Kazunori Kuwana

To investigate the differences in the vaporization of liquid hydrogen (LH2) under different heat input conditions from the ground, LH2 was poured into two cylindrical vessels. One vessel was made of concrete with an inner diameter of 0.5 m and a reservoir depth of 0.25 m, and the other vessel was made of mortar with the same inner diameter and the same reservoir depth, with 0.15 m of gravel or soil at the bottom. The temperature at each point in these vessels was measured using T-type thermocouples. The weight of LH2 was measured to study the differences in heat input and vaporization rate. The vaporization rate was the fastest for gravel, followed by concrete and then soil. The experiment with the gravel layer confirmed that liquid air was produced after the vaporization of LH2. In this paper, we reported the vaporization process of LH2 under transient heat transfer.

The smoldering spread and extinction behavior are significantly impacted by both wind velocity and angle. However, this combined effect has not been extensively quantified. This study investigates the influence of the wind angle, ranging from θ0 = 0° (concurrent smoldering spread) to θ0 = 180° (opposed smoldering spread), which is defined as the angle between the directions of the forced airflow and the reaction propagation, on blow-off limit and smoldering spread rate of 2.5 mm incense sticks using a rotating apparatus. The experimental results indicate an increase in smoldering rates and blow-off wind velocity as θ0 decreases. The maximum average smoldering rate is about 2.5 cm/min at θ0 = 0°, while it is less than 1 cm/min at θ0 = 180°. The blow-off wind velocity is 15 m/s at θ0 = 0° and 5 m/s at θ0 = 180°. The blow-off limit was discussed based on Damköhler number (Da). A simplified heat transfer analysis, coupled with computational fluid dynamics simulation, was used to calculate the smoldering rate. The simple model captures the experimental trend of the increasing smoldering rate at low wind velocity. A higher smoldering temperature results in a higher smoldering rate. Additionally, the higher smoldering rate observed at θ0 = 0° was reproduced, emphasizing the significant role of heat transfer. To maintain a steady smoldering rate, all the heat produced by the reactions must be removed, which occurs most efficiently at θ0 = 0°, leading to the highest smoldering rate.

Fire whirls, also known as fire tornadoes, represent an extraordinarily severe fire phenomenon, exemplified by the catastrophic events shortly after the great Kanto earthquake in 1923. This research aims to propose an early-warning analysis employing image processing techniques for predicting before the fire-whirl formation. Scaled-down experiments were conducted using two speed-adjustable fans to control the wind movement and generate fire whirls. Through image processing, a set of input flame images is transformed into the evolution of flame height as output data, which serves as the basis of the signal processing. The difference between whether fire whirls occur or not can be detected from the flickering of the flame-height signal. Without delivering external winds, minor changes in noise components are observed at subsequent times, showing no signs of fire whirls. On the other hand, the noise component preceding the fire whirl occurrence highlights a significant increase in standard deviation and autocorrelation, attributing to a slower recovery rate from a perturbed state near a transition to a fire whirl. In this paper, a dynamical marker is constructed as a composite metric of smoothed flame height, standard deviation, and autocorrelation coefficient at lag 1 of the noise component, showing upward trends prior to fire whirl formation. The effectiveness of the dynamical marker as a warning sign for predicting fire-whirl occurrences is validated through experiments of three different wind speeds with the alarm threshold of +3σ to mitigate an unnecessary false detection.

Theoretical predictions of dust-explosion characteristics, such as the minimum explosive concentration (MEC), can help improve the accuracy of quantitative risk assessment. This paper presents a continuum model to simulate one-dimensional dust-cloud combustion. It is found that the dilution of dust by thermal expansion reduces the propagation speed of a combustion wave through a dust cloud. At the same time, the delay in the increase of particle velocity due to relaxation time counteracts the dilution effect. Therefore, neglecting the relaxation time results in the underestimation of propagation speed. In the absence of heat loss, extinction occurs when the adiabatic combustion temperature decreases to the ignition temperature; MEC is independent of particle size in such cases. With the presence of heat loss, on the other hand, extinction occurs at a higher dust concentration; MEC increases with particle size. Accurate evaluation of ignition temperature is key to the quantitative prediction of MEC. A particular focus is given to gravity effects, reflecting recent experimental efforts to obtain fundamental insights into propagation mechanisms without being affected by gravity. In upward propagation, particles fall toward the reaction front, enhancing combustion and increasing the propagation speed. The situation in downward propagation is the opposite, and extinction occurs when the gravity level is beyond a critical value. In upward propagation, MEC can decrease with an increase in particle size, owing to the influence of gravity.

Fire whirls occur in urban and wildland fires, intensifying the local burning rate and generating long-distance firebrands. A striking feature of fire whirls is their increased flame heights, and this article provides a review of previous efforts to understand how the height of a fire whirl is determined. This paper mainly discusses four factors that influence fire-whirl height: burning rate, strong vorticity, turbulence reduction, and vortex breakdown. It is shown that each influence can be understood based on a simple constant-density mixture-fraction model. In the constant-density approximation, the flame shape can be analyzed in a prescribed flow field. This paper considers a one-celled Burgers vortex, a two-celled Sullivan vortex, and a strong-vorticity flow in which the axial velocity near the axis of rotation is faster than that in the peripheral region.