Jerzy Chomiak

Turbulent flame speed and thickness: phenomenology, evaluation, and application in multi-dimensional simulations

Due to their fundamental importance for premixed combustion theory, turbulent flame speed and thickness were a subject of a large number of investigations for many decades. The paper reviews the research and extensively discusses the still unresolved issues in an attempt to define a foundation for evaluating different combustion models and defining a simple approach to multi-dimensional computations of premixed turbulent combustion. The approach consists of the use of an algebraic expression for the local turbulent flame speed in order to close the averaged balance equations describing the combustion process. Several models have been suggested utilizing this approach and the first laboratory and industrial applications of them have shown encouraging results. These successful applications motivate a thorough discussion and further development of the approach. Models utilizing the approach are reviewed and two issues are emphasized. First, certain models focus on the combustion regime characterized by a growing mean flame brush thickness, whereas other models are associated with fully developed flames of asymptotically stationary structure. Second, many different expressions for flame speed are invoked by different models. Thus, the behavior of the mean flame brush thickness and flame speed should be analyzed in order to provide a more solid phenomenological base for the approach and this is the main goal of the paper. Moreover, such an analysis also aims at selecting experimentally well-established trends and, thus, contributes to the development of a database necessary for testing various models of premixed turbulent combustion. Sources of errors in measurements of turbulent flame speeds are discussed and strong quantitative scatter of the published data is demonstrated. Nevertheless, turbulent flame speed, St, is shown to be a phenomenologically meaningful quantity, because various experimental investigations indicate the same qualitative trends in the behavior of St at moderate turbulence. The following trends: (1) an increase in St by rms turbulent velocity u′; (2) an increase in St and dSt/du′ by the laminar burning velocity; and (3) an increase in St by pressure despite the decrease in the laminar burning velocity, are well-established and can be used for testing various models of premixed turbulent combustion. Moreover, certain experimental results indicate a decrease in St by molecular transfer coefficients, other things being equal, and this trend may also be used for testing models. A number of various expressions for St, available in the literature, are tested against well-established trends, but only a few expressions are shown to be able to predict all the basic trends. An analysis of numerous experimental data obtained by various teams under different conditions indicates that a self-similar regime of premixed turbulent combustion, characterized by growing mean flame brush thickness, δt, and by the universal dimensionless spatial profile of the progress variable across the brush, occurs in most laboratory and industrial burners. The development of δt is mainly controlled by turbulent diffusion. Only certain models are able to describe this regime. From the group of models evaluated positively, the Flame Speed Closure (FSC) model is highlighted since: (1) it corresponds to the regime of growing mean flame brush thickness; and (2) it utilizes an experimentally well-supported expression for turbulent flame speed. Various numerical tests of the model, performed by numerous teams under substantially different conditions are summarized. Further development and validation of the model and it applications are reviewed. Finally, the paper shows that, after decades of long research, a simple, robust, conceptually straightforward, and extensively validated premixed combustion simulation tool is available for applications now.

Flame quenching by turbulence

Abstract Quenching of laminar flames entering a region of intense turbulence without mean flow was studied by instantaneous temperature, global and local chemiluminescence measurements, and fast Schlieren photography. The experiments were performed in a constant-pressure, square 50 × 50 mm tube for flammability limit studies for two integral scales of turbulence, namely, 2.14 and 6 mm over the rms turbulent velocity range of 0.1-1.2 and 0.5-3.4 m/s, respectively. Lean and rich methane and propane air mixtures and ammonia-air mixtures in the entire burning range were studied. It was shown that premixed flames are quenched by turbulence for a critical value of Karlovitz-Kovasznay criterion K = ( u′ L )( δ l ν l ) of the order of 10–20, where u′ denotes the rms turbulent velocity, L the integral scale of turbulence, δl the laminar flame thickness, and νl the laminar burning velocity.

