摘要:

The transient internal processes of heat conduction and liquid surface regression are investigated theoretically. The volume fraction of solid particles is considered large enough so that the overall droplet diameter is constant. Vaporization occurs at the regressing surface of an inner sphere of solid particles and liquid, which is surrounded by an outer porous shell of solid particles that grows in thickness because of the surface regression. Singular perturbation expansions are used to obtain approximate solutions for the temperature profiles of the inner sphere and the regression velocity of its surface. The expansion parameter is the ratio of energy required to raise the inner sphere to the liquid vaporization temperature, to that required for liquid vaporization. For small values of the parameter: (i) The inner sphere is heated by the inward diffusion of energy from the regressing surface, (ii) the decrease in the cube of the diameter of the inner sphere is approximately linear with time, (iii) an approximate expression for the vaporization time is linearly proportional to both the liquid volume fraction and the square of the overall droplet diameter. For large values of the parameter: (i) The inner sphere is heated by the inward propagation of a thermal wave front located at the regressing surface, (ii) an approximate expression for the vaporization time is proportional to the liquid volume fraction raised to an exponent less than unity and the square of the overall droplet diameter

ISSN:0010-2202    

DOI:10.1080/00102208608959796

出版商:Taylor & Francis Group    

年代:1986

数据来源: Taylor

摘要:

The puff-jet, a mechanically assisted ignition system, produces a turbulent ignition kernel that enhances the combustion of methane, a slow burning fuel. Detailed mapping of the puff-jet ignition system in a motored internal combustion engine has confirmed preliminary results. Compared with a spark gap ignition system, the puff-jet decreased both the ignition delay and the burn duration of methane charges in a motored single cylinder internal combustion engine and extended the lean misfire limit. Output power from the same engine run continuously on lean air-methane mixtures was increased using the puff-jet system because of the ameliorated combustion characteristics using that ignition system rather than a conventional igniter. Further, the output power from the engine running continuously with the puff-jet was almost the same as that achieved with a plasma jet igniter, without the concomitant high energy demand or electrode wear

ISSN:0010-2202    

DOI:10.1080/00102208608959797

出版商:Taylor & Francis Group    

年代:1986

数据来源: Taylor

摘要:

We examine the stationary spherical diffusion flame attached to a fuel drop, and consider the consequences of an instantaneous reduction of the Damkohler number below the extinction threshold, so that steady vigorous burning is no longer possible. Extinction occurs in two stages that are described by a combination of analysis and numerical calculation, valid in the limit of infinite activation energy. The first stage is characterized by a small but very rapid drop in the flame temperature, and a corresponding small but rapid density increase within the reaction zone or flame sheet. The associated increase in mass within the sheet is fed by an unsteady 0(1) velocity field, induced beyond the sheet, and superimposed on the initial 0(1) velocity defined by the steady state. This induced velocity is responsible, in due course, for the drawing in of fluid from infinity towards the collapsing flame. The second stage takes over once reaction becomes negligible, and is characterized by the dispersion of the temperature field through diffusion and advection. This is a rather slow process

ISSN:0010-2202    

DOI:10.1080/00102208608959798

出版商:Taylor & Francis Group    

年代:1986

数据来源: Taylor

摘要:

The dynamic behaviour of an intrinsically stable premixed flame front, propagating downwards in a weakly turbulent incoming flow is investigated for an arbitrary gas expansion ratio. The complete hydrodynamic problem, in which the flame appears as a free boundary, is solved analytically in the linear approximation. In this case the mutual interaction between combustion and turbulence is fully described theoretically. The response of the flame is found as a function of both the frequency and wave number content of the incoming turbulence. In addition to the gas expansion ratio, only two other parameters appear in the theory, the Markstein number (characterizing the curved flame structure) and the Froude number (based on the flame speed and the flame thickness) Sensitivity of the turbulent flame speed to the characteristics of the incoming flow as well as to these numbers is considered. The theoretical results are illustrated by comparison with some recent laboratory experiments

ISSN:0010-2202    

DOI:10.1080/00102208608959799

出版商:Taylor & Francis Group    

年代:1986

数据来源: Taylor

摘要:

Unsteady motions in combustion chambers have previously been treated with an approximate analysis in which an acoustic field is represented as a collection of coupled nonlinear oscillators constructed in one-to-one correspondence to the acoustic modes. Two parameters characterize the linear behavior of each oscillator; a single parameter arises from the nonlinear acoustics carried out to second order in small fluctuations. The formal results are used here as the basis for studying the existence and stability of limit cycles for longitudinal modes. Owing to the special structure of the equations, explicit and precise conclusions can be reached. Existence and stability depend only on the parameters defining the linear motions. The nonlinear gas-dynamics influence the amplitudes of motion in the limit cycle. At least one of the acoustic modes must be linearly unstable to produce a nontrivial limit cycle. Generally, energy flows both up and down among the modes, but there are exceptional cases when limit cycles exist only if the fundamental mode is unstable. Explicit results are given for the special cases of two and three modes; the analysis is extendible to any number of modes

ISSN:0010-2202    

DOI:10.1080/00102208608959800

出版商:Taylor & Francis Group    

年代:1986

数据来源: Taylor

摘要:

This paper is concerned the calculation of flame structure of one-dimensional laminar premixed flames using the technique of operator-splitting. The technique utilizes an explicit method of solution with one step Euler for chemistry and a novel probabilistic scheme for diffusion. The relationship between diffusion phenomenon and Gauss-Markoff process is exploited to obtain an unconditionally stable explicit difference scheme for diffusion. The method has been applied to (a) a model problem, (b) hydrazine decomposition, (c) a hydrogen-oxygen system with 28 reactions with constant Dρ2approximation, and (d) a hydrogen-oxygen system (28 reactions) with trace diffusion approximation. Certain interesting aspects of behaviour of the solution with non-unity Lewis number are brought out in the case of hydrazine flame. The results of computation in the most complex case are shown to compare very favourably with those of Warnatz, both in terms of accuracy of results as well as computational time, thus showing that explicit methods can be effective in flame computations. Also computations using the Gear-Hindmarsh for chemistry and the present approach for diffusion have been carried out and comparison of the two methods is presented

ISSN:0010-2202    

DOI:10.1080/00102208608959801

出版商:Taylor & Francis Group    

年代:1986

数据来源: Taylor

摘要:

A model is developed for the pyrolysis and combustion of wood, explicitly including the fibrous heterogeneity of the substance. The heterogeneity enters the equations through the fiber diameter and the interfiber material treated as undergoing charring. The emphasis is on the condensed phase. The vapor phase details are not included in this paper. The energy flux needed to drive the pyrolysis is externally prescribed; an externally heated slab or a slab burning under its own flame resulting from the products of pyrolysis mixing with an oxidizer (air) provide two examples. The method of matched asymptotic expansions is used to analytically derive a closed form expression for the time independent rate of regression of the charring plane, including the nonlinear Arrhenius degradation term for the pyrolysis. The differences between predictions from previous homogeneous models and this heterogeneous model are discussed. Some remarks concerning the apparent negative temperature rate region indicated by many thermogravimetric analyses and the fibrous nature of wood are presented

ISSN:0010-2202    

DOI:10.1080/00102208608959802

出版商:Taylor & Francis Group    

年代:1986

数据来源: Taylor

摘要:

A model equation is constructed to describe the evolution of concentration statistics downstream of a uniform grid at which two reacting species are introduced, one on each half of the grid. The two species undergo an irreversible reaction to produce a product. It is assumed that the reaction does not alter the density field of the carrying fluid sufficiently to induce buoyancy modification of the turbulence. The model equation is solved numerically for specific initial conditions and reaction kinetics, including the zero-reaction rate case. The latter problem reduces to an analogy with the half-heated grid flow for which there is adequate experimental data. Model predictions are generated for reactive flow cases for which no experimental data exist

ISSN:0010-2202    

DOI:10.1080/00102208608959803

出版商:Taylor & Francis Group    

年代:1986

数据来源: Taylor

摘要:

Ignition of a polydisperse single-component fuel spray is studied. The physical model consists of a mist of air and fuel droplets contained in a tube. The left end of the tube acts as the ignition source. The polydisperse character of the spray is represented by discrete and finite size distribution. Other notable features of the two-phase ignition model are that the temperature in the droplet interior is resolved spatially as well as temporally, that the inter-phase slip is considered, that a one-step reaction scheme with non-unity exponents of fuel and oxygen concentrations is employed, and that the interesting physical phenomenon is resolved on the scale of the spacing between droplets. An Eulerian-Lagrangian hybrid scheme is used to solve the two-phase equations. The one-dimensional unsteady equations are integrated to obtain the ignition time delays and ignition energies. Influence of initial droplet size, size distribution, overall equivalence ratio, fuel type, and the locations of the nearest droplets to the ignition source is examined. The results clearly demonstrate that the Sauter mean diameter is not capable of representing the ignition characteristics of a polydisperse spray. The polydisperse spray ignition can, however, be well correlated to an equivalent monodisperse spray by using a mean diameter based on the total surface area of the spray. This observation is confirmed over a range of fuel volatility, initial droplet sizes, size distribution, and overall equivalence ratios. For a given size distribution, fuel volatility and overall equivalence ratios, the values of initial droplet sizes for the minimum ignition delays are also obtained. The results also indicate a strong dependence of ignition delays upon the location of the nearest droplets to the ignition source, which underscores the statistical character of the spray ignition process

ISSN:0010-2202    

DOI:10.1080/00102208608959804

出版商:Taylor & Francis Group    

年代:1986

数据来源: Taylor

摘要:

Temperature distributions have been measured in axisymmetric ethylene-air diffusion flames using high spatial resolution coherent anti-Stokes Raman spectroscopy. As ethylene flow increased and the flame approached a smoke-point condition, the temperatures attained in the upper part of the flame were reduced by about 300 K below maximum radial temperatures low in the flame. Addition of diluent Na to ethylene caused a reduction in temperature low in the flame but increased temperature higher in the flame. Maximum temperatures attained in all ethylene flames were between 0.84 and 0.89 of respective adiabatic flame temperature (AFT) The upper temperature of the near-smoke-point flame was only 0.76 of AFT. Results are compared with the generalized flame front model of Mitchell. Mie scattering measurements are also discussed. Brief studies with propane and a nonsooting CO flame are reported; maximum axial and radial temperatures were between 0.84 and 0.87 of AFT. Results indicate the importance of thermal loss from soot radiation, radial transport processes and fuel pyrolysis. Nonluminous radiation and finite reaction rates are other possible factors. Overall results highlight how the upper luminous part of the sooting ethylene flames is likely above the primary flame front and is a soot burnout zone. For such cases peak axial temperature tended to occur below the luminous tips, an indication also of the location of the true end of the flame. A broad axial and radial zone near peak axial temperature was established in the upper part of all flames

ISSN:0010-2202    

DOI:10.1080/00102208608959805

出版商:Taylor & Francis Group    

年代:1986

数据来源: Taylor