Yuji Hasemi

THERMAL MODELING OF UPWARD WALL FLAME SPREAD.

This paper describes an engineering model of the upward turbulent flame spread along a vertical combustible surface based on a concept of ignition and flame spread as a result of inert heating of the solid to an ignition temperature. Experiments were made by using porous line burners to represent the wall flame heat transfer as a function of heat release rate and pyrolysis length. An exploratory analysis was made to correlate flame spread properties with thermometric material properties based on this model. This analysis seems to be consistent with current experimental work on turbulent wall flames.

Surface temperature at ignition of wooden based slabs

Up to now, a sufficiently developed ignition theory is not available for wood slabs and other combustible building materials in the form of layers. Very often, the concept of surface ignition temperature as a material characteristic is used, especially when related to fire modelling. Measurements of surface temperature at ignition during cone calorimeter tests for several plywoods and particle board in the horizontal and vertical configurations are presented here. It was found that the surface temperature at ignition depends not only on the material properties but also strongly on the experimental conditions (irradiation, sample configuration). There is evidence that two different mechanisms control ignition in the horizontal and vertical orientations of the samples.

STUDY ON FLAME HEIGHT OF MERGED FLAME FROM MULTIPLE FIRE SOURCES

A series of experiments to study merged flame from multiple fire sources was carried out. The porous 15-cm2 burner was used as a unit burner and propane was employed as a fuel. Many burners with various heat release rates were placed in a square configuration with various separation distances. Flame height was measured using video images and temperature distribution with height was measured using thermocouples. The numerical results from the Fire Dynamics Simulator (FDS) of the National Institute of Standards and Technology was compared with the experimental data, and the comparison results show that FDS correctly simulates the merged flame from multiple fire sources. In addition, an empirical model to estimate the merged flame height was developed considering the effect of separation distance. The validity of this model was confirmed from the comparison with the simulation results, and excellent agreement is found between the calculated merged flame height from the model and the numerical ones. To study the detailed structure of merged flame, 3 × 3 fire source configuration was taken as an example to quantitatively show oxygen mass fraction profiles and velocity fields, which were also given to indicate the formation mechanism of the merged flame.

Unsteady-state Upward Flame Spreading Velocity Along Vertical Combustible Solid And Influence Of External Radiation On The Flame Spread

To use wood as a building interior material, it is necessary to understand the fire-resistant qualities of wood. This paper describes the experiments conducted to investigate the ignitability and combustibility of wooden interior materials. In general, wood has complicated structure and composition. Moreover, the structures and constituent elements from different types of trees differ. Therefore, the ignitability and combustibility of wood are greatly affected by the physical and chemical properties. In this paper, we focuses on density, the most important physical index of wood, because density has an extremely intricate relationship with the other physical properties of wood and is the most representative index of the ignitability and combustibility of wood. With an aim of expressing the fire-resistant qualities of wood with density, the relationship between density and the ignitability and combustibility of wood was investigated. Assuming that when the surface temperature of the wood approached the ignition point it would ignite, ignition problems can be lumped together and dealt as thermal problems of the wood. Based on an equation of thermal conductivity, a quantitative model was prepared to represent time of ignition. Several other assumptions were made, and the model was simplified. Cone calorimeter tests were conducted on 8 types of wood to investigate the relationship between the ignitability and combustibility of these materials on the one hand, and density on the other. From the experimental results, the model coefficients were derived. The model obtained was expressed only in terms of wood density and heat flux, allowing us to make highly accurate predictions of time of ignition. Although the total heat release (THR) due to combustion showed a positive correlation with density, it was apparent that other factors besides density were also influential.

Similarity solutions and applications to turbulent upward flame spread on noncharring materials

Abstract The primary achievement in this work has been the discovery that turbulent upward flame spread on noncharring materials (for pyrolysis lengths less than 1.8 m) can be directly predicted by using measurable flammability parameters. These parameters are: a characteristic length scale which is proportional to a turbulent combustion and mixing related length scale parameter ( q ″ net ( ΔH c ΔH v ))2, a pyrolysis or ignition time τp, and a parameter which determines the transient pyrolysis history of a non-charring material: λ = L c ΔT p = ratio of the latent heat to the sensible heat of the pyrolysis temperature of the material. In the length scale parameter, q ″ net is the total net heat flux from the flames to the wall (i.e., total heat flux minus reradiation losses), ΔHc is the heat of combustion and ΔHc is an effective heat of gasification for the material. The pyrolysis or ignition time depends (for thermally thick conditions) on the material thermal inertia, the pyrolysis temperature, and the total heat flux from the flames to the wall, q ″ fw . The present discovery was made possible by using both a numerical simulation, developed earlier, and exact similarity solutions, which are developed in this work. The predictions of the analysis have been validated by comparison with upward flame spread experiments on PMMA. The present results are directly applicable for pyrolysis lengths less than 1.8 m over which experiments in practical materials show that the total (radiative and convective) heat flux to the wall from the flames is a function of the height normalized by the flame height ( Z Z f ) having a maximum value that is nearly constant for many materials; this profile is approximated in the work by a uniform profile of constant heat flux over the flame length, without loss of generality or violation of the physical situation. As the pyrolysis length increases (> ∼ 1.8 m), radiation dominates and a different total wall heat flux distribution applies. For this case a numerical simulation, such as FMRCs upward Flame Spread and Growth (FSG) code, can be used to predict upward flame spread rates while the present correlations can provide an upper bound for the flame spread rate.

Upward Flame Spread along a Vertical Solid for Transient Local Heat Release Rate

Theoretical and experimental analysis of turbulent upward flame spread along a vertical solid is presented A conventional flame spread model based on a linearized flame height approximation is generalized to incorporate burnout effects. The model is verified against full-scale flame spread tests using material heat release data obtained from intermediate-scale tests in which the time history of surface heat flux during the flame spread tests was reproduced.

EXPERIMENTAL FLAME CORRELATIONS AND DIMENSIONAL RELATIONS IN TURBULENT CEILING FIRES

Flame size and heat flux correlations were obtained by experiments for circular turbulent flame sheets developing from a downward injection source beneath an unconfined inert ceiling, and are compared against those for one-dimensional ceiling flames. These correlations show proportionality of the flame sheet area to heat release rate and the representation of flame heat transfer as a function of the distance from the source normalised by flame length. Heat release rate per unit flame sheet area in circular flames is found to be significantly smaller than that in one-dimensional flames. It suggests a weaker entrainment of ambient air to circular flames than to one-dimensional flames. Total heat flux to the ceiling surface from the flame sheet is less than 30 kW/m 2 and is not enough to accelerate flame spread. This suggests the importance of preheating of a combustible ceiling by a hot gas layer for the fast fire spread generally observed in real and experimental room fires. Dimensional analysis based on the experimental results suggests the proportionality of horizontal velocity to the distance from the upstream end of the burning surface, and faster velocity in one-dimensional flames than in circular flames.

Relationship between heat of combustion, lignin content and burning weight loss

The measurements of effective heat of combustion obtained from the Cone Calorimeter test for several wood composites (different types of plywood and particle board) at horizontal configuration are presented. Comparison of the average effective heat of combustion at different irradiation shows no correlation to gross heat of combustion measured in the oxygen bomb calorimeter. It was also found, that for the materials studied, there is no statistically significant correlation of heat of combustion to lignin content, but on the other hand, there is an evidence of correlation to the burning weight loss of the samples, but further investigation is necessary. Additionally, the effective heat of combustion is shown as a function of time for different external radiant heat-flux level for the chosen materials. Similar profiles have been found for remaining samples. Two different types of such curves can be distinguished.

MOTOR SCHEMA-BASED CELLULAR AUTOMATON MODEL FOR PEDESTRIAN DYNAMICS

A new cellular automaton model for pedestrian dynamics based on motor schema is presented. Each pedestrian is treated as an intelligent mobile robot, and motor schemas including move-to-goal, avoid-away and avoid-around drive pedestrians to interact with their environment. We investigate the phenomenon of many pedestrians with different move velocities escaping from a room. The results show that the pedestrian with high velocity have predominance in competitive evacuation, if we only consider repulsion from or avoiding around other pedestrians, and interaction with each other leads to disordered evacuation, i.e., decreased evacuation efficiency. Extensions of the model using learning algorithms for controlling pedestrians, i.e., reinforcement learning, neural network and genetic algorithms, etc. are noted.

Prediction of the formation of backdraft in a compartment based on large eddy simulation

Purpose – To investigate the fluid structure of gravity current in backdraft consisted of the hot gas and the ambient air, to predict the ignition time for backdraft and to study the effect of opening geometries on the ignition time.Design/methodology/approach – Numerical models based on large eddy simulation in fire dynamics simulator are adopted to study the ignition time.Findings – The density (temperature) profiles and velocity fields from the numerical simulation show the typical fluid structure of gravity current, i.e. the slightly raised head, the billows formed behind the head and the lobes and clefts at the leading edge. The increased mixing of gravity current by the ceiling opening geometries comparing to the mixing by the end opening geometries is a result of the three‐dimensional flow. The non‐dimensional velocity presented here is independent of the different normalized density differences, and only depends on the different opening geometries. From this result, it is feasible to predict the i...