Toshisuke Hirano

Controlling Mechanisms of Flame Spread

Abstract Recent advances in the experimental study of the mechanisms controlling the spread of flames over the surface of combustible solids are summarized in this work. The heat transfer and gas phase chemical kinetic aspects of the flame spread process are addressed separately for the spread of flames in oxidizing flows that oppose or concur with the direction of propagation. The realization that, in most practical situations, the spread of fire in opposed gas flows occurs at near extinction or non-propagating conditions is particularly significant. Under these circumstances, gas phase chemical kinetics plays a critical role and it must be considered if realistic descriptions of the flame spread process are attempted. In the concurrent mode of flame spread, heat transfer from the flame to the unburnt fuel appears to be the primary controlling mechanism. Although gas phase chemcial kinetics is unimportant in the flame spreading process, it is important in the establishment and extension of the diffusion ...

Postulations of flame spread mechanisms

Abstract An experimental study was conducted to explore the flame spread mechanism over thin solid fuel sheets. Flame spread rates over paper at various inclined angles were measured, and Schlieren photography was used to qualitatively assess heat transfer to the unburnt material in front of the pyrolysis zone. Two different types of flame spread were observed. One is te downward flame spread observed in the range of −90 to −30 deg from the horizontal. In this region, flame spread rate was almost constant with time, although it increased slightly with increasing angle. The other type of spread observed was the accelerative upward flame spread at angles of zero to 90 deg. Flame spread at angles from −30 deg to zero seemed unsatable and increased by repetitive acceleration and deceleration In the range of inclined angles from −90 to −30 deg, heat transfer from the flame zone to the unburnt material seemed to take place mainly through the gas phase in the region 0.2 ∼ 0.4 cm in front of the pyrolysis zone. In this case, the direction of the gas stream could be considered to oppose that of the flame spread. In the case of upward flame spread, the unburnt material in front of the pyrolysis zone seemed to be heated by convection of the bottom side, where the direction of the gas stream was obviously parallel with that of flame spread.

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...

Modeling of vented hydrogen-air deflagrations and correlations for vent sizing

Abstract Modeling of hydrogen-air deflagrations on the base of advanced lumped parameter theory and comparison with experiments in closed and vented large scale vessels have been carried out. Burning velocity and overall thermokinetic index for hydrogen-air mixtures with hydrogen concentrations of 20.0–41.7% by volume and at elevated temperature 373.15 K were determined. The slight decrease of overall thermokinetic index with equivalence ratio in enriched by hydrogen mixtures has been revealed, that is inverse to observed for hydrocarbon-air systems. It has been determined that flame stretch during vented deflagration constitutes about 1.5–2.2 for investigated conditions. The Le Chatelier-Brown principle analog, revealed previously for vented hydrocarbon-air deflagrations, has been verified for hydrogen-air systems. It has been shown that suggested correlation for the deflagration-outflow-interaction number, χ/μ, in dependence on vessel scale and Bradley number is right for both hydrocarbon- and hydrogen-air mixtures. It has been concluded that gained data on vented hydrogen-air deflagrations obey the same general physical regularities that were revealed previously for hydrocarbon-air systems.

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.

Temperature profile across the combustion zone propagating through an iron particle cloud

Abstract Knowledge of the mechanism of combustion zone propagation during dust explosion is of great importance to prevent damage caused by accidental dust explosions. In this study, the temperature profile across the combustion zone propagating through an iron particle cloud is measured experimentally by a thermocouple to elucidate the propagation mechanism. The measured temperature starts to increase slowly at a position about 5 mm ahead of the leading edge of the combustion zone, increases quickly at a position about 3 mm ahead of the leading edge, reaches a maximum value near the end of the combustion zone, and then decreases. As the iron particle concentration increases, the maximum temperature increases at lower concentration, takes a maximum value, and then decreases at higher concentration. The relation between the propagation velocity of the combustion zone and the maximum temperature is also examined. It is found that the propagation velocity has a linear relationship with the maximum temperature. This result suggests that the conductive heat transfer is dominant in the propagation process of the combustion zone through an iron particle cloud.

Effect of turbulence on vaporization, mixing, and combustion of liquid-fuel sprays

The effect of turbulence properties on spray flame characteristics has been investigated experimentally in detail. A fine scale fluctuation was imposed on a spray by setting a grid in front of the spray nozzle. This simple way of changing the turbulence characteristics was proved to be a very effective way of increasing evaporation rate of the spray. It was found that the faster evaporation does not necessarily lead to faster combustion. As the turbulence characteristics change, evaporated fuel does not burn instantly but the flame whose characteristics are similar to those of a gaseous diffusion flame rather than to those of a heterogeneous spray flame can be observed. The results indicate that with the increase of evaporation rate, mixing of gaseous fuel and air becomes a controlling process of combustion. In the case of a jet mixing with the ambient air, the mixing between heterogeneous phases is more efficient than that between two homogeneous species. This fact is well known from the study of particle-laden jets. In this study its effects in reacting heterogeneous flows are shown.

Concentration profile of particles across a flame propagating through an iron particle cloud

Abstract The number density profile of particles across a flame propagating through an iron particle cloud has been examined experimentally. The iron particles were suspended in air and ignited by an electric spark. Measurements were performed using high-speed photomicrography combined with laser light scattering technique. It is shown that for relatively large (agglomerated) particles the number density of iron particles changes in the range of x smaller than 11.0 mm, where x is the distance from the leading edge of the combustion zone. The number density increases with the decrease of x in the range 0.6 ≤ x ≤ 11.0 mm, reaches a maximum at x ≈ 0.6 mm, and then decreases. The maximum value of the number density is about 2.6 times larger than that at the region far ahead of the flame ( x >11.0 mm). This increase in the number density of particles must cause a change of the lower flammability limit. By assuming that the increase in the number density is caused by the velocity difference of particles from surrounding gas flow, the profile of the number density of particles has been estimated on the basis of measured velocities of particles. The estimated number density profile of particles agrees well with that of the measured profile. The increase in the number density of particles just ahead of the flame will appear not only in iron particle cloud but also in any two-phase combustion systems, such as combustible particle cloud, combustible spray and so on.

Combustion Behavior of Iron Particles Suspended in Air

Abstract The combustion zone propagating through an iron particle cloud and the combustion behavior of individual iron panicles have been examined by using high-speed photomicrographs. Propagation of the combustion zone of 4˜5 mm in width was observed as the movement of a luminous zone which consists of burning iron particles. In the region just behind the leading edge, burning particles of various diameters are examined. As the distance from the leading edge becomes larger, smaller particles are fading away, and then only large particles are observed to remain luminous in the region where the distance is larger than 2 mm. Each iron particle bums at the combustion zone without gas phase flame. The burn-out time (the duration of light emission) is proportional to the diameter of iron particle when the particle diameter is not so large. It agrees well with the result of a simple analysis. As the particle diameter becomes larger, the burn-out time becomes much larger than that predicted by the simple analysis.

Tomakomai Large Scale Crude Oil Fire Experiments

This paper summarizes the results of large-scale crude oil fire experiments conducted in Tomakomai, Japan, in 1998 to obtain information that could be applied to the development of firefighting strategies for, and the design of, huge petroleum storages. Arabian light-equivalent crude oil was burned in pans 5-, 10-, and 20-m in diameter. Most of the experiments were performed under favorable conditions. Measured data include external radiation, infrared image of the flame, flame temperature, gas concentration inside the flame, and other burning characteristics. The height of the strongest radiant emittance was H/D=0.1 to 0.2, where D=pan diameter and H=height from the initial fuel surface, and a kind of fireball appeared occasionally at the intermittant flame zone. Emitted smoke particles were sampled on the ground and observed with a scanning electron micrograph, and the distribution of the diameters of primary smoke particles was examined. The average diameter of primary smoke particles is 53.0 (±10.5)nm. The dependence of burning characteristics and flame structure on pan diameter is discussed. The flame height of the 20-m diameter pan fire is 1.9 (±0.3) •D. The burning rate increases as the pan diameters increase, but the radiative fraction decreases as pan diameter increases.