Leading Flame Edge Research Articles

Flame spread over crude oil sludge

Fire spread over crude oil sludge has been studied to explore the fire growth process in the cargo bay of a crude oil tanker during cutting and welding repairs. The aspects of flame spread have been examined in detail and the variation of flame spread phenomena with the fraction of n -hexane (typical volatile component) in sludge is presented. The flame spread rate is shown to increase as the n -hexane fraction in sludge increases. The temperature distributions in the sludge as the flame spreads over it were derived from temperature-time diagrams. The distance of heat penetration behind the leading, flame edge decreases with the increase of flame spread rate. By comparing the flame spread rates over the sludge to those over single component liquid or solid combustibles, it is inferred that the dominant mode of heat transfer for flame spread over sludge is conduction through the still unliquefied sludge. In considering the behavior of a volatile component in the condensed phase of a mixture with nonvolatile components, the rate determining step of vaporization of the volatile component is taken to be its diffusion through the condensed phase. A flammable layer cannot be established over sludge at room temperature even when the sludge temperature is above the flash point measured by a close cup method. The increase of the spread rate with the fraction of volatile component of the sludge is attributed to the appreciable increase of the vapor fraction near the leading flame edge due to the temperature increase.

Flame propagation across a liquid fuel in an air stream

The behavior of flames propagating across methanol in opposed and concurrent air streams was examined using high-speed schlieren photography, and the propagation mechanisms of these flames are discussed. For the free stream velocity U f gradually decreases as the absolute value |U| of U increases. The absolute value |U c | of the critical value U c at which V f becomes 0 is found to be much larger than the flame propagation velocity V f 0 at U=0. While for U>0, i.e., in the case of the concurrent air stream, V f remains almost constant until U becomes equal to V f 0 , and becomes nearly equal to U in the range of U>V f 0 . In the case of U f 0 , the effects of the free stream on the flow in front of the leading edge of the flame seem to be reduced to a great extent, so that the flame can propagate without a remarkable decrease in V f even in the opposed air stream of a much higher velocity than V f 0 . On the other hand in the case of U>V f 0 , the flame propagation seems to be closely related to the behavior of the hot gas overhanging the methanol surface in fron of the leading edge of the flame. The heat from the overhanging hot gas is expected to increase the methanol vapor concentration in front of the leading flame edge and ignite it. Thus, an inclined flame can follow the hot gas even when the initial methanol temperature T 1 is lower than that of the flash point. The relation (ϱu/ϱy) sc 2 =B·S L 2 ·δ 0 is found to be valid for T 1 >15°C, in which ( ϱu/ϱy) sc is the critical velocity gradient at the methanol surface in the approach flow, B a constant, S L the burning velocity at the leading flame edge, and δ 0 the flammable layer thickness at U=0.

Gas Movements in Front of Flames Propagating Across Methanol

Abstract The gas velocity profiles in front of the leading edges of flames propagating across methanol at initial temperatures Ti from -5°C to 35°C were measured by using high-speed schlieren photography combined with a hot gas tracer technique. For Tl much lower than the flash point Tlf, any appreciable gas movement could not be observed in front of the leading flame edge. Therefore, as considered in previous studies, preheating in this case was supposed to be mainly caused by convection in the liquid phase. For Tl slightly lower than Tlf, the maximum value of the extrapolated gas velocity across the flame front was found to be a fairly high value. This result could be consistently intepreted by considering the increase of the methanol vapor concentration in front of the leading flame edge due to preheating. For Tl above Tlf, the experimentally predicted aspects of the gas movements near the leading flame edges were found to coincide with the theoretically predicted aspects for the flame propagation thro...

A further study on effects of external thermal radiation on flame spread over paper

An experimental study has been conducted to elucidate the mechanism by which the spread rate of a stable, downward-spreading flame over paper increases with the increase of the radiant flux from an external heat source. The gas velocity and temperature profiles near the leading flame edges under various external radiant fluxes were examined by using particle tracer techniques and fine wire therm ocouples.Velocity profiles in front of the leading flame edge were greatly influenced by the external thermal radiation. As the external radiant flux was increased, the gas-stream velocity upstream of the leading flame edge increased and the deceleration of the gas stream approaching the leading flame edge became intense. Temperature profiles near the leading flame edge also were greatly influenced by the external thermal radiation. At the station 0.2 cm ahead of the pyrolysis front the temperature gradient at the paper surface in the direction normal to it decreased as the external radiant flux was increased, while at the station just ahead of the pyrolysis front the temperature gradient increased with the external radiant flux. Although the preheat zone width decreased with the increase of external radiant flux, the net heat flow to the unburned material through the gas phase increased. The increase of the flame spread rate with increasing external radiant flux could be attributed not only to the increase of the unburned material temperature but also to the increase of the rate of gas-phase heat transfer from the flame zone to the unburned material.

Instability of downward flame spread over paper in an air stream

An experimental study has been conducted to elucidate the mechanism causing the instability of downward flame spread in an air stream. Following detailed observations of unstable flame spread phenomena, the flow fields near unstably spreading flames and the temperature profiles across the preheat zones were examined using particle tracer techniques and fine wire thermocouples. When the free-stream velocity was above that of the stable flame-spread limit, a series of local blow offs were observed during the spread. In this case, most leading flame edges were inclined or bended, and the blow offs occurred mainly at the locations where both inclined angle of the leading flame edge from the horizontal surface and curvature of the leading flame edge were small. Measured temperature-time diagrams at a given distance from the paper surface were almost similar if flames spread stably across the thermocouple junction. The flame spread phenomena including blow offs at the burning zone with a straight leading flame edge could be considered to depend mainly on the velocity component of the free-stream normal to the leading flame edge but scarcely on the velocity profile in the boundary layer of the approach flow.

Gas velocity and temperature profiles of a diffusion flame stabilized in the stream over liquid fuel

The gas velocity and temperature profiles across the laminar boundary layer with a diffusion flame estblished over methanol or ethanol were measured with the free stream, of air parallel to the liquid-fuel surface. The flame stabilizing mechanism and fuel consumption rate are discussed. The results show that the maximum velocity appearing near the blue-flame zone, where the gas stream is accelerated, increases downstream and exceeds the free-stream velocity at a point about 0.2 cm from the leading edge of the fuel vessel. The temperature at the blue-flame zone is found to increase downstream about 1.5 cm from the leading edge of the fuel vessel and then to decrease slightly still farther downstream. The fuel consumption rate is observed to increase monotonically with the increase of the free-stream velocity. It is shown that in order to elucidate the flame stabilizing mechanism, the velocity profile change due to the flame reaction must be taken into account. The diffusion flame over the liquid fuel can be considered to remain stable until the leading flame edge shifts beyond the leading edge of the fuel vessel due to the increase of the free stream velocity.

Effects of radiation and convection on gas velocity and temperature profiles of flames spreading over paper

The effects of radiation and convection on the mechanism of flame spread over a thin combustible solid have been studied. The gas velocity and temperature profiles near flames spreading downward over paper were measured using particle tracer techniques and fine-wire thermocouples. The air stream moving vertically upward was decelerated as it approached the leading edge of a stably spreading flame, and a lower velocity region appeared near the paper surface in front of the leading flame edge. When a low-velocity air stream flowed vertically downward, vortices appeared near the spreading flame. The temperature profiles near a stably spreading flame indicated that a large amount of heat flowed to the unburned material in a narrow region adjacent to the pyrolysis front. When the air flowed vertically downward, hot gas flowed along the paper surface in front of the pyrolysis front. The increase of the flame spread rate with the increase of the radiative heat flux was attributed mainly to the increase of the surface temperature due to radiative heating. The flame spread rate was shown to be closely related to the velocity profile just in front of the leading edge of the spreading flame.

Aerodynamic and thermal structures of the laminarboundary layer over a flat plate with a diffusion flame

The influence of the flame reaction on the aerodynamic and thermal structures of the laminar, two-dimensional boundary layer over a flat plate has been studied experimentally. The diffusion flame was produced by injecting fuel gas (methane or propane) from a porous plate surface into a uniform air stream flowing along the plate. The results show that acceleration of the gas stream is observed, even at the station closeto the leading flame edge located just above the edge of the porous plate. The temperature in the blue-flame zone is found to increase with the distance from this leading edge. Changes in the velocity and temperature profiles are also observed in the boundary layer upstream of the leading flame edge. The pressure distribution has been calculated using the measured velocity and temperatureprofiles. The pressure distortion on the air-stream side of the flame zone can be atributed to the rapid increase in the thickness of the laminar boundary layer with a diffusion flame. The velocity overshoot near the flame zone may be caused by this pressure distortion. It is shown that the aerodynamic structure of the boundary layer changes considerably in the presence of a diffusion flame, and that even in the laminar boundary layer over a flat plate, the pressure cannot be considered to be uniform if a diffusion flame is established in it.