Koyu Satoh

Experimental Research on Burning Rate, Vertical Velocity and Radiation of Medium-Scale Fire Whirls

Fire whirls are often reported in forest and urban fires with devastating damage to life and property. This work conducted experiments using a medium-scale facility to study the initiation mechanism, the vertical velocity and radiative heat transfer of fire whirls. Heptane was used as the fuel in the experiments. The variations of burning rates versus time indicate that the drag force on the root of the flame plays an important role in the initiation and decay of a fire whirl. Analyses show that the pressure difference due to whirling, characterized by the circulation 

CFD Study of Huge Oil Depot Fires - Generation of Fire Merging and Fire Whirl in (7 x 7) Arrayed Oil Tanks

One of the largest industrial fire disasters may occur in oil tank depots which store large amounts of oil. Many previous studies on the fire safety of oil tank depots have been related to the fire propagation from one single oil tank fire to the adjacent tank via radiation. However, single oil tank fire may cause a fire whirl in windy conditions, entraining much more ambient air and enhancing flame radiation, which may increase the possibility of fire propagation toward the neighboring tanks. In addition, when an oil depot storing a large amount of oil in tanks is subject to destructive earthquakes, merging fires and fire whirls may be generated, leading to disastrous consequences. In this work, the authors examined the fire merging and fire whirl behaviors in multiple huge oil tank fires by CFD simulations using FDS v4. The constant heat release rate model was employed and the effects of tank-to-tank distance, wind speed and heat release rate were examined. It was found that these parameters are important to cause fire merging and fire whirls, and at the same time, the conditions to cause fire merging and fire whirls lie in a limited range. Some relevant correlations were established. The results are expected to be useful for mitigating the disasters due to fire merging and fire whirls.

Neutral Plane and Length Scale of Spill Fire Plume Considering the Effects of Cross-Ventilation

This study sought to explore the neutral plane and length scale of spill fire plume considering the effect of cross-ventilation. The neutral plane heights for the downwind and upwind openings were firstly formulated, which suggested that the neutral plane heights vary monotonically with the crosswind velocity. The formulation was verified using the experimental data by two different methods. It was verified that with increasing crosswind velocity, the decrease of neutral plane height will induce rapid increase in the burning intensity of spill fire plume, as well as the convective heat flow rate, the flame length, and the original temperature. A new mathematical model consisting of a new length scale was developed as an extension from the models in literature for no-wind cases to cover the cases with various cross-ventilation conditions. It was indicated that the new length scale can be used to correlate the axial temperature data of spill fire plume under various cross-ventilation conditions.

Numerical Study of Characteristics of Burning Phenomena in Equidistant Square Arrayed n-Heptane Fires

Merging of large-scale city fires and forest fires causes rapid acceleration of fire growth. Once a merging fire occurs, it becomes more difficult to suppress, with greater potential damages. In particular, merging fires may induce fire whirls in windy conditions. However, the details of interactions in multiple fires that cause fire merging have not been fully clarified. For the interactions in multiple fires, the inter-fire distance among fires greatly affects the merging phenomenon. The objective of this paper is to examine the detailed merging conditions, particularly the burning rate increase and total heat release rate, by numerical simulation of reduced scale fires. The burning behavior of n-heptane in n × n fire arrays is examined, using the fire simulation software, FDS by NIST. In addition, another simple model is employed. The number of array matrix, n, is varied, together with the inter-fire distance. The simulation results show that there are considerable differences between both simulations and experiments. However, the differences between the simpler simulation Method II and experiments are fewer than the simulation Method I. The following possibilities are considered: (1) The oil pan size affects the difference, but the results between simulations and experiments are so large. (2) The grid size for simulations may have some effects on the simulation results due to the resolution, (3) the experimental results may not always be precise, since the burning rates in the experiments are measured by the burn-out time and (4) the wind caused by merging fires may reduce the radiative heat flux to the adjacent fuel. The relationship between flame length and burning rate and the relationship between flame length and radiative heat flux are well-correlated.Copyright

Preliminary Study of Fire Spread in Cities and Forests, Using PMMA Specimen as a Fuel in CFD Simulations

It is important to examine the behavior of forest fires and city fires to mitigate the property damages and victims by fires. There have been many previous studies on forest fires where the fire spreading patterns were investigated, utilizing artificial satellite pictures of forest fires, together with the use of corresponding weather data and GIS data. On the other hand, large area city fires are very scarce in the world, particularly in modern cities where high-rise concrete buildings are constructed with sufficient open spaces. Thus, the examples of city fires to be referred are few and detailed investigations of city fires are limited. However, there have still been existing old cities where traditional houses built with flammable material such as wood, maybe historically important, only separated with very small open spacing. Fires may freely spread in those cities, once a big earthquake happens there and then water supply for the fire brigade is damaged in the worst case along with the effect of strong wind. There are some fundamental differences between the forest fires and city fires, as the fuel may distribute either continuously or discretely. For instance, in forest fires, the dead fallen leaves, dry grasses and trees are distributed continuously on the ground, while the wooden houses in cities are discretely distributed with some separation of open spacing, such as roads and gardens. Therefore, the wooden houses neighboring the burning houses with some separation are heated by radiation and flames to elevate the temperatures, thus causing the ignition, and finally reaching a large city fire. The authors have studied the forest fire spread and are planning to start a laboratory experiment of city fire spreading. In the preliminary investigation, a numerical study is made to correlate with the laboratory experiment of city fire propagation, utilizing the three-dimensional CFD simulations. Based on the detailed experimental analysis, the authors are attempting to modify the three dimensional CFD code to predict the forest fires and city fires more precisely, taking into account the thermal heating and ignition processes. In this study, some fundamental information on the city fire propagation has been obtained, particularly to know the safe open spacing distances between the houses in the cities and also the wind speed.Copyright

CFD Study of Large City Fires in Windy Conditions Relevant to Aerial Firefighting

If a big earthquake happens to occur in a city with wooden houses in windy conditions, city fires could cause enormous damages, while firefighting operations will be extremely limited. Aerial firefighting with water dumping is expected; however, it has not been commonly used in fighting earthquake-induced large city fires. An important problem is that the information concerning fire front combustion and circumstances over fire areas is unknown. Such information is highly needed for flight safety and strategy. Due to the difficulty of large real-scale experiments, information on large city fires is limited. CFD studies of large city fires in windy conditions were made, but the details have not yet been fully clarified. This paper studies the fire flows near the fire front and over fire areas using CFD simulations, for which the radiation and temperatures in windy conditions are particularly examined. The characteristics of radiative heat flux, tilt angle, temperature, and wind velocity under different parameters are investigated. The results show that if (3 × 3) burning houses near the fire front are successfully extinguished, the radiative heat flux reaching the houses downwind can be reduced drastically. The temperature at 300 m from the ground will be lower than 100 °C, and the radiation will also be very low, which may provide instructions for the safety of aerial firefighting.

CFD Study of a Fire Whirl of Huge Oil Tank: Burning Rate, Flame Length, Distributions of n-Heptane and Oxygen in a Fire Whirl

The number of huge oil storage tanks is increasing in the world. If a fire occurs in one of these tanks, it is very difficult to suppress. Additionally, if a fire whirl occurs in an oil tank fire, it is extremely dangerous for firefighters to extinguish the fire. The authors have numerically studied huge fire whirls in a large oil tank depot and predicted the generation of those fire whirls. Here, another study is attempted to clarify the details of huge fire whirl in a large oil tank, using two kinds of fire whirl generation channels in CFD simulations using the software, FDS by NIST. Details of burning rates, velocities of whirling flames, radiative heat flux, heat release rates and whirling cycles are examined, using oil tanks with the diameters of 0.2 to 80 m. In oil tanks with a diameter of 80 m, a tall fire whirl is generated. The height is about 1000 m. In this study of oil tanks fires with small to large diameters, it has been found that fire whirl lengths are about 8 to 11 times of the oil tank diameter. The maximum radiative heat flux due to a fire whirl in 80 m diameter oil tanks exceeds 100 kW/m2. Since the maximum radiation is found at twice the distance of oil tank diameters from the tank centers, adjacent oil tanks may be ignited. This study has also examined a method used to prevent fire whirl generation in huge oil tanks.Copyright

CFD Simulations of Urban and Wildland Fire Spread Among Discrete Fuels Under Effect of Wind

It is important to investigate the urban and wildland fire behavior to mitigate the fire hazards. There have been many studies on such fires, but the need of real time fire simulations has recent increased and a demand to predict fire spread patterns in urban and wildland regions for decision-making strategies against fires has emerged. However, the knowledge of fire spread behavior is still insufficient, particularly for the condition of discrete fuel distributions. Under this condition the fire spread behavior shows high complexity due to the significant interactions between the radiation, conduction and convection heat transfer, especially under significant ambient wind effects. This paper investigates urban and wildland fire spread behavior by utilizing CFD simulations for two types of fuels under the effect of wind. A 15×15 square array, consisting of 225 fuel sources, is used to simulate the discrete fuel distribution, with varying fuel spacing and wind speed. The simulation method is similar to that used in our previous study, but with different ignition heaters. The comparison of the simulated results for the reduced and real scale models is reasonable, as verified by the similarity law. The critical fire spread distance, the wind effect upon fire spread, and the variation of fire spread rate for the two types of fuels are extensively investigated.Copyright