摘要:

The influence of the Lewis number on turbulent flame front geometry is investigated in a premixed turbulent stagnation point flame. A laser tomography technique is used to obtain the flame shape, a fractal analysis of the multiscale flame edges is performed and the distribution of local flame front curvature is determined. Lean H2/Air and C3H8/Air mixtures with similar laminar burning rates were investigated with Lewis numbers of 0·33 and 1·85 respectively. At the conditions studied the laminar H2/Air mixture is unstable and a cellular structure is observed. Turbulence in the reactant stream is generated by a perforated plate and the turbulent length scale (3 mm) and intensity (7%) at the nozzle exit are fixed. The equivalence ratio is set so that the laminar burning velocity is the same for all the cases. The results show clearly that the turbulent flame surface area is dependent on the Lewis number. For a Lewis number less than unity surface area production is observed. The shape of the flame front curvature distribution is not found to be very sensitive to the Lewis number. For the H2/Air mixture the distribution is skewed toward the positive values indicating the presence of cusps while for the C3H8/Air mixture the distribution is more symmetrical. In both cases the average curvature is found to be zero, and if the local burning speed varies linearly with curvature, the local positive and negative burning velocity variations due to curvature will balance.

ISSN:0010-2202    

DOI:10.1080/00102209308907644

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

The authors have developed the fundamentals of qualitative combustion theory involving multi-component gasless compositions on the basis of interaction effects in a multi-phase region. Using a three-component system as a model example it has been shown that the multi-phase behavior leads to a considerable expansion of combustion diversity in one and the same system due to exothermic and endothermic stages of matter redistribution between the compounds being produced, and also between one-phase and two-phase layers of a diffusive zone. In such a system one can, by varying the composition of the starting mixture, obtain various interaction regimes corresponding to (a) high or low rates of heat release, (b) monotonic or nonmonotonic temperature profiles due to matter transformation, and (c) the existence of several independent waves. For example, the second wave may be separated from the first by an endothermic stage whose zone consists of several thousand warmed-up layers.

ISSN:0010-2202    

DOI:10.1080/00102209308907645

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

Results and analysis of the data acquired in an experiment on laminar flame spreading at microgravity conducted on a recent Space Shuttle mission are presented. In the experiment, a thin fuel (ashless filter paper) was burnt in a quiescent environment in a 50% O2/50% N2mixture at a total pressure’ of 152 kPa (1·5 atm). The entire experiment from ignition to extinction was recorded using two 16 mm cine-cameras, and the temperature of the fuel surface and at two locations in the gas phase were measured using Pt/Pt-Rh thermocouples.

ISSN:0010-2202    

DOI:10.1080/00102209308907646

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

Recent work with Lorentz-induced mixing of planar diffusion flames has required an understanding of their behavior when subjected to electric fields. Extensive work with electric currents in premixed flames has been reported, but the current-carrying properties of planar diffusion flames have not previously been characterized. In this work, a cesium-seeded methane/oxygen diffusion flame is investigated under various conditions. A d.c. electric field is applied transverse to the flow direction but parallel to the flame sheet. Two distinct modes of discharge are observed, one sustaining currents an order of magnitude larger than the other. The transition between the modes is described, and the conditions supporting each type of discharge are reported. Comparisons are made with similar observations reported in the literature for premixed flames, and differences are contrasted in light of fundamental differences in the characters of the flames. The effects of electrode gap and location downstream of the burner exit plane are examined. Predictions of the expected temperature rise during each arc mode are made based on theoretical considerations. Optically measured temperatures in the discharge region are reported and compared with predicted values.

ISSN:0010-2202    

DOI:10.1080/00102209308907647

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

The value of knowing residence time distributions (RTD's) in mass burn incinerators and the serious limitation imposed by the use of gas volume-flow rate based average residence lime with regard to these incinerators are recognized. The utility for understanding macromixing of having detailed RTD-information about the incinerator overfire region is demonstrated by estimating RTD's using transient numerical simulation of tracer injection into the 2-D, isothermal flow field of a typical mass burn incinerator configuration. Several numerical experiments are conducted to determine the effect of overfire jets on the RTD's of tracer flows introduced into different grate streams. The results indicate that valuable diagnostic information on the state of mixedness in the overfire region can be extracted from these RTD's by mapping them onto a macromixing scale whose extremes are the RTD of a plug flow and that of a stirred tank. Further, they not only underscore the critical role played by jets in achieving desirable stales of macromixing in the overfire region but also point to the significance of the geometry effects. Lastly, the opportunity created by the RTD studies for experimental validation of incinerator overfire mixing simulations is identified.

ISSN:0010-2202    

DOI:10.1080/00102209308907648

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

We have performed both laser-saturated fluorescence (LSF) and linear laser-induced fluorescence (LIF) measurements of NO in lean and rich atmospheric-pressure C2H6/O2/N2flames. Unlike previous LSF measurements of OH, NH, and CH, the LSF measurements of NO require a broadband detection scheme, and thus we include a comprehensive theory for broadband LSF. Saturation of NO is found to be easily attainable at atmospheric pressure. When high laser energies are used to insure saturation of NO, background fluorescence often occurs from additional flame species; hence, a subtraction technique is introduced to eliminate this fluorescence from the NO signal. Calibration of both the LSF and LIF techniques was accomplished by doping lean flames with known quantities of NO. A comparison of the LSF and LIF signals from postflame gases at ~ 1700 K as a function of equivalence ratio suggests that the influence of stoichiometry on fluorescence quenching is nearly negligible. Finally, we discuss the relative merits of the LSF and LIF methods, which both give a detection limit of ~1 ppm in atmospheric-pressure flames.

ISSN:0010-2202    

DOI:10.1080/00102209308907649

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

A new method has been developed for enhancing bulk mixing in a planar diffusion flame. The fuel stream is seeded with cesium to raise the electrical conductivity at the reactant interface. A current is passed through the flame, and by applying an orthogonal magnetic field a Lorentz body force is induced perpendicular to the flame sheet. Sinusoidal oscillation of the current is used to wiggle the flame sheet at the imposed frequency and promote bulk mixing of the reactants through a disruption of the laminar behavior of the flow. High-speed video images have been analyzed to estimate induced lateral flame sheet velocities. Experimental results compare well with velocities predicted from a simple theoretical analysis.

ISSN:0010-2202    

DOI:10.1080/00102209308907650

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

The extinction of a premixed flame propagating in a stagnation-point flow under the influence of an inert spray is studied using activation energy asymptotics. A completely prevaporized and partially prevaporized spray are identified on the basis of a critical value of the initial droplet size for completing the vaporization process at the flame. Parameters for flame extinction consist of the liquid loading and the droplet size of the spray indicating the internal heat loss associated with liquid vaporization, and the flow stretch which strengthens and weakens the burning strength of the Le < 1 and Le > 1 flame, respectively. The Le < 1 flame can be extinguished when it endures a small flow stretch and experiences a partially prevaporized inert spray with the liquid loading and the droplet size large enough. Furthermore, flame extinction of the Le < 1 flame by the inert spray is characterized by a S-shaped curve. Conversely, results for the Le > 1 flame show that flame extinction is possible for the flame with and without the inert spray; and that there is an abrupt change on extinction characteristics in response to varying the droplet size. Finally, the flow stretch and the inert spray are responsible for extinction of the Le < 1 flame identified by a C-shaped curve and a W-shaped curve, respectively.

ISSN:0010-2202    

DOI:10.1080/00102209308907651

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

An investigation is made into the response of the mass burning rate of a premixed flame to small but sharp changes in pressure. In this paper a fast time scale is considered such that

ISSN:0010-2202    

DOI:10.1080/00102209308907652

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

A zone model, which incorporates a recently developed horizontal ceiling/floor vent flow algorithm, was written to simulate a compartment fire ventilated only through an opening in the ceiling. Development of this algorithm makes it possible to characterize vertical flow through a horizontal vent in those scenarios where the standard vent flow model is invalid, i.e. where the difference in pressure across the horizontal vent is at or near zero and the density of the gas above the vent is greater than the gas below the vent. Model predictions are shown to be comparable to reduced scale experimental results.

ISSN:0010-2202    

DOI:10.1080/00102209308907653

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor