摘要:

The effect of lateral stream separation distance on the dynamics of flame propagation and thermal structure of a simplified spray flame has been studied. The flame was made up of 300µm average diameter hexane droplets injected through ten droplet streams in a plane, horizontally separated by a distance varying from 1 to 6 mm. Sequential photography was used to document the flame front motion and micro-thermocouples were used to perform measurements of gas-phase temperatures. The reactive flow was characterized by an inlet pre-ignition zone, followed by a bluish partially-premixed flame which acted as the ignition source of the fuel streams. Further downstream, a pattern of yellowish diffusion flames surrounding individual streams or groups of them was established, depending on the lateral separation of the streams. As the lateral spacing of the streams was increased, the vertical region swept by the flame front increased due to an augmentation in the flame propagation unsteadiness associated with larger variations in the local fuel-vapor concentration. Increased lateral spacing resulted in higher temperature fluctuations and lower average temperature gradients across the flame front.

ISSN:0010-2202    

DOI:10.1080/00102209008951637

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

An explicit expression for the parametric dependence of the rate of wind-aided flame spread across a thick horizontal fuel slab is obtained by approximate analysis. The slab, semi-infinite in length and depth, is taken to gasify by sublimation upon heating from ambient temperature to pyrolysis temperature. The evolved fuel vapor is burned in a vigorous gas-phase diffusion flame with oxygen from the laminar air stream. The hot combusion-product gases flow over the downwind portions of the surface of the slab, to preheat more of the slab to the pyrolysis temperature. The rate at which the pyrolysis-front position propagates downwind is identified with the rate of the flame spread; this rate is the key information sought from solution of this nonstandard Stefan-type problem. The two-dimensional, unsteady phenomenon admits a similarity solution, such that the solution is expressible in terms of (1) a Blasius-type independent variable (familiar from boundary-layer studies), and (2) the ratio of streamwise distance to time. For a flow with Prandtl and Lewis-Semenov numbers equal to unity, the parametric dependence of the spread rate is obtained by use of similarity; by adoption of the approximation that the modification of the thermal profile in the gas phase for the preheating zone, owing to heat transfer to the solid, is modest enough that the heat content (above ambient) of a column of gas is but little altered after the column traverses the pyrolysis-front position (which progresses downwind at a slower speed than the effective transport speed of the column); and by use of properties of the known steady-state solution (which holds for a fully involved slab, and which is taken to be established instantaneously at the leading edge of the slab). The rate of flame spread is found typically to vary linearly with the ratio of the characteristic conductance of the gas phase to that of the solid phase, but its dependence on the value of the ambient oxygen mass fraction is more intricate.

ISSN:0010-2202    

DOI:10.1080/00102209008951638

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

Nonlinear pressure oscillations in combustion chambers have been studied using an approximate analysis which treats the unsteady motions as a collection of coupled nonlinear oscillators, each corresponding to an acoustic mode of the chamber. Based on spatial and temporal averaging of the conservation equations, the formulation provides a convenient means for analyzing the nonlinear behavior of acoustic wave motions. The work addresses the linear and nonlinear characteristics of transverse acoustic waves in a cylindrical chamber, with attention focused on the conditions for the existence and stability of limit cycles. As a result of the special structure of the equations, both general and precise conclusions can be reached. Explicit results are given for the special cases of two and three standing modes. Spinning wave motions will be treated in a subsequent paper.

ISSN:0010-2202    

DOI:10.1080/00102209008951639

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

In this paper we propose modificalions to our original model of surface growth which reconciles the empirical result that surface growth is proportional to the soot surface area in "normal" (unseeded) flames but not in flames that have been seeded. The new model proposes: (i) The controlling experimenlally measured variable determining the rate of surface growth is the amount of soot generated in the inception stage and delivered to the growth stage, (ii) Growth stops at long times because of changes in the reactivity of the soot particles, not because of depletion of gas phase growth species. (iii) Acetylene and - to a lesser extent - diacetylene are the principal growth species in all premixed flames, although PAH may dominate during particle inception, and other species may dominate in diffusion flames, (iv) Surface growth occurs on reactive sites such as defects or edges which are lost by a temperature-dependent annealing process. The number of active sites is not directly affected by coagulation, surface growth, or gas phase chemistry. The initial surface area is actually a surrogate for the number of active sites.

ISSN:0010-2202    

DOI:10.1080/00102209008951640

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

Numerical simulation is used to study mixing of a passive scalar in a spatially-developing shear layer at high Reynolds number. The numerical method is based on the discretization of the vorticity and scalar gradients into finite-area elements and the transport of these elements along particle trajectories. Results show that mixing is governed by the entrainment of fluid from both streams into the large structures generated by the roil-up of the vorticity layer. Local value of scalar concentration oscillates, due to the passage of these structures, between values limited by the Peclet number. Instantaneous scalar profiles exhibit mixing asymmetry and the skewness of concentration fraction within the eddies in favor of the high-speed stream. Mixing statistics of a passive scalar agree well with the experimental measurements of Masutani and Bowman in a two-dimensional shear layer, and emphasize the effect of molecular diffusion on mixing. The rate of burning in a single step Arrhenius chemical reactions between the two streams increases due to mixing enhancement, overcoming the decrease due to the strain field generated by roll-up. Local product concentration is everywhere proportional to the vorticity, suggesting a new formula for turbulent combustion modeling.

ISSN:0010-2202    

DOI:10.1080/00102209008951641

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

A corona discharge with catalytical coatings on its plane electrodes has been studied as a technique for NOxreduction in gas mixtures. The underlying concept is that the combination of a gas discharge with calalytically active electrode coatings may allow the activation of the thermodynamically favourable NO-decay, e.g. into the 2NO → N2+ 02reaction channel. Widely differing materials such as metal oxide catalysts, conductive polymers, Si3N4-passivation and nitro enamon were tested as electrode coatings. With given corona polarity the different coatings gave widely differing DeNOx-efficiencies, the values of which are distributed over an interval of about two orders of magnitude. The largest energy yield this far obtained is 4mg of reduced NO per dissipated kilojoule of electrical energy. Further experiments will aim at improving this value by a factor of five at least, so that a typical coal power plant‚s flue gas could be cleaned of its NO from, for example, 1200 ppm to 200 ppm by employing about 5% of its electrical power.

ISSN:0010-2202    

DOI:10.1080/00102209008951642

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

A method of incinerating dichlorodifluoromethane (CFC-12) in the premised and diffusion flames has been developed. In the case of premixed CFC-12/methane/air flames, the burning velocity decreases with addition of CFC-12, but the amount of unreacted CFC-12 in Ihe burned gas is lower than the detection limit of the gas-chromatograph for molar ratios of CFC-12/methane smaller than around 0.2 when methane/air ratio is stoichiometric. The degree of destruction of CFC-12 decreases with increase of the CFC-12/methane ratio. No soot formation is observed in the range examined for the premixed flame experiments. On the other hand, in the case of CFC-12/methane diffusion flames in air, a large amount of soot formation is observed, although CFC-12 is also destructed almost completely in the range examined. These results indicate that the incineration melhod is useful and practical for the destruction of chloro-fluorocarbons.

ISSN:0010-2202    

DOI:10.1080/00102209008951643

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

An interpretation of the ratio of the rate of creation of products in strained and unstrained laminar flames is shown to provide a convenient estimate for laminar flame speed for use in combustion calculations in which the flame is treated as a discontinuity. The ratio in question is available for a variety of laminar flames as a result of previous analyses or can be computed for a particular chemical system.

ISSN:0010-2202    

DOI:10.1080/00102209008951644

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor