A number of experimental researches on dust explosion have been published with measured data of the explosive characteristics such as the ignition temperature, the flame propagation velocity, the upper and lower limit explosible concentrations and the rate of pressure rise. In this study the above-mentioned explosive characteristics are dealt with systematically based on a simple model of uniformly dispersed dust cloud consisting of particles of the same size, from the viewpoint of heat transfer process. Comparing the computed results with the empirical law and the experimental data, it is assured that the explosive characteristics can be predicted theoretically. In addition, the analysis of the area for the pressure relief venting is conducted to minimize the damage, leading to a new dimensionless vent ratio.
A new tapping system has been developed to measure bulk densities of powdery materials and to study their compaction characteristics. The new tapping system has an electronic monitor of impact acceleration to observe the peak value of impact acceleration, that is, the intensity of tapping action. The impact acceleration of a conventional tapping machine is not monitored quantitatively and generally higher than 200 G. The adjustable range of the new tapping system is from 3 to 500 G. Tapping height adjustment of a conventional tapping machine is done without any appropriate change of colliding materials. Such an adjustment causes very ambiguous covariation of tapping intensity and tapping energy quantity. Some confusions of these different factors have been making it difficult to analyse and discuss the effects of tapping height. The peak value of impact acceleration and the tapping energy quantity of the new tapping system can be varied independently through the combination of tapping height adjustment and cushioning material selection. Four limestone powders, five white alundum powders and two Kanto loam powders were used as testing powders. Their specific surfaces, SW , were measured by an air permeability method. Tapping tests were done for each powder under various tapping conditions. Higher tapping energy in the sense of quantity causes faster compaction process but does not affect the terminal tapping density, ρ∞, which varies widely with SW and tapping acceleration, A. The terminal tapping density of finer powder is lower than that of coarser powder. Higher impact acceleration causes higher terminal density, but the effect of A seems to be saturated when A becomes sufficiently high. The density difference owing to the size difference becomes smaller with an increase in A. Tapping tests with lower impact acceleration are much more informative than those with higher acceleration.
Some problems that the author has encountered in size measurement of submicron particles in air and in water have been outlined. When particle size is smaller than around 0.5μm, the mean velocity caused by Brownian motion of a particle, which cannot be repressed by any usual means, becomes comparable with the gravitational settling velocity in air and exceeds the velocity in water. When the number concentration of particles is higher than about 108 particles/cm3 , Brownian coagulation, which cannot be repressed in air and in water unless the use of appropriate dispersion agents becomes significant. Size measurement by sedimentation methods in one or both of these cases gives an erroneous result. Another difficulty arises in size measurement of aggregate particles composed of submicron primary particles. When one may want to know the size distribution of aggregates as they are, sedimentation in air is effective. For measurement of primary particle size composing aggregates, on the other hand, sedimentation in water is effective since deaggregation in water is much easier than in air. As to submicron liquid droplets there is a problem of stability. Even if they have low vapor pressure such as oil, they can easily evaporate to decrease in size by the Kelvin effect which becomes significant as the droplet size decreases.
Interrelations among shearing stress, applied vertical stress (by external load), actual vertical stress at shearing plane, vertical displacement (powder bed expansion or contraction) and shearing displacement have been extensively investigated for a number of powders under various test conditions such as the method of preconsolidation, the shearing cell diameter, the initial void fraction of powder beds and so on, by utilizing a simple shear tester capable of testing under both constant-load and constant-volume conditions and of measuring the actual vertical stresses (6 points) in the vicinity of shearing plane, the shearing stress and the vertical displacement of a powder bed simultaneously. It has been concluded that; if the internal friction factor having a universal validity and fairly independent of the test conditions is to be obtained, it is essential to measure the mean actual vertical stress in the vicinity of shearing plane, or it is indispensable otherwise to measure at least the vertical displacement of powder beds along with the shearing displacement and to make sure that the vertical displacement is being kept as small as possible during a course of shearing.
To make clear the problems in the measurement of the adhesion force of powder, new measuring principles and methods were developed, in which emphasis was laid on extending measuring ranges especially to high temperature condition. As measuring methods for the adhesion force of single particles, vibration and impaction separation methods were developed, and it was found that in separating a particle, the rolling motion by tangential force was very important, for which a fundamental equation was derived. The application of this tangential force to the adhesion measurement was proposed. Some additional measurements were carried out at high temperature to investigate the effect of temperature on the adhesion force of single particles.
γ-Fe2O3 was vibration ball-milled with varying amplitude, α, from 8 to 24mm and grinding time. The enthalpy increase, ΔH*, due to mechanical treatment was measured by a differential scanning calorimeter. Pretreated sample was subsequently heated at 693K for 60min, and the fractional transformation into α-phase, a1, was obtained. Both ΔH* and a1 increased monotonically with grinding time with leveling off at longer milling time. The tendency of a level off was different, however, between ΔH* and a1. ΔH* increased monotonically with increasing amplitude whereas a1 showed a maximum at α = 10mm. The correlation between a1 and the lattice disturbance, the latter being obtained from the relative intensity of X-ray diffraction peaks, was not uniquely determined, but depended on the amplitude. This suggests that the same lattice disturbance does not always lead to the same degree of acceleration of the subsequent reaction. The reason was discussed on the basis of the content of ΔH* and the nuclei-growth mechanism of γ→α transformation of Fe2O3.
Some characteristic changes of ultra-fine powders obtained by a recently developed dry grinding system called UMF were introduced together with the performance of the grinding system. UMF has made it possible to grind not only the general minerals but also some hard substances like zirconium oxide, silicon nitride etc. down to submicron powder in dry condition. In addition, the grinding capacity was greater than that expected from conventional mills taking into account product fineness. The differences of some powder characteristics between the ground product and the raw material were confirmed, using an electron microscope, thermogravimetry, differential thermal analysis and X-ray diffraction analysis for several materials including calcium carbonate, mica, barium titanate and so on.
Since last century, a number of investigations were reported on the peculiar sound producing beach or desert sands. The so-called musical sand, which emits a musical sound when walked on, can be found in several parts of the world. A long time ago, many Japanese beaches were known as musical one. However, recently almost all the beaches have lost the peculiar properties with pollution attributed to various human activities. Various fine dusts and oily materials adhered to the surface of the sand grains. If additional influx of polluted materials had been stopped for a very long period of time, they would have been cleaned up by the mechanical washing action of sea wave, so-called beach action, in natural environment. Laboratory experiments were conducted in an attempt to simulate the natural process. For this purpose, gyratory motion washer was used, because the movement speed and pattern of water and sand were nearly similar to beach action. The washed sand had excellent sound producing properties and pronounced not only in the air but also in the water. The perfectly cleaned up sand was characterized by very high friction coefficient of the sand layer, so the penetration test of the sand layer showed clear stepwise characteristics, resulting from significant difference of static and dynamic friction coefficient. Moreover, from the correspondence of the wave form of sound pressure and stress in sand layer, an explanation of sound producing mechanism was described.