Haixiang Chen

Thermogravimetric analysis of peat decomposition under different oxygen concentrations

Smoldering combustion of peat is of global concern as a natural hazard to consume sequestered carbon and form wide-area haze. It is affected by thermal decomposition kinetics of peat and the diffusion and availability of oxygen. In this work, thermal decomposition behavior of peat was investigated using thermogravimetric analysis under the atmosphere with different oxygen concentrations. The results showed that thermal decomposition process of peat could be divided into three stages: dehydration, oxidative pyrolysis of organic matters into volatiles and char, and oxidation of the generated char. The apparent activation energies of peat decomposition under different oxygen concentrations were calculated by model-free methods of Kissinger, FWO, Starink, Gyulai, and Friedman. A two-step reaction model was proposed to describe thermal decomposition kinetics of peat (excluding dehydration stage) and the effect of oxygen concentration on the kinetic parameters was discussed. These results provide basic data for smoldering modeling of peat.

Interaction between flaming and smouldering in hot-particle ignition of forest fuels and effects of moisture and wind

Ignition of natural fuels by hot metal particles from powerlines, welding and mechanical processes may initiate wildfires. In this work, a hot steel spherical particle (6–14 mm and 600–1100°C) was dropped onto pine needles with a fuel moisture content (FMC) of 6–32% and wind speed of 0–4 m s–1. Several ignition phenomena including direct flaming, smouldering and smouldering-to-flaming transition were observed. The critical particle temperature for sustained ignition was found to decrease with the particle size (d) and increase with FMC as (°C), and the maximum heating efficiency of particle was found to be . As the particle size increases, the influence of FMC becomes weaker. The flaming ignition delay times for both direct flaming and smouldering-to-flaming transition were measured, and decreased with particle temperature and wind speed, but increased with FMC. The proposed heat-transfer analysis explains the ignition limit and delay time, and suggests that the hot particle acts as both heating and pilot sources like a small flame for direct flaming ignition, but only acts as a heating source for smouldering. This study deepens the fundamental understanding of hot-particle ignition, and may help provide a first step to understanding the mechanism behind firebrand ignition.

Two-step Consecutive Reaction Model of Biomass Thermal Decomposition by DSC

The thermal decomposition of one kind of biomass in air has been investigated by differential scanning calorimetry (DSC) analyzer. The results indicate that the heating process of samples from ambient temperature to 923 K at low heating rates shows two obvious exothermic peaks. According to the decomposition mechanism, the first exothermic step is attributed to oxidative degradation of hemicellulose and cellulose, and the second exothermic step is attributed to lignin degradation and char oxidation. The reaction model of the studied biomass thermal decomposition has been studied by iso-conversional methods and optimization computation. The results suggest that the two-step consecutive reaction model is suitable to describe the exotherm of biomass thermal decomposition in air.

Effect of slope on spread of a linear flame front over a pine needle fuel bed: experiments and modelling

This paper experimentally evaluates the effect of slope on spread of a linear flame front over a pine needle fuel bed in still air. The slope angle of the fuel bed varied from 0 to 32°. The fuel mass consumption in flaming fire spread, temperature over the fuel bed, velocities of the flow around the flame front and heat fluxes (total and radiant) near the end of the fuel bed were measured. The mass loss rate and rate of fire spread both increased with increasing slope, whereas the fuel consumption efficiency varied in the opposite way. It was shown that a weak reverse inflow and an upslope wind (induced by the flame itself) exist respectively ahead of and behind the flame front, and their significant difference in velocity (causing a pressure difference) plays an essential role in the forward tilting of the flame front. This mechanism promotes burning, especially on higher slopes. Natural convective cooling has a remarkable effect on the fuel pre-heating in the spread of linear flame fronts under slope conditions. A fire spread model for a linear flame front was developed to consider the natural convective cooling and the fuel consumption efficiency. The model agrees well with the experimental data on fire spread rate. Its reliability, especially for higher slopes, was verified by comparison with other models.

Experimental Study on Cross-ventilation Compartment Fire in the Wind Environment

When fire occurs in the rooms of high-rise buildings, the strong ambient wind will play an important role in fire spread and smoke movement behavior. However, wind effect on compartment fire in cross ventilation condition has not been fully studied so far. In the present study, an effort has been made to study crossventilation compartment fire in the wind environment through experimental investigations. The experimental fire was generated by 250ml (10cm×10cm tray burner) or 500ml (20cm×20cm tray burner) nheptane on the floor of a cube enclosure with two opposite vents on the walls. The inside and outside gas temperature profiles at different vertical and horizontal locations were recorded by two thermocouple matrixes. The ambient wind velocity was set to zero, 1.5m/s and 3m/s. It is observed that the ambient wind has two contradictory effects on the compartment fire: promoting fire severity by more oxygen supplying and cooling the fire by heat removing and combustible gases diluting. The spilled-out flame/plume extends horizontally farther with the increase of wind speed. It is found that the compartment fire with 500ml fuel reaches post-flashover stage while that with 250ml doesn’t. The wind effect is obviously observed in larger fires while not significant in smaller fires.

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.

Progress Of Fundamental Fire Research In China

A fire-year research project “Fire Dynamics and Fundamentals of Fire Protection” (20022007) has been sponsored by the China NKBRSF (National Key Basic Research Special Funds), in order to gain deep understandings of natural fire phenomena and whereby facilitate the development of practical fire safety design/engineering, management and firefighting technologies. The project has been carried out for more than two years since 2002, with successful establishment of a multidisciplinary collaboration network of most famous China universities and institutions in the field of fundamental fire research. This review summarizes the new significant progress of the project during 2002-2004. A prospect of fundamental fire research in China is also presented.

A method to assess the wildfire induced breakdown of high-voltage transmission lines

Wildfire is responsible for many breakdown faults of high voltage transmission lines. How to evaluate the risk of wildfire induced breakdown is of concern to the line managers. This paper presents a risk assessment method of wildfire induced breakdown of high voltage transmission lines, by combination of the wildfire model and the breakdown mechanism. The method is consisted of three modules. The wildfire module predicts the fire characteristic parameters, the breakdown module provides the electric field strength of breakdown, and the risk assessment module evaluates the breakdown risk according to two typical breakdown scenarios. A case study gives a preliminary evidence of the methods validity. Since the method is module based, it can be updated easily and assess the breakdown risk induced by different fire types.

Heat Loss and Kinetic Effects on Extinction and Critical Self-Sustained Propagation of Forced Forward Smoldering

This work studied the forced forward smoldering mechanism of foam by a transient one-dimensional numerical model. A three-step reaction scheme, including foam oxidation and pyrolysis, char oxidation was adopted. Based on the first principles of mass, momentum, and energy, the mass fractions of solid and gas species as well as temperature evolutions in self-propagation regime were examined. Foam oxidation and pyrolysis fronts were found to be ahead of char oxidation front where oxygen is not completely consumed. The critical kinetic parameters for self-sustained smoldering were determined. It was found that pyrolysis endothermic reaction, heat losses, and limited kinetics of char oxidation are favorable for smoldering extinction. Especially, smoldering extinction is more sensitive to kinetic parameters of char oxidation reaction than that of other two reactions. The halfway quenching of smoldering is mainly due to the weakening of char oxidation reaction, which cannot offset the pyrolysis endothermicity and heat loss to environment.

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