Transient and geometrical effects in expanding turbulent flames

To study turbulent combustion, experiments with expanding, statistically spherical flames ignited by a spark are widely used. The goal of the work is to show that certain trends in the behavior of turbulent flame speed 5, observed in such experiments, are substantially affected by the curvature of the mean flame brush and by the ignition conditions. For this purpose, simulations of expanding, spherical, premised flames were performed using the k - ϵ turbulence model and the Turbulent Flame Speed Closure of the balance equation for a progress variable. Three major trends have been observed in the simulations. First, the analysis of various physical mechanisms controlling the increase of St, has shown that the time-dependence of the mean heat release rate, invoked by the model, is of substantial importance for small kernels only. For moderately large flames, the development of St, is mainly controlled by the relaxation of the reduction effect of the mean flame curvature on the flame speed. The second manifestation of the mean curvature mechanism is the opposite effects of the turbulent length scale L on the speed of asymptotically stationary, planar flames and of moderately large, statistically spherical flames. In the spherical case, a stronger reduction of the flame speed of small kernels is observed in turbulence with a larger scale. As the kernel grows, the reduction effect relaxes and the dependence of St, on L reverses. Third, when the ignition energy is close to the critical value igniting the turbulent mixture, a regime of kernel expansion characterized by substantially reduced flame speed and burning velocity can occur even in relatively large, statistically spherical turbulent flames. The physical cause of this memory effect consists in the formation of a highly dispersed kernel followed by slow after-burning, When the spark energy is kept constant, the increase in turbulent velocity u′ increases the critical ignition energy and the transformation to the aforementioned regime occurs. This mechanism can contribute to the decrease of St, with u′, observed in many experiments. Finally, the suppression of counter-gradient diffusion in spherical flames is discussed at the end of the paper.

Modeling variable density effects in turbulent flames—Some basic considerations

Abstract The paper discusses the basic physical phenomena involved in pressure-density interactions, and presents models of pressure-velocity, pressure-scalar, baroclinic and dilatation effects for variable density low Mach-number turbulence. Their implementation in the κ - e framework is then described and their performance evaluated. The models assume that both scalar transport and turbulence generation arising from pressure-density interactions in flames are caused by the motion of large scale turbulent thermals superposed on the normal turbulence mechanism. The velocity of the thermals is related directly to the mean pressure gradient and local density differences in the flames. It is furthermore assumed that the correction for dilatation effects in the φ - e system can be determined from the constraint of conservation of the angular momentum of turbulence per unit mass. Simple corrections of the κ - e system are proposed for fast chemistry diffusion and premixed flames subject to variable pressure gradients, which offer substantial improvements in the predictions of the flames. Some problems remain, particularly in predictions of turbulence in premixed flames, owing to large scale instabilities of the flames observed in the experiments.

Flame propagation along a fine vortex tube

ABSTRACT Interaction between a premixed flame and a fine vortex tube perpendicular to the flame is simulated numerically in order to study the effects of the rotating velocity and tube diameter on flame propagation along the vortex tube. The vortex tube has an initial circumferential velocity ranging from 1.8 to 36 times the laminar burning velocity and an initial core diameter ranging from 0.18 to 1.71 times the flame thickness. It is shown that a premixed flame can be accelerated along the vortex tube axis, and that the propagation velocity is proportional to the maximum circumferential velocity of the vortex tube. It is also shown that the influence of the vortex on the flame propagation can be neglected when the Reynolds number based on the vortex tube diameter and maximum circumferential velocity is below 10. The proportionality factor between the propagation velocity and circumferential velocity increases above the critical number with azimuthal speed and reaches about one at the Reynolds number of ...

Lewis Number Effects in Premixed Turbulent Combustion and Highly Perturbed Laminar Flames

Abstract Various perturbed laminar flames are numerically simulated by reducing combustion chemistry to a single reaction. The following flame configurations are addressed: expanding spherical, cylindrical, and symmetrical planar flames; converging spherical flame; expanding cylindrical and symmetrical planar flames affected by external steady or time-dependent strain rate; steady strained cylindrical and symmetrical planar flames. The results (1) show that a simple linear relation between the local consumption velocity and flame stretch rate is valid only for weakly perturbed laminar flames; (2) highlight the importance of transient effects; and (3) show that, in the case of a small Lewis number, the highest local combustion rate is reached in the expanding spherical flame ignited by the hot pocket of the critical radius. This highest local combustion rate is successfully used to describe the extensive Karpovs experimental data base on turbulent burning velocities for mixtures characterized by substanti...

Basic considerations in the turbulent flame propagation in premixed gases

Abstract A general discussion is given of some fundamental problems of turbulent flame propagation in premixed gases. The following subjects are considered in greater detail: Stability of laminar flames in turbulent flow, shear wave-flame interaction, flame generated turbulence, influence of small scale turbulence on flame propagation and structure of turbulent flames at high Reynolds numbers. The principal object of this study is to describe the basic physical facts which have to be taken into consideration in the modeling of turbulent flames in gases without giving a detailed survey of all the research that has been carried out in the field.

