摘要:

The performance of a dynamic subgrid model for the turbulent burning speed of a premixed flame is investigated for a series of idealized test cases where the flame front is wrinkled by a multiple-scale shear flow; a rigorous asymptotic subgrid model is also implemented for comparison. Explicit formulae for the flame wrinkled shape and turbulent speed are available to generate a reference database. The role of the subgrid wrinkling models is to achieve the same overall flame shape and propagation speed in a simulation where only the largest scales of the flow are explicitly accounted for. Very good results are obtained when the subgrid burning speed enhancement is estimated using the asymptotic subgrid model. On the other hand, the dynamic model attempts to exploit the scaling observable in the simulation to extrapolate the turbulent burning speed enhancement in the original system. The performance of this strategy is adequate for some regimes but poor for others; the source of the problem is traced back to the existence of a scaling transition that occurs as the flame propagating speed is adjusted during the large-eddy simulation. A modification to the scaling of the enhanced burning is implemented to account for the existence of the two distinct scaling ranges; it improves significantly the predictions of the dynamic model away from the transition, but results in the near-critical range remain predictably very poor compared with the rigorous asymptotic model results. These conclusions based ona prioriperformance for the reference steady data are confirmed by comparing unsteady large-eddy and direct simulations. Results based on rigorous mathematical tools are possible here because of the separation of length scales in the special class of idealized flow fields used in this study: their relevance to more realistic flows is also discussed.

ISSN:1364-7830    

DOI:10.1088/1364-7830/4/4/301

出版商:Taylor & Francis    

年代:2000

数据来源: Taylor

摘要:

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.

ISSN:1364-7830    

DOI:10.1088/1364-7830/4/4/302

出版商:Taylor & Francis    

年代:2000

数据来源: Taylor

摘要:

Spark ignition, as the first step during the combustion in Otto engines, has a profound impact on the further development of the flame kernel. Over the last ten years growing concern for environment protection, including low emission of pollutants has increased the interest in the numerical simulation of ignition phenomena to guarantee successful flame kernel development even for lean mixtures.

ISSN:1364-7830    

DOI:10.1088/1364-7830/4/4/303

出版商:Taylor & Francis    

年代:2000

数据来源: Taylor

摘要:

A counterflow diffusion flame model is studied revealing that, at least as a part of the quenching boundary is approached in parameter space at low-enough Lewis numbers, an edge of a diffusion flame, or triple flame, has a propagation speed that still advances the burning solution into regions that are not burning. In crossing the quenching boundary, the advancing flame edge remains a robust part of the solution but the flame behind the edge is found to break up into periodic regions, resembling ‘tubes’ of burning and non-burning, accompanied by the appearance of an oscillatory component in the speed of propagation of the edge. In crossing a second boundary the propagation speed of the flame edge disappears altogether. The only unbounded, non-periodic stationary solution then consists of an isolated flame tube, although stationary periodic flame tubes can also exist under the same conditions. In passing back through parameter space, starting with a single flame tube already present, there is no sign of hysteresis and the oscillatory edge propagation reappears at the same point where it disappears. On the other hand, in continuing forwards across a third, final boundary the flame tube is extinguished leaving no combustion whatever. Boundaries in parameter space where different solutions arise are mapped out.

ISSN:1364-7830    

DOI:10.1088/1364-7830/4/4/304

出版商:Taylor & Francis    

年代:2000

数据来源: Taylor

摘要:

Radiation heat loss has an important impact on near-limit flames. It has been shown that radiation heat loss can make stretched CH4–air flames extinguish at a lower stretch rate. Numerical calculations of counterflow premixed H2–air flames were conducted using an accurate description of the chemical kinetics and transport properties. Radiation heat loss was considered. The results show that in addition to the stretch extinction limit, radiation heat loss also allows the lower and middle equivalence ratio counterflow premixed H2–air flames to extinguish at a lower stretch rate. For middle equivalence ratio counterflow H2–air flames, the closed temperature profiles become distorted O–shaped curves due to the lower Lewis number, being different from those of CH4–air flames. For higher equivalence ratio counterflow premixed H2–air flames, there are two stable flame branches—a normal flame branch and a weak flame branch. When the equivalence ratio is greater than a critical value, the closed temperature profile curve of every equivalence ratio flame opens and the normal flame curve can be extended to zero stretch rate. The calculation of the concentration limit of one-dimensional planar premixed H2–air flames was also conducted. The results show that the critical equivalence ratio corresponds to the concentration limit of the one-dimensional planar premixed H2–air flame. The extension of the flammable region due to the stretch is amplified for the counterflow H2–air flame because of its much lower Lewis number than that of the counterflow CH4–air flame.

ISSN:1364-7830    

DOI:10.1088/1364-7830/4/4/305

出版商:Taylor & Francis    

年代:2000

数据来源: Taylor

摘要:

A numerical approach based on the enthalpy method is proposed for solving generalized phase-change problems. The method is applied to predict pyrolysis and ignition of polymeric combustible materials. In contrast to the traditional approach, here both enthalpy and temperature are treated as independent variables, and the conservation equations are solved simultaneously in conjunction with the constitutive equations. Also, the formulation of the constitutive equations for the phase change is not necessarily the same for all of the possible phases, but can be chosen independently according to the characteristics of the physical problem and the requirements of the numerical analysis of each respective phase. Thus with this new approach, which we refer to as the enthalpy-temperature hybrid method, the enthalpy method is applicable to the generalized phase-change problems regardless of the form of the constitutive equations. The proposed method is first applied to a one-dimensional classical freezing problem for verification. It is found that the numerical results for the temperature history and the position of the phase-change interface agree well with the analytic solution existing in the literature. The method is then applied to the numerical simulation of the pyrolysis and ignition of a composite material with a polymer as the matrix and fibreglass as the filling material. Three models of oxygen distribution in the molten layer are considered to explore the melting and oxygen effects on the polymer pyrolysis. Numerical calculation shows that high oxygen concentrations in the molten layer enhance the pyrolysis reaction, resulting in a larger amount of pyrolysate, but in lower surface temperatures of the sample. It also shows that the distribution of oxygen in the molten layer has a strong effect on the pyrolysate rate, and therefore on ignition and combustion of the polymers. Comparison with available experimental data indicates that a model of oxygen distribution in the molten layer that is limited to a thin layer near the surface best describes the ignition process for a homogeneously blended polypropylene/fibreglass composite.

ISSN:1364-7830    

DOI:10.1088/1364-7830/4/4/306

出版商:Taylor & Francis    

年代:2000

数据来源: Taylor

摘要:

For combustion in a homogeneous flow reactor stochastic fluctuations of the Damköhler number are analysed. The chemistry is described by a one-step first-order reaction with Arrhenius kinetics. The case of an adiabatic system is studied allowing for a reduction to a single equation for the temperature. A parameter of that equation is the Damköhler number which is separated into a mean and a stochastic term carrying white noise characteristics. This allows for an exact treatment of the probability density function in terms of a Fokker-Planck equation. In the case of steady statistics the Fokker–Planck equation is solved exactly in terms of exponential integrals. It is shown that the probability density function usually admits mono- and bi-modal behaviour but even tri-modal behaviour is observed depending on the activation energy, the heat of combustion, the mean Damköhler number and the variance of the Damköhler number. The most common features are mono-modal characteristics, which indicate that the system is strongly attracted either to the unburned or the burned state. Two cases of Damköhler number variations are studied. The first one corresponds to the classical combustion situation, where two steady states exist and transitions between them can be triggered by Damköhler number fluctuations. The other corresponds to a new mode of combustion, called ‘mild’ combustion, where the steady-state solution depends monotonically on the Damköhler number, such that ignition and extinction events are suppressed.

ISSN:1364-7830    

DOI:10.1088/1364-7830/4/4/307

出版商:Taylor & Francis    

年代:2000

数据来源: Taylor

摘要:

Gaseous flame balls and their stability to symmetric disturbances are studied numerically and asymptotically, for large activation temperature, within a porous medium that serves only to exchange heat with the gas. Heat losses to a distant ambient environment, affecting only the gas, are taken to be radiative in nature and are represented using two alternative models. One of these treats the heat loss as being constant in the burnt gases and linearizes the radiative law in the unburnt gas (as has been studied elsewhere without the presence of a solid). The other does not distinguish between burnt and unburnt gas and is a continuous dimensionless form of Stefan's law, having a linear part that dominates close to ambient temperatures and a fourth power that dominates at higher temperatures.

ISSN:1364-7830    

DOI:10.1088/1364-7830/4/4/308

出版商:Taylor & Francis    

年代:2000

数据来源: Taylor

摘要:

We present an adaptive algorithm for low Mach number reacting flows with complex chemistry. Our approach uses a form of the low Mach number equations that discretely conserves both mass and energy. The discretization methodology is based on a robust projection formulation that accommodates large density contrasts. The algorithm uses an operator-split treatment of stiff reaction terms and includes effects of differential diffusion. The basic computational approach is embedded in an adaptive projection framework that uses structured hierarchical grids with subcycling in time that preserves the discrete conservation properties of the underlying single-grid algorithm. We present numerical examples illustrating the performance of the method on both premixed and non-premixed flames.

ISSN:1364-7830    

DOI:10.1088/1364-7830/4/4/309

出版商:Taylor & Francis    

年代:2000

数据来源: Taylor

摘要:

Very-long-time numerical simulations of an idealized pulsating detonation with one irreversible reaction having an Arrhenius form are performed using a hierarchical adaptive second-order Godunov-type scheme. The initial data are given by the steady solution and the truncation error produces the perturbation to trigger the instability. The detonation is allowed to run for thousands of half-reaction times of the underlying steady wave to ensure that the final amplitudes and periods of the nonlinear oscillations are achieved. Thorough resolution studies are performed for various representative regimes of the instability. It is shown that to obtain quantitatively good solutions over 50 numerical grid points in the half-reaction length of the steady detonation are required, while to obtain a converged solution over 100 points are required, even near the stability boundary. This is much higher resolution than has generally been used in previous papers in either one or two dimensions. Resolutions of less than approximately 20 points per half-reaction length give very poor predictions of the periods and amplitudes near the stability boundary or entirely spurious solutions for more unstable detonations. The evolution of the converged solutions as the activation energy increases, and the detonation becomes more unstable, is also investigated.

ISSN:1364-7830    

DOI:10.1088/1364-7830/4/4/310

出版商:Taylor & Francis    

年代:2000

数据来源: Taylor