ABSTRACT Coalfield fires represent a critical global challenge, causing significant energy loss and posing serious environmental risks. The fracture network within coal seams, as the principal pathway for oxygen migration, plays a pivotal role in governing fire initiation and propagation. This study focuses on the widely distributed asymmetric Y-shaped fracture structures in coal seams and develops a thermo-hydro-chemical multiphysics coupling model that integrates heat transfer, fluid flow, gas diffusion, and oxidation reaction kinetics to simulate the onset and evolution of coal seam fires. Finite element simulations are conducted to systematically examine the influences of fracture intersection angles, matrix diffusion coefficients, and oxygen concentrations on temperature field evolution and heat source migration mechanisms. The results reveal that the migration of temperature extremum points reflects heat source evolution, and the critical time for heat source migration decreases with increasing intersection angle – for instance, from approximately 35 days at 30° to 27 days at 90°. Increasing the matrix diffusion coefficient enhances oxygen penetration depth (from ~5 mm to ~12 mm) and enabling the bifurcation zone to dominate heat accumulation more rapidly, thereby shortening the critical time. Oxygen concentration shows a positive correlation with temperature extremum, reducing oxygen concentration from 21% to 9% delays the critical migration time from approximately 28 days to 51 days, and at 3% concentration, no heat source migration occurs within the simulation period. These findings deepen the understanding of coal fire development mechanisms and provide a quantitative theoretical foundation for precise prediction and targeted prevention of coal seam fires.

ABSTRACT In this study, the mixtures obtained by adding the volume of 5%, 10%, 15%, 20% and 25% hydrogen to natural gas were burned in a Bunsen burner. For this purpose, a thermal camera, video camera, and gas analyzer were utilized to investigate that of changes in detail. Besides, flame sizes and areas were measured by converting flame photographs from RGB format to HSV color space using a MATLAB-based method. It was observed that as the hydrogen ratio in the mixture increased; the flame length shortened, the flame temperature increased, CO and NO2 emissions increased slightly, and NO emissions decreased. In the case of 25% hydrogen addition, an increase in flame temperature of 53°C was detected compared to 100% natural gas. Moreover, the area covered by the premixed flame decreased by 55.6% and the flame length decreased by 18 mm at the same case. The changes in flame dimensions affect the heat transfer in the combustion chamber, and the low temperatures occurring in the combustion chamber cause an increase in CO and NO2 emissions. On the other hand, when the combustion chamber diameter was reduced from 250 mm to 130 mm, it was observed that CO emissions decreased and NO2 emissions increased as the hydrogen ratio was increased from 0% to 25%. When natural gas is enriched with hydrogen and burned, the relationship between flame size and the combustion chamber has been shown to affect emissions. These experimental results have shown that matching flame and combustion chamber sizes is important for emissions when using hydrogen-enriched natural gas mixtures.

ABSTRACT This study explored the complex dynamics of moving fire sources within tunnels, focusing on the influence of variable ventilation velocities on heat release rate, flame inclination angle, and tunnel ceiling temperature. The experiments were carried out at a vehicle speed of 1.0 m/s, with the longitudinal ventilation velocity being adjusted from 0.5 m/s to 3.0 m/s. The flame inclination angle, as a transient parameter, responded consistently when relative velocities remained unchanged, regardless of fuel type or heat release rate. The research revealed that the peak maximum ceiling temperature occurred at a ventilation velocity of 1.0 m/s, attributed to heightened flame radiation. Moreover, the direction of flame tilting, whether upstream or downstream, altered the timing of maximum temperature occurrences. And the influence of ventilation velocity on tunnel ceiling temperature rise proved less significant compared to the movement of the fire source itself. Furthermore, the study validated the application of the vehicle speed correction factor for enhancing the accuracy of models predicting the maximum temperature rise in moving fire scenarios. This work deepened the understanding of tunnel fires influenced by the interaction between moving fire sources and longitudinal ventilation.

ABSTRACT Non-stationary and non-linear processes govern thermo-acoustic instability in a combustion or propulsion device, which involves coupling between the chamber acoustics and the heat-release oscillations. In this study, we demonstrate the efficacy of combining Empirical Mode Decomposition (EMD), Hilbert-Huang Transform (HHT), and non-linear time-series analysis to reveal the underlying oscillatory modes and the evolution of system dynamics. The experimental configuration consisted of a confined, partially premixed methane-air flame, in which the influence of swirl strength on the flame characteristics and naturally excited thermo-acoustic instabilities was investigated. The objective is to understand the swirl-flame combustion characteristics and the thermo-acoustic aspects of the system at different swirl intensities. The EMD-based analysis reveals that the flame characteristics, the temporal nature of the heat and pressure oscillations, and their coupling are closely linked to the strength of the swirling flow. For a fixed global equivalence ratio ( ϕ g ), the burner exhibited self-excited acoustic instability, accompanied by reduced combustion efficiency and a shorter flame standoff distance, at high swirl strengths. With changing swirl strength, the dynamics of the acoustic pressure changed from limit cycle oscillations and intermittency to a quasi-stable state. The study shows that analyzing thermo-acoustic instability using a combination of EMD and HHT can successfully isolate and reveal valuable information about the temporal features of the instability from non-stationary, non-linear acoustic pressure and heat release oscillation signals. The proposed EMD–Hilbert framework can be extended to practical systems, such as gas turbines and rocket combustors for early detection and characterization of thermo-acoustic instabilities.

ABSTRACT Coal combustion has recently faced immense negative publicity because of its potential impact on climate change. This negativity has obscured the fact that coal combustion remains a significant contributor to global energy utilization and is likely to remain so for some time. It is prudent, therefore, to continue finding the most environmentally sensible implementations for the burning of coal. This includes both efficient combustion and the effective reuse of coal fly ash. Modifications of coal boilers for NOx control made maintaining low residual carbon in boiler fly ash more difficult, and determining the appropriate char reactivity to use in simulating coal-fired combustors is a critical characteristic for optimal design. This paper describes a residual carbon-in-ash study. Experiments are conducted in a tubular, downward-fired, refractory-lined, laminar entrained flow reactor (EFR) that provides the coal with up to a 2 s residence time under conditions of pilot-scale boilers. Two types of coals are used in order to affirm the predictions of a simple kinetics-diffusion (KD) model from FLUENT® and a detailed oxidation char burnout kinetics (CBK) model. A thermo-gravimetric analyzer (TGA) provides comparative oxidation understanding. The three-dimensional KD model included convective and radiative heat transfer at the boundaries, along with heterogeneous and homogeneous combustion chemistry, to evaluate char burnout kinetics validated with the experiments. The results show that the KD model can successfully achieve essentially equivalent carbon in ash predictions, based on loss on ignition (LOI), at the late stages of char burnout as does the more sophisticated CBK model.

ABSTRACT To investigate the influence mechanism of acid/alkali erosion on coal spontaneous combustion (CSC), using C600 high-precision microcalorimeter and in-situ FTIR, we analyzed the oxidative thermal characteristics, critical temperature points, and dynamic evolution of functional groups during coal heating process under acid/alkali treatment. Correlation analysis was performed to quantitatively assess the correlation degrees among these parameters. The results demonstrate that under both ambient temperature and heating conditions, acid/alkali-treated coal exhibits increased functional group content, enhanced reactivity and greater heat release, with characteristic temperature t2 shifting forward. Acidic treatment demonstrates the most significant enhancement. Within the acidic regime, this promotion effect exhibits a strong negative correlation with solution pH (R = -0.97). Alkaline treatment is weaker than acidic but stronger than neutral treatment, which nevertheless maintains higher reactivity than raw coal. Furthermore, with prolonged erosion duration, acid/alkali treated coal exhibited significantly increased functional group quantity, reactivity, and heat release (pH4-90 sample showed 81% higher heat release than raw coal). In conclusion, acid/alkali erosion substantially enhances coal’s spontaneous combustion tendency, with acidic environments demonstrating the most prominent effect. These results establish the theoretical foundation for the prevention and control of spontaneous combustion in water-immersed coal within goaf areas.

ABSTRACT Methane/air and propane/air turbulent premixed flames were investigated in a slot Bunsen burner at fuel-to-air equivalence ratios 0.7 ≤ φ≤ 1.3, inlet velocities 2.6 m⋅s−1 ≤ U in ≤ 7.0 m⋅s−1, and nondimensional turbulence intensities 0.4 ≤ u’/S L ≤ 2.5. Experiments were performed at atmospheric pressure and included simultaneous planar laser-induced fluorescence (PLIF) of the CH radical and particle image velocimetry (PIV) for assessing the instantaneous flame front and the flow field, respectively. In methane flames with a fixed φ, the turbulent flame speeds S T /S L exhibited a nonmonotonic dependence on u’/S L when U in increased with increasing u’/S L . The S T /S L initially increased, then attained a maximum at a specific value of u’/S L that depended on φ, and finally decreased with a further increase of u’/S L . This dependence was qualitatively different from a related one reported in the literature for stoichiometric methane/air Bunsen flames, where U in was maintained constant as u’/S L increased. Furthermore, the present nonmonotonic dependence was observed for u’/S L as low as 0.55, values considerably lower than the u’/S L ≈3 in past literature. In propane flames with a fixed φ, the S T /S L displayed a monotonically increasing dependence on u’/S L when U in was increasing with increasing u’/S L . Contrary to the methane flames, the propane S T /S L increased with increasing equivalence ratio from φ = 0.8 to 1.2 due to preferential diffusion effects. Moreover, in the propane flames and for 0.4 ≤ u’/S L ≤ 1.4, the S T /S L were mainly controlled by U in rather than u’/S L . Analysis of the flame front curvature and the skewness of the curvature distributions attested the absence of Darrieus–Landau (DL) hydrodynamic instabilities. The increase in the propane turbulent burning velocities at rich stoichiometries was mainly attributed to an enhancement of the local burning rates rather than to an increase in flame surface wrinkling.

ABSTRACT Replacing nitramine explosives like RDX with high-energy compound of CL-20 in composite propellant formulations results in undesirable combustion characteristics, including a sharp increase in burn rate at high pressures and elevated pressure exponents. To address this issue, this study aims to mitigate the pressure sensitivity of the burn rate while preserving high energy density of solid propellants. Two types of energetic burn rate reduction catalysts – graphene-based carbonylhydrazine complexes (GO-CHZ-N, N = Co, Ni) and triaminoguanidine-glyoxal complexes (TAGP-M, M = Ba, K) – were incorporated into CL-20 crystals via spray drying method to achieve precise catalytic modification, consequently the required additive loaded in propellant formulations for tuning combustion behaviors can be reduced. The investigation focused on elucidating the influence of these burn rate reduction catalysts on the thermal decomposition behavior of CL-20, which determines the combustion performance of propellant significantly. Differential scanning calorimetry (DSC) analyses indicated a significant positive catalytic effect in the thermal decomposition of CL-20, with TAGP-Ba exhibiting the most pronounced effect. Furthermore, the CL-20/catalyst composites were used to coat aluminum powders, improving combustion efficiency and mitigating agglomeration associated with low burn rates in conventional formulations. Four types of catalytically tailored Al@CL-20 composites with high combustion efficiency were successfully fabricated. Notably, the catalysts exhibited an inhibitory effect on the thermal decomposition of the Al@CL-20 composites, with TAGP-K showing the strongest suppressive performance. In addition, catalysts regulate the combustion behavior by influencing the decomposition process of CL-20. The incorporation of TAGP-Ba and TAGP-K was observed to diminish the particle ejection during combustion, through the flame structure captured by the high-speed camera. Within the tested pressure range, the Al@CL-20-Ba and Al@CL-20-Co composites demonstrated an effective burn rate deceleration and low-pressure sensitivity, highlighting their potential for tailoring combustion behavior in advanced solid propellants.

ABSTRACT Accurate identification of coal temperature and visualization of temperature field in hidden space such as coal bunker, coal pile and goaf are the key to accurately prevent and manage coal spontaneous combustion. Therefore, following the acoustic temperature measurement theory, we design and build an experimental system to conduct acoustic measurement of loose coal temperature. An acoustic emission experiment on loose coal was conducted, in which a cubic correlation–based time-delay estimation method was developed for acoustic flight time measurement. Meanwhile, acoustic reconstruction process in two-dimensional temperature field of hidden heat source of loose coal, acoustic transceiver arrangement, division of area to be measured and the evaluation method of reconstruction quality are studied. The reconstruction algorithm model of loose coal temperature field is proposed according to the least square method, while model accuracy is validated. According to our findings, the estimated acoustic flight time value using the three-time delay estimation algorithm is within the error range, and the stability is good. With the increase of loose coal particle size, the error value of temperature field reconstruction increases slowly. The average errors for two-dimensional temperature field reconstruction of loose coal in three diverse particle sizes at 180°C are 1.33%, 1.83% and 2.42%, respectively. Among them, the reconstruction quality and stability of two-dimensional temperature field of coal spontaneous combustion in the 0.6–1.5 cm particle size are the best. Our findings lay the theoretical foundation for developing acoustic coal temperature measurement technologies.