摘要:

The stability of a plane, premixed flame is re-examined for finite activation energies. This stability problem must be solved numerically; however, the calculations are performed in the spirit of the infinite activation energy theory. Numerical difficulties in determining the steady solution for both adiabatic and non-adiabatic flames are identified and resolved. The stability equations are solved using the compound matrix method. The theory and calculations presented resolve all discrepancies between the infinite activation energy results and numerical calculations reported previously.

ISSN:1364-7830    

DOI:10.1088/1364-7830/3/4/301

出版商:Taylor & Francis    

年代:1999

数据来源: Taylor

摘要:

Hydrodynamic (Landau) instability in combustion is typically associated with the onset of wrinkling of a flame surface, corresponding to the formation of steady cellular structures as the stability threshold is crossed. In the context of liquid-propellant combustion, such instability has recently been shown to occur for critical values of the pressure sensitivity of the burning rate and the disturbance wavenumber, significantly generalizing previous classical results for this problem that assumed a constant normal burning rate. Additionally, however, a pulsating form of hydrodynamic instability has been shown to occur as well, corresponding to the onset of temporal oscillations in the location of the liquid/gas interface. In the present work, we consider the realistic influence of a non-zero temperature sensitivity in the local burning rate on both types of stability thresholds. It is found that for sufficiently small values of this parameter, there exists a stable range of pressure sensitivities for steady, planar burning such that the classical cellular form of hydrodynamic instability and the more recent pulsating form of hydrodynamic instability can each occur as the corresponding stability threshold is crossed. For larger thermal sensitivities, however, the pulsating stability boundary evolves into a C-shaped curve in the (disturbance-wavenumber, pressure-sensitivity) plane, indicating loss of stability to pulsating perturbations for all sufficiently large disturbance wavelengths. It is thus concluded, based on characteristic parameter values, that an equally likely form of hydrodynamic instability in liquid-propellant combustion is of a non-steady, long-wave nature, distinct from the steady, cellular form originally predicted by Landau.

ISSN:1364-7830    

DOI:10.1088/1364-7830/3/4/302

出版商:Taylor & Francis    

年代:1999

数据来源: Taylor

摘要:

The aim of this work is to propose a new model for turbulent flows, called the fractal model (FM), applicable both in a Reynolds averaged Navier–Stokes (RANS) and a large-eddy simulation (LES) formulation, with the ultimate goal of applying it to simulate turbulent combustion irrelevant of its mode (premixed or non-premixed). The model is able to turn itself off in the laminar zones of the flow, and in particular near walls. It is based on the fractal theory. It describes the physics of the smaller spatial scales and therefore represents a small-scales model.

ISSN:1364-7830    

DOI:10.1088/1364-7830/3/4/303

出版商:Taylor & Francis    

年代:1999

数据来源: Taylor

摘要:

Axisymmetric laminar methane–air Bunsen flames are computed for two equivalence ratios: lean (Φ=0.776), in which the traditional Bunsen cone forms above the burner; and rich (Φ=1.243), in which the premixed Bunsen cone is accompanied by a diffusion flame halo located further downstream. Because the extremely large gradients at premixed flame fronts greatly exceed those in diffusion flames, their resolution requires a more sophisticated adaptive numerical method than those ordinarily applied to diffusion flames. The local rectangular refinement (LRR) solution-adaptive gridding method produces robust unstructured rectangular grids, utilizes multiple-scale finite-difference discretizations, and incorporates Newton's method to solve elliptic partial differential equation systems simultaneously. The LRR method is applied to the vorticity–velocity formulation of the fully elliptic governing equations, in conjunction with detailed chemistry, multicomponent transport and an optically-thin radiation model. The computed lean flame is lifted above the burner, and this liftoff is verified experimentally. For both lean and rich flames, grid spacing greatly influences the Bunsen cone's position, which only stabilizes with adequate refinement. In the rich configuration, the oxygen-free region above the Bunsen cone inhibits the complete decay of CH4, thus indirectly initiating the diffusion flame halo where CO oxidizes to CO2. In general, the results computed by the LRR method agree quite well with those obtained on equivalently refined conventional grids, yet the former require less than half the computational resources.

ISSN:1364-7830    

DOI:10.1088/1364-7830/3/4/304

出版商:Taylor & Francis    

年代:1999

数据来源: Taylor

摘要:

Conditional moment closure (CMC) predictions for a turbulent piloted jet diffusion flame of methanol in air at velocities of 66.2 and 90.3 m s−1are presented. Predictions are compared with the experimental joint Raman-Rayleigh-LIF results of Masriet aland laminar flamelet calculations. Three comprehensive chemical mechanisms (SKELETAL, GRI-Mech and SUBGRI) are used to represent the chemistry of the methanol flame. The SKELETAL mechanism shows the best agreement among the various mechanisms employed. It is found that the SUBGRI mechanism reduces computational cost in terms of memory and CPU time without compromising results where the focus is on the main reactive chemistry.

