Forest fires play a crucial role in the replacement of plant communities. However, climate change may significantly affect tree regeneration after severe wildfires, transform ecosystems and cause huge economic losses. Pneumatic fire extinguishers have become the primary portable firefighting equipment in China's mountainous and roadless regions due to their high mobility and simple operation. However, prolonged operation often leads to engine overheating, which reduces rotational speed, resulting in lower jet velocity, increased air outlet temperatures, and decreased firefighting efficiency. Extensive studies examine the interactions between air flow and gas-phase combustion, but mainly focus on the effects of ambient wind on pool fire combustion. The understanding of their dynamic mechanisms during engine overheating, which causes reduced air jet velocity and elevated air flow temperature, is still poorly understood. To address these gaps, this study numerically investigates the suppression effect of high-speed air jets from pneumatic fire extinguishers on gas-phase combustion, aiming to provide methodological references and data support for optimizing pneumatic fire extinguisher design and improving pneumatic firefighting strategies.

This study employed the scale-adaptive simulation (SAS) turbulence model and eddy dissipation model to simulate the gas-phase combustion of n-heptane pool fires under the influence of air jets. The SAS turbulence model, based on the modifications of the k-L turbulence equation, blends the advantages of the Reynolds-averaged Navier-Stokes (RANS) and large-eddy simulation (LES) models. It can dynamically adjust its turbulence length scale to balance the modeling and resolution of turbulence stress transport. In this study, the SAS turbulence model was used to simulate n-heptane pool fires in air jets. The oil pan height was increased by 100 mm to minimize the gap between the simulation and experimental results. The simulation of high-speed jet interaction with gas-phase combustion involves two methods: computational domain coupling of the pneumatic fire extinguisher and n-heptane combustion for data transfer, and boundary condition transfer from the fan outlet to serve as the jet inlet conditions in the n-heptane combustion computational domain. For computational efficiency, the second method was chosen. The stable n-heptane gas-phase combustion simulation results were used as the initial flow field, and the RANS results at the fan outlet (average velocity of approximately 80 m/s) were extracted and implanted into the interaction computational domain inlet.

The height increases of the oil pan improved model accuracy, keeping the SAS temperature error within an acceptable range. Results showed that high-speed airflow effectively suppressed gas-phase combustion. As the jet velocity increased, the flame shape underwent remarkable changes. At low velocities, the flame maintained a stable and continuous structure within a concentrated combustion area. However, as the velocity of the jet exceeded a certain threshold, it strongly impacted and disrupted the flame, which was stretched, distorted and ultimately extinguished. Furthermore, within the studied range, changes in the jet temperature had minimal impact on the gas-phase combustion of n-heptane. Regardless of the temperature, the flame structure and combustion efficiency remained similar at the same jet velocity. This is likely because physical dilution of the jet masks the sensitivity of the gas-phase combustion rate to the temperature. When the jet velocity dominates, minor temperature fluctuations cannot significantly alter the combustion process.

This study presents a simulation approach for analyzing gas-phase combustion suppression of n-heptane pool fires using high-speed airflow. The key findings are as follows: (1) jet velocity is the dominant factor in flame suppression. As the velocity increases, the kinetic energy of a jet increases, effectively diluting combustible gases and disturbing the flow field. This hinders fuel-oxidizer mixing and leads to flame breakup and suppression. However, increased jet velocities may cause fuel splashing in actual firefighting, so an optimal jet velocity range needs to be determined. (2) Within the studied range, the jet temperature exerts minimal impact on combustion. At the same velocity, jets with different temperatures produce similar flame structures and efficiencies, thus indicating that temperature regulation is not critical for suppression under these conditions. (3) The SAS turbulence model is effective for simulating air jet-gas-phase combustion interactions as it balances computational accuracy and cost, outperforming the RANS and LES models. Thus, it is suitable for further firefighting simulation studies. Overall, this study provides simulation methods and data references for optimizing pneumatic fire extinguishers. Future studies should focus more on combustion suppression effects under complex conditions and refine simulation methods to better suit actual firefighting scenarios.

The accurate combustion simulation of wood is essential for improving fire safety in architectural and wildland contexts. Existing studies, which predominantly rely on fire dynamics simulators (FDS), face considerable limitations, particularly in terms of grid adaptability for curved geometries and the oversimplification of pyrolysis models. These limitations often result in substantial deviations from experimental data, thereby reducing the reliability of fire safety predictions. This study develops a comprehensive simulation framework for wood ignition using ANSYS Fluent to address the above gaps. This framework is validated through controlled experiments to improve its predictive accuracy for fire dynamics.

The experimental phase of this study employed small cylindrical Finnish pine wood blocks, each with a diameter and length of 30 mm. The wood blocks had an average moisture content of 8.68% and an apparent density of 460.27 kg/m³. Thermogravimetric analysis (TGA) was conducted to quantify wood moisture content, which was found to be 8.87%, and pyrolysis conversion rate, which reached 0.745 at 500 ℃. Ignition tests were performed under a heptane flame, revealing mass loss ratios of 20%-50% within just 2 min. This remarkable mass loss was attributed to surface charring and the development of internal pyrolysis gradients. Combustion was further characterized by three distinct stages: an evaporation stage (Stage Ⅰ) marked by slow mass loss; a rapid pyrolysis stage (Stage Ⅱ) defined by accelerated degradation; and a slow mass decline stage (Stage Ⅲ), wherein the accumulation of a char layer inhibited further reactions. Postcombustion analysis highlighted the formation of a uniform 5 mm char layer, with internal conversion rate gradients showing a surface value of 19.14% and low internal values. These gradients were influenced by gas permeability and temperature distribution within the wood. In the numerical simulation phase, ANSYS Fluent was employed to model the complex multiphase processes involved in wood ignition. User-defined functions (UDFs) were developed to incorporate drying and pyrolysis. Wood components were simplified into moisture and organic matter, with porosity values of 0.676-0.679 derived from cell wall density measurements. Pyrolysis kinetics were modeled using a modified Arrhenius model, integrating parameters obtained from TGA. A virtual heat-exchange layer was introduced to adjust surface heating rates, effectively mimicking the insulating effect of water vapor observed in experiments. A rotating slip-grid method ensured the uniform heating of the wood sample. Meanwhile, the large eddy simulation was employed to capture the turbulent combustion of heptane. Radiation effects were modeled using the discrete ordinates approach, which was coupled with energy equations to account for stage changes and chemical reactions.

The key innovations of this study include the development of a spatially resolved conversion rate gradient model for char layers with the thickness x, expressed as α_s(x)=e^-0.28-x, and dynamic porosity adjustments to reflect gas transport limitations within the wood. Simulation results demonstrate strong agreement with experimental mass losses, thereby validating the proposed method. This study reveals that surface charring substantially decelerates pyrolysis by reducing gas permeability, whereas internal temperature gradients govern the cessation of reactions within the wood.

This work establishes a robust Fluent-based framework for simulating wood ignition, effectively overcoming the limitations of FDS through advanced mesh resolution and detailed pyrolysis modeling. By integrating experimental data into UDFs, the method established herein enhances predictive capabilities for fire spread in structural and environmental fire scenarios. Future research could focus on expanding the model to incorporate heterogeneous secondary reactions, thereby further bridging the gap between simulations and real-world fire behavior.