摘要:

This work investigated transient processes in gas-phase combustion over a porous sphere. According to a numerical model, the ignition criterion,dg/dt = 0, at the spherical surface is verified to be appropriate for the entire flow field that includes both the front and wake regions of the sphere. The flame developed stage is defined to be when the surface heat flux becomes invariant with time for all orientations. The effects of Damkohler and Lewis numbers on the ignition delay and the period of flame development are examined. For a small Lewis number or a large Damkohler number, ignition initiates near the forward stagnation point, and a stable enveloped flame is formed. Conversely, for a large Lewis number or a small Damkohler number, ignition takes place in the wake region and the developed flame is a wake flame.

ISSN:0010-2202    

DOI:10.1080/00102209408935343

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

Measurements with dilute gaseous mixtures of hydrocarbons and oxygen in a nitrogen carrier gas have shown that the workfunction of a Pt film in interaction with the gas phase changes abruptly and reversibly by approximately 0.7 eV at elevated temperature as the concentrations of the chemically active species are varied through their stoichiometric ratio. This effect provides a new mechanism for sensing the stoichiometric air-to-fuel ratio (A/F) in exhaust or flue gas applications. The phenomenon of the thermionic emission of alkali metal ions in which the rate of emission increases strongly with the workfunction of the emitting surface offers an electrical means for detecting the workfunction change as well as a simple structure for realizing a practical device.

ISSN:0010-2202    

DOI:10.1080/00102209408935344

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

In this paper, we present a study concerning the special role of the thermal conduction effect on the transition from self-explosion to detonation occurring in the ignition by a pocket of hot reactive gas. At the initial moment, a uniform hot reactive pocket is assumed to be surrounded by cold reactive mixtures. The detailed evolution process from the initial conditions is simulated numerically taking into account chemical reactions, molecular transport effects and compressible effect for one dimensional geometry. It is found that thermal conduction may play a key role in the transition process for the case of intermediate initial temperature of hot pockets. When the initial temperature is just above the thermal extinction limit, a premixed flame is ignited after a diffusive explosion; and when the initial temperature is very high, it is also a premixed flame that is ignited after a constant density explosion. In these two cases, the produced overpressure is much smaller than that produced by the corresponding Chapman-Jouguet detonation. While for a case of initial temperature between these two cases, the thermal conduction effect may establish a temperature gradient leading to the initiation of detonations and the production of a very large overpressure.

ISSN:0010-2202    

DOI:10.1080/00102209408935345

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

The combustion of electrostatic sprays of heptane in laminar counterflow diffusion flames was experimentally studied by measuring droplet size and velocity distributions, as well as the gas-phase temperature. A detailed examination of the evolution of droplet size distribution as droplets approach the flame shows that, if substantial evaporation occurs before droplets interact with the flame, an initially monodisperse size distribution becomes bimodal. A secondary sharp peak in the size histogram develops in correspondence of diameters about one order of magnitude smaller than the mean. No evaporation mechanism can account for the development of such bimodality, that can be explained only in terms of a disintegration of droplets into finer fragments of size much smaller than that of the parent. Other evidence in support of this interpretation is offered by the measurements of droplet size-velocity correlation and velocity component distributions, showing that, as a consequence of the ejection process, the droplets responsible for the secondary peak have velocities uncorrelated with the mean flow. The fission is induced by the electric charge. When a droplet evaporates, in fact, the electric charge density on the droplet surface increases while the droplet shrinks, until the so-called Rayleigh limit is reached at which point the repulsion of electric charges overcomes the surface tension cohesive force, ultimately leading to a disintegration into finer fragments. We here report on the first observation of such fissions in combustion environments.

ISSN:0010-2202    

DOI:10.1080/00102209408935346

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

A statistical approach is introduced for describing influences of turbulence on high-frequency combustion instability in liquid-propellent rocket motors. The Rayleigh criterion is generalized to account for turbulence-related spatial variations of the combustion response. Special attention is given to the distinguished limit in which the acoustic oscillations are rapid compared with the turbulent fluctuations, which in turn are rapid compared with the linear growth rate of the instability. It is shown that nonlinear acoustics of the instabilities need to be considered to describe the influences of turbulence. When this nonlinearity involves supercritical bifurcation, the turbulence lends to decrease the most probable intensity of the instability. When it involves subcritical bifurcation, the turbulence can produce a bimodal probability density function for the intensity of the instability, with appreciable probabilities of high-amplitude and low-amplitude acoustic oscillations but small probabilities of oscillations of intermediate amplitudes. These phenomena can have a bearing on erratic pressure-amplitude bursts sometimes observed in liquid-propellant engines.

ISSN:0010-2202    

DOI:10.1080/00102209408935347

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

The ignition and combustion processes of coal-water slurry (CWS) in a prechamber type diesel engine were investigated in this work. Specific engine and fuel parameters studied included: (I) needle lift pressure; (ii) inlet air temperature; (iii) fuel injection timing; and (iv) percent coal loading in the slurry fuel. Analysis of the experimental data utilized a numerical simulation of the combustion process using submodels for the key physical and chemical processes. Energy release schedules were determined using a one-zone thermodynamic analysis with measured cylinder pressure as a function of time. Successful operation of the engine using the coal slurry required modifications to the engine and support systems. These modifications are briefly discussed in this paper. The major conclusions of the work include: (I) higher needle lift pressures led to shorter ignition delay times for the CWS fuel; (ii) increasing the inlet air temperature shortened ignition delay times and improved burning rates of the slurry; and (iii) dilution of the slurry with water can significantly affect the combustion processes of the fuel.

ISSN:0010-2202    

DOI:10.1080/00102209408935348

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

Luminosity of pool fires from methanol/additive mixtures is investigated theoretically and experimentally. Phase diagrams of solutions are used to explain the transient behavior of flame luminosity of binary fuel mixtures. A model, based on solution theory, is developed to quantify the flame luminosity of methanol/additive binary fuel mixtures from the luminosities of the individual components and an interaction coefficient. The theoretical results agree well with the experimental data. The luminosity of methanol additive mixtures is found to be a function of the mixture's carbon/hydrogen (C/H) ratio. Flame luminosity increases with increasing mixture C/H ratio to a maximum (occurring around C/H = 0.35), and then decreases slightly beyond that point. Additives that form positive azeotropes with methanol and that have high C/H ratio are found to be the best candidates for enhancing the luminosity of methanol flames.

ISSN:0010-2202    

DOI:10.1080/00102209408935349

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

The unbumed gas region of wrinkled laminar flame stabilized on a Bunsen burner was visualized by a laser tomography and the fractal analysis was applied to the boundaries of unbumed gas to characterize the geometry of wrinkled laminar flame. The fractal dimension of the wrinkled laminar flame front is independent of any scale of the unbumed mixture turbulence. It increases with the turbulence intensity from 2.20 to 2.32 and approaches at high intensity turbulence to 2.36 which is the fractal dimension of the non-reactive turbulent flow. The existence of inner and outer cut-offs is clearly shown. Both cut-ofls are also independent of any scale of unbumed mixture turbulence. The inner cut-off coincides with the scale of wrinkles caused by the laminar flame instability and the outer cut-off is determined by the flame base diameter.

ISSN:0010-2202    

DOI:10.1080/00102209408935350

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

Direct numerical simulations for turbulent diffusion flames were conducted to clarify the relationship between the flow parameters and the fractal parameters such as fractal dimension and inner cutoff of the flame front. The fractal dimension increases with Reynolds number Re, and reaches the value of 1.44 at Rex = 252. The inner cutoff, calculated by the box counting method, is two to three times that of the enstrophy based microscale and is very close to the scale of maximum enstrophy dissipation. To construct the fractal dynamic subgrid-scale model the probability density function of the scalar gradient on the flame front was calculated. The behavior of the filtered PDF was also investigated. Filtered mean scalar gradient on the flame front shows a cutoff, which is about two times that of the inner cutoff of the flame front. The results we have obtained show that large eddy simulations for turbulent diffusion flames by using the fractal dynamic subgrid-scale model may be possible.

ISSN:0010-2202    

DOI:10.1080/00102209408935351

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

The detailed three-dimensional structure of the flame front disturbance induced by a weak pressure wave has been studied experimentally using a small combustion chamber. The flame front disturbance was recorded by a newly developed high-speed Schiieren system, which gives a view from the direction normal to the flame front. The results were compared with those induced by preferential diffusion mechanisms. The image of a flame front disturbance induced by acceleration caused by weak pressure waves was found to be circular when it was observed from the direction normal to the flame front. This image was much different from that induced by the preferential diffusion mechanisms, which was observed as a net composed of thick lines. These images of the flame front disturbances were compared with those observed in our previous studies, in which the disturbances were observed from the direction normal to the flame propagating direction. Consequently, the flame front disturbances induced by acceleration were inferred to be horn-shaped, while those induced by the preferential diffusion mechanisms were inferred to be ridge-shaped. It is postulated that this difference of the flame front shapes should be attributed to the difference of the mechanisms which, sustain or enhance the disturbances.

ISSN:0010-2202    

DOI:10.1080/00102209408935352

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor