摘要:

Images of mixture fraction, temperature, scalar dissipation rates and OH concentrations in turbulent diffusion flames of methane-air and hydrogen-carbon dioxide are presented. The images are derived from Rayleigh scattering, fuel Raman scattering and OH-LIF. The images reveal that the reaction zones in these flames are strongly affected by the turbulence within them and that they become broadly distributed as the velocity is increased. The reaction zone width, as estimated from the measured OH profiles, is found to increase with the increase in jet Reynolds number of the flames. Local turbulence affects the OH profiles and causes a variation in the OH concentrations with little apparent variation in the corresponding mixture fraction and temperature images. This is seen in flames which are far from blow off and is not thought to be a local extinction effect but the direct influence of turbulence in the reaction zone. High scalar dissipation rates are not measured in the reaction zone where unburnt samples are encountered. Local nonburning may be due to a lack of ignition of premixed fluid or to local quenching by large eddy entrainment of cold fluid. The measured scalar dissipation rates do not increase significantly with an increase in the jet velocity. This may be due to the decrease in the gas diffusivity with decreasing temperature which offsets the increase in the spatial mixture fraction gradient. The scalar dissipation rates are believed to reduce in regions of local nonburning due to the associated reduction in the temperature.

ISSN:0010-2202    

DOI:10.1080/00102209808952042

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

The ignition of a solid exposed to a step in surface temperature, including the effect of the curvature, is analyzed by means of large activation energy asymptotics in the whole range of values of the Damköhler number,Da, which lead to a thermal runaway. For very large values ofDathe chemical reaction takes place in a surface boundary layer. The evolution of the temperature in this layer is described by a universal problem. Its solution contains a mathematical singularity, which identifies the ignition event occuring at an ignition time much smaller than the conduction time through the solid. WhenDais large but of the same order that the square of the nondimensional activation energy, the ignition time becomes of the order of the conduction time. The structure of the ignition problem is identical to that in the previous case, but its solution depends on the body size and shape. In both cases an initial quasisteady stage is followed by a transition stage in which the nonsteady effects must be retained. Finally, forDaof order unity the chemical reaction extends over the whole solid, and the ignition process can be described in terms of a first inert heating stage and a second reacting stage in which the temperature evolves according a nonsteady Frank-Kamenetskii equation.

ISSN:0010-2202    

DOI:10.1080/00102209808952035

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

This paper deals with the generation of new peaks in the pressure spectrum in controlled combustors in experimental investigations of active control of thermoacoustic instabilities. Typically, the reported experiments have demonstrated that the dominant thermoacoustic instability can be suppressed, but secondary peaks at different frequencies which were not excited in the uncontrolled combustor appear. We develop a physically-based model of an actively controlled premixed laminar combustor which takes into account (a) laminar flame kinematics, (b) linear acoustic dynamics with coupling between the acoustic modes, and (c) actuation using side-mounted and end-mounted loudspeakers. Using this model, we analyze some of the experimental controllers proposed in the literature and explain the origin of secondary peaks observed in these studies. Secondary peaks are created while using these controllers due to resonant coupling between various mechanisms in the combustor that are distinct from those responsible for thermoacoustic resonance and mechanisms in the controller other than those that enable suppression of thermoacoustic instability. Other than at acoustic frequencies, premixed laminar combustors respond at lower frequencies due to flame dynamics, and at higher frequencies due to antiresonance. The experimental controllers are usually implemented using analog electronic circuitry whose components are designed so as to provide the functionalities of a phase-shifter, a filter, and an amplifier. Since analog filters tend to provide a phase compensation over a wide range of frequencies and not just at the isolated (unstable) frequency, they can initiate resonances by coupling with various mechanisms present in the combustor. Low frequency secondary peaks are typically due to coupling between the flame dynamics and the filter components in the controller, while high frequency peaks are due to either the interaction between various components of the active controller themselves, or the interaction between the controller components and antiresonance. Such phenomena clearly underscore the need for an active control design which accounts for the combustor dynamics over a range of frequencies with the goal of obtaining the desired performance over this entire range.

ISSN:0010-2202    

DOI:10.1080/00102209808952036

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

The effects of atmosphere composition on the rate of opposed-flow flame spread (Sf) over thin solid fuel beds at microgravity (μg) were measured and compared to earth gravity (lg) results and theoretical predictions. Two modifications to standard atmospheres were considered. First, the effects of sub-flammability-limit concentrations of a gaseous fuel (CO or CH4) were measured and compared to an existing theoretical model that was extended to ug conditions. The agreement between the model and experiment is reasonable considering the simplicity of the model. Notably, both model and experiment show that the effect of added gaseous fuel is greater at μg than lg. Secondly, the effect of diluent type onSf-was studied by comparing results using He, N2, Ar, CO2and SF6diluents. It was found that, in agreement with prior studies in N2diluent, for He, N2or Ar diluents,Sfwas larger at 1g than μg. In contrast, for CO2diluent,Sfwas slightly lower at 1g than at μg and for SF6diluent,Sfwas much lower at 1g than μg. Moreover, unlike He, N2and Ar, for CO2and especially SF6diluents the minimum O2concentration required to support flame spread was lower at μg than 1g. For SF6, the minimum O2concentration at μg was even lower than the upward (concurrent-flow) limit. This unusual behavior is proposed to be a result of reabsorption of radiation emitted from the gases, which is significant only for gases with small mean absorption lengths.

ISSN:0010-2202    

DOI:10.1080/00102209808952037

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

We consider planar, uniformly propagating combustion waves driven by the filtration of gas containing an oxidizer which reacts with the combustible porous medium through which it moves. We find that these waves are typically unstable with respect to hydrodynamic perturbations. For both forward (cofiow) and reverse (counterflow) filtration combustion (FC), in which the direction of gas flow is the same as or opposite to the direction of propagation of the combustion wave, respectively, the basic mechanism leading to instability is the reduction of the resistance to flow in the region of the combustion products, due to an increase of the porosity in that region. Another destabilizing effect in forward FC is the production of gaseous products in the reaction. In reverse FC this effect is stabilizing. We also describe an alternative mode of propagation, in the form of a finger propagating with constant velocity. The finger region occupied by the combustion products is separated from the unburned region by a front, in which chemical reactions and heat and mass transport occur. We show that the finger solution of the combustion problem can be characterized as a solution of a Saffman-Taylor (ST) problem, originally formulated to describe the displacement of one fluid by another having a smaller viscosity, in a porous medium or in a Hele-Shaw configuration. The ST problem is known to possess a family of finger solutions, with each member characterized by its own velocity and each occupying a different fraction of the porous channel through which it propagates. We propose a criterion to select the correct member of the family of solutions, based on consideration of the ST problem itself, rather than on modifications of the problem,e.g., by adding surface tension to the model and then taking the limit of vanishing surface tension.

ISSN:0010-2202    

DOI:10.1080/00102209808952038

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

An approach for modeling differential diffusion effects in turbulent nonreacting and reacting jets with stochastic mixing models is proposed. This approach has been applied to the modified Curl's mixing model and the Interaction-by-Exchange-with-the-Mean (IEM) mixing model used in the joint scalar Probability Density Function (PDF) approach. For comparison, a more elaborate mixing model based on the Linear-Eddy Model (LEM) has been incorporated into the PDF approach. Numerical results are assessed by comparisons with experimental data by Smithet al. (1995a, b) with a fuel mixture or 36%H2and 64%CO2and the data by Meieret al. (1996b) with a fuel mixture of 50%H2and 50%N2. The predicted and measured differential diffusion effects arc examined extensively with reference to computed laminar opposed flow flames. For nonreacting jets, the numerical results and data agree reasonably well. However, less agreement is found in the numerical results and the jet flame data obtained by Smithet al. (1995b). For the 50%N2+50%H2turbulent jet flames, the numerical results agree with the data better at locations far downstream than in the near field. The observed discrepancies may be due to the re-laminarization of the flow in the near field. Moreover, results obtained with the modified Curl's and the IEM model with differential diffusion effects are in good agreement with those obtained with the Linear-Eddy Model which is computationally expensive for high Reynolds number flows.

ISSN:0010-2202    

DOI:10.1080/00102209808952039

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

Staged combustion has been accepted as an effective way to reduce NOxemission. Experimental research is difficult because of the complexity of the chemical reactions related with nitrogen. Based on Miller and Bowman's overall elementary reaction model, NO formation and destruction under fuel staging is calculated in this paper. The accordance of the calculation results with experimental data show that the above model is good for predicting NOxformation and destruction. Sensitivity analysis shows that C, CH, CH2and HCCO play an important role in NO destruction and reducing under fuel staging, NO generated in the primary zone can be reduced effectively by staged combustion. The main factors that affect NO reducing are the air-fuel ratio in the primary combustion zone, the combustion temperature in the reburning zone and the mass factor of the reburning fuel in the overall fuel. The best air-fuel ratio in the primary combustion zone for NO reducing in the reburning zone is 1. The raising of the temperature in the reburning zone is good for the reducing of NO. The best mass fraction of the reburning fuel in the overall fuel for NO destruction is 30%.

ISSN:0010-2202    

DOI:10.1080/00102209808952040

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

Over a long period of time, in the research of catalysis burning mechanism in double-base propellant, it is well known that many kinds of metal compounds can increase the burning rate of propellant, but only lead compound can make burning rate-pressure curve show super-rate burning, plateau and mesa burning. It is also known that adding a little fine grain of copper compound and carbon black into double-base propellant with lead salt can play a role of assistant catalysis. That is to say, it can gain better catalysis efficiency than lead being used alone and increase burning rate further without destroying the plateau effect (Singh and Rao, 1988; Hewkinet al., 1971; Kubotaet al., 1971 and Cai, 1987).

ISSN:0010-2202    

DOI:10.1080/00102209808952041

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor