In pulverizing systems, elevated ambient temperatures make coal dust deposited on hot surfaces prone to spontaneous ignition; once entrained by airflow into the surrounding space, such deposits may further evolve into coal dust cloud explosions, posing major risks to safe operations. The objective of this study is to reveal the critical conditions and kinetic characteristics under which self-ignition of deposited coal dust initiates explosions in such systems.

In this study, a coal dust combustion—explosion experimental platform was combined with Fluent numerical simulations to conduct a systematic investigation of the self-ignition, ejection, and explosion processes of deposited coal dust under heated environments, as well as behavioral characteristics of these processes. First, a coal dust combustion—explosion experimental system capable of precise control of ambient temperature and hot-plate temperature was established; on this basis, the internal temperature distribution of coal dust layers at different hot-surface temperatures and the critical conditions for self-ignition were examined. Second, ejection tests were conducted with dust-layer center temperatures of 260-380 ℃ to analyze the formation, ignition, and explosion of deposited coal dust clouds. Finally, based on the experimental results, a Fluent-based numerical model was developed for the post-lofting processes, including moisture evaporation, devolatilization, gasphase combustion, and char combustion. The particle trajectories, velocities, and temperature evolution during lofting were analyzed, and the critical oxygen concentration and dust concentration required to induce an explosion were determined.

Experimental results show that the hotplate temperature required for thermal runaway decreases with increasing dust-layer thickness. For a 4 mm coal dust layer, the critical hotplate temperature for self-ignition is 255 ℃; when the thickness increases to 10 mm, this temperature drops to 225 ℃. When the internal temperature of a self-ignited dust layer lies within 275-395 ℃ (corresponding to a center temperature of 300-340 ℃), the resulting coal dust cloud can trigger an explosion. The lofting process can be divided into three stages—rapid ejection, decelerating diffusion, and free diffusion—with explosions occurring mainly in the latter two; the maximum particle velocity is approximately 60 m/s, and the peak temperature is approximately 2400 ℃. Mechanistically, volatiles released from coal particles undergo homogeneous combustion outside the particles, whereas char undergoes heterogeneous combustion within the particle interior; these simultaneous phenomena significantly elevate the flame-core temperature. As the ambient temperature increases, both the critical coal dust mass concentration and the critical oxygen concentration for explosion decrease: when the ambient temperature rises from 120 ℃ to 200 ℃, the critical dust cloud concentration decreases from 380 to 95 g/m³, and the critical oxygen concentration decreases from 21% to 13%.

Combustible gases (e.g., CO) generated during the self-ignition stage accumulate and are subsequently ignited by high-temperature particles, initiating gas-phase combustion. The resulting heat release then ignites suspended coal dust particles, triggering solid-phase combustion. This sequence constitutes a critical pathway by which self-ignition escalates into an explosion. These findings provide a theoretical basis for explosion prevention and control in pulverizing systems and offer practical guidance for risk mitigation measures, including hot-surface temperature control, dust-layer thickness management, ventilation and oxygen concentration limits, and operational strategies that minimize the lofting of preheated deposits.

To elucidate the evolution patterns and hazard characteristics of overheating-induced short-circuit faults in energized conductors under external thermal radiation, this study systematically investigated critical heat flux, characteristic temperatures, fault initiation time, insulation resistance evolution, short-circuit current-voltage waveform characteristics, and arc energy variation. The objective was to identify key parameters for the early warning of electrical fires during conductor short-circuit failure under varying thermal radiation intensities. The findings aim to provide experimental evidence for risk identification and assessment of electrical circuit failures in high-temperature environments.

Using an electrical fault simulation apparatus, stable thermal radiation intensities of 23—32kW·m^-2 were applied to ZR-RVVB conductors operating under rated current conditions. Parameters including surface and internal temperatures, insulation resistance, pre-and post-short-circuit current and voltage waveforms, and fault occurrence times were recorded simultaneously. Thermal failure stages were defined using temperature-time curves. Short-circuit types were classified through waveform and time-frequency domain analyses, and short-circuit arc energy was calculated based on voltage-current integration. Comparative analyses were conducted to determine parameter variation patterns across different thermal radiation intensities.

Experimental findings indicated that 23kW·m^-2 represents the minimum critical thermal radiation intensity that causes short-circuit failures in ZR-RVVB conductors under rated current conditions. With increasing thermal radiation intensity, the conductor temperature rise showed four stages: transient thermal shock, accelerated pyrolysis, critical failure, and thermal steady state. The initial pyrolysis temperature (T₁), peak temperature (T₂), short-circuit trigger temperature (T_sc), and steady-state temperature (T₃) increased approximately linearly with increasing heat flux. Meanwhile, the duration of each stage decreased with increasing heat flux, showing a power-law relationship. This reduction is associated with faster heating and accelerated insulation degradation under higher thermal radiation intensities. Notably, the short-circuit trigger time shortened from ~1053.4s to 172.4s. At a heat flux of 32kW·m^-2, the insulation resistance dropped rapidly to ~0GΩ within 180s. Overall, insulation resistance declined significantly with increasing thermal radiation intensity. A reduction below ~1GΩ signaled imminent insulation failure. Fault mechanisms transitioned from metallic short circuits to carbonization path-type and arc-type faults as the thermal radiation intensity increased. At a heat flux of 25kW·m^-2, metallic short circuits were the dominant failure mode, accounting for ~70% of failures. When the heat flux exceeded 26kW·m^-2, the frequency of carbonization path-type faults increased significantly, peaking near 31kW·m^-2. Time-frequency energy analysis indicated that arc-type short circuits exhibited the highest high-frequency energy characteristic parameters, with a high-frequency energy peak (HHF) of 0.860 and a high-frequency energy ratio (RHF) of 0.416, both of which were significantly higher than those of the other two fault types. Energy released after short-circuit increased significantly with increasing thermal radiation intensity; arc-type faults released the highest energy (approximately ~9324.89J), followed by carbonization path-type faults. Meanwhile, metallic short circuits released the least energy. This indicated that higher thermal radiation intensities lead to greater short-circuit energy release and increased destructive potential.

This study characterized the temperature rise behavior, insulation resistance evolution, fault type transitions, and energy release characteristics of energized conductors under varying thermal radiation intensities. The findings provide a foundation for rapid short-circuit fault classification and the development of early-warning models.

The study of fire spread behavior in wires holds significant importance for guiding fire safety measures in variable pressure environments such as nuclear power plants, aerospace applications, hyperbaric oxygen chambers, and high-altitude areas. Currently, there is a lack of comprehensive research focused on variable pressure environments widely applied in spacecraft, high-altitude regions, nuclear power plants, and civilian hyperbaric oxygen chambers. Therefore, this study investigates the fire spread and molten droplet behavior of polyethylene (PE) wires under variable pressure conditions ranging from 40 to 500kPa using a self-built experimental platform.

This study selected typical thermoplastic PE wires as the research subject and constructed a fire spread experimental platform to investigate the effects of variable pressure conditions (40-500kPa) and oxygen volume fractions of 21% and 30% on fire spread behavior. Simulations were conducted using the solidification/melting model in Fluent software to analyze the melting and dynamic motion of molten material suspended on a metal plate.

1) Within the 40-100kPa range, a low-luminosity blue flame appears at the base of the flame and fades as pressure increases. For an oxygen volume fraction of 21%, the bottom blue flame disappears after 100kPa. For a 30% oxygen volume fraction, it vanishes at 60kPa. At pressures from 100kPa to 500 kPa, the blue region at the bottom of the flame disappears, flame brightness intensifies. The top flame color changes from bright yellow to orange, and soot production increases. Increasing the oxygen volume fraction from 21% to 30% reduces the orange region at the flame's top and decreases black soot in the upper section, reshaping the flame into a more triangular form. During this stage, the fire spread rate and mass loss rate increase significantly; 2) As pressure increases, the flame width of the PE wire decreases. At 500kPa, the flame width measures 2.0 and 2.2cm for oxygen volume fractions of 21% and 30%, respectively. Flame height increases with pressure, peaking at 500kPa. At oxygen volume fractions of 21% and 30%, the maximum flame heights are 3.7 and 4.8cm, respectively; 3) At an oxygen volume fraction of 21%, molten dripping occurs in the 40-80kPa range. However, at an oxygen volume fraction of 30%, molten dripping ceases above 60kPa. Simulations reveal that molten droplets form 4.4s after PE is heated and separate from the main body under gravity with a maximum velocity of approximately 22cm/s. Surrounding airflow exhibits a spiral motion during droplet detachment.

This study primarily reveals the fire spread and molten droplet behavior of PE wires under different pressure conditions, providing a foundation for predicting and preventing fire development in PE wires found in variable pressure environments.