Lizhong Yang

The pyrolysis and ignition of charring materials under an external heat flux

Abstract Experimental measurements have tested the effect of an external heat flux on the pyrolysis and combustion of charring materials using a cone calorimeter and a radiation platform, particularly with a small heat flux. Differences in the pyrolysis and burning of wood under strong and weak heat fluxes are discussed. Also, a modified model of pyrolysis for charring materials in a fire is proposed. In this model some special factors that affect pyrolysis, such as heat loss by convection and radiation caused by the surface temperature rising and also shrinkage of the char’s external surface, are considered. The pyrolysis of wood and the time to ignition is predicted using the model, which is also used to explain the mechanism of the pyrolysis of wood exposed to weak external fluxes for long times.

Experimental study of back-draft in a compartment with openings of different geometries

Abstract This paper presents the results of reduced-scale experimental tests to study back-draft in a reduced-scale compartment (1.2 m × 0.6 m × 0.6 m), fitted with six different geometries for the opening on one end-wall and with two geometries for the opening in the ceiling. The experimental variables included the flow rate, the time during which the fuel was burned, and the geometries of the inlet. The quantities recorded before the back-draft included temperature and the concentrations of oxygen, carbon dioxide, and carbon monoxide. To quantify the effect of back-draft, the gas velocities in the inlet and also the pressures in the compartment were measured. The effects of different inlet geometries on the occurrence of back-draft are discussed. This study shows that the mass fraction of total hydrocarbons (i.e, the un-burned methane in this study), whose critical value varies with the geometry of the inlet, is a key parameter determining the occurrence of back-draft.

Simulation of evacuation behaviors in fire using spacial grid

Abstract A two-dimensional Cellular Automata (CA) model to demonstrate the special phenomena of occupants evacuating from fire room is presented. A set of simple but effective models is proposed to investigate the effect of fire smoke on route choice. The concept of danger grade is introduced, and occupants select the target cell according to the value of danger grade at each time step. Some technique is introduced to substitute the human intelligence, such as premeditation. The simulation results show that human evacuation is influenced greatly by both human visual field and building exit.

Investigation of combined stairs elevators evacuation strategies for high rise buildings based on simulation

Abstract Based on computer modeling and simulation, the problem of evacuation strategies that utilize a combination of stairs and elevators for high rise buildings is investigated in this paper. First, by using the Pathfinder software, a 28-storey building model with stairs and elevators is established. Then, based on the obtained model, several evacuation scenarios are designed for researching this issue. The simulation results show that the optimal percentages of the occupants evacuated by the elevators, when achieving the shortest evacuation time, is almost not related to the number of evacuated persons and floors. Furthermore, the evacuation time decrease achieved by increasing the elevator maximum speed is not as much as that obtained by increasing the number of elevators. Moreover, when evacuating different age groups, it is found that selecting the aged (slow-moving) people, who may create congestion in the stairs, to be evacuated by elevators can speed up the evacuation efficiently, and selecting the children to be evacuated by elevators can improve the utilization rate of the elevators. Finally, the evacuation with a priority order from lower floors to upper floors is considered. Results show that the evacuation with a priority order from lower floors to upper floors can effectively decrease the congestion on the stairs caused by the aged people, and achieve a faster evacuation rate during most of the evacuation process, although the total evacuation time increases a little.

Experimental Studies on the Flammability and Fire Hazards of Photovoltaic Modules

Many of the photovoltaic (PV) systems on buildings are of sufficiently high voltages, with potential to cause or promote fires. However, research about photovoltaic fires is insufficient. This paper focuses on the flammability and fire hazards of photovoltaic modules. Bench-scale experiments based on polycrystalline silicon PV modules have been conducted using a cone calorimeter. Several parameters including ignition time (tig), mass loss, heat release rate (HRR), carbon monoxide (CO) and carbon dioxide (CO2) concentration, were investigated. The fire behaviours, fire hazards and toxicity of gases released by PV modules are assessed based on experimental results. The results show that PV modules under tests are inflammable with the critical heat flux of 26 kW/m2. This work will lead to better understanding on photovoltaic fires and how to help authorities determine the appropriate fire safety provisions for controlling photovoltaic fires.

INTERACTIONS OF PEDESTRIANS INTERLACED IN T-SHAPED STRUCTURE USING A MODIFIED MULTI-FIELD CELLULAR AUTOMATON

Little work has been done before in the study of separating pedestrian flow interlaced. Under open boundaries, the interaction of separating pedestrian flow interlaced in a T-shaped structure was simulated, using a modified multi-field cellular automaton updating synchronously. The free-jammed phase transition diagram of pedestrian flow and principles of the pedestrian interference were obtained. The movement of pedestrians is free flow in the low entrance density. While it is a complete jammed flow with the entrance density increasing to a certain level and little difference existing between the left moving probability and the right moving probability. Thus, the dominant factor influencing pedestrian flow is the interference of opposite pedestrian flows due to changing movement directions. And it is changing to an incomplete jammed flow with this difference increasing. Thus, the dominant factor is changing to the interference of the coincident pedestrian flow and the limitation of the bottleneck.

Experimental Study of the Influences of Orientation and Altitude on Pyrolysis and Ignition of Wood

On account of limited attention paid to the influence of orientation and altitude in previous research, the pyrolysis and ignition characteristics of wood in different orientations and altitudes were investigated experimentally. Two orientations were explored: vertical and horizontal. In addition, experiments were conducted in Lhasa and in Hefei, which are at very different altitudes. Ignition time, mass loss rate, and surface temperature are discussed. Compared with those in the horizontal orientation, shorter ignition times, faster mass loss rates, and lower surface temperatures were observed in the vertical orientation. In addition, there are shorter ignition times, faster mass loss rates, and higher surface temperatures in Lhasa (high altitude) than in Hefei (low altitude).On account of limited attention paid to the influence of orientation and altitude in previous research, the pyrolysis and ignition characteristics of wood in different orientations and altitudes were investigated experimentally. Two orientations were explored: vertical and horizontal. In addition, experiments were conducted in Lhasa and in Hefei, which are at very different altitudes. Ignition time, mass loss rate, and surface temperature are discussed. Compared with those in the horizontal orientation, shorter ignition times, faster mass loss rates, and lower surface temperatures were observed in the vertical orientation. In addition, there are shorter ignition times, faster mass loss rates, and higher surface temperatures in Lhasa (high altitude) than in Hefei (low altitude).

An Integral Model for Wood Auto-ignition under Variable Heat Flux:

A one-dimensional integral model that predicts the auto-ignition time of wood under linearly increasing radiant heat flux is developed. The solution to the integral model for the ignition time is compared with measured data. Reasonable comparative results are shown for the ignition time of wood exposed to different heat flux increase rates (HFIR). Both the solutions and the measured data also show that the ignition time can be expressed as a power function and the power exponent is about 0.67.

Turbulent smoke flow in evacuation staircases during a high-rise residential building fire

Purpose – The purpose of this paper is to study the behavior of smoke flow in a typical high-rise residential building fire in six common smoke control systems. Design/methodology/approach – The pressure, temperature and CO2 concentration were used to trace the motion of turbulent smoke flow through CFD. Findings – It is found that the hot smoke could rise up and spread into the indoor space on the upper floors through the staircase. When the pressure in the evacuation staircase is higher, it would be more difficult for the smoke to enter the staircase and transport vertically. On the other hand, the smoke would soon transport to the indoor space on the upper floors horizontally. During this process, the smoke shows a more disorder horizontal transport under the sole effect of thermal buoyancy than the co-existence of thermal buoyancy and the air inlet. Research limitations/implications – Because of the chosen research approach, the research results may need to be tested by further experiments. Practical ...

Experimental and theoretical analysis of acceleration of a gas flame propagating over a dust deposit

In the present work, we studied experimentally and theoretically the propagation of a methane-air flameand its interaction with a coal dust deposit. The experiments were conducted in a horizontal flame-acceleration tube, 1.48 m long and 89×89 mm square cross section. Photodiodes and a digital video camera were used to detect flame propagation. Flame propagation was analyzed with a one-dimensional flow model and a turbulent-burning-rate model. The results indicate that there are three distinguishable stages and an interesting propagation pattern of three transient fronts when a gaseous flame propagates over a dust deposit. In the first stage, the gas flame accelerates gradually until the dust deposits are dispersed into the air and heated by the gaseous products so that the dust ignites. In the second stage, a flame of three transient fronts—a leading gas flame, a forward dust flame, and a backward dust flame—was observed. At the last stage, a complex gas-dust flame forms when the dust flame is integrated into the gaseous flame. The results also show that, under the present conditions, the flame is accelerated slightly by the dust deposits but dramatically by obstacles. The terminal flame speed with obstacles A (blockage ratio, 0.05) or obstacle B (blockage ratio, 0.10) is about 4 to 16 times of that without obstacles. Because the dispersion of the dust is dominated by gas flow, the dust generally flies in the same direction as the flame propagation when the flame propagates from the closed end of the tube. However, the dust flies in the opposite direction when the flame travels from the open end of the tube.