Turbulent burning velocity and speed of developing, curved, and strained flames

The problem of a physically meaningful definition of speed and burning velocity of a developing turbulentpremixed flame of a finite thickness is studied analytically and numerically in planar and spherical cases. Analytical studies are based on the well-documented self-similarity of the normalized profiles of the mean density across the turbulent flame brush. Numerical simulations have been performed with the flame speed closure model of turbulent combustion. The goals of the study are to develop methods for determining two reference surfaces: (1) a flame speed surface, that is, a surface the speed of which is controlled by the burning rate integrated across the brush but is not directly affected by the rate of flame thickness growth, and (2) a burning velocity surface, that is, a surface the area of which multiplied by the flame speed defined above characterizes the aforementioned burning rate. For planar flames, the former surface is defined and proven to be an isoscalar one. For spherical flames,expressions for determining both surfaces are derived, but these surfaces are different and they are not isoscalar ones. Simulations have shown that (1) these features are not well pronounced under typical conditions and (2) when investigating spherical flames, one may associate flame speed and burning velocity with the same isoscalar surface. A method for evaluating the unburned mixture velocity, which is needed to convert the observed speed of expanding spherical flames to the speed with respect to unburned mixture, is developed. The method is shown to be applicable to measurements of turbulent flame speeds in stagnation flows also. In all the cases studied, a reference value of the progress variable is found to be a roughly invariant (with respect to flame geometry and development) characteristic determining flame speed and burning velocity surfaces.

Combustion characteristics of diesel sprays from equivalent nozzles with sharp and rounded inlet geometries

The ignition delay, flame structure, temperature, and soot distribution in a diesel spray injected at 80 MPa in a high-temperature (830 K) and high-pressure (6 MPa) quiescent air was studied for two nozzles, one with 0% hydrogrinding (HG) and another with 20% HG. HG in diesel nozzles is the process of forcing an abrasive fluid through the nozzles with sharp inlets; the abrasive fluid wears the sharp inlet edge of the spray holes until a prescribed flow rate is achieved. The percentage of HG used in this article is a measure of an increase in the volume flow rate after the HG process in a low-pressure flow test. The difference is substantial. For convenience, at some instances 0% HG is referred to as the sharp inlet and 20% HG as the rounded inlet. Spray impingement studies were made to evaluate the time-resolved spray momentum, nozzle discharge coefficient, and turbulence kinetic energy to characterize the nozzle internal flow effects on spray combustion. Equivalent nozzles were selected such that the momentum rates of the spray from both nozzles, as determined by the spray impingement, were the same. This was obtained by increasing the orifice diameter of the nozzle with 0% HG to compensate for the higher friction losses and lower discharge coefficient of the nozzle. The differences in discharge coefficient indicate that the flows inside the nozzles have different turbulence and cavitation levels. Inspite of the strong differences in internal flow, the sprays, which had the same momentum rate, behaved identically. In particular, the spray dispersion, penetration, ignition delay, combustion temperatures, flame volumes, soot concentration, and liftoff distances were almost the same for both sprays. Also, the use of noncircular injection orifices was shown not to change the combustion and emission performance of a diesel engine when the momentum of the fuel jets is the same. The work thus shows that diesel spray combustion is fully controlled by the spray momentum and that for realistic injection and combustion conditions the internal nozzle flow structure does not matter as long as it does not change the momentum.

Evaluation of Ignition Improvers for Methane Autoignition

The objective of this study was to estimate the efficiency of methane autoignition promotion by testing different ignition improvers including hydrogen, H2, hydrogen peroxide, H2O2, ozone,O3 and dimethyl ether, DME, (CH3)2O. This was accomplished by computing ignition delays for CH4/O2/Ar or N2 mixtures of various compositions, concentrations of the promoters, pressures and temperatures. Ignition delay times for additive-free mixtures were used for tuning a methane oxidation mechanism consisting of 185 reversible elementary reactions between 32 species. A selection of the reaction rate parameters available in the standard databases was made to optimize the agreement between simulation and experimental results for one particular set of test conditions (reference mixture) by refining the rate parameters of the most sensitive stages revealed by sensitivity analysis. The agreement achieved between model predictions and shock tube experimental data is very good. To investigate the effect of dimethyl ether on m...