摘要:

Flame spread and blowoff in an opposed air stream over a thermally-thin solid fuel is studied theoretically. The model includes the quasi-steady, two dimensional Navier-Slokes/ momentum, energy and species equations with one-step overall chemical reaction and second-order, finite-rate Arrhenius kinetics in gas phase. In a reference frame attached to the flame front, the flame spread rate v¯f) becomes an eigenvalue for this problem. The solid phase equations become steady, consisting of an energy balance coupled with the heat flux from the gas phase and a mass balance including Arrhenius pyrolysis kinetics. The parametric study is based on a variable Damkohler number (Da) which is a function of opposed flow velocity (u∞ ). The spread rate v¯fand the flame size are reduced and the flame becomes weaker as Da is decreased or u∞ is increased. A blowoff limit is reached when Da is lowered to a critical value. Heat conduction in the solid fuel contributes to higher VF and is the dominant process near the blowoff limit. The flame structures for both far and near-limit flames are presented graphically. Ahead of the flame front, a flow recirculation zone is found for every case of compulalion. The structural analysis shows that the flame has both premixed- and diffusion-flame characteristics. Finally, the comparison between the two cases shows the effect of flame stretch by increasing the opposed flow velocity.

ISSN:0010-2202    

DOI:10.1080/00102209008951603

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

In this paper we examine the effect of stretch on the stability of premixed flames. The stability problem is posed within the asymptotic limits of large activation energy and of disturbances of wavelength much larger than the flame thickness. Only disturbances that are independent of the direction of flow divergence are considered here. It is shown that stretch stabilizes the long wavelength disturbances which are otherwise unstable as a result of the Darrieus-Landau instability. On the other hand it is known that diffusion may have a stabilizing influence on the short wavelength disturbances. Thus, for a sufficiently large strain rate, a flame may become absolutely stable even in the absence of gravity or other stabilizing effects. We identify here the critical value of the strain rate above which stability may be attained and determine the stability boundaries in terms of a scaled Lewis number and of stretch, for representative values of thermal expansion.

ISSN:0010-2202    

DOI:10.1080/00102209008951604

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

The appearance of time-dependent cellular structures in premixed flame propagation is known to occur in a parameter regime usually associated with the onset of steady cellular flame formation. We consider this phenomenon in the context of a model for downward flame propagation in vertical channels. In this model, the nonlinear evolution problem for the flame front describes flame propagation near the cellular instability threshold and predicts spinning solutions that arise from a certain class of steady, nonaxisymmetric mode interactions. However, unlike many combustion instabilities which can be anticipated from a linear stability analysis, a nonlinear stability analysis is required to predict these time-dependent solutions, which appear as either a secondary or tertiary infinite-period bifurcation from a bimodal cellular flame. In addition, it is shown that each admissible mode interaction is associated with a family of nonsteady flames and that each member of the family of spinning flames associated with the first pair of modes is orbitally stable.

ISSN:0010-2202    

DOI:10.1080/00102209008951605

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

The Frank-Kamenetskii theory of the self-heating of a reactive slab is extended to include volumetric heat losses. The method of non-dimensionalising the heat loss highlights the important role of the Frank-Kamenetskii parameter in the heat dissipation term. Two types of heat loss are considered: firstly that without any dependence on temperature and secondly that with a global (and linear) temperature dependence. It is shown that in either case there is a critical non-dimensional heaf-loss rate beyond which thermal runaway will not occur. This value corresponds to when the actual heat loss is a certain small fraction of the volumetric heat generation. The fraction depends, through the Frank-Kamenetskii parameter, on the constants governing the chemical reaction— in particular the activation energy and heat of reaction.

ISSN:0010-2202    

DOI:10.1080/00102209008951606

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

This short communication applies a recently developed theory of moderate frequency flame-acoustic interaction to the classical experimental configuration of a flame resonating in a tube.It has been well known ever since the early experiments of Lord Raylcigh, that a flame (or just a hot porous gauzes— as used by Rijke) produces a loud “ gauze tone” due to the effect of the impedance of the flame/ gauze system on the progress of the acoustic waves up and down the tube. It is the significant change in the phase of the velocity of the acoustic disturbance which is all important in the resonance which occurs, The change in velocity is governed by a “ velocity transfer function” V which in general will be a function of the type of flame and geometry used.

ISSN:0010-2202    

DOI:10.1080/00102209008951607

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor