Fire behaviors along timber linings affixed to tunnel walls in mines

Timber linings are applied as primary supports in the tunnel fault and fracture zones of mines. These linings are essential to prevent broken rock from falling during the occurrence of exogenous fires. In this study, experiments and numerical simulations were carried out using a fire dynamics simulator to investigate the flame-spread rate, flame characteristics, smoke movement, and spread process of timber-lining fires under different wind speeds of 0, 0.25, 0.5, and 0.75 m/s. It was found that cross-section flame spreading follows the three-stage sidewall-ceiling-sidewall pattern. Moreover, the average flame-spread rate increases along the vertical flame-spreading direction and decreases when the flame reaches the timber-lining corners. Moreover, the flame lengths underneath the timber-lining ceiling in the x-direction are longer than those in the y-direction. As the wind speed increases, the normalized flame lengths R(f) in the two directions decrease, and the maximum temperature underneath the ceiling decreases. In addition, the maximum temperature in the three tunnel sections of interest is first recorded in the tunnel cross-section in the initial fire stage. Higher wind speeds correspond to farther distances of the maximum-temperature points of the three timber-lining sections from the fire source.

Experimental investigations on two-sided upward flame spreading over thin flax fabric under the influence of restricted distance

Up to 2021, most previous work focused on upward flame spread over thin solid fuel completely attached to objects or with both sides freely exposed to the air, but did not take the restricted distance (distance between fuel and objects) effects into account. In this paper, the restricted distance effects on upward flame spread over thin solid fuels were investigated using 0.65 mm thick, 120 cm tall and 6.0 cm wide flax fabric sheets under various restricted distances of 1.0–3.5 cm. The essential parameters were monitored and analyzed simultaneously, including flame length, pyrolysis spread rate, surface temperature and ignition time. The main conclusions drawn are as follows: when the restricted distance is no more than 1.5 cm, the flame length on the unrestricted side is larger than that on the restricted side, whereas the variation exhibits the opposite trend when the restricted distance is beyond 1.5 cm. As the restricted distance increases from 1.0 to 3.5 cm, the flame length and flame spread rate first increase and then decrease, reaching a maximum value at 3.0 cm restricted distance, whereas the ignition time shows the opposite trend. The decrease rate of the surface temperature with the distance from the pyrolysis front first drops and then rises as the restricted distance increases, which qualitatively characterizes that the heat flux received by the virgin surface first increases and then decreases with restricted distance. The non-monotonic trends of heat flux received by the virgin surface and consequently the flame spread rate as a function of restricted distance are due to the combined restricted distance effects of the chimney effect, wall radiation and restricting oxygen supply. The results of this paper are not only helpful in better understanding the upward flame spread over a thin flax fabric under restricted distance, but also provide some basic data for fire prevention of thin solid fuels.

Using an Artificial Neural Network to Predict Flame Spread across Electrical Wires

There is currently a global-scale transition from fossil fuel energy technologies towards increasing use of electrically driven energy technologies, especially transportation and heat, fueled by renewable energy sources, which is making fire safety in electrically powered systems increasingly important. The work presented here provides a coherent understanding of flame spread parametric trends and associated fire safety issues in electrical systems for structural, transportation, and space applications. This understanding was obtained through use of an artificial neural network (ANN) that was trained to predict the flame spread rate along “laboratory” wires of different sizes and compositions (copper, nichrome, iron, and stainless-steel tube cores and HDPE, LDPE, and ETFE insulation sheaths) and exposed to different ambient conditions (varying flows, pressure, oxygen concentration, orientation, and gravitational strength). For these predictions, a comprehensive data base of 1200 data points was created by incorporating flame spread rate results from both in-house experiments (400 data points) as well external experiments from other sources (800 data points). The predictions from the ANN showed that it is possible to merge together various data sets, including results from horizontal, inclined, vertical, and microgravity experiments, and obtain unified predictive results. While these initial results are very encouraging with an overall average error rate of 14%, they also show that future improvements to the ANN could still be made to increase prediction accuracy.

Thermodynamic and Kinetic Characteristics of Combustion of Discrete Polymethyl Methacrylate Plates with Different Spacings in Concave Building Facades

Polymethyl methacrylate plates are widely applied to buildings, producing significant fire hazards. It lacks a theoretical basis for the fire risk assessment of polymethyl methacrylate in concave building facades. Therefore, experimental methods are used to investigate combustion characteristics of discrete polymethyl methacrylate plates in a concave building facade. Influences of fuel coverage and structure factor are investigated, which is scant in previous works. When structure factor is invariable, average flame height increases first and then decreases as fuel coverage increases, and the turning point is between 0.64 and 0.76. In total, three different patterns of pyrolysis front propagation are first observed for different fuel coverages. Flame spread rate first increases and then decreases as fuel coverage rises, and the turning point is also between 0.64 and 0.76. When fuel coverage is invariable, the flame spread rate first increases and then decreases with increasing structure factor, and the turning point is 1.2. A model for predicting the flame spread rate of discrete polymethyl methacrylate is also developed. The predicted values are consistent with experimental results. Fuel spread rate of discrete polymethyl methacrylate rises as the fuel coverage increases. The above results are beneficial for thermal hazard evaluation and fire safety design of polymethyl methacrylate used in buildings.

Experimental study on flame propagation over overload-wire under varying inclination Angle

To better understand the process of fire caused by conducting wire, based on the study of overload of the low-voltage wire, the theoretical analysis of flame spread mechanism of overload-wire was proposed, and the functional relationship between flame shape characteristics and flame spread speed, current, and inclination angle was studied. The results show that: (1) the theoretical model of flame propagation can well reflect the changes of thermodynamic parameters in the process of flame propagation, and it is in better agreement with the experimental results. (2) When the current value is constant, with the increase of the inclination angle of the wire (0°-90°), the flame is elongated along the wire direction, the width of the flame base increases, and the angle between the flame front and the wire decreases. When the inclination angle is fixed, with the increase of the inclination angle of the conductor, the flame shape becomes more "high and wide" and the flame height increases at the same time. (3) When the current is constant, the flame spread rate increases with the increase of wire inclination angle; when the inclination angle is constant, the flame spread rate decreases sharply with the increase of current.

Experimental Study on Flammability and Flame Spread Characteristics of Polyvinyl Chloride (PVC) Cable

Polyvinyl chloride (PVC) is widely applied in cables as insulation materials, which are vital for operation and control of industrial processes. However, PVC cables fires frequently occur, arousing public concern. Therefore, experimental methods are used to study flammability and flame-spread characteristics of PVC cable in this paper. Influences of cable structure and number are investigated, which is scanty in previous works. As cable core number of single cable or cable number of multiple cables rises, average flame height and width increase while the increment decreases. Formulas concerning dimensionless flame height and single cable diameter (or total width of multiple cables) are obtained. The former is negatively correlated with the latter. For single cable, convective heat transfer is dominant, and flame-spread rate decreases as cable core number increases. Cable maximum temperature, which drops first and then rises as cable core number increases, is observed in the cable core area. For multiple cable, the flame-spread rate increases as cable number increases. As the cable number rises, the length of pyrolysis and combustion zone increases while the maximum temperature of cable surface decreases. This work is beneficial to fire hazard evaluation and safety design of PVC cables.

Effects of porosity and area density on upward flame spread characteristics over thin flax fabric

Few investigations have systematically addressed the porosity effects of upward flame spread over fabric fuels, although the porosity is a special property for fabric fuels. The present paper studies the porosity and area density effects on upward flame spreading using 160.0 cm tall and 8.0 cm wide flax fabric samples with various porosities and area densities. The flame shape, flame length, flame spread rate, ignition time, standoff distance and surface temperature distribution are obtained and analyzed. The major findings are summarized as follows: as the porosity increases and corresponding area density declines, the flame spread rate and flame length increase, whereas the ignition time decreases, which is because the oxygen can reach the fuel surface in the pyrolysis region more easily and, subsequently, the heat flux received by the virgin fuels increases. The two parameters of flame standoff distance and surface temperature in the preheating region can be applied to characterize the heat flux received by the virgin fuels. Generally, when the porosity increases and the corresponding area density decreases, the flame standoff distance and the surface temperature at the same distance from pyrolysis front increase, which reveals that the heat flux received by the virgin surface increases.

Numerical Study of the Effects of Confinement on Concurrent-Flow Flame Spread in Microgravity

The objective of this work is to investigate the aerodynamics and thermal interactions between a spreading flame and the surrounding walls as well as their effects on fire behaviors. A three-dimensional transient computational fluid dynamics (CFD) combustion model is used to simulate concurrent-flow flame spread over a thin solid sample in a narrow flow duct. The height of the flow duct is the main parameter. The numerical results predict a quenching height for the flow duct below which the flame fails to spread. For duct heights sufficiently larger than the quenching height, the flame reaches a steady spreading state before the sample is fully consumed. The flame spread rate and the pyrolysis length at steady-state first increase and then decrease when the flow duct height decreases. The detailed gas and solid profiles show that flow confinement has multiple effects on the flame spread process. On one hand, it accelerates flow during thermal expansion from combustion, intensifying the flame. On the other hand, increasing flow confinement reduces the oxygen supply to the flame and increases conductive heat loss to the walls, both of which weaken the flame. These competing effects result in the aforementioned nonmonotonic trend of flame spread rate as duct height varies. Near the quenching duct height, the transient model reveals that the flame exhibits oscillation in length, flame temperature, and flame structure. This phenomenon is suspected to be due to thermodiffusive instability.

Concurrent-Flow Flame Spread Over a Thin Solid in a Narrow Confined Space in Microgravity

A numerical study is pursued to investigate the aerodynamics and thermal interactions between a spreading flame and the surrounding walls as well as their effects on fire behaviors. This is done in support of upcoming microgravity experiments aboard the International Space Station. For the numerical study, a three-dimensional transient Computational Fluid Dynamics combustion model is used to simulate concurrent-flow flame spread over a thin solid sample in a narrow flow duct. The height of the flow duct is the main parameter. The numerical results predict a quenching height for the flow duct below which the flame fails to spread. For duct heights sufficiently larger than the quenching height, the flame reaches a steady spreading state before the sample is fully consumed. The flame spread rate and the pyrolysis length at steady state first increase and then decrease when the flow duct height decreases. The detailed gas and solid profiles show that flow confinement has competing effects on the flame spread process. On one hand, it accelerates flow during thermal expansion from combustion, intensifying the flame. On the other hand, increasing flow confinement reduces the oxygen supply to the flame and increases conductive heat loss to the walls, both of which weaken the flame. These competing effects result in the aforementioned non-monotonic trend of flame spread rate as duct height varies. This work relates to upcoming microgravity experiments, in which flat thin samples will be burned in a low-speed concurrent flow using a small flow duct aboard the International Space Station. Two baffles will be installed parallel to the fuel sample (one on each side of the sample) to create an effective reduction in the height of the flow duct. The concept and setup of the experiments are presented in this work.