摘要:

Numerical simulations of transient sprays were made based on an Eulerian gas and a Lagrangian drop formulation coupled with chemical reactions for the gas and for soot.Computed profiles of axial and radial velocity,species and soot concentration,gas temperature,local or total heat release rate are presented to understand the overall and the internal structure of the combusting spray.Some of the computed results are compared with experiments taken in a rapid compression machine.The results indicate the following. (1) The computations predict the ignition delay,the transient configuration of the spray flame and drop penetration distance reasonably well.(2) The drops evaporate in the early stage of the spray combustion and the drop penetration distance is predicted well by taking account of the secondary breakup of the drops in the evaporating sprays. The secondary breakup is liable to occur at high pressure and high temperature atmosphere.(3) Ignition occurs at a hot spot in the off-axis region and is followed by the rapid spread of the combustion and the fast release of heat. (4) A flame front is formed downstream from the nozzle exit. Near the flame front, the local heat release rate is very intense where premixed combustion occurs. Gaseous turbulent diffusion dominates combustion of the fuel in the core of spray.

ISSN:0010-2202    

DOI:10.1080/00102209108924075

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

A model of a laminar Burke-Schumann type of diffusion flame is developed in which the fuel is supplied in the form of droplets having an arbitrary initial velocity distribution. The spray of droplets is described using a sectional approach. The governing equations are normalized and various nondimensional parameters associated with the spray and the motion of the droplets therein are identified. Analytic solutions are presented for different assumed droplet velocity distributions. Effects of droplet velocity, dimensionless drag and vaporization coefficients and spray angle parameter on flame height, width and type are illustrated in a discussion of a detailed parametric analysis based on numerous calculations.

ISSN:0010-2202    

DOI:10.1080/00102209108924076

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

The propagation for spread rate lrofwind-aided firespreadacross an array of very-small-diarne-ter, discrete fuel elements is sought, as a step toward the objective of predicting the advance in time of a firefront through either strewn debris in a heavily-blast-damaged scenario or through brush-and-grass-type wildlands. Here, the quasisteady rate (if one exists) is sought for conditions under which there is both (1) a wind whose mean speed U is constant and whose direction is constant, and (2) a horizontal bed with a macroscopically uniform fuel distribution m. More precisely, m is the mass of fuel (per unit planform area of the bed) consumed with firefront passage; here the fuel elements are thin. so that, for conditions under which fire propagates at all, the fuel consumed is identical with the fuel loading initially present. In accord with laboratory-scale experiments in a specially dedicated firetunnel, to be reported separately, analysis suggests thai I'r varies with (U/m)1/2 over a wide (but obviously finite) range of the parameters V and m. Also in accord with observations, the model adopted postulates a steadily translating firefront of finite streamwise expanse sandwiched between a thoroughly burned-out upwind expanse of the bed and a downwind expanse which is undergoing preheating from the ambient temperature to the onset-of-pyrolysis temperature. The analytic modeling further suggests the inference that fresh fuel is preheated to sustain the firespread by convection/diffusion. Only if the thin-fuel loading m approaches the rather large value of I'ery roughly 2 g/cm2 is radiative preheating likely to playa role comparable to that of convective/diffusive preheating.

ISSN:0010-2202    

DOI:10.1080/00102209108924077

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

The large-eddy simulation (LES) model previously developed for cold flow in an axisymmetric ramjet combustor is extended to include a model for premixed fuel combustion. The combustion model explicitly uses a subgrid-scale local turbulent flame speed in the governing equation. This model not only reduces the computational effort, when compared to a model with detailed finite-rate chemical kinetics, but also avoids the potential error in the amount of heat release causd by numerical diffusion. Both stable and unstable combustion are simulated with evidence of decaying and growing pressure oscillations, respectively. The observed flow features on unstable combustion. such as a propagating hooked flame and its phase relation with the pressure oscillation, agree with experimental observations. These simulations clearly establish that the LES model contains the essential physics of combustion instability in a ramjet. Although the simulations show general qualitative features of combustion instability, further research is required to take into consideration the important effects of subgrid-scale turbulence on combustion instability. Extensive simulations using the resulting LES model may then lead to a better understanding of the physical processes involved in combustion instability.

ISSN:0010-2202    

DOI:10.1080/00102209108924078

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

Tien's model for unsteady gas phase analysis has been adopted for the oscillatory burning of homogeneous solid propellants. The gaseous flame is assumed to be one dimensional, premixed and laminar. Three overall chemical processes simulating the fizz zone and the final flame zone (including dark and luminous zones) are considered to take place in the gas phase. The propellant is assume to decompose according to the Arrenhius law with no condensed phase reaction. The strong gas resonance obtained by T'ien was recovered. The three chemical reaction analysis resulted in an extended flame zone and produced an acoustic admittance which differed from that obtained from Tien's one-step chemical reaction at both low and high frequencies. There appear two peaks: one. at a lower frequency, corresponds to a resonance occurring in the condensed phase to amplify the pressure oscillation; the other, near the gas-phase characteristic frequency, is in the direction to damp the pressure oscillation.

ISSN:0010-2202    

DOI:10.1080/00102209108924079

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

In this study the gas phase ignition model of a pyrolyzing combustible solid has been proposed in a one-dimensional slab geometry when it is initially exposed to a radiatively active high-temperature source. Only the pyrolysis product was assumed to abosrb and emit the radiation in the model. By applying it to the combustible polymethylmethacrylate (PMMA), the effect of radiation on the thermal history and ensuing ignition delay has been determined. The thermally radiative heating was found to be more influential than the conductive heating for activating PMMA ignition. An experimental observation of the temperature decrease (as being away from the fuel surface) in the radiant ignition of PMMA was also confirmed in this theoretical study. Whereas the emissivity of the hot source was observed to playa significant role in the ignition process, a change in distance between the fuel surface and the hot source made no appreciable change in the resulting ignition delay, because the radiation is predominant over conduction.

ISSN:0010-2202    

DOI:10.1080/00102209108924080

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

Extinction of the counterflow diffusion flame in high pressures has been studied experimentally up to the ambient pressure of 10 MPa. Oxidizer was air and fuels were methane, ethane. carbon monoxide, carbon monoxide/hydrogen mixtures. Experimental results have shown that the critical stagnation velocity gradients for the methane and ethane are about constant for pressures up to about 2 MPa and 1 MPa,respectively, beyond which they decrease with the increase of pressure. For the carbon monoxide, the critical stagnation velocity gradient increases with pressure, reaches a maximum at about 1 MPa, and decreases. The increase of the hydrogen content in the carbon monoxide/hydrogen mixture results in increase of the critical stagnation velocity gradient for a certain value of pressure below 1 MPa, beyond which it does not affect when the hydrogen is below about 0.7%. The apparent pressure exponent of the overall reaction in the flame were obtained from the flame extinction data. For the methane and ethane,the apparent pressure exponents are one at pressures below about 2 MPa for methane and 1 MPa for ethane, beyond which they decrease. For the carbon monoxide, the apparent pressure exponent is two at pressure below about 1 MPa, beyond which it decreases. When the hydrogen content is less than about 0.7%, the increase of the hydrogen content results in decrease of the apparent pressure exponent under the pressure below about 1 MPa. When the pressure is larger than 1 MPa, the hydrogen conlent below 0.7% dose not affect the overall chemical reaction rate of carbon monoxide and air.

ISSN:0010-2202    

DOI:10.1080/00102209108924081

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

Flamelet models for turbulent combustion provide an approach (via the laminar flame) to include detailed flame chemistry into fluid dynamic simulations of IC engines. The flamelet model postulates that a turbulent flame is a statistical distribution of premixed, laminar flames. However. turbulence affects the laminar flames through strain (a) and curvature. The effect of positive strains (outflow condition) on counter-flow flames is evaluated by integrating the species destruction (or formation) through the flame.This integral is the net reaction per area of the flame and is compared to the integral for the unstrained flame. Strain reduces the net reaction rate per unit area. Flame stretch is a measure of the reduction in fuel consumption due to turbulent strain. Flame stretch is a property tending to slow turbulent flame speeds by the reduction in fuel consumption.

ISSN:0010-2202    

DOI:10.1080/00102209108924082

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

Two distinct heavy fuel oil (HFO) sprays were generated by a twin-fluid atomiser, for differing atomising air to fuel ratios and investigated under non-reacting conditions in a spray test rig in which their radial mass distribution and mean droplet size and distribution were determined. Both sprays exhibit solid cone type distributions in their atomisation pattern—with that of the lower atomising air to fuel ratio having a more sharply defined central peak. Increasing the atomising air to fuel ratio reduces the droplet size distribution, producing a spray with a greater uniformity in its radial variation. The sprays, however,subtend approximately the same angle. The atornisation tests were followed by combustion measurements in a large-scale laboratory cylindrical furnace for both sprays. Measurements of mean gas temperature, species concentrations (02, CO2, CO and unburnt hydrocarbons) and incident wall radiation nux are reported. The two flames exhibit distinct vapourisation rate histories which may be attributed to differences in droplet size and spray momentum. The data presented are intended to provide a better understanding of the relationship between spray quality and combustion performance and to assist the validation of mathematical models for prediction of HFO combustion.

ISSN:0010-2202    

DOI:10.1080/00102209108924083

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

The ignition and behavior of candle flames was observed photographically in free-fall (drop tower) tests under 19%-25% 02concentrations in a nitrogen-diluted, I atm environment. A few tests were also conducted with helium dilution and with ignition in normal gravity. Compared to normal gravity. the microgravity flames were of lower power and greater sphericity. When a candle burning in normal gravity was released into microgravity, the flame shape changed immediately from the classical tear-drop to a sphere; the visible soot diminished throughout the test time. Tests with a longer duration of microgravity are being pursued in order to study extinction behavior, flame interactions, and burning rates.

ISSN:0010-2202    

DOI:10.1080/00102209108924084

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor