摘要:

The problem of turbulent premixed flames is considered within the framework of the field equations describing flames as level surfaces for scalar fields and stochastic control theory for Markov diffusion processes. It is shown that all field equations can be interpreted as a dynamic programming partial differential equation of second order (Hamilton - Jacobi - Bellman PDE) and the corresponding scalar fields can be regarded as the value functions. The explicit formulae for the scalar fields as a minimum/maximum of a functional integral have been derived and the most interesting result is that the running cost function is the same as a Lagrangian function for a relativistic particle moving in an external field and the particle velocity plays the role of a control function.

ISSN:1364-7830    

DOI:10.1080/713665234

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

A general theory of diffusional-thermal instability for diffusion flames is developed by considering the diffusion-flame regime of activation-energy asymptotics. Attention is focused on near-extinction flames in a distinguished limit in which Lewis numbers deviate from unity by a small amount. This instability analysis differs from that of premixed flames in that two orders of the inner reaction-zone analyses are required to obtain the dispersion relation. The results, illustrated for a one-dimensional convective diffusion flame as a model, exhibit two types of unstable solution branches, depending on whether Lewis number is less than or greater than unity. For flames with Lewis numbers sufficiently less than unity, a cellular instability is predicted, which can give rise to stripe patterns of the flame-quenching zones with maximum growth rate occuring at a finite wavelength comparable with the thickness of the mixing layer. The result for the critical Lewis number shows that the tendency toward cellular instability diminishes as the Peclét number of the flame decreases. On the other hand, for flames with Lewis numbers sufficiently greater than unity, a pulsating instability is predicted, which occurs most strongly when the Peclét number is small. For this type of instability, the planar disturbance is found to be most unstable with a real grow rate, and a conjugate pair of complex solutions bifurcates from the turning point of the real-solution branch and extends to higher wave numbers. An increase of the reaction intensity is found to stabilize the flame at all wavelengths. Employing the Peclét number as a small parameter, an approximate dispersion relation is derived as a quadratic equation, which correctly predicts all of the qualitative characteristics of the instability.

ISSN:1364-7830    

DOI:10.1080/713665228

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

A computational technique is described and demonstrated that can decrease by three orders of magnitude the computer time required to treat detailed chemistry in reactive flow calculations. The method is based on the in situadaptive tabulation (ISAT) of the accessed region of the composition space - the adaptation being to control the tabulation errors. Test calculations are performed for non-premixed methane - air combustion in a statistically-homogeneous turbulent reactor, using a kinetic mechanism with 16 species and 41 reactions. The results show excellent control of the tabulation errors with respect to a specified error tolerance; and a speed-up factor of about 1000 is obtained compared to the direct approach of numerically integrating the reaction equations. In the context of PDF methods, the ISAT technique makes feasible the use of detailed kinetic mechanisms in calculations of turbulent combustion. The technique can also be used with reduced mechanisms, and in other approaches for calculating reactive flows (e.g. finite difference methods).

ISSN:1364-7830    

DOI:10.1080/713665229

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

An analysis is presented of the viscous - inviscid interaction region around the tip of a flame spreading over the surface of a solid fuel in a forced laminar high Reynolds number air stream that opposes the flame propagation. Through the interaction, the vaporization of the solid and the thermal expansion of the gas originate an adverse pressure gradient upstream of the flame tip, which leads to a decrease of the shear acting on the small region controlling the flame spread rate. Under certain realistic conditions this adverse pressure gradient may separate the boundary layer upstream of the vaporizing region of the solid, leading to a new mode of flame spread with a higher spread rate determined by the flow in the whole interaction region.

ISSN:1364-7830    

DOI:10.1080/713665230

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

A piloted turbulent natural-gas diffusion flame is investigated numerically using a 2D elliptic Monte Carlo algorithm to solve for the joint probability density function (PDF) of velocity and composition. Results from simulations are compared to detailed experimental data: measurements of temperature statistics, data on mean velocity and turbulence characteristics and data on OH. Conserved-scalar/constrained-equilibrium chemistry calculations were performed using three different models for scalar micro-mixing: the interaction by exchange with the mean (IEM) model, a coalescence/dispersion (C/D) model and a mapping closure model. All three models yield good agreement with the experimental data for the mean temperature. Temperature standard deviation and PDF shapes are generally predicted well by the C/D and mapping closure models, whereas the IEM model gives qualitatively incorrect results in parts of the domain. It is concluded that the choice of micro-mixing model can have a strong influence on the quality of the predictions. The same flame was also simulated using reduced chemical kinetics obtained from the intrinsic low-dimensional manifold (ILDM) approach. Comparison with the constrained-equilibrium results shows that the shape of the OH concentration profiles is recovered better in the ILDM simulation, and that the ILDM reduced chemical kinetics can correctly predict super-equilibrium OH.

ISSN:1364-7830    

DOI:10.1080/713665231

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

It is analysed theoretically how the stirring of an exothermically reacting fluid layer affects its thermal explosion limits. Analytical and numerical analyses reveal that, in accordance with the intuitive expectations, the short-scale stirring makes the thermal explosion more difficult through the increased heat transfer to the boundaries. However, under the long-scale stirring, promoting formation of hot spots, transition to the explosion may be facilitated rather than hampered.

ISSN:1364-7830    

DOI:10.1080/713665232

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

This paper explains the methodology used to develop a high-resolution, multi-dimensional Euler solver that is capable of handling non-ideal equation of state and stiff chemical source terms. We have developed a pointwise implementation that has computational advantages for our intended applications, as opposed to a finite volume implementation. Our solver allows for the placement of internal reflective boundaries and the standard inflow and outflow and reflective boundaries at the edge of the domain. We discuss the spatial discretization and the temporal integration schemes, upwinding and flux splitting and the combined use of the Lax - Friedrichs and Roe schemes to solve for the required fluxes. A complete description of the pointwise internal boundary method is given. An overall summary of a representative code structure is given. We provide details on the verification of our integrated set of algorithms that resulted in an application code. We demonstrate the order of convergence for test problems. Two example applications from measurement of detonation shock dynamics and deflagration to detonation transition in porous energetic materials are presented.

ISSN:1364-7830    

DOI:10.1080/713665233

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor