摘要:

This paper presents a computational study of ignition in nonpremixed counterflowing CO/H2versus heated air. To understand the chemical and dynamical mechanisms governing CO/H2ignition, we have examined the following features: the effect of H2concentration on the system's ignition characteristics, the dominant chemical kinetics just prior to ignition, the role of convection and diffusion with respect to chemical kinetics, and the dependence of ignition temperature on pressure variation. Three ignition regimes were identified, based upon the influence of H2concentration, which we have designated: 1) H2-dominating, 2) transition and 3) H2-catalyzing. In the H2dominated regime, relevant for high H2concentrations, the controlling ignition kinetics involve that of H2chemistry with minimal CO influence. In the transition regime, the participation of CO kinetics, especially through its exothermicity, allows the CO/H2system to be ignitable even when the analogous N2/H2system ceases to be ignitable. Sensitivity analysis shows that it is the competition between H and HO2chemistry in the two systems that determines whether or not the system is ignitable. Finally, in the H2-catalyzed regime, relevant for low concentrations, the dominant kinetics are found to include both H2and CO chemistry. Furthermore, by studying the effects of pressure on the ignition temperature in the of H2-catalyzed regime, three iginition limits corresponding to those of H2explosion limits are identifed, as is reasonable to expect.

ISSN:0010-2202    

DOI:10.1080/00102209708935684

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

We study the thermal propagation of a reaction wave through a condensed medium in the case when the thermal conductivity of the mixture may change in a jumpwise fashion at some critical temperature. This can occur, e.g., when the actual heat transfer processes are accounted for as a heat conduction process with an effective thermal conductivity. Then a change in the actual mechanism of heat transfer at some point in the reaction wave may result in a significantly different effective thermal conductivity. Our model also allows for the thermal conductivity of the product to be different from that for the initial reactants. We have determined the structure of uniformly propagating reaction waves and analyzed their stability. We have shown, in particular, that if the thermal conductivity is increased in a region just ahead of the reaction zone then the wave is either stabilized on destabilized depending on whether the width of this region is below or above a critical value.

ISSN:0010-2202    

DOI:10.1080/00102209708935685

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

The liftoff and blowout characteristics of laminar flames in a partially premixed jet are studied for propane and n-butane fuels mixed with air. As the flow rate increases, flame lifts off from a nozzle attached flame and liftoff height increases highly nonlinearly with flow rate, and then blowout occurs. The jet velocities at liftoff and at blowout decrease linearly with the increase in air dilution and are independent of nozzle diameter. The liftoff height at blowout is proportional to the square of nozzle diameter and to the square of fuel mass fraction. Correlations for the liftoff heights and blowout conditions are derived by using a cold jet theory for velocity and species concentrations, based on the tribrachial nature of lifted flames. That is, at the flame anchoring point, the flame has the characteristics of a stoichiometric flame and its propagation speed balances axial flow velocity. The experimental findings of liftoff and blowout are successfully predicted.

ISSN:0010-2202    

DOI:10.1080/00102209708935686

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

Methane-oxygen, methane-nitrous oxide and carbon monoxide-nitrous oxide counterflow diffusion flames have been studied at low pressures. Species concentrations have been measured using probe extraction and gas chromatography, and temperatures have been measured using thermo-couples. Measurements have been compared to predictions of a computer model. The chemical kinetic mechanism input into these models has been found to be successful in predicting essential features of the structures of these flames. However, improvement are still needed to predict some of the structural features of the flames more accurately. For the methane-nitrous oxide flame, rate generation analysis was performed to identify which chemistry is most essential in predicting the methane-nitrous oxide flame structure. The results of this analysis were compared to rate generation analysis of a premixed methane-nitrous oxide flame. The differenoes between these analyses have been noted. The rate generation analysis results can be used to help guide the formulation of chemical kinetic mechanisms of reduced size, which are required by global propellant modelers.

ISSN:0010-2202    

DOI:10.1080/00102209708935687

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

Analysis of previously reported experimental results has shown that the formation of diheptyl-peroxide from n-heptane in a motored C.F.R. engine, although it involved free radicals, was not a chain reaction. It was possible to calculate an activation energy which was really the sum of the activation energy of the two steps in the formation of diheptylperoxide.

ISSN:0010-2202    

DOI:10.1080/00102209708935688

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

Tar yield collection from the pyrolysis of relatively large particles of biomass are investigated using the model of Miller and Bellan (1997).A variety of feedstocks are considered by varying the ratios of cellulose, hemicellulose and lignin within the biomass. Effectsof secondary tar reactions, quenching, temperature, particle size and carrier gas are assessed. Secondary tar reactions occuring in both the particle's interior and the exterior boundary layer strongly reduce the potential amount of tar available for collection compared to the maximum given by kinetic predictions. The primary effect of these reactions is the existence of an optimal reactor temperature range for maximizing tar yields. This range is a function of both the quenching location and the initial particle size. For rapid qvenching near the particle surface, tar collection is maximized at high temperatures for small particles, and at low temperatures for large particles. For delayed quenching, low temperatures slow the secondary reactions and provide larger tar yields for all particle sizes investigated. Tar yields are also dependent on the choice of the inert carrier gas;

ISSN:0010-2202    

DOI:10.1080/00102209708935689

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

The mechanism of flame spread over thermally thick solid fuels in an opposing flow is explored. The pathways of forward heat transfer are computed with mathematical models of increasing complexity in all regimes. New formulas are developed that reproduce numerical results for the dominant forward heat transfer pathways in the thermal and chemical kinetic regimes and compare well with experimental data.

ISSN:0010-2202    

DOI:10.1080/00102209708935690

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

This article examines the structure near a cold wall of the flame-tip region of a triple or tribrachial flame. A transformation of coordinates is employed that enables the energy equation to be integrated across the premixed flame (PF) are, subject to an upstream matching condition. An approximate matching condition is derived that enables a differential equation to be derived for the PF arc shape. The solution of this equation produces flame contours X(Z) which may be grafted onto an orthogonal χ-Z grid as the function χ=X(Z). Comparison between the calculated and numerically computed flame shapes is favorable.

ISSN:0010-2202    

DOI:10.1080/00102209708935691

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

Direct numerical simulations of turbulent nonpremixcd hydrocarbon ‘flames’ are carried out with a constant density approximation. Numerical resolution and stiffness problems which are inherent in oxidation of hydrocarbon fuels are overcome by using a systematically derived two-step mechanism. The rate constant of the elementary fuel consuming step is carefully relaxed to make the fuel consumption layer amenable to numerical resolution. The validity of relaxing this rate constant is demonstrated. A detailed discussion of various aspects of the two-step chemistry modeling is given. It is shown that the strategy of using the two-step mechanism with relaxed rate constants does not compromise flame structure and the associated underlying physical processes. Thus, the two-step mechanism is found to be more suitable than a four-step or a one-step mechanism to directly simulate turbulent combustion of hydrocarbon fuels.

ISSN:0010-2202    

DOI:10.1080/00102209708935692

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

ISSN:0010-2202    

DOI:10.1080/00102209708935693

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor