摘要:

High speed visual observations of free turbulent jet diffusion flames are presented. Computer graphic volume rendering, whereby many frames from a movie sequence are displayed simultaneously, is used to provide a unique view of the flow evolution. Two fuels were used, acetylene and ethylene, to span Ihe range from the momentum-driven to the buoyancy-driven regime. The data show that, for all flow regimes, the flames exhibit a large-scale organized motion in the far-field of the jet as manifested by a progression of organized structures up through the length of the visible flame. Occasional pairings of structures are also observed. These structures eventually become the flame tip, which burns out in a quasi-periodic manner. Under no conditions was a random, small-scale breakup and disappearance of the flame tip ever observed. In all cases the speed of the organized structure is seen to be constant and equal to 12 ± 2° of the jet exit velocity. The constancy of speed occurs in spite of the axial decay of the centerline velocity and is believed to result, in part, from the requirement of the flame to burn from the outer edges of the jet towards the jet centerline with increasing downstream distance, thus compensating for axial velocity decay. The characteristic time of the flame tip burnout event, for all flow regimes, is found to be in good agreement with the local large-scale time estimated from the visible jet width and the centerline velocity. We conclude that the organized component of the jet motion exists for most jet flames reported in the literature, and is responsible for flame tip oscillation, but that its role has been overlooked.

ISSN:0010-2202    

DOI:10.1080/00102209108951708

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

A detailed experimental study of the synthesis of transition metal nitrides via combustion reactions between gaseous nitrogen and the respective metal powders at elevated pressures is presented. Two representative combustion reactions were studied: between tantalum and nitrogen, which proceeds without the formation of any liquid phase, and between titanium and nitrogen, which is accompanied by melling of the reactant titanium powder. The effects of nitrogen pressure, metal particle size, porosity of the metal sample and dilution of the solid phase on the combustion characteristics were studied and optimum conditions for the synthesis of the respective nitrides were determined. Values of the apparent activation energies for the two reactions were extracted, based on combustion characteristics measurements.

ISSN:0010-2202    

DOI:10.1080/00102209108951709

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

Spark-ignition of propane/air mixtures in a constant volume combustion bomb at 200 kPa pressure has been studied within an explored time window covering four decades (0.2μs−2.0m)

ISSN:0010-2202    

DOI:10.1080/00102209108951710

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

Microgravity tests varying oxygen concentration and forced flow velocity have examined the importance of transport processes on flame spread over very thin solid fuels. Flame spread rates, solid phase temperature profiles and flame appearance for these tests are measured. A flame spread map is presented which indicates three distinct regions where different mechanisms control the flame spread process. In the near-quenching region (very low characteristic relative velocities) a new controlling mechanism for flame spread—oxidizer transport-limited chemical reaction—is proposed. In the near-limit, blowoff region, high opposed flow velocities impose residence time limitations on the flame spread process. A critical characteristic relative velocity line between the two near-limit regions defines conditions which result in maximum fiammability both in terms of a peak flame spread rate and minimum oxygen concentration for steady burning. In the third region, away from both near-limit regions, the flame spread behavior, which can accurately be descibcd by a thermal theory, is controlled by gas-phase conduction.

ISSN:0010-2202    

DOI:10.1080/00102209108951711

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

The electrical ignition of a liquid gun propellant was studied in configurations suitable for medium caliber regenerative liquid propellant guns. Two igniter prototypes were developed and tested. Test results from the first igniter prototype showed that it was feasible to ignite the liquid propellant without the formation of an arc. The design of the second igniter reduced the amount of energy required for ignition and was more practical for direct application into a gun fixture. The pressure-time history in both the igniter and a test chamber was used to determine expected igniter performance in a gun fixture. The electric discharge was found to consist of two distinct phases. The first phase consisted of an initial region, dominated by ohmic heating, followed by a second region that was most likely dominated by electrolysis effects. The electrical characteristics of the initial region were predicted by a simple LRC circuit model that was based on the ohmic heating of the liquid propellant. The second phase was the arc portion of the discharge, which was not normally present in tests with the first igniter.

ISSN:0010-2202    

DOI:10.1080/00102209108951712

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

A quasisteady multidimensional Stefan problem is treated by means of similarity, asymptotic methods, and numerical analysis. The continuous penetration of a solid “erodee” (a thick broad slab of ice) is effected by the gradual consumption of a solid “eroder” (composed of lithium). The ice erodee melts and the (liquid) water diffuses to the lithium eroder, where an exothermic, indefinitely rapid, diffusion-controlled surface reaction occurs. The products of the reaction convey some of the chemically-generated heat to the surface of the ice erodee. so that further melting may occur, and the process may proceed. The analysis seeks to estimate the quasisteady rates of consumption of the erodee and eroder, for the (convenient) quasi-selfsimilar case of confocal parabolas defining the boundaries or the fluid layer between the two solids. The analysis is simplified by observing that the flow is of lubrication-theory type: creeping, primarily unidimensional motion owing to the thinness of the fluid layer between the eroder and erodee, with diffusion of heat and mass predominantly in the direction perpendicular to the principal motion. Because the lithium eroder would float on a water layer, a weight must be added to the eroder to maintain close proximity of eroder and erodee. and the analysis is completed by consideration of the force balance on the eroder.

ISSN:0010-2202    

DOI:10.1080/00102209108951713

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

The study of strained flames is of great interest for turbulent combustion modelling. The structure and properties of these flames are usually determined with either very simple but unrealistic one-step or modeled multiple-step kinetics or with large detailed chemical mechanisms. Recent studies initiated by Peters indicate that it may be possible to employ reduced mechanisms and still obtain accurate solutions. It is our objective to evaluate this approach in the context of propane-air flames. The reduced scheme of Peters et al. is first tested. A systematic analysis of the successive steps leading to reduced scheme is conducted. An augmented reduced mechanism which contains new species OH and O and involves additional equilibrium reactions for H2, 02and CO2is proposed. All the computations are compared with respect lo solutions obtained with an initial complex scheme due to Warnatz. The results obtained at atmospheric pressure are generally satisfactory. It is observed that the differences between the augmented reduced mechanism and the complex scheme vary with the parameters such as equivalence ratio and pressure. Our reduced scheme comes closest to the complex scheme for rich conditions while the reduced mechanism of Peters is more appropriate for lean flames. As pressure is increased, the results differ more markedly from the complex scheme. While flame structures are still well predicted, the flame stand off distance is not well retrieved and as a consequence the crititcal extinction value of the strain rate is not very accurate.

ISSN:0010-2202    

DOI:10.1080/00102209108951714

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

In the present experimental study, some examples of hysteresis phenomena in combustion systems are described. In particular, the onset of acoustic instability of premixed flames in stagnation-point flow is investigated. It is found that for a given nozzle speed, transition from acoustic to non-acoustic burning regime is history dependent. The critical molar fuel concentrationsXFUandXFLcorresponding to the onset and termination of acoustic flames as a function of the mean exit velocity of the gas at the nozzle are determined which show the hysteresis band. In the study, additional example of such hysteresis effects in terms of the extinction and re-establishment of combustion of premixed flames in the stagnation-point flow is examined. Finally, the hysteresis effects associated with the onset and termination of rotation of polyhedral butane Bunsen flames as a function of the velocity of a co-flowing nitrogen stream is investigated. The possible significance of the results to the modeling of flamelet dynamics in turbulent combustion is discussed.

ISSN:0010-2202    

DOI:10.1080/00102209108951715

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

ISSN:0010-2202    

DOI:10.1080/00102209108951716

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor