Flow velocity measurement methods for weak flame plumes face several challenges due to the complex dynamics involved. Flame plumes, driven by buoyancy forces, are inherently turbulent, with the plume motion accompanied by the entrainment of the surrounding air. The boundary between the plume and the environment continuously evolves in space, making it difficult to capture the plume's true flow characteristics. Past plume velocity measurement methods rely on intrusive methods, using tools such as Pitot tubes, smoke probes, and hot-wire anemometers. These methods disrupt the plume flow field and cannot accurately reflect the undisturbed temporal and spatial characteristics of the entire plume, thereby limiting their applicability for a detailed analysis of weak flame plumes.

To address these challenges, we employed the schlieren imaging technique to visualize flame plumes. This nonintrusive visualization technique allowed the capture of the flow field induced by buoyancy forces at a high resolution. Industrial cameras were used to record the ignition process and flame plume dynamics at varying heights and oil pan diameters. By analyzing the schlieren images, we aimed to overcome the limitations of traditional measurement methods. In this study, we derived a simplified two-dimensional Navier-Stokes equation to develop an optimized optical flow (OF) algorithm tailored for velocity measurements in flow fields. The proposed algorithm was applied to the schlieren images of flame plumes, showing significant improvements over conventional OF methods.

The key advancements of the optimized algorithm are as follows. (1) Enhanced sensitivity and precision: The optimized algorithm produces smoother displacement fields and more uniform vorticity fields. This enables the detection of finer vortex structures that are often overlooked by conventional OF methods. By improving the resolution and accuracy of the calculated flow field, the algorithm provides a more detailed representation of the flame plume's dynamics. (2) Rapid convergence: During the velocity calculation process, the optimized algorithm achieves rapid convergence. The energy residual after the first iteration is reduced to less than 10^-2, and the energy residual in the final OF field remains below 10^-5. This indicates that the proposed algorithm achieves a high accuracy in fewer iterations, making it computationally efficient. (3) Improved robustness in experimental validation: In flame plume velocity measurement experiments, the optimized algorithm demonstrates superior robustness compared with conventional OF methods. A dimensional analysis of the results shows a significant improvement in the fit between the predicted and measured values. Specifically, the coefficient of determination (R²) increases from 0.90 for the conventional OF method to 0.98 for the optimized algorithm. Additionally, the measured results are in close agreement with the Heskestad model results. While conventional OF methods show an average error range of -20%-30% in the plume region, the optimized algorithm reduces this error range to -5%-20%. This reduction highlights the enhanced accuracy and reliability of the optimized algorithm.

Overall, the proposed algorithm provides a more stable, accurate, and efficient approach for measuring the velocity of weak flame plumes. By addressing the limitations of conventional OF measurement techniques and aligning more closely with theoretical prediction models, this study offers a valuable contribution to the flame plume analysis. These findings pave the way for the improved understanding and modeling of fire dynamics, with potential applications in fire safety engineering and combustion research.

As a green energy resource, hydrogen-doped compressed natural gas (HCNG) is progressively gaining attention. A potential large-scale method of hydrogen transportation is mixing hydrogen with natural gas using the natural gas supply pipeline. However, owing to the difference in the properties of natural gas and hydrogen mixture, leakage and explosion accidents possibly occur during the transportation and utilization of HCNG. To ensure the safe promotion and utilization of HCNG, it is necessary to mix inert gas suppressants into HCNG. Therefore, this study aims to investigate the explosion characteristics of HCNG in a 0.5 m×0.5 m×0.5 m square explosion vessel. The suppressing explosion experiments are conducted using the explosion suppressants carbon dioxide and heptafluoropropane.

First, a study on the explosion characteristics of HCNG with a hydrogen volume fraction in the range of 0 % to 50 % is conducted. Subsequently, the HCNG with the 50 % volume fraction hydrogen is selected to investigate the explosion suppression based on the suppressants, carbon dioxide and heptafluoropropane. The profiles of critical explosion parameters such as maximum explosion overpressure, maximum overpressure delay time, and flame propagation velocity are analyzed under different volume fractions of suppressants.

Results show that the maximum explosion overpressure and flame propagation velocity increase with increasing hydrogen doping ratio. With increasing volume fraction of suppression gas, the maximum explosion overpressure rapidly decreases, and the delay time to reach the maximum overpressure is prolonged. The high-speed image acquisition results indicate significant stretching of the flame surface. The spherical flame propagation velocity is considerably decreased by the suppressant. Subsequently, the typical volume fraction interval of the two suppressants is selected for comparing their explosion suppression effects. The explosion suppression effects of carbon dioxide and heptafluoropropane on the HCNG are quantified.

The suppression effect of heptafluoropropane is found to be generally comparable with that of carbon dioxide at twice the volume fraction. Complete explosion suppression is attained using 10 % volume fraction heptafluoropropane or 20 % volume fraction carbon dioxide, and the maximum explosion overpressure is decreased by 98.0 % and 94.4 %, respectively. The reasons for the suppression effect of heptafluoropropane being considerably superior to that of carbon dioxide are discussed based on the physical and chemical differences between the two suppressants. In particular, heptafluoropropane demonstrates better abilities of oxygen isolation, combination of explosive reactive radicals, and absorption of reaction heat than those of carbon dioxide. The current results can serve as a guide for improving the transportation safety of HCNG.