Combustion Zone Research Articles

Generation and persistency of combustion-derived environmentally persistent free radicals from phenolic compounds over a Fe2O3/SiO2 surface

Combustion of organic solid wastes releases phenolic compounds which can act as precursors in the formation of environmentally persistent free radicals (EPFRs) in the post-flame, cooling zone of waste combustion. The study investigated the generation mechanism of EPFRs from phenolic compounds catalyzed by transition metals in air atmosphere under simulated combustion conditions. Representative combustion-derived phenolic compounds were used, and SiO2 particulates containing different mass ratio of Fe2O3 were synthesized as carriers. EPFRs formed had g-factors between 1.9998 and 2.0066, indicating phenoxyl-, cyclopentadienyl-, and semiquinone-type radicals, along with paramagnetic F-centers. The promotion effect of phenolic compounds on EPFR formation during heating decreased as catechol > hydroquinone > phenol > p-cresol. This trend is related to hydroxyl groups and activation energy. In particular, catechol chemically adsorbed on Fe2O3 at 600 K led to the formation of EPFRs with relatively high spin concentrations (up to 1.28 × 1017 spin/g). Higher Fe2O3 concentrations promoted the transformation of phenoxyl-type radicals into cyclopentadienyl-type and paramagnetic F-centers. However, as the Fe2O3 loading increased from 1.25% to 5%, the density of EPFRs decreased. The findings related to the influence of various precursors and Fe2O3 concentration on EPFR formation provide valuable insights for estimating EPFR generation and associated risk during combustion processes.

Effects of vacuum degree on the detonation characteristics and energy release laws of RDX explosive

To explore the detonation characteristics and energy release laws of RDX explosives under varying degrees of vacuum, the influence of vacuum degree on the fireball characteristic parameters, shock wave propagation laws and detonation performance of columnar RDX charges were studied using the air-blast experiments and AUTODYN simulations, and the fireball temperature distributions were mapped with the colorimetric thermometry technique. The experimental results indicated that as the degree of vacuum increased, the combustion duration and zone area size of the explosive fireball were both extended, while its temperature decay rate was slowed down, making the afterburning effect more pronounced. Furthermore, the peak overpressure and positive impulse of the explosion shock wave showed a downward trend, with the reductions of 56.1 % and 53.5 %, respectively, with the increasing vacuum degree. Numerical simulation was also used to reveal the energy release laws of RDX explosive, and the comparison between the experimental and numerical results showed a good degree of concordance with an average error of 6.45 %. The results of the study could provide a reference for expanding the explosion theory of explosives in plateau environment and improving the performance of explosion-proof equipment.

Effect of Temperatures on Flame Propagation of Diethyl Ether Spray Explosion

ABSTRACT The study aims to identify the flame propagation in the diethyl ether (DEE) spray explosion through mathematical analysis by processing the gray-level histogram of the flame images. Different ambient and material temperatures’ effect on flame propagation and structure was investigated using a high-speed camera in a 20-L explosion vessel. The results showed that the overall duration could be divided into combustion and deflagration processes. With the ambient temperature increased, the combustion duration showed a trend of first decreasing, then increasing, and then decreasing again. The combustion zone first increased and then decreased. The explosion duration showed an overall decreasing trend. The explosion duration was longer than the combustion duration, ranging from 152.75 to 307 ms. The overall combustion and explosion duration trend was consistent with the explosion duration. When the ambient temperature was 308.15 K, the combustion duration reached its minimum value of 13.25 ms, and the equivalent radius of the combustion zone reached its maximum value of 52.89 mm. With the increase in material temperature, both the combustion and explosion durations showed a trend of first decreasing and then increasing, with combustion durations ranging from 12.42 to 16.25 ms and explosion durations ranging from 106 to 117.5 ms, respectively. When the material temperature was 308.15 K, the combustion zone expansion rate reached its maximum value of 10.20 m/s, and the explosion zone expansion rate reached its maximum value of 13.36 m/s. The effect of ambient temperature on the development of combustion zone is greater than that of material temperature, which is mainly reflected in the obvious change of equivalent radius.

NUMERICAL JUSTIFICATION OF THE LABORATORY TECHNIQUE FOR TESTING OF FIRE-EXTINGUISHING POWDERS

Improvement of firefighting means and methods for measuring their effectiveness are important tasks in the