摘要:

Direct numerical simulations (DNS) are conducted of a preheated planar methane jet diffusion flame for various flow conditions. The fuel stream is a mixture of methane and nitrogen, and the oxidizer stream is air. The chemistry is modeled via the 1-step global mechanism of Bhui-Pham (1992). The flame behavior is assessed for various oxidizer stream temperatures, fuel stream velocities and nitrogen dilutions of the fuel stream. Consistent with experimental results, the root mean square (rms) values of temperature show two local maxima and a local minima on either side of the jet centerline and the probability density function (PDF) of temperature displays bimodality within the intermittent flow regions. Analyses of the post-ignition region of the flame in mixture fraction space indicate that as the conditional average values of the temperature increase downstream, those of the reaction rate decrease. The near-field characteristics of the flame are strongly influenced by the dilution of the fuel stream. An increase in the fuel dilution results in the increase in flame-vortex interactions, flame thickness and finite-rate chemistry effects. Peak values of the tangential strain rate and the curvature, calculated on the flame surface, are also increased. The correlations between the scalar dissipation rate and the strain rate improve significantly when the interactions between the flame and the vorticity field increase. The analyses of the fiowfield show that the laminar flamelet model compares favorably with the DNS in the regions where the flame curvature is small.

ISSN:0010-2202    

DOI:10.1080/00102200008924213

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

An experimental investigation of methane oxidation in the presence of NO and NO2has been made in an isothermal plug-flow reactor at 750-1250K. The temperature for on-set of oxidation was lowered by 250 K in the presence of NO or NO2at residence times of 200 ms. At shorter residence times (140 ms) this enhancement effect is reduced for NO but maintained for NO2. Furthermore two temperature regimes of oxidation separated by an intermediate regime where only little oxidation lakes place exist at residence times of 140 ms, if NO is the only nitrogen oxide initially present. The results were explained by the competition between three reaction paths from CH3to CH2O. A direct high temperature path (A), a two-step NO2enhanced low temperature path (B) and a slow three step NO enhanced path (C), which may produce NO2to activate path B. The negative temperature coefricient behaviour was explained by a competition between paths (A) and (C). A previously published reaction set was modified to take these reaction patterns into account.

ISSN:0010-2202    

DOI:10.1080/00102200008924214

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

A new air bag inflator based on the combustion reactions of methane-oxygen mixtures has been developed and modeled. The performance of this inflator was evaluated in terms of pressure-lime relationships inside the inflator and in a receiving tank simulating an air bag as well as the temperature-time relationship in the tank. A theoretical model has been developed to simulate the transient pressure, temperature and mass flow rate from the inflator to the tank. The model is based on the change in the internal energy inside the inflator and the receiving lank as the mass flows from the inflator to the tank. The model utilizes the Chemical Equilibrium Compositions and Applications code developed by NASA to estimate the equilibrium conditions in the inflator and the mass fractions of the product species. The model predicts the pressures and temperatures inside the inflator and the tank as a function of time. The predicted results were in good agreement with the experimental results. This model can predict transient pressures and temperatures with an accuracy of ±15%. The present modeling approach can be applied to a range of combustible gas mixtures, including hydrocarbon-oxygen, hydrogen-air and others.

ISSN:0010-2202    

DOI:10.1080/00102200008924215

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

The multiple flame structures for a pair of droplets in tandem in hot convective flows are analyzed numerically, by employing a body-fitted technique in association with a non-orthogonal curvilinear coordinate system. For selected droplet spacing and Reynolds numbers, three flame structures, including the double, triple, and tetra-brachial flames, are identified at specific ambient temperatures and droplet size ratios. As the droplet size ratio is small, the numerical results indicate that a double-flame structure composed of the front fuel-lean premixed flame and the rear diffusion flame develops at lower ambient temperature. Increasing the droplet size ratio results in the two flames experiencing the separation, then merger, and eventually destruction. Hence a triple flame consisting of both fuel-rich and fuel-lean premixed flames, as well as a diffusion flame appears. When the ambient temperature is higher, for the double-flame the front flame departs from the rear one as a result of the increased flame speed. On the other hand, because the flame moves upstream and the blowing effect of the trailing droplet grows, the triple-flame converts into a tetra-brachial flame at the larger droplet size ratio. The formation of these flames is attributed to the coupling of fuel vapor, cooling effect, and oxidizer depletion stemming from the leading droplet, and these observations can provide a more accurate modeling on droplets behaviors in spray.

ISSN:0010-2202    

DOI:10.1080/00102200008924216

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

Centerline and radial profiles of gas temperature and C3 to CI2 hydrocarbon concentrations were measured in nitrogen-diluted methane and ethylene coflowing nonpremixed flames. The methane or ethylene volumetric flowrates were kept constant while the nitrogen flowrate in the fuel mixture was systematically varied. In both series of flames, the lemperatures in the pyrolysis zone were shifted to higher heights in the flame by increases in the overall flame length, and were reduced both by the decrease in the adiabatic flame temperature and by a mechanism that likely involves decreased heat transfer from the flame front to the centerline. The maximum centerline concentrations of every measured non-fuel hydrocarbon monotonically decreased and shifted to higher heights in the flame as dilution was increased. The maximum mole fractions of most linear hydrocarbons were roughly proportional to the hydrocarbon mole fraction in the un reacted fuel mixture, while the maximum mole fractions of aromatic hydrocarbons varied roughly as the square of that quantity. The decrease in the initial methane or ethylene concentration in the fuel appears to be the primary cause of the reductions in the maximum nonfuel hydrocarbon concentrations. For all hydrocarbons the shape of the centerline mole fraction profiles, including the location of the maximum concentration and of the species'disappearance, correlate well with temperature.

ISSN:0010-2202    

DOI:10.1080/00102200008924217

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

This study numerically investigates the ignition behaviors of vertically oriented cellulosic materials subjected to a radiant heat flux in a normal gravitational field. The entire process is delineated into two distinct stages. In the heating up stage, the maximum temperature increases with time but at a decreasing rate because of the pyrolysis reaction. The flame development stage consists of ignition and transition processes. In the ignition process, the maximum temperature in gas phase increases dramatically within a short period of time because a large amount of heat is generated from chemical reaction of the accumulative, flammable mixture. The flame is in a transition from a premixed flame to a diffusion one, except for the small region around the flame front. For the effect of varying the heating duration on ignition behavior, prolonging the imposed radiative heat time leads the ignition from a transition one to a persisting ignition. The effect of varying the solid fuel thickness indicates that the ignition delay lime increases with an increase of solid fuel thickness while 5S<1.802. For ].802<6S<3.244, the ignition delay times remain constant. Finally, if the external heating rate is the same order as the heal diffuse rate, the ignition delay time increases with an increase in solid fuel thickness.

ISSN:0010-2202    

DOI:10.1080/00102200008924218

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

Engine and laboratory experiments are performed to understand the formation of deposits during ignition delay. An experimental Diesel engine allows to observe deposit formation as a global phenomenon including homogeneous and heterogeneous reactions as well as interaction between liquid and gaseous phases. A CFR engine is used to work under the same global conditions but without the liquid phase. A flow reactor enables to investigate the effects of defined and controlled temperatures and fuel/air ratios.

ISSN:0010-2202    

DOI:10.1080/00102200008924219

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor