摘要:

The numerical procedure of combustion modelling by the boundary–domain integral method (BDIM) is presented, based on detailed chemical and physical processes of the premixed gaseous fuel. The boundary element method is employed to solve the conservation equations of overall mass, momentum, energy and mass species. The law of mass action is applied to determine the chemical reaction rates representing the basis of the source terms in the conservation equations for mass species. A freely propagating flame including the forced ignition of a homogenous hydrogen–air mixture is calculated as an example problem of the numerical simulation of the combustion processes.

ISSN:1364-7830    

DOI:10.1088/1364-7830/3/1/001

出版商:Taylor & Francis    

年代:1999

数据来源: Taylor

摘要:

Momentum and energy integral analysis of planar and axisymmetric laminar, buoyant diffusion flames is presented with the objective of describing the flame properties and ambient air entrainment. The analysis follows the traditional momentum/energy integral technique utilizing the Howarth-Dorodnitzyn transformation for variable density flows and description of the velocity profiles through the flame region in the transformed constant-density coordinate system. The assumptions utilized in the formulation include the boundary layer and thin-flame approximations, a Prandtl and Schmidt number of unity and the product of molecular diffusivity-density squared being constant throughout the field. It is shown that a similarity solution exists for a vertical planar flame formed between a semi-infinite region of fuel in contact with a semi-infinite region of oxidizer and the results are presented in non-dimensional form in this case and are compared with their asymptotic values. For the steady laminar axisymmetric buoyant diffusion flames originating from circular burners, numerical solutions are obtained for different diameter burners. It is found that ambient air entrainment is predominantly determined by the air consumption at the flame front and only an additional small portion of mass (about 5% of that consumed at the flame front) is induced in the flame region. When compared with the measured air entrainment in buoyant diffusion flames, the computed entrainment rates are found to be significantly smaller than the experimental values. This points to the fact that unsteady large-scale engulfment of ambient air into fire plumes dominates over the diffusive and buoyant boundary-layer transport of air around a fire plume.

ISSN:1364-7830    

DOI:10.1088/1364-7830/3/1/002

出版商:Taylor & Francis    

年代:1999

数据来源: Taylor

摘要:

A theory of ignition is presented to analyse the effect of porosity on the time to ignition of a semi-infinite porous energetic solid subjected to a constant energy flux. An asymptotic perturbation analysis, based on the smallness of the gas-to-solid density ratio and the largeness of the activation energy, is utilized to describe the inert and transition stages leading to thermal runaway. As in the classical study of a nonporous solid, the transition stage consists of three spatial regions in the limit of large activation energy: a thin reactive–diffusive layer adjacent to the exposed surface of the material where chemical effects are first felt, a somewhat thicker transient–diffusive zone and, finally, an inert region where the temperature field is still governed solely by conductive heat transfer. Solutions in each region are constructed at each order with respect to the density-ratio parameter and matched to one another using asymptotic matching principles. It is found that the effects of porosity provide a leading-order reduction in the time to ignition relative to that for the nonporous problem, arising from the reduced amount of solid material that must be heated and the difference in thermal conductivities of the solid and gaseous phases. A positive correction to the leading-order ignition-delay time, however, is provided by the convective flow of gas out of the solid, which stems from the effects of thermal expansion and removes energy from the system. The latter phenomenon is absent from the corresponding calculation for the nonporous problem and produces a number of modifications at the next order in the analysis arising from the relative transport effects associated with the gas flow.

ISSN:1364-7830    

DOI:10.1088/1364-7830/3/1/003

出版商:Taylor & Francis    

年代:1999

数据来源: Taylor

摘要:

The asymptotic structure of laminar, non-premixed methane flames is analysed using a reduced four-step chemical-kinetic mechanism. Chemical reactions are presumed to take place in two layers: the inner layer and the oxidation layer. In the inner layer the fuel reacts with radicals and the main compounds formed are the intermediate species CO and H2. These intermediate species are oxidized in the oxidation layer. The structure of the oxidation layer is described by two second-order differential equations: one for CO and the other for H2. Two limiting cases are considered. At one limit the global step CO+H2⇌CO2+H2is presumed to maintain partial equilibrium everywhere in the oxidation layer except in a thin layer adjacent to the inner layer. At the other limit the steady-state approximation is introduced for H2everywhere in the oxidation layer except in a thin layer adjacent to the inner layer. This limit, called ‘slow CO oxidation’, has not been analysed previously. The structure of the inner layer is described by two second-order differential equations: one for the fuel and the other for the H radicals. This is a significant improvement over previous models in which either a steady-state approximation is introduced for the H radicals in the inner layer, or the reaction between the fuel and radicals is presumed to be very fast. The chain-breaking elementary reaction CH3+H+M→CH4+M is found to have a significant influence on the structure of the inner layer and on the scalar dissipation rates at extinction. The influence of this reaction was either neglected in previous models or was included as a perturbation to the principal elementary reactions taking place to the leading order in the inner layer. Using the results of the asymptotic analysis the scalar dissipation rates at extinction are calculated at a pressure of 1 bar. They are found to agree well with those calculated numerically using a chemical-kinetic mechanism made up of elementary reactions.

ISSN:1364-7830    

DOI:10.1088/1364-7830/3/1/004

出版商:Taylor & Francis    

年代:1999

数据来源: Taylor

摘要:

Comparisons between direct numerical simulation (DNS) of detonation and detonation shock dynamics (DSD) is made. The theory of DSD defines the motion of the detonation shock in terms of the intrinsic geometry of the shock surface, in particular for condensed phase explosives the shock normal velocity,Dn, the normal acceleration,[Ddot]n, and the total curvature, κ. In particular, the properties of three intrinsic front evolution laws are studied and compared. These are (i) constant speed detonation (Huygens construction), (ii) curvature-dependent speed propagation (κ relation) and (iii) curvature- and speed-dependent acceleration ([Ddot]n–Dn–κ relation). We show that it is possible to measure shock dynamics directly from simulation of the reactive Euler equations and that subsequent numerical solution of the intrinsic partial differential equation for the shock motion (e.g. a[Ddot]n–Dn–κ relation) reproduces the computed shock motion with high precision.

ISSN:1364-7830    

DOI:10.1088/1364-7830/3/1/005

出版商:Taylor & Francis    

年代:1999

数据来源: Taylor

摘要:

The behaviour of energetic solids subjected to simple shear loading is modelled to predict ignition. The model is transient, one dimensional and includes effects of thermal diffusion, plastic work and exothermic reaction with one-step irreversible Arrhenius kinetics. A common power-law constitutive model for shear stress which accounts for strain hardening, strain-rate hardening and thermal softening is used. For the energetic solid composite LX-14 subjected to an average strain rate of, the model predicts reactive shear localization after an induction time of approximately t=5 ms, in which regions of high strain rate () are confined to thin spatial zones. An approximate thermal explosion theory which enforces spatially homogeneous stress, temperature and reaction progress as well as accounting for the early-time dominance of plastic work over exothermic reaction allows the development of simple analytic expressions for the temporal evolution of all variables. The simple expressions give predictions which agree well with both: (a) numerical predictions of a spatially homogeneous theory which allows simultaneous influences of reaction and plastic work and (b) numerical predictions of a spatially inhomogeneous theory which accounts for spatial stress distributions, thermal diffusion and spatially localized reaction. In particular, the induction time to ignition is captured accurately by the approximate theory; hence, while reactive shear localization may accompany an ignition event, ignition causality is most dependent on plastic work done during the time of spatially homogeneous shear.

ISSN:1364-7830    

DOI:10.1088/1364-7830/3/1/006

出版商:Taylor & Francis    

年代:1999

数据来源: Taylor

摘要:

Diffusional–thermal instability is analysed for near-extinction counterflow diffusion flames to examine the instability characteristics of strained diffusion flamelets in turbulent flames, with the additional intention of providing a guideline to future experimental investigations. Attention is focused on the linear stability of the instability patterns appearing in the unstrained direction of two-dimensional counterflow diffusion flames, which is treated by the near-equilibrium regime of activation-energy asymptotics with Lewis numbers close to unity. The effects of unequal Lewis numbers for fuel and oxidizer are also taken into account by introducing an effective Lewis number. The resulting formulation describing linear stability of the harmonically decomposed disturbances turns out to be identical to the formulation derived previously for equal fuel and oxidizer Lewis numbers. For effective Lewis numbers less than unity, cellular instability is predicted for the entire range of the equivalence ratio, and the threshold Lewis number maintains a value slightly less than unity. On the other hand, for effective Lewis numbers sufficiently greater than unity, two types of oscillatory instabilities are found. As the effective Lewis number increases from unity, a travelling instability is first encountered for a range of finite wavelengths, and a pulsating instability emerges immediately above the travelling instability. These two types of oscillatory instabilities are predicted only for equivalence ratios sufficiently greater than unity because the threshold Lewis numbers for these instabilities are found to be infinity at unity equivalence ratio. For large values of the equivalence ratio, which is typical of most hydrocarbon flames, oscillatory instabilities are predicted only for flames burning extremely heavy hydrocarbon fuels or for flames heavily diluted by light inert gases.

ISSN:1364-7830    

DOI:10.1088/1364-7830/3/1/007

出版商:Taylor & Francis    

年代:1999

数据来源: Taylor

摘要:

An analysis is presented of the steady two-dimensional spread of a flame down a vertical surface of a thermally thick solid fuel, under the assumptions that the gas-phase reaction is infinitely fast and that vaporization of the solid occurs at a constant temperature. A condition of regularity of the solid phase temperature, the gas phase mixture fraction and enthalpy, and the mass vaporization flux at the vaporization front is proposed to determine the flame spread rate. Numerical solutions computed using this condition show that the spread rate decreases with decreasing heat of reaction and becomes zero for a certain heat of reaction. An order-of-magnitude analysis is given for the case when heat conduction in the solid dominates heat conduction in the gas.

ISSN:1364-7830    

DOI:10.1088/1364-7830/3/1/008

出版商:Taylor & Francis    

年代:1999

数据来源: Taylor

摘要:

A gridless technique for the solution of the integral form of the radiative heat flux equation for emitting and absorbing media is presented. The technique was developed to yield solutions of participating media radiative heat transfer in a manner compatible with numerical simulation of the flow and temperature field by gridless vortex and transport element methods, respectively. Additional utility for subgrid analyses, in conjunction with direct numerical simulation or reduced dimensional mixing and turbulence models is noted. Results are compared to established and newly developed closed-form solutions for transport with uniform properties in a planar media. Significant errors in alternative techniques that include simplifications of the governing equations are illustrated by direct comparison. Radiative transfer analyses of highly resolved temperature fields representative of single and multiple (150) flame sheets are performed. Multipole fast solvers for the exponential integrals in the radiative transfer equations are developed and are shown to yield reductions in CPU time of an order of magnitude for modest problem sizes (N∼O(1000)) and two orders of magnitude for large problems (N∼O(10000)). Additional improvements in computational efficiency are provided since the gridless formulation only requires a numerical representation of the portion of the domain with significant temperature gradients and the results are provided in the form of gradients of energy deposition.*This work was performed in part at Sandia National Laboratories, a multiprogram laboratory operated by Sandia Corporation, a Lockheed-Martin Company, for theUSDepartment of Energy under contract no DE-AC04-94AL85000.

ISSN:1364-7830    

DOI:10.1088/1364-7830/3/1/009

出版商:Taylor & Francis    

年代:1999

数据来源: Taylor

摘要:

This paper is concerned with the numerical simulation of two-dimensional flame balls. We describe a Galerkin-type discretization in an adaptive basis of orthogonal wavelets. The solution is represented by means of a reduced basis being adapted in each time step. This algorithm is applied to compute the evolution of circular and elliptic thermodiffusive flames. In particular, we study the influence of the Lewis number, the strength of radiative losses and of the initial radius. The results show different scenarios. We find repeated splitting of the flame ball which is studied in some detail, extinction after excessive growth and also obtain quasi-steady flame balls.

ISSN:1364-7830    

DOI:10.1088/1364-7830/3/1/010

出版商:Taylor & Francis    

年代:1999

数据来源: Taylor