ISSN:1364-7830    

DOI:10.1088/1364-7830/3/4/305

出版商:Taylor & Francis    

年代:1999

数据来源: Taylor

摘要:

The interaction of a premixed methane-air flame with a two-dimensional counter-rotating vortex pair is studied under stoichiometric and rich conditions using a detailed C1C2chemical mechanism. The flame structure and transient response are examined, both at curved cusps and on the vortex-pair centreline. Differences between the two flames are observed in the unsteady behaviour of species mole fractions and production rates. In contrast with earlier one-dimensional opposed-jet flame data, the present results show that the rich flame exhibits a faster response to unsteady strain-rate disturbances than does the stoichiometric flame. Analysis of the results suggest this may be due to the increased dependence of the flame on H, and the decreased role of OH, under rich conditions. Results are also presented from an experimental V-flame vortex-pair interaction study. Measured peak CH and OH data are also found to exhibit a faster flame response under rich conditions.

ISSN:1364-7830    

DOI:10.1088/1364-7830/3/4/306

出版商:Taylor & Francis    

年代:1999

数据来源: Taylor

摘要:

Recent models of straight diffusion flame edges are extended to consider the effect of a curved edge forming the perimeter of an axisymmetric ‘hole’, where a burning flame surrounds a quenched inner region. For ‘free’ flame-holes (without a heat sink near the axis), at small Damköhler number (Da), the holes grow if the initial radius is large but shrink if it is small. For largeDa, the holes shrink for any initial radius. Thus, free flame-holes are not stable for anyDa, which is consistent with experimental observations. When the flame-hole is ‘anchored’ by a heat sink near to the axis, stationary holes of finite radius can exist for sufficiently highDa, but the solutions revert to ‘free’ hole behaviour for radii sufficiently larger than the heat sink radius. Based on these results, it is suggested that quasi-stationary flame-holes are not likely to be a common feature of turbulent diffusion flames, except possibly when large lateral gradients ofDaare present due to intense vortices passing through the flame front.

ISSN:1364-7830    

DOI:10.1088/1364-7830/3/4/307

出版商:Taylor & Francis    

年代:1999

数据来源: Taylor

摘要:

The focus of this paper is on numerical modelling of methanol liquid pool fires. A mathematical model is first developed to describe the evaporation and burning of a two-dimensional or axisymmetric pool containing pure liquid methanol. Then, the complete set of unsteady, compressible Navier-Stokes equations for reactive flows are solved in the gas phase to describe the convection of the fuel gases away from the pool surface, diffusion of the gases into the surrounding air and the oxidation of the fuel into product species. Heat transfer into the liquid pool and the metal container through conduction, convection and radiation are modelled by solving a modified form of the energy equation. Clausius–Clapeyron relationships are invoked to model the evaporation rate of a two-dimensional pool of pure liquid methanol. The governing equations along with appropriate boundary and interface conditions are solved using the flux-corrected transport algorithm. Numerical results exhibit a flame structure that compares well with experimental observations. Temperature profiles and burning rates were found to compare favourably with experimental data from single- and three-compartment laboratory burners. The model predicts a puffing frequency of approximately 12 Hz for a 1 cm diameter methanol pool in the absence of any air co-flow. It is also observed that increasing the air co-flow velocity helps in stabilizing the diffusion flame, by pushing the vortical structures away from the flame region.

ISSN:1364-7830    

DOI:10.1088/1364-7830/3/4/308

出版商:Taylor & Francis    

年代:1999

数据来源: Taylor

摘要:

The effect of thermal radiation on the dynamics of a thermal explosion of a flammable gas mixture with the addition of volatile fuel droplets is studied. This is based on an original physical model of self-ignition. The thermal radiation energy exchange between the evaporating surface of the fuel droplets and burning gas is described using the P-1 model with Marshak boundary conditions. The original system of equations describing the effects of heating, evaporation and the combustion of fuel droplets is simplified to enable their analysis using asymptotic methods. The mathematical formulation is eventually reduced to a singularly perturbed system of ordinary differential equations. This allows us to apply the advanced geometric asymptotic technique (integral manifold method) for the qualitative analysis of the behaviour of the solution. Possible types of dynamic behaviour of the system are classified and parametric regions of their existence are determined analytically. The main attention is concentrated on the situations where delays might occur before the final ignition. Our study is focused on the impact of thermal radiation on the delay time. The dimensionless parameter responsible for the impact of thermal radiation is singled out and analysed. The dependence of the delay characteristics on the physical parameters of the problem under consideration is analysed. An explicit expression for the minimum time delay of the thermal explosion of fuel droplets in the presence of thermal radiation is derived and applied to the thermal explosion ofn-decane and tetralin droplets. It is pointed out that the effects of thermal radiation can be significant, especially at high temperatures, and cannot be ignored in the analysis of this phenomenon.

ISSN:1364-7830    

DOI:10.1088/1364-7830/3/4/309

出版商:Taylor & Francis    

年代:1999

数据来源: Taylor