Combustion Behaviors And Flame-Moving Mechanism Of Fire Whirls

As a special fire behavior, the fire whirl is often reported in forest and urban fires with devastating damages to life and property. In essence, it is a swirling flame due to the nonlinear interaction between general fire plumes and vortices in the surroundings. As compared to general pool fires, fire whirls are characterized by the significant increases in burning rate, flame height, upward speed, flame temperature and entrainment in the ground layer, with great potentials for strong flame radiation and long-distance spotting fire. Thus, a fire whirl can significantly accelerate fire spread rate. In real fires, a fire whirl is difficult to be predicted because of the complex mechanism of the vorticity generation and its effect on the fire plume. By use of a fire whirl apparatus consisting of a square enclosure, this thesis presents a study on the combustion behavior of steady fire whirls, including the burning rate, flame height, fire plume characteristic and flame radiation property. In addition, the moving-type fire whirl is extensively analyzed by the scale-model experiment in order to investigate the interaction between line fire and cross wind. The condition and mechanism of for the formation of concentrated vortex in a line fire and the moving of the fire whirl flame are discussed.Firstly, the drag force and eddy structure of fire whirls are explored, and the semi-empirical correlations for the burning rate and flame height are established. The drag force is generated by the fire whirl vortex acting on the liquid fuel surface, which plays an important role in the initiation and decay of fire whirls. The flame near the ground boundary layer involves the eddy structure similar to a couple of vortex ring, while above the ground boundary layer the eddy is suppressed to a large extent. It is clarified that flow circulation helps increase both the fuel-flame contact area and the actual fuel surface area, leading to the increase of burning rate. Analysis verifies that the ratio of the fire whirl flame height to the flame height without circulation (i. e. that of a general pool fire) can be solely correlated with the flow circulation, thereby a new formulation for flame height is established, which successfully decouples the burning rate and flow circulation.Secondly, the fire whirl plume is explored, and some semi-empirical correlations for the upward speed in the centerline, radial temperature distribution, and mass flow rate are developed. The classical plume theory and dimension analysis are used to analyze the experimental data. Analysis indicates that the dissipation of vertical momentum in the radial direction is reduced by flow circulation, resulting in the increase of the upward speed with increasing flow circulation. The fuel-rich core in the flame body of fire whirl significantly affects the radial temperature distribution in the continuous flame region, and the flame body can be described by the combination of a cylinder and a cone. The flow circulation significantly suppresses the fire plume radius, and decreases its increasing rate with the vertical distance. It is also demonstrated that the fire whirl flame involves laminarized regions in its lower section, coexisting with turbulent regions in the upper portion. The flow circulation enhances the air entrainment in the ground layer by altering the radial velocity profile and increasing the radial velocity. In the lower section of the flaming region, the significant decrease of the mixture between combustion products and surrounding air dominates the effect of flow circulation on the flame height.Thirdly, the fire whirl flame radiation is also explored, and a semi-empirical correlation and a thermal radiation model for radiant heat flux are established. The radiant heat fluxes are measured in the vertical and radial directions. By dimension analysis, the dimensionless radiant heat flux versus dimensionless distance is derived by fitting the experimental data. Based on the measured heat flux, the radiative fraction of fire whirls is calculated to be nearly42%,1.4times as large as that for general pool fires. The radiative heat feedback fraction is shown to increase with pool diameter, similarly as general pool fires. A correlation for excess flame temperature coupling excess entrainment fraction and radiative fraction is derived by simplifying the mass and energy conservation equations, which helps clarify that fire whirls maintain higher excess temperature than general pool fires due to the effect of less excess entrainment fraction. By extensively considering the volumetric emission and absorption of flame mixture, a new formula is proposed to characterize the vertical profile of the flame surface emissive power of fire whirls, by which a revised solid flame model is presented for predicting the radiant heat flux from the flame.Finally, the formation condition and mechanism of the fire whirl concentrated vortex and the moving flame are explored by analyzing the interaction between line fire and cross wind. The moving-style fire whirl is well known for its appearance during the wildland fire of Brazil in2010. Experimental analysis indicates that a line fire near the ground, a reasonable attack angle between the line fire and the cross wind, and wind speed within a critical range are the three essential conditions for the formation of fire whirl in a line fire. By examining the advection and bending of vorticity, it is also deduced that the concentrated vortex of fire whirl results from the coupling of the line fire plume and the horizontal vortex line near the ground surface. By assuming the solid-body rotation of fire whirl flame, a possible mechanism of moving fire whirl is proposed, which states that the flame moving is mainly controlled by the drag force, lift force and ground friction.