摘要:

A simple reaction rate model which eliminates much of the non-linearity associated with the Arrhenius model is suggested. The model is applied to one-dimensional flame problems: the unstrained flame at unity and non-unity Lewis numbers and the strained flame at unity Lewis number. Exact expressions for the temperature and species profiles and consumption rates are obtained; in thestrained flame, these require numerical evaluation. The solutions demonstrate that the model agrees in all essentials with the Arrhenius model and is much simpler mathematically. For the strained flame problem, the exact solution involves integral functions. Analytical temperature profiles and the consumption rate can be obtained for low and high strain rate. Many of the results obtained via activation energy asymptotics and numerical solution for the Arrhenius model are reproduced by the current model with much less analytical complexity. In the strained flame solutions one can identify two regimes of the flame response to strain. At low strain, we find migration of the flame front downstream as the strain increases. At high strain rates, the direction of migration is reversed. The consumption rate as a function of strain rate is in excellent agreement with the numerical solutions of Darabiha et at, This agreement suggests that the mathematical form of the model is not important to the global flame response and simplification of reaction rate models is justifiable for many flame problems. Moreover, the analytical tractability of the proposed model makes it a good pedagogical tool.

ISSN:0010-2202    

DOI:10.1080/00102209308924116

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

Development of Scramjet engines requires a detailed knowledge of the coupling between supersonic flow and chemical kinetics of combustion. In this paper we consider a stationary uniform, laminar supersonic mixing layer of hydrogen and air and investigate some conditions under which ignition occurs. More specifically, the influences of viscous dissipation effects, initial temperature and/or velocity gradients are investigated. This problem, where detailed chemistry for H2-air is used, is solved numerically. The structure of the reactive mixing layer is shown to consist of an induction region, a thermal runawayregion, a region where both premixed and diffusion flames coexist, and a pure diffusion flame region. The exact length of the induction zone is found to depend sharply on the upstream boundary conditions and more precisely on the intensity of viscous dissipation together with amplitude and direction of transverse temperature and velocity gradients.

ISSN:0010-2202    

DOI:10.1080/00102209308924117

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

Thermophoresis, meaning particle drift down a local gas temperature gradient, is now known to be important to many combustion-related technologies. Until now, however, no direct experimental determinations of primary and aggregated particle thermophoretic diffusivities, αTD, in high temperature combustion environments have been reported. To perform such measurements, we selected a seeded laminar counterflow diffusion flame (CDF) operated at low strain-rate as a well-defined combustion system, offering at the same time a low velocity and high temperature gradient environment. We established a CH4/ O2Inert opposed jet diffusion flame in which the gaseous fuel/oxygen ratio, and the diluent flow rates were adjusted to obtain a flat, stable flame, approximately coincident with the gas stagnation plane (GSP). Particles fed to or formed on either or both sides of the GSP move toward this plane until the local axial velocity is exactly counterbalanced by the thermophoretic velocity. As a result of this dynamic "equilibrium" condition, a particle stagnation plane (PSP) is established on one or both sides of the GSP,resulting in the formation of a readily observable "dust-free" zone. Dramatic confirmation of this phenomenon is offered by using laser-sheet visualization of the region, which reveals a thick dark zone, the dust-free volume, that contrasts with the bright particle-laden regions. This "phase separation" phenomenon allowed us to determine TiO2particle thermophoretic diffusivities by: i) measuring the temperature field using fine thermocouples; ii) measuring the thickness of the dark zone (i.e. PSP-positions) using laser light scattering; and iii) measuring/computing the axial gaseous convective velocity at the particle stagnation plane(s). Experiments and calculations indicate quantitative agreement between these measurements and kinetic theory predictions for isolated spheres at Knp≫ I in the case of CH4/O2N2diffusion flames. Replacement of N2with He as diluent resulted in a much thicker and more readily measurable particle-free layer, but yielded only qualitative agreement with the theory, because of uncertainties in the gas composition in the flame, as well as possible contributions from simultaneous diffusiophoretic mechanisms. In some 'sooting' diffusion flames, and in 'synthesis' flames in which particles are the desired products, both laminar and turbulent, it is shown that thermophoresis can influence particle residence times in the decisive region where nucleation, growth, coagulation, sintering and oxidation occur, as well as particle temperatures, which influence particle morphology and radiative heat loads. We briefly discuss the non-premixed combustion conditions under which these thennophoretically-induced effects are likely to be appreciable.

ISSN:0010-2202    

DOI:10.1080/00102209308924118

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

Using Lie group theory, a self-similar solution. representing a diffusion flame in which both cross stream and streamwise diffusion is included is obtained. The solution arises in problems related to conjugate heat transfer and pollutant dispersion applications that are governed by the same partial differential equation. Results including flame shape, flame height dependence on Peelet number and overall stoichiometry, are compared with other known solutions. A unique feature of the solution is the existence of a maximum in the size of the back diffusion region as a function of Peclet number. This could be important in very low speed burner applications.

ISSN:0010-2202    

DOI:10.1080/00102209308924119

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

The conventional methods of simplified kinetics modeling through the use of partial-equilibrium and quasi-steady approximations are reviewed and critiqued. The method of computational singular pertutbation (CSP) is then presented with special emphasis on the interpretation of CSP data to obtain physical insights on massively complex reaction systems. A simple example is used to demonstrate how CSP deals with complex chemical kinetics problems without the benefits of intuition and experience.

ISSN:0010-2202    

DOI:10.1080/00102209308924120

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

A method using a group of universal curves for calculating the burning rate of coal char / carbon particles in air is presented in this paper, and it is very simple and useful for the engineering calculation. Both the heterogeneous reactions on char / carbon surface and the gas-phase reaction around the particles are taken into account in this method, which can be used not only for the cases of kinetics or bulk diffusion rate-controlling, but also for cases of diffusion-kinetics limitation. A new dimensionless parameter, Fb, called "burning rate-controlling parameter" has also been proposed, and it can quantitatively determine which resistance is controlling.

ISSN:0010-2202    

DOI:10.1080/00102209308924121

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor