摘要:

A new theoretical study of counterflow spray diffusion flames is presented. The fuel is introduced into the system in the form of liquid droplets suspended homogeneously in an inert environment. Analytic solutions are developed for the entire range of injection velocities, including large rates of injection for which asymptotic expansions based on the reciprocal of the injection velocity are involved. The sensitivity of flame position and temperature to combined transport, vaporization and liquid fuel spray-related and injection rate effects is analyzed in a comprehensive study of calculated results. These results were obtained assuming an infinite chemical Damkohler number so that the flame appears as a thin front. Additional numerically computed results with finite rate chemical kinetics show that the presence of the spray of vaporizing droplets tends to make flame extinguishment easier and ignition more difficult than in the single gas-phase counterflow diffusion flame situation.

ISSN:0010-2202    

DOI:10.1080/00102209308907659

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

The transversely averaged description of an axisymmetric convective column over a maintained source of weak buoyancy, given by Morton, Taylor, and Turner (1956), is extended to encompass the tall updraft over a large-area fire, with provision for large density differences in a rather arbitrarily stratified, moist ambient. The plume zone (from the completion-of-burning height to the onset-of-condensation height for water vapor), and the capping cloud (from the onset-of-condensation height to the altitude at which the updraft stagnates), are treated such that the solution may be patched to a previously described model for the flaming zone (from the pyrolyzing-fuel-bed surface to the completion-of-burning height). The relative contributions of combustion-generated and entrained-ambient water vapor to in-column moisture content are systematically investigated, along with the dilution with altitude of smoke density of the convective column. According to results from the simple model, the density discrepancy between in-column fluid and ambient fluid at the same altitude typically vanishes in the unsaturated plume, so a capping cloud is found to be a region of negative buoyancy. The desirability is noted of cloud-chamber experiments to examine the efficiency of rainoul/washout of smoke for cases in which sufficiently strong convection results in the rapid growth of condensed water droplets (and/or ice particles) to precipitating sizes. Such cases are not encompassed by the modeling, which is limited to the case of small droplets that are lofted by the updraft

ISSN:0010-2202    

DOI:10.1080/00102209308907660

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

ISSN:0010-2202    

DOI:10.1080/00102209308907661

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

The action of ambient pressure variation upon buoyant laminar jet diffusion flames is studied experimentally and theoretically at low Reynolds number. For the first time, the flame height and the surface area are measured as a function of ambient pressure at different flow rates and for different burner diameters. Two distinct regimes have been found: one in which the flame flickers and the other in which it does not. For steady flames, the flame surface area increases significantly with reduction of the ambient pressure, while the height increases slightly or remains nearly constant. For flickering flames, the flame height decreases with reduction of pressure, which finally results in a decrease in the surface area of the flame. A simple theoretical model based on a cylindrical flame geometry which considers both the radial diffusion of species and the buoyancy effects, is proposed to predict the trend of surface area variation for steady flames. For steady flames, both reductions of gravity levels and ambient pressures can reduce the oxidizer flux entering the flame front, thereby increasing the flame surface area. It is shown that the local flux of oxidizer is proportional to the term (gP2)1/4. The flame flickering can be suppressed by reducing the ambient pressure. Comparisons between the effects of pressure reduction and gravity reduction upon the flame shape and the hydrodynamic behavior are also mentioned.

ISSN:0010-2202    

DOI:10.1080/00102209308907662

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

Computed flame motion through and between swirling eddies exhibits a maximum advancement rate which is related to the time duration of flame motion between eddies. This eddy spatial structure effect upon the apparent turbulent flame speed appears to be similar to the square-root dependence observed in wrinkled flamelet data. The rate-limiting behavior at one eddy length-scale can be removed by inclusion of smaller eddies which reside between the larger eddies. This large-eddy, small-eddy concept yields a recursion relation and repeated functional iteration can be done to approximate a desired flame speed relation. As an example, an iteration to produce STIn ST= u' is given for the range of u' observed in liquid flames. Currently, the iteration process is a post-diction of flame speed, but if a universality can be developed, then a predictive theory of turbulent flame propagation might be achieved.

ISSN:0010-2202    

DOI:10.1080/00102209308907663

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

The rates of gasification by CO2have been determined for three laboratory produced chars derived from U.K. coals with varying ash contents. Data for the temperature range 900 to 1233K were obtained using an isothermal thermogravimetric technique. Reaction rates were determined in atmospheres of either CO2, O2or CO2/O2mixtures for which the CO2partial pressure varied between 10 to 101 kPa. Pre-exponential factors and activation energies for the elementary reaction steps were evaluated using the Langmuir-Hinshelwood expression. High temperature data for the range from 1400 to 1800K were obtained by projecting the char particles through a flat methane-air flame. Particle temperatures in the flame were measured using a two-colour pyrometer system and the rates of char gasification at various CO2partial pressures were calculated from energy balance equations and then used to provide limited information on the char/CO2reaction rate above 1400K.

ISSN:0010-2202    

DOI:10.1080/00102209308907664

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

A study of finite amplitude instabilities occurring in cellular premixed flames is undertaken, both in the case of 2D and 3D flames. We use two different models: the complete equations of isobaric flames with one global exothermic reaction and a modified Michelson-Sivashinsky equation (including gravity effects). Different instabilities are found that can be explained in terms of Eckhaus type instabilities of cellular patterns. An oscillatory instability also exists, which produces self turbulizing flames

ISSN:0010-2202    

DOI:10.1080/00102209308907665

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

This work considers an existing system of differential equations based on Huygens' Principle that can, under some assumptions, trace the position in time of the perimeter of a large wildland fire. These equations in general require a sophisticated numerical solution, however it is demonstrated that if fuel and meteorological conditions are functions of time only, then an analytic solution can be obtained to the equations. The solution is in the form of a definite integral involving the affecting variables expressed as functions of time. For the special case of only time dependent wind velocity, the solution is presented in terms of probability distributions. The solution allows an analysis of its properties giving an insight into the behaviour of wildland fire.

ISSN:0010-2202    

DOI:10.1080/00102209308907666

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

A hollow-cone kerosene spray with a nominal cone angle of 30 degrees from a pressure-swirl fuel atomizer was used in a swirl-stabilized combustor. Swirling air flow with a calculated swirl number of 0.36 is generated with a swirl plate having an exit air velocity vector of 30 degrees with respect to the chamber axis. Effects of swirl and dilution air flow rates on the shape and stability of the flame are investigated. A Phase Doppler Particle Analyzer (PDPA) is used to measure drop size, mean and rms values of axial drop velocity, fuel volume flux, drop velocity and size distributions, and size-classified drop velocity profiles for two cases of with and without combustion and at six different axial locations from the nozzle. A thermocouple is used to measure the uncorrected average temperature at these axial positions. For the no-combustion case all air and fuel flow rates were kept at the same values as the combusting spray condition. A Laser Doppler Velocimeter (LDV) is employed to measure the axial gas velocities and investigate the flew field in the combustor without fuel injection.

ISSN:0010-2202    

DOI:10.1080/00102209308907667

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

A simple analysis and measurements are presented, which show that adiabatic premixed laminar flames can be stabilized on a flat flame burner, especially designed for this purpose. The physical properties of these flames are identical to those of flat freely propagating flames. The adiabatic state can be accomplished in practice when the burner plate temperature is well above the temperature of the unburnt mixture. The net heat loss of the flame to the burner is zero (i.e. the flame is adiabatic) when the measured radial temperature profile of the burner plate is uniform. These flames are particularly suitable for comparison with theoretical or numerical flat flame studies.

ISSN:0010-2202    

DOI:10.1080/00102209308907668

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor