摘要:

A numerical investigation of structures of laminar liquid oxygen/hydrogen spray diffusion flames in the counterflow configuration is performed for high subcritical, cryogenic conditions. The dilute spray is modeled using Lagrangian equations; both mono- and bidisperse sprays are considered. The Eulerian formulation of the gas phase equations includes detailed transport and detailed chemical reactions which enables the investigation of both the inner flame structure and the extinction of these flames. The model includes transport coefficients for the gas phase for cryogenic temperatures and elevated pressure. Moreover, the formulation of thermodynamic equilibrium at the droplet surface includes both temperature and pressure dependence of the binary mixture, and it replaces the commonly used Raoult's law. The study presents parametric dependencies of the flame structure on equivalence ratio, strain rate, and dispersity of the spray. It appears that the Sauter mean diameter is a good approximation for describing the flame structure for the bidisperse spray flames at low strain.

ISSN:0010-2202    

DOI:10.1080/00102200008947334

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

For reactive two-phase flows a single probability density function (PDF) is defined for the totality of all gas-phase and liquid-phase random variables. This single unconditional PDF - or equivalent^ its gas-conditional and liquid-conditional PDF together - allows a complete statistical description and hence a full modelling of reactive, laminar or turbulent, two-phase flows. For the special case of two-phase sprays, for both the gas phase and the liquid phase general PDF-transport equations are given in which the terms concerning the local and instantaneous interaction between the two phases are in closed form but certain turbulence terms would require modelling.

ISSN:0010-2202    

DOI:10.1080/00102200008947335

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

The present study is a new step in the attempt to extend an approach used in the modeling of gaseous turbulent combustion to spray turbulent combustion. Our attention is focused on the new random fluctuations created by the presence of the spray. The ideas developed here are based on the discussion presented by Borghi (1995). A local study of the equations allows us to find a closure for the new correlation term induced by spray evaporation in the equation for the variance of the mixture fraction. This was done without new assumptions, just by using the stochastic approach already developed in the KIVA II code to describe spray vaporization. We also show the importance of a new variable, Z,, which describes the initial state of an evaporated fluid particle. An equation for the PDF of this variable and for its mean value is also written in closed form. For the moment, its contribution to the mixture fraction PDF is taken into account through its average value. Finally we have tested our model with an experiment where one set of experimental conditions is close to the infinitely fast chemistry and another set of conditions demonstrates the effect of finite chemical kinetics. We have found a qualitative agreement between the measured data and our computational results. Further development should be done, for instance coupling this model with an atomization model to avoid any assumption about the first stages of the evaporation and combustion process.

ISSN:0010-2202    

DOI:10.1080/00102200008947336

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

In this work the interaction between turbulence and premixed combustion is studied by means of direct numerical simulations using a level set equation based on the scalar G that describes the motion of the flame front represented by an iso-surface G = G0. The G-equation contains terms accounting for flame propagation, flame curvature and gas expansion effects. The flame front is transported and wrinkled by the turbulent flow field and propagates in a direction normal to itself with its laminar burning velocity. The turbulent flow field is based on the constant density Navier-Stokes equations.

ISSN:0010-2202    

DOI:10.1080/00102200008947337

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

In this paper, a numerical study of turbulent premixed flames of propane and air is presented. The experimental configuration investigated (Besson el al, 1999) is a planar channel thai incorporates a sudden expansion in a form of two symmetrical backward-facing steps. Two lean mixtures of propane and air are injected in the upper and lower halves of the upstream part of the channel, separated by a plate and in the present work, the fuel-air ratios of these two streams are different. The numerical model includes a second-order closure for Reynolds stresses and uses a modified Bray-Moss-Libby theory to model a reactive turbulent flow with variable stoichiometry.

ISSN:0010-2202    

DOI:10.1080/00102200008947338

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

A computational model for turbulent premixed gaseous combustion is investigated, where the combustion process is modelled in terms of a single transport equation for a reaction progress variable c. Turbulent closure of the source term of the progress variable is based on a model, where the turbulent flame speed is used in an extension to a field variable. In order to check the model, numerical results are compared with experimental data from a turbulent premixed V-shaped flame, where the conditions of the approaching turbulent flow and of the chemical processes have been varied separately. Regarding the simple structure of this model, it is found to predict the flame shape and flame width sufficiently well. Additionally, three other relations from the literature for the turbulent flame speed variable have been tested with in this approach, showing that experimentally determined flame speed relations have to be reduced in order to be used within this flame speed closure. Furthermore, the influence of the formal structure of the reaction term (¯ωc∼ |∇¯c|) is compared with that of two other possible approaches (¯ωc∼ ¯c · (1 − ¯c)/Ly, and ¯ωc∼ min[(l − ¯c),¯c,γ]). While the experimental flame shape has straight boundaries, for the parabolic approach a concave bounded flame shape is found, if the length scale Ly is hold constant. This can be understood by analyzing the reaction rate integral across the flame brush.

ISSN:0010-2202    

DOI:10.1080/00102200008947339

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

This paper presents a computational algorithm to predict highly-transient flame behavior in counter-flow situations. The first objective of the paper is to extend the transient counterflow problem to incorporate some gasdynamic compressibility effects, yet retain the desirable similarity structure. By relaxing assumptions in earlier formulations, the computational algorithms can deliver high accuracy even in periods of extremely rapid transients, like combustion ignition. The algorithms are demonstrated on two combustion-ignition problems for methane-air, counterflow, nonpremixed flames. The first concerns the ignition transient in a steady strain field. The second concerns the effects of a high-frequency oscillatory strain field on the ignition process. The results reveal that, when the mean strain rate is near the steady ignition limit, the ignition process is highly sensitive to the details of the strain-rate fluctuations.

ISSN:0010-2202    

DOI:10.1080/00102200008947340

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

We consider the interaction of vortices of different size and strength (vorticity) and a diffusion flame of N2-diluted H2and air stabilized on an opposed-jet burner. In our direct numerical simulations, which take into account the effects of detailed chemistry and transport, we demonstrate the effects of flame curvature of opposite orientations by placing a vortex on either the air or the fuel side of the diffusion flame. When the flame curvature is convex towards the fuel stream, the flame burns more intensely even at a scalar dissipation rate,X, close to the critical (extinction) value,Xcof the flat one-dimensional diffusion flame (for the same composition of the reactant streams). On the other hand, if the flame curvature is convex towards the air stream, the flame weakens and in some cases extinguishes even when the localXis significantly lower thanXc. Depending on the curvature orientation, the extinction scalar dissipation rate can vary considerably. This observation raises questions on the common use of a single extinction scalar dissipation rate in many turbulent diffusion flame simulations. Our results indicate that the role of curvature in the transient flame-vortex interaction is similar to what was observed in previous studies of steady curved flames. We also observe that the reignition process following local extinction is two-dimensional. The reignition process observed in our simulations may not be described well by flamelet models (steady or transient) that are based on one-dimensional formulations.

ISSN:0010-2202    

DOI:10.1080/00102200008947341

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

The unsteady flamelet model is applied in a numerical simulation of soot formation in a turbulent C2H4jet diffusion flame. A kinetically based soot model is used, which relies on a detailed kinetic mechanism to describe the formation of small polycyclic aromatic hydrocarbons. To describe the formation, growth, and oxidation of soot particles, flamelet equations for the statistical moments of the panicle size distribution are derived. Since the effective Lewis number of large panicles tends to infinity, a formulation is given, which allows the investigation of the effect of different diffusion coefficients of the particles on the soot formation process. The results of the calculation are compared to experimental data, showing very good agreement for the temperature, which is shown to depend strongly on soot and gas radiation. The predicted soot volume fraction compares reasonably well with the measured data, if differential diffusion of the panicles is considered, Calculations with unity particle Lewis numbers show similar results, but overpredicts the soot volume fraction in the rich part of the flame.

ISSN:0010-2202    

DOI:10.1080/00102200008947342

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

The paper presents an original work in which a hybrid turbulent combustion model, based upon a stochastic evaluation of the Joint Scalar Probability Density Function (PDF), is used in conjunction with a skeletal soot model and a detailed kinetic mechanism for fuel oxidation. First, the Probabilistic Eulerian Lagrangian turbulent combustion model, which is theoretically able to describe chemical reactions occurring in a turbulent flow for a wide range of Damkohler numbers, is presented and justified. Then, the chemistry model which accounts for soot formation and oxidation is exposed and validated in rich ethylene premixed laminar flames. This modelling approach coupling turbulence and chemistry is eventually applied to predict soot levels in a turbulent jet diffusion flame of ethylene burning in still air. Results are in good agreement with experimental data and the peak value of soot volume fraction on the centreline is fairly well described even though an accurate radiative heat transfer model is still necessary to be more predictive on mean temperature levels.

ISSN:0010-2202    

DOI:10.1080/00102200008947343

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor