Ritsu Dobashi

Kinetic modeling of thermal decomposition of natural cellulosic materials in air atmosphere

Abstract The wood and leaf samples of eight species are examined by non-isothermal means to determine the mass loss kinetics of the thermal decomposition with linear temperature programming in air atmosphere. A simple kinetic description, named in this work as ‘First Order Pseudo Bi-component Separate-stage Model (PBSM-O1)’, is developed based on the experimental results and integral analysis method. The model assumes that the mass loss process of any wood or leaf sample consists of three steps. The first step corresponds to the water evaporation, and the subsequent two mass loss steps are mainly due to two major pseudo components. The two pseudo components decompose respectively at two separate temperature regions, other than at the global temperature region as used in the previous developed models by other authors. The global mass loss rate of the sample is looked on as controlled respectively by the reactions of the two components respectively during the lower and higher temperature ranges. The kinetics of the two components are found to abide by the first order equation, which gives the best fits to the experimental data compared with other model functions. The advantages of PBSM-O1 are discussed by comparing it with other kinetic models. PBSM-O1 is additionally validated by the reasonable agreement between the experimental and calculated results.

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.

Experimental study on gas explosion behavior in enclosure

Abstract Effects on gas explosion behavior of gas flow turbulence, combustible gas concentration distribution, and flame front instability, which are the most probable causes that disturb the propagating flame front during gas explosions, have been examined. Qualitative aspects of these effects had already been examined in our previous experimental studies. In this paper, data measured in these studies were analyzed quantitatively. It is shown that the gas flow turbulence increases the flame propagating velocity and this makes the pressure rise rapid. When the gas flow turbulence exists the pressure rise from the initial pressure is observed to be proportional to about the 3.6th power of the time t from ignition, whilst if no gas movement exists it is proportional to the 3rd power of t . When the concentration of a combustible gas was non-uniform, the gas explosion behavior strongly depends on the concentration distribution. The flame front becomes unstable by acceleration of the gas induced by a following pressure wave propagating toward the unburned gas side and generated flame front disturbance grows quickly, so that the flame propagating velocity increases rapidly. Then a fast pressure rise is observed and it is proportional to the 6.4–6.8th power of t .

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.

Mechanisms of flame propagation through combustible particle clouds

Abstract An experimental study has been conducted on the mechanisms of flame propagation through combustible solid particle clouds of 1-octadecanol. The combustible particle cloud is ignited in its centre by an electric spark, and the growth of flame kernel is observed with a CCD video camera. The direct light emission and schlieren images of propagating flame and the laser light scattering images of particles have been simultaneously recorded. After ignition, a flame kernel is observed to grow with a yellow luminous zone whose outline is of an irregular shape. At the same time, a smooth shaped schlieren front is observed to propagate at 4–8 mm ahead of the outline of the yellow luminous zone. Inside the schlieren front, dispersed blue flame spots appear but no smaller particles can be seen, and only bigger particles are observed in the border region near the schlieren front. Across the schlieren front, smaller particles (most of them are about 10–20 μm in diameter) rapidly gasify just behind the schlieren front, while the gasification of particles with a diameter larger than 80 μm is delayed and the vapour lumps formed behind the schlieren front ignite to form circular dispersed blue flames. It has also been revealed that the average propagation velocity of the schlieren front increases with the number density of smaller particles, while it is scarcely affected by the mean diameter of combustible particle clouds. This fact implies that flame propagation is mainly supported by the combustion of smaller particles gasifying across the schlieren front.

The burning of large n-heptane droplets in microgravity

Experimental results are presented on the burning and sooting behavior of large n -heptane droplets in air at atmospheric pressure under microgravity conditions. The experiments were performed at the Japanese Microgravity Center (JAMIC) 10 s dropshaft in Hokkaido, Japan. Soot volume fraction, burning rate, flame standoff, and luminosity were measured for droplets of 2.6 mm and 2.9 mm in initial diameter. These are the largest droplets for which soot volume fraction measurements have ever been performed. Previous measurements of soot volume fractions for n -heptane droplets, confined to smaller droplet sizes of less than 1.8 mm, indicated that maximum soot volume fraction increased monotonically with initial droplet size. The new results demonstrate for the first time that sooting tendency is reduced for large droplets as it has been speculated previously but never confirmed experimentally. The lower soot volume fractions for the larger droplets were also accompanied by higher burning rates. The observed phenomenon is believed to be caused by the dimensional influence on radiative heat losses from the flame. Numerical calculations confirm that soot radiation affects the droplet burning behavior.

ACCURATE MEASUREMENT OF THERMOPHORETIC EFFECT IN MICROGRAVITY

The behavior of particles under thermophoretic effect was examined experimentally by getting rid of the effect of natural convection utilizing microgravity environment. The microgravity environment was realized by using a drop tower facility. In this system, measurement fields with temperature gradient were established between two metal plates. In order to provide a steep temperature gradient within a limited experimental duration, the preheated metal plate was forced to approach the cold plate quickly after the microgravity environment was established. Temperature distribution and the behavior of individual particles were observed simultaneously. The fields in an almost steady state with monotonic temperature gradient were obtained. In these fields, no other forces than thermophoretic force which might induce the particles movement were confirmed to exist. The fields ideal for the measurement of the thermophoretic effect were obtained in our experiments. The measured thermophoretic velocities were found ...