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Dimensionless groups suggested by the mathematical modelling of subsonic fuel jet flames, and extensive experimental data, have been reasonably successsful in correlating dimensionles flame heights and flame lift-off distances in terms of a dimensionless flow number. This approach is extended to the more exacting correlations that define regimes of flame quenching, blow-off, and lifted flames. Experimental data from these diverse sources are analysed, and the bounds of these regimes are expressed in terms of the critical minimum jet pipe diameter to avoid blow-off, normalised by the laminar flame thickness for the maximum burning velocity mixture, and the flow number. The regimes extend from low Reynolds number laminar flows in hypodermic tubes to high Reynolds number choked flows, with supersonic shocks. Data are well corrrelated in the subsonic regime for a range of hydrocarbon gases, in which critical pipe diameters for the avoidance of blow-off increase with flow number. Matters are more complex in the extended choked flow regime, in which there are less data. This regime of supersonic flow and shock waves is one of improved fuel/air mixing and enhanced reactivity, to such an extent that the critical pipe diameter, after reaching a maximum, decreases. Data are presented in this regime, and indeed over the full range of conditions, for methane, propane and hydrogen jet flames. Hydrogen exhibits more reactive characteristics than the hydrocarbons. In terms of the correlating parameters, whereas laminar flame thickness is related to that of the non-reacting preheat zone, such a zone is difficult to define with hydrogen, as a consequence of the upstream diffusion of H atoms, and this aspect is discusssed. [ABSTRACT FROM AUTHOR]

The paper first reviews the mode of generation of fire whirls, their properties, and operational regimes, under well-controlled experimental conditions. The situation is different with wildfires. These are uncontrolled and less well understood. A modified analytical approach is described for these conditions. This is based on global energy levels, per unit ground area, for different fuels, and their associated rates of fire spread, under different conditions. These enable regimes of possible fire whirl generation in wildfires to be defined. The associated accuracy is not that of a laboratory flame, but it is probably practically sufficient. Only in two instances were atmospheric fires experimentally controlled. In the remainder, the rate of fire spread was a more meaningful parameter than a burning velocity, and the critical lateral velocity was seldom known accurately. Nevertheless, it was possible to relate it to the heat release rate, with both in dimensionless form. [ABSTRACT FROM AUTHOR]

The lifted flame behavior of inclined turbulent jets, considering the relative angle between the fuel jet momentum and flame buoyancy was investigated experimentally by varying the inclination angle of nonpremixed fuel jets. Variations of lift-off length from the flame base to the nozzle exit was quantified experimentally with nozzles of various diameters (2, 3, and 5 mm) and inclination angles (range of −90° to 90°). The data was analyzed based on the experimental finding of upstream preheating effect depending on inclination angles. Major findings are as follows: (1) The lift-off length (h) increases linearly with the increase in initial fuel jet velocity (u e) at a fixed inclination angle. The proportionality slope κ of the linear relationship between h versus u e decreases appreciably with jet inclination angle for the negatively inclined flames; while for the positively inclined flames, the lift-off length decreases relatively weakly. (2) Physical analysis on the flow characteristics of inclined jets was conducted, and the preheating effect was proposed based on the combustion behaviors, especially for the negatively inclined jet flames. The preheating temperatures of unburned fuel/air mixtures at the flame base and nozzle exit were experimentally quantified, revealing that the negatively inclination angle can have a significant influence on the preheating temperatures. (3) Based on the proposed preheating mechanism, a physical model accounting for the effect of jet inclination angle was developed to quantify the lift-off length of inclined jet flames. The proposed model successfully represented lift-off lengths at all the experimental conditions with various inclination angles and nozzle diameters. The present findings provide new data set and a reasonable physical model for lifted flame behavior of inclined turbulent jet flames, revealing the effect of the relative angle between fuel jet momentum and flame buoyancy. [ABSTRACT FROM AUTHOR]

An extensive review and re-thinking of jet flame heights and structure, extending into the choked/supersonic regime is presented, with discussion of the limitations of previous flame height correlations. Completely new dimensionless correlations for the plume heights, lift-off distances, and mean flame surface densities of atmospheric jet flames, in the absence of a cross wind, are presented. It was found that the same flow rate parameter could be used to correlate both plume heights and flame lift-off distances. These are related to the flame structure, jet flame instability, and flame extinction stretch rates, as revealed by complementary experiments and computational studies. The correlations are based on a vast experimental data base, covering 880 flame heights. They encompass pool fires and flares, as well as choked and unchoked jet flames of CH 4 , C 2 H 2 , C 2 H 4 , C 3 H 8 , C 4 H 10 and H 2 , over a wide range of conditions. Supply pressures range from 0.06 to 90 MPa, discharge diameters from 4 × 10 −4 to 1.32 m, and flame heights from 0.08 to 110 m. The computational studies enabled reaction zone volumes to be estimated, as a proportion of the plume volumes, measured from flame photographs, and temperature contours. This enabled mean flame surface densities to be estimated, together with mean volumetric heat releases rates. There is evidence of a “saturation” mean surface density and increases in turbulent burn rates being accomplished by near pro rata increases in the overall volume of reacting mixture. [ABSTRACT FROM AUTHOR]