摘要:

A single laser source and a single intensified CCD camera have been used to image temperature and the hydroxyl radical, OH in turbulent nonpremixed flames. Images are simultaneous and quantitative. Temperature is derived from Rayleigh scattering. Laser Induced Fluorescence (LlF) is used to image OH. Correction of the LIF image for quenching effects is done using a combination of laminar flame calculations and the temperature measurements. Images for flames of the following fuels are presented: CH3OH, C2H5OH, CNG (91% CH4by volume) and CNG-H2(31% CNG, 69% H2by volume). The OH profile is used as a measure of reaction zone thickness. Number densities of OH are found to be in superflamelet levels for all flames investigated here. Temperatures agree well with laminar flame computations. Flames show evidence of partial or full premixing of the fluid in the reignition zone at high velocities downstream of regions of high local extinction. The OH profiles are seen to increase in width and this is consistant with an increase in calculated reaction zone width and an increase in stoichiometric mixture fraction. An increase in Reynolds number alone does not change the OH profile thickness.

ISSN:0010-2202    

DOI:10.1080/00102209708935603

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

The chemical structure of stoichiometric methane–oxygen–argon flames seeded or not with 1% heptafluoropropane (CF3CHFCF3) was measured by the molecular beam-mass spectrometry technique. Both flames were stabilized on a water-cooled flat-flame burner under low pressure (4.2 kPa). Mole fraction profiles were computed by a simulation code as well. Modeling of the unseeded flame was performed with an updated version of a mechanism issued from Warnatz. Two submechanisms were considered to model the chemistry of fluorinated species: (i) a mechanism proposed by Westbrook to model flame inhibition by CF3Br, (ii) a modified version of (i) validated recently by Sanogo in a modeling study of the effect of C2F6on a methane flame. Both submechanisms were compared on the basis of their reaction pathways. They have in common a key role played by CF2in the consumption of the fluorinated additive. The consumption of this radical forms CF that is consumed very slowly with Westbrook mechanism,in contradiction with experimental results. A better agreement is obtained with the modified version so that this study extends and confirms its validation.

ISSN:0010-2202    

DOI:10.1080/00102209708935604

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

Modeling of Partially-Stirred-Reactors (PaSR) by stochastic simulation is carried out for investigating the influence of unmixedness on thermo-chemical properties. First, the joint scalar probability density function (pdf) of the PaSR is derived using a control volume analysis to provide theory for development of stochastic simulation procedures. Second, through numerical exploration and analysis, an improved stochastic algorithm is developed to eliminate the dependence of solution on time step. The limitations on age distribution from the stochastic simulation are identified. For nonpremixed combustion with either the modified Curl's mixing model or the Linear-Mean-Square-Estimation (LMSE) mixing model, analytic expressions are derived for the unmixedness in terms of residence time and mixing frequency. Numerical simulations have been performed revealing that the unmixed nature has profound influence on ignition delay and NO formation in PaSR with hydrogen combustion. The performance of recently developed reduced chemistry for hydrogen combustion is evaluated by comparisons with detailed chemistry. The predicted NO formation is found sensitive to mixing model.

ISSN:0010-2202    

DOI:10.1080/00102209708935605

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

A one-dimensional model for a sink-stabilized spherical flame is proposed and analyzed. It is shown that the stabilization occurs only within a certain range of the sink flow rates, beyond which the flame sulfers either blowoff or flashback. It is suggested that the system may be helpful for understanding the flame stabilization in the flow wake past a blulf body, the principal motivation for the present study.

ISSN:0010-2202    

DOI:10.1080/00102209708935606

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

The use of Fourier transform infrared absorption spectroscopy to obtain species and temperature profiles within solid-propellant flames is described. Measurements were performed at a spectral resolution of 2 cm-1and a spatial resolution of 1 mm within the flame zone of an RDX-composite propellant. A nonlinear, least-squares routine was developed to deduce the absolute species concentrations of CO, NO, H2O, CO2HCN, CH4and N2O as well as the rotational temperature. The deduced temperature profile revealed the presence of darkzone/ luminous flame-zone structures. The mole fraction of CO increased while the mole fraction of HCN decreased from the beginning of the dark zone to the luminous flame zone. Data on the spatial variation of species concentration and temperature within these zones are important for the development and validation of chemical kinetic mechanisms describing the combustion of solid propellants.

ISSN:0010-2202    

DOI:10.1080/00102209708935607

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

Predictions and measurements of a swirling unstaged, high NOx, pulverized coal flame are presented. The predictions have been obtained by means of relatively simple models for turbulence, turbulent combustion and turbulent nitric oxide chemistry. Turbulence is modeled using the standard k-epsilon model. The turbulent combustion model incorporates a two-step reaction scheme together with an eddy break-up model. In the NO- chemistry model, thermal-NO and fuel-NO chemical reaction rates are statistically averaged over the fluctuating temperature using the Beta probability density function. Radiation is computed by means of the Discrete Transfer Radiation model. A constant linear absorption coefficient is assumed while scattering is not taken into account. The paper contains a unique, comprehensive comparison between predictions and measurements of a pulverized coal flame issued from an industrial-scale burner. The coal fired was Gottelborn hvBb coal. Attention is paid to velocities, turbulence quantities, temperatures, coal burnout levels and species concentrations of oxygen, carbon monoxide, carbon dioxide, nitrogen oxides precursors and nitrogen oxides. Radiative heat fluxes near the furnace refractory wall and the heat extraction of the cooling loops are addressed also.

ISSN:0010-2202    

DOI:10.1080/00102209708935608

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

A rigorous thesmochemical analysis of the pyrolysis of aromatic chlorides shows that destruction efficiencies in excess of 99.999 percent are possible in a reducing atmosphere at temperatures near 1000 K. This process used for the destruction of chlorinated organic ompounds yields non-halogenated hydrocarbons and hydrochloric acid as reaction products. Qualitative batch experiments on chloromethane and polychlorinated biphenyls (PCBs) showed that such efficiencies could be obtained in residence times below 7 minutes at 1273 K. Qualitative continuous flow experiments demonstrated that these efficiencies are actually obtained in residence times less than 1.8 seconds for chloromethane and 4.4 seconds for the PCB mixture Arochlor 1254. A simple kinetic analysis shows that the Aroclor 1254 radicals are produced by the decomposition of hydrocarbon solvents [n-hexane and iso-octane) used to provide the reducing atmosphere. The difference in residence times between batch and continuous flow reactors is explained by temperature gradient effects in batch reactors and the associated heat flux limiting rate phenomenon due to endothermic reaction steps.

ISSN:0010-2202    

DOI:10.1080/00102209708935609

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

An experimental study is conducted to evaluate the effectiveness of sodium bicarbonate (NaHCO3) powder in extinguishing flames burning hydrocarbon fuels. Experiments are performed on flames stabilized in a counterflow burner. Powders with various particle sizes are used. The minimum amount of NaHC03powder required to extinguish these flames is measured and compared with previous measurements using CF3Br and N2. The results show NaHCO3, to be much more effective than CF3Br and N2, in extinguishing the flames. The calculated equilibrium temperatures of NaHCO3inhibited flames close to extinction show that NaHCO3, inhibits the flame by chemical mechanisms.

ISSN:0010-2202    

DOI:10.1080/00102209708935610

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

Ignition of non–premixed boundary–layer flows over a hot wedge–type porous wall with fuel injection is analyzed mathematically and numerically. The free–stream fluid is a diluted oxidizer such as air. An ignition criterion corresponding to the condition for lower vertical tangency on an S-shaped ignition–extinction response curve is derived by the method of large activation energy asymptotics. This ignition criterion depends on all relevant system parameters such as flow conditions, thermophysical properties (Pr, SCoand Scf) and the overall chemical kinetic mechanism of reactants. The relative sensitivity of system parameters affecting the ignition is investigated. In particular, the influence of system parameters on the transition from the kinetically controlled to the diffusionally controlled ignition mechanism is studied. Numerical calculations are also performed to compare with the asymptotic results. The asymptotic solutions are found to be in good qualitative agreement with numerical solutions.

ISSN:0010-2202    

DOI:10.1080/00102209708935611

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

Catalytic combustion of methane can generate thermal gradients that are large enough to shatter monoliths during several transient operating modes. This paper presents simulations from a 2-D transient numercial model of the transport and surface chemistry in a passage in a catalytic monolith, including convection and diffusion in the gas phase and heterogeneous reactions on the monolith walls. Rates of surface methane oxidation are based on reaction kinetics for a supported Pd'O catalyst assigned from differential reactor measurements. A thermal stress analysis package uses the predicted temperature profiles to calculate stress formation profiles in the monolith. The model neglects homogeneous reactions, but nevertheless describes the essential features of transient operating modes that generate the largest thermal stresses. Simulations are reported for inlet methane concentrations from 3 to 5 vol% in air and inlet temperatures from 300 to saoae. Results illustrate dynamic stress formation during combustor warm-up, cool-down and cycling of the inlet methane concentration. including cases with thermal stresses as high as 630 MPa, which exceed the fracture strength of typical monolith materials such as mullite and cordierite. The highest thermal stresses form perpendicular to the flow direction during warm-up transients. and would tend to crack the monolith walls along their axes.

ISSN:0010-2202    

DOI:10.1080/00102209708935612

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor