摘要:

The wave equation for pressure waves is considered in the presence of a strong temperature gradient due to the interference of a flame. The characteristic length scale of the pressure wave is of the order of the typical diffusion length scale in the combustion zone so that the temperature gradient has a significant effect on the progress of the wave. In this paper the progress of pressure pulse and step function type inputs interacting with the flame is considered, and in particular the effect of the steep temperature rise is monitored for different pulse thicknesses and varying steepness of pressure rise (in the case of step function type inputs). It is shown that sharp pressure changes (where in rough terms “sharp” means that the major change takes place well within half the 99% temperature rise distance of the initial steady temperature profile) induce quite a strong deformation to the reflected pressure wave which should be possible to observe experimentally.

ISSN:0010-2202    

DOI:10.1080/00102209308907630

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

The reaction CH3+OH plays an important role for the modelling of laminar burning velocities of CH4-air flames. Experimental data on its rate coefficient are scarce and the importance of this reaction is shown by rate coefficients obtained recently from theoretical models. For CH3OH-air flames the reaction CH2OH+H→ products is likewise important and to a lesser extent so is the reaction CH3O+H. At higher temperatures no experimental data exist on these reactions and it has been tried in recent modelling studies to calculate rate coefficients for CH3O+H from those of CH3+OH by using equilibrium considerations. This approach if applied to a multiple channel reaction is generally dangerous, and indeed in this particular case by confusing the CH3O and CH2OH exit channels a rather misleading rate expression was obtained for the reaction CH3O+H→.CH3+OH.

ISSN:0010-2202    

DOI:10.1080/00102209308907631

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

A family of Probability Density Functions (PDF's) generated byJohnson-EdgeworthTranslation (JET) is used for statistical modeling of the mixing of an initially binary scalar in isotropic turbulence. The frequencies obtained by this translation are shown to satisfy some of the characteristics of the PDF's generated by theAmplitude MappingClosure (AMC) (Kraichnan, 1989; Chenet al., 1989). In fact, the solution obtained by one of the members of this family is shown to be identical to that developed by the AMC (Pope, 1991). Due to this similarity and due to the demonstrated capabilities of the AMC, a justification is provided for the use of other members of JET frequencies for the modeling of the binary mixing problem. This similarity also furnishes the reasoning for the applicability of thePearson Family(PF) of frequencies for modeling of the same phenomena. The mathematical requirements associated with the applications of JET in the modeling of the binary mixing problem are provided, and all the results are compared with data generated by Direct Numerical Simulations (DNS). These comparisons indicate that theLogit-Normalfrequency portrays some subtle features of the mixing problem better than the other closures. However, none of the models considered (JET, AMC, and PF) are capable of predicting the evolution of the conditional expected dissipation and/or the conditional expected diffusion of the scalar field in accordance with DNS. It is demonstrated that this is due to the incapability of the models to account for the variations of the scalar bounds as the mixing proceeds. A remedy is suggested for overcoming this problem which can be useful in probability modeling of turbulent mixing, especially when accompanied by chemical reactions. While in the context of a single-point description the evolution of the scalar bounds cannot be predicted, the qualitative analytical-computational results portray a physically plausible behavior.

ISSN:0010-2202    

DOI:10.1080/00102209308907632

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

Chemical structures of atmospheric-pressure, fuel rich (sooting) premixed, laminar, flat flames of C6H5Cl and CH4have been investigated using a flat flame burner. Species mole fraction profiles were determined by using micro-probe sampling coupled to on-line gas chromatography/mass spectrometry. In addition to those of reactants, mole fraction profiles have been determined for CO, CO2, HCl, H2O, H2, C2H6, C2H4, C2H2, C4H2, C4H4, C5H6, C6H6, C6H5CH3, C6H5C2H3, C6H5C2H, C6H5OH, ClC6H4OH, ClC6H4CH3, C6H4Cl2, and C10H8. In a fuel lean flame of pure C6H5Cl highly chlorinated hydrocarbons such as CCl4, Cl2C6H3OH, C5HCl4, C2HCl3, C6H3Cl3and C2Cl4were also observed. The roles of these species, in view of the current ideas of the mechanism of combustion of C6H5Cl are discussed.

ISSN:0010-2202    

DOI:10.1080/00102209308907633

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

In many combustion applications, the strain rate determines the flame response. Experimental evidence and theoretical arguments indicate that the strain rate defines the conditions of ignition in many flow configurations. Ignition in time-variable strained flows is analysed in this article. The response of a reactive element submitted to a time dependent strain rate is considered with numerical and asymptotic methods. The results obtained for this generic problem are then used to predict the ignition of non-premixed shear flows formed by hot and cold reactants. The plane mixing layer and coaxial jet formed by streams of hydrogen and hot air are specifically considered. The spatial distribution of the typical strain rate is first determined and a transformation into a local frame of reference is used to describe a spatially evolving strain rate as a time-dependent quantity. This provides the complete strain rate history for an initial reactive interface. Asymptotic analysis gives the ignition time which is then transformed back into an ignition distance. A good agreement is found between theoretical results and four test cases pertaining to supersonic combustion experiments.

ISSN:0010-2202    

DOI:10.1080/00102209308907634

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

An experimental study is carried out of the effect on the propagation of a smolder reaction through the interior of a porous fuel of a forced flow of oxidizer opposing the direction of smolder propagation. The potential effect of buoyancy in the process is also analyzed by conducting the experiments in the upward and downward propagation, and comparing the respective results. The experiments are conducted with a high void fraction flexible polyurethane foam as fuel and air as oxidizer, in a geometry that approximately produces a one-dimensional smolder propagation. Measurements are performed of the smolder reaction propagation velocity and temperature as a function of the location in the sample interior, the foam and air initial temperature, the direction of propagation, and the air flow velocity. For both downward and upward smoldering three zones with distinct smolder characteristics are identified along the foam sample. An initial zone near the igniter were the smolder process is influenced by heat from the igniter, an intermediate zone where smolder is free from external effects, and a third zone near the sample end that is affected by the external environment. The smolder velocity data are correlated in terms of a nondimensional smolder velocity derived from a theoretical model of the process previously developed. The analysis of the results confirm that the smolder process is controlled by the competition between the supply of oxidizer and the transfer of heat to and from the reaction zone. At low flow velocities oxygen depletion is the dominant factor controlling the smolder process, and the smolder velocity and temperatures are relatively small. Increasing the flow velocity strengthens the smolder reaction due to the oxygen addition resulting in increased smolder velocities and temperatures. These parameters, however, reach a maximum and as the air velocity is increased further the smolder reaction becomes weaker and eventually dies out due to convective cooling.

ISSN:0010-2202    

DOI:10.1080/00102209308907635

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

Acetaldehyde combustion process in CSTR has been simulated by means of a model capable of describing the complex dynamic behavior experimentally observed, including periodic cool flames, single and complex waveform periodic ignitions and jumps.

ISSN:0010-2202    

DOI:10.1080/00102209308907636

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

The influence of a vortex on a gaseous diffusion reaction is examined. The vortex is taken to be two dimensional, and the species are initially assumed to occupy adjacent half spaces. In the flame-sheet limit, thermal expansion and the effects of variable diffusion are taken into account. A global similarity solution exists for this problem, and a simple expression for the solution is determined in the asymptotic limit of large Schmidt number. The problem is also analyzed for finite-rate chemistry, appropriate for an isothermal, bimolecular reaction. The problem depends upon three parameters, Reynolds number, Schmidt number and the equivalence ratio, with the Damköhler number equal to the dimensionless time. The structure of the reaction region normal to the flame front is examined as a function of time. The evolution of the reaction to a state relation, dependent only upon the mixture-fraction variable, is demonstrated as the Damköhler number becomes large.

ISSN:0010-2202    

DOI:10.1080/00102209308907637

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

Minimum ignition energies have been calculated for alkane-air mixtures at standard ambient conditions. The governing conservation equations have been solved using approximate transport properties and chemistry. A six-species, four-step global reaction scheme for combustion of alkane hydrocarbons up to butane (Jones and Lindstedt, 1988) was employed. Radiative heat loss due to band emission from the ignition kernel which is included in the calculations was found to be negligible at equivalence ratios where ignition energy is a minimum. The important effect of energy input radius on minimum ignition energy is discussed in some detail for a propane-air mixture. After establishing a methodology for finding the minimum ignition energy for a specific fuel and equivalence ratio, results are presented for methane through butane over a wide range of equivalence ratios. A comparison is made with experimental results, and the similarities and differences are discussed.

ISSN:0010-2202    

DOI:10.1080/00102209308907638

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

By using direct numerical simulations (DNS), the accuracy of the Conditional Moment Closure Model (CMC) is tested for a nonpremixed reaction in decaying, spatially homogeneous turbulence. Simple one step, second order, irreversible, isothermal chemistry is used. The model equation is solved in two ways:(a) assuming that the mixture fraction and its dissipation are statistically independent, (b) accounting for their statistical dependence via a shape factor. In both cases (a) and (b) the predicted mean of the fuel mass fraction agreed well with the DNS data. Predictions obtained using approximation (b) were somewhat better than those using approximation (a).

ISSN:0010-2202    

DOI:10.1080/00102209308907639

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor