摘要:

The Coherent Flamelet Model (CFM) is applied to symmetric counterflow turbulent premixed flames of Kostiuket al.(1993b). The flame source term is set proportional to the sum of the mean and turbulent rate of strain while flame quenching is modeled by an additional multiplication factor to the flame source term. The turbulent rate of strain is set proportional to the turbulent intensity to match the correlation for the turbulent burning velocity by Abdel-Gayedet al.(1987). Mixing with ambient air is assumed to be proportional to the mass fraction of the burned and unburned mixture in each computational cell. The predicted flame position and turbulent flow field coincide well with the experimental observations. The nozzle exit velocity at extinction shows qualitative agreement for variation of the equivalence ratio, nozzle separation distance and exit turbulence condition.

ISSN:0010-2202    

DOI:10.1080/00102209808952060

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

According to the special feature of the flow outside a char particle under forced convection conditions (Re < 150), a simplified model for calculating the burning rate of a char particle is presented in this paper. The burning processes of frontal and rear hemispheres are discussed, respectively. The effects of both flow and ash formation are considered in the study. Based on this model, a numerical simulation of the burning process is conducted for different type of char. Good agreement of the calculated results with corresponding experiment data demonstrates that this model is reasonable for the case with Re ≤ 150.

ISSN:0010-2202    

DOI:10.1080/00102209808952061

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

Detailed measurements of mixture fraction, flame temperature, soot volume fraction and spectrally resolved thermal radiation are reported in turbulent kerosine spray flames, burning at elevated temperatures and pressures (up to 13 bar). Flames are supported on an internal mixing, air-assist injector and confined within a cylindrical pressure vessel. Contrasting results, arising from the analysis of data on kerosine spray flames burning at various operating pressures, air-fuel ratios and fuel flow rates, are presented which provide new insight into the influence of operating pressure on soot formation and radiative heat transfer in practically relevant configurations. The experimental results between 1 and 13 bar suggest an underlying pressure dependence of the soot chemistry which is broadly linear. In addition, however, fuel atomisation and injector air-fuel ratio have a pronounced effect on the sooting propensity of the spray flames, which can be stronger than the influence of the operating pressure itself. They have a particularly strong influence on the rates of oxidative attack on soot particles and precursors and on the spectral distribution of thermal radiation between particulate and gaseous emissions. Continuum radiation from clouds of dispersed soot particles in the flame provides the major contribution to the total flame radiation in these comparatively fuel-rich kerosine spray flames with gas band radiation proving of only minor importance. The total radiation intensity from the flame then closely mirrors the trend in soot formation rates with respect to the dependence on operating conditions.

ISSN:0010-2202    

DOI:10.1080/00102209808952062

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

The transition from particle flamelet burning to premixed flame burning of ammonium perchlorate (AP)/hydrocarbon (HC) binder propellants is identified with detachment of the oxidizer/fuel (O/F) flamelet from the stoichiometric tip of the diffusion field over the AP particle. The conditions (pressure, particle area, local stoichiometry) for detachment are revealed by singular regions of the burning rate versus pressure curves for selected bimodal propellants.

ISSN:0010-2202    

DOI:10.1080/00102209808952063

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

The pyrolytic decomposition of triethyl phosphate (TEP) has been studied at one atmosphere, from 706 to 854 K, in an isothermal quartz-lined, 4.5-cm-ID turbulent flow reactor with residence times between 15 and 85 ms. TEP was initially present at levels between 30 and 125 ppm, in N2. CO was used as an internal standard for mixing. Fourier-transform infrared spectrometry and gas chromatography/mass spectrometry preceded by conversion to trimethylsilyl derivatives were used to measure TEP and its products in samples extracted through quartz sampling tubes. Ethene, ethanol, diethyl phosphate, monoethyl phosphate, and orthophosphoric acid were observed as products. A multistep decomposition mechanism has been proposed, involving parallel pathways producing ethanol and ethene. Overall uni-molecular rate parameters for TEP decomposition have been obtained. Wall reactions affect the ethanol-to-ethene ratio, and may also affect the TEP decomposition rate.

ISSN:0010-2202    

DOI:10.1080/00102209808952064

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

A simplified three-step model of soot inception has been employed with high activation energy asymptotics to study soot inception in nonpremixed counterflow systems with emphasis on understanding the effects of hydrodynamics and transport. The resulting scheme yields three zones: (1) a fuel oxidation zone wherein the fuel and oxidizer react to form product as well as a radical R, (e.g., H), (2) a soot/precursor formation zone where the radical R reacts with fuel to form “soot/precursor” S, and (3) a soot/precursor consumption zone where S reacts with the oxidizer to form product. The kinetic scheme, although greatly simplified, allows the coupling between soot inception and flame structure to be assessed. The results yield flame temperature, flame location, and a soot/precursor index S1as functions of Damkohler number for S formation. The soot/precursor index indicates the amount of S at the boundary of the formation region. The flame temperature indirectly indicates the total amount of S integrated over the formation region because as S is formed less heat release is available.

ISSN:0010-2202    

DOI:10.1080/00102209808952065

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

Cyclohexane oxidation has been studied in a jet-stirred reactor in the temperature range of 750 to 1100 K. at 10 atm. Major and minor species profiles have been obtained by probe sampling and GC analysis. A chemical kinetic reaction mechanism developed from previous studies on smaller hydrocarbons is used to reproduced the experimental data. It has been updated and validated for CI to C5 submechanisms. Good agreement is obtained between computed and measured mole fractions. The major reaction paths of cyclohexane consumption and the formation and the consumption routes of the main products have been identified for our experimental conditions.

ISSN:0010-2202    

DOI:10.1080/00102209808952066

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

A simple modelling easily usable in the industrial environment has been previously described in order to predict the evolution of the combustion characteristics of solid-gas mixtures in a closed vessel. The aim of this work consists in extending the model development to a wide range of dust suspensions in the general case of vented explosions and to test the model behaviour to get a risk evaluation and improve safety conditions in industrial plants. A simple representation of the combustion phenomena based on energy transfers between particles and the action of specific molecules is presented. The pressure venting due to the vent breaking is calculated from thermodynamical characteristics given by the model and taking into account the mass rate of discharge of the different products deduced from the standard orifice equations. The application conditions determine the fuel ratio of the used mixtures, the nature or the chemical kinetics and the calculation of a universal set of parameters. The proposed development is compared to Bartknechl's experimental results and shows the model representativeness for varied dust suspensions (cornstarch, cellulose and aminophenazone) in very large use conditions such as the concentration in a rich or a lean mixture, the vessel volume or its geometry and the choice of the vent area or the venting pressure. The first results indicate correct tendencies in the predictions and it seems interesting to precise this evaluation in the future with vented gaseous mixtures.

ISSN:0010-2202    

DOI:10.1080/00102209808952067

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

Presented is an asymptotic analysis of premixed laminar flames in opposed streams under circumstances of fast chemistry and/or low rates of strain and of high Reynolds numbers. In this case there exist three distinct reaction layers, two premixed and one nonpremixed, the latter centered on the stagnation plane. These layers separate inviscid, chemically inert regions with constant gas properties. The state variables in each region are determined by the reactants exiting from each jet. If the location of the two premixed reaction layers is known, the velocity distributions in the four regions can be readily calculated. An activation energy analysis of the structure of the two premixed reaction layers determines the Damköhler parameter associated with their locations. Comparison of the asymptotic analysis is made with a recently reported large scale computation of triple flames. Agreement with the distributions of the velocity components is satisfactory; however, as a consequence of the idealized chemistry treated here the temperature distributions in the two high temperature regions are only roughly correct. The analysis shows the influence of changes in jet velocities and equivalence ratio of the fuel rich reactant stream on the location of the premixed reaction layers and on the velocity distributions. The former changes establish the limiting behavior of triple flames corresponding on the one hand to the premixed reaction layers being in the neighborhood of the exit planes and on the other lo all three layers being embedded in a viscous layer centered on the stagnation plane.

ISSN:0010-2202    

DOI:10.1080/00102209808952068

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

Thermoacoustic instability was investigated and controlled in an experimental low-emission swirl stabilized combustor, in which the acoustic boundary conditions were modified to obtain combustion instability. Several axisymmetric and helical unstable modes were identified for fully premixed and partially premixed/diffusion combustion. These unstable modes were associated with flow instabilities related to the recirculation region on the combustor axis and shear layer instabilities at the sudden expansion (dump plane). The spatial locations of the intense combustion regions associated with the different unstable modes were visualized by phase locked images of OH chemiluminescence. The axisymmetric mode showed large variation of the heat release during one cycle, while the helical modes showed variations in the radial location of maximal heat release. A closed loop active control system was employed to suppress the thermoacoustic pressure oscillations and to reduce NOxemissions. Microphone and OH emission sensors were utilized to monitor the combustion process and provide input to the control system. Acoustic actuation was utilized to modulate the air flow to aflect the mixing process and the combustion. Suppression levels of up to 5 dB in the pressure oscillations and a concomitant reduction of NOxemissions were obtained using an acoustic power of less than 0.002% of the combustion power. The microphone based controller was slightly more efficient than the OH-based controller. This was due to the reduced coherence of the combusting large-scale structures which resulted in a deterioration of the OH signal when the controller became effective.

ISSN:0010-2202    

DOI:10.1080/00102209808952069

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor