the formation of islands of unburnt mixture. In this paper, the model (in the isotropic formulation (IFM)) is used in conjunction with a time-dependent LES (but with the limitations of an isotropic model) approach and is validated through a three-dimensional axisymmetric diffusion flame studied experimentally by Correa and Gulati and numerically by many researchers. The time-dependent results obtained are in good agreement with the experiments. Moreover, the IFM solution offers a possible explanation for the stabilization process of this flame, which undergoes local stretching of the order of 46 000 s^?1.     

Numerical simulation of asymmetric non-steady-state turbulent flows around supersonic sharp cones

Results are presented of a numerical simulation of turbulent flows around pointed circular supersonic cones at angles of attack α≥3θ_c, where θ_c is the cone half-angle. The problem is solved in the framework of a locally conical approximation of a set of Reynolds’ equations written in terms of mass-averaged quantities with the use of a differential one-parametric turbulence model. The numerical solutions are obtained with the help of an implicit finite-difference unidirectional scheme, with the focus on violation of the symmetry and stationarity of the separated cross flow.     

Numerical simulation of momentumless turbulent wake dynamics in linearly stratified medium

This paper presents comparison of two numerical models of the momentumless turbulent wake dynamics behind a body of revolution in a linearly stratified medium, namely, the model based on direct (DNS) numerical integration of Navier–Stokes equations in the Oberbeck–Boussinesq approximation and the mathematical model with application of a semi-empirical turbulence model of the third order. The results of calculations by these two models agree with the known experimental data.     

Three-dimensional direct numerical simulation of turbulent channel flow catalytic combustion of hydrogen over platinum

The turbulent catalytic combustion of a fuel-lean hydrogen/air mixture (equivalence ratio ? = 0.24) was investigated by means of three-dimensional direct numerical simulation (DNS) in a platinum-coated plane channel with a prescribed wall temperature of 960 K and an incoming Reynolds number, based on the channel height, of 5700. Heat transfer from the hot catalytic walls laminarized the flow, as manifested by the progressive suppression of the high vorticity components of the flow aligned parallel to the channel walls at increasing streamwise distances. The impact of turbulence suppression on the mass transfer towards or away from the catalytic wall was subsequently assessed. Far upstream where high turbulence fluctuations persisted, the instantaneous local transverse gradient of the limiting hydrogen reactant (a quantity proportional to the catalytic reaction rate) as well as the instantaneous hydrogen concentration at the wall exhibited strong fluctuations by up to 300%, a result of finite-rate chemistry induced by the high inrush events towards the catalytic walls. Fourier analysis of the reaction rate fluctuations yielded peak frequencies of less than 1 kHz, values comparable to the thermal response frequencies of typical materials in commercial catalytic geometries. This has direct implications on the thermal stress of the reactor walls as well as on the decoupling between flow and solid thermal modeling currently used in practical catalytic reactors. Far downstream, the dampening of turbulence resulted in weaker hydrogen concentration fluctuations with nearly symmetric distributions. Finally, computed transverse turbulent species fluxes indicated inherent weaknesses of near-wall turbulence models in describing turbulent transport of species with disparate molecular diffusivities.     

Paradigms in turbulent combustion research

The development of the basic conceptual viewpoints, or paradigms, for turbulent combustion in gases over the last 50 years is reviewed. Significant progress has been made. Recent successes in the prediction of pollutant species and extinction/re-ignition phenomena in non-premixed flames are seen as the result of close interaction between experimentalists, theoreticians, and modellers. Premixed turbulent flames seem to be dependent on a much wider range of factors, and predictive capabilities are not so advanced. Implications for large eddy simulation (LES) and partially premixed combustion are outlined.     

Numerical simulation of the 3-dimensional combustion of aluminized heterogeneous propellants

We report the first 3-dimensional simulations of aluminized propellant combustion, accounting for heat conduction in the solid, combustion in the gas-phase, and coupling of these via the irregularly moving propellant surface, one that can not be defined by a single-valued height function. The simulations are used to examine the dynamics of aluminum particles in the near-neighborhood of the surface after detachment, and to provide an estimate of the time to ignition of the particles, and their speed and height above the surface at ignition. In addition, we examine the temperature history of the particles during their rise to the surface, determine whether they melt or not, and in this way test Cohen’s well-known melting criterion. And, we discuss a simple model which provides insights into how aluminum particles floating on a binder melt layer would migrate because of surface tension effects, and calculate an average migration distance that is consistent with previous agglomeration studies.