ISSN:0010-2202    

DOI:10.1080/00102209508951945

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

Experiments have been conducted to study the transport and chemical kinetics mechanisms controlling the spread of flames over the surface of a thick solid fuel in an oxidizing gas flow moving in the direction of flame propagation (concurrent or flow assisted flame spread). In the experiments, the solid fuel (PMMA) is unchanged, but the oxidizer gas flow velocity, turbulence intensity and oxygen concentration, are systematically varied to determine their effect on the flame spread process. This is done by measuring the rate of flame spread, flame length, surface heat flux, exhaust gas temperature, products of combustion and soot.

ISSN:0010-2202    

DOI:10.1080/00102209508951915

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

Single wood pieces in quiescent air were ignited by an electric coil which was removed shortly after. There was found some critical size, specimens smaller than which would be ignited, burn with a flame and, produce a carbonaceous residue (sometimes glowing). Thin pieces burned more easily than thick ones but the duration of flaming combustion depended on the specimen size. Pieces thicker than the critical size did not sustain a flame. Simplified theoretical analysis based on pure one-dimensional transient heat conduction gave a satisfactory description of the evolution of the pyrolysis process and the surface gas flux. There also existed a minimum heating rate that led to piloted ignition; the heating process with a rate lower than this minimum would simply result in carbonization without flame. Calculations on piloted ignition of thin samples showed that thinner pieces need higher heating rates than thicker ones, which agreed with experimental measurements.

ISSN:0010-2202    

DOI:10.1080/00102209508951916

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

Predictions and measurements of a swirling unstaged, high NOx, natural gas flame are presented. The predictions have been obtained by means of relatively simple models for turbulence, turbulent combustion, radiation and turbulent nitric oxide chemistry. Turbulence is modeled by means of the standard K-epsilon model. The turbulent combustion model incorporates a two-step reaction scheme together with an eddy break-up model. Radiation is computed by means of a four-flux method and the Exponential Wide Band Model is used to calculate local radiative properties. In the NO-chemistry model, thermal-NO and prompt-NO chemical reaction rates are statistically averaged using the Beta probability density function. The paper contains a unique comparison between measurements and predictions of a natural gas flame issued from an industrial-scale burner. Special attention is given to establishing the importance of thermal boundary conditions on both the energy balance and calculated NO emissions. The accuracy of the model predictions, including NO and CO emissions is established by carrying out appropriate sensitivity studies.

ISSN:0010-2202    

DOI:10.1080/00102209508951917

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

A mathematical model is presented for the swirling unstaged natural gas flame. The performance of the model is critically assessed by comparison with a detailed set of experimental measurements. The model makes use of a conserved scalar approach. Turbulence is modelled using k-epsilon model with turbulence chemistry interaction accounted for using a two parameter presumed p.d.f. Radiative heat transfer is treated by means of the discrete transfer method and chemistry models making use of the “mixed is burnt” and partial equilibrium assumptions are evaluated. Numerical solution is by means of a collocated finite volume approach in a general non-orthogonal curvilinear coordinate system. It is seen that predictions to reasonable engineering accuracy can be obtained, with the partial equilibrium model offering some advantage over fast irreversible chemistry. Recommendations for future development are given.

ISSN:0010-2202    

DOI:10.1080/00102209508951918

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

An expression for the propagation velocity of a reaction front in a porous solid material is obtained by using a one-dimensional energy conservation equation. The reaction is divided in two distinct fronts.an ignition front where gasification of large part of the fuel occursand a flamingfront where gas phase oxidation takes place. Forced flow and reaction move in opposite directions, so, from a frame fixed to the front, fuel and air will enter the reaction from one side and products will leave through the other side. The propagation velocity obtained from this analysis is applicable to fuels with a high fraction of volatile materials, such as those characteristic of municipal waste products. The analysis is divided in two parts, the calculation of the propagation velocity and the determination of a critical ignition temperature. For the first part, heat transfer towards the virgin fuel, by convection, conduction and radiation, controls the propagation of the reaction front and an expression for the propagation velocity is obtained as a function of a characteristic ignition temperature. For a control volume at the ignition front the Semenov thermal explosion theory is used to determine the characteristic ignition temperature. The expressions are correlated with experimental data obtained from burning solid mixtures that are characteristic of municipal waste products. The materials used are, therefore, highly heterogeneous in both their composition and geometrical characteristics. Experiments were conducted with cardboard, wood and a mixture of cardboard, wood and plastic in a counter-flow packed bed reactor and it was observed that the results correlated well with the theoretical expressions. The good agreement of a fundamental theory and experiments conducted with materials of practical importance, not only clarifies the mechanisms controlling this type of combustion but also contribute to determine the validity of such an approach when applied to heterogeneous materials.

ISSN:0010-2202    

DOI:10.1080/00102209508951919

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

The oxidation of Spherocarb carbon particles is studied in a research-scale fluidized bed reactor (FBR) over the temperature range of 630K-130OK at atmospheric pressure. The reactor uses nitrogen carrier gas to which various amounts of oxidizer (from 6%-30% by volume) are added. Carbon loading (spherical 60/80 mesh spherocarb particles) is kept low so that the reactor remains nearly isothermal overall. A procedure for a non-intrusive approximation of average particle temperature rise was developed and indicated that particle temperatures do not rise more than 20 K above the bed temperature. Oxidation rates are determined by measuring the bed temperature and the time dependent formation of gas-phase carbon oxides. Assuming a first order dependence on oxidizer concentration, overall rates of oxidation for various pure oxidizers in nitrogen are determined. The overall activation energy for O2(37 ± 2 kcal/mole) corresponds well with previously published values. Overall activation energies for CO2(61±9 kcal/mole), and H2O (82 ± 8 kcal/mole) are considerably larger than those for oxygen, and overall rates were found to be the same order of magnitude as those for oxygen at 1500 K and 1380 K, respectively. The overall activation energy for N2O (33 ± 3 kcal/mole) is similar to that for oxygen with a rate about 75% slower.

ISSN:0010-2202    

DOI:10.1080/00102209508951920

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

A chemical reaction mechanism for methane combustion was developed by a method different in principle from previous efforts that start from detailed chemistry and use steady-state and other approximations. It describes, accurately and with the same parameter set, such different phenomena as flames and high and intermediate temperature ignition. The approach used for chemistry simplification resolves the main features of hydrocarbon oxidation into steps, each with simple kinetic sense, describing thermal decomposition of fuel, true and degenerate chain branching, propagation, termination, and the formation of final products. Rate coefficient expressions for elementary reactions were taken from the literature, while those of generalized reactions were optimized using an automated regression procedure against ignition and flame speed data. The predicted composition and pressure dependences for ignition and combustion of methane and methane-ethane mixtures are in good agreement with experiment.

ISSN:0010-2202    

DOI:10.1080/00102209508951921

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

Combustion phenomena of metal powders in contact with NaNO3used as propellants have been studied in order to find out burning rate variations at different oxidiser levels. Commonly used metals like Mg, Al, Zr and B (a metalloid) have been considered for the present study. It is found that the burning rate of the propellants largely depends on mixture ratio of the fuel and oxidiser. The burning rate of Mg-NaNO3and Zr-NaNO3propellants decreases with increasing concentration of NaNO3in contrast to the energy content of the propellants, On the otherhand the burning rate as well as the energy content of Al-NaNO3and B-NaNO3propellants increases with increasing NaNO3concentration.

ISSN:0010-2202    

DOI:10.1080/00102209508951922

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

An important circumstance that is often encountered in enclosure fires is that of heat and mass transfer across a horizontal vent. Such vents are present in enclosed regions such as rooms in multi-leveled buildings and in ships. The transport arises due to the finite density and pressure differences that usually exist across such vents. It is important to determine the resulting flow rate and the nature of the flow, particularly whether the flow is unidirectional and the direction in which it occurs or whether it is bidirectional across the vent, since the growth and spread of the fire are dependent on these characteristics. This paper presents a study of this heat and mass transfer problem, employing air at different temperature levels to provide the buoyancy. A laser sheet is used for visualizing the flow of smoke through the vent. Quantitative results are obtained on the resulting flow rates for wide ranges of the governing variables. It is also found that transient effects are important in most cases and temperature data are used to study the oscillations in the flow and to determine the transition from one regime to the other. Some correlations are derived from the measurements to characterize the dependence of the flow rate across the vent on the governing variables, such as pressure and temperature differences.

ISSN:0010-2202    

DOI:10.1080/00102209508951923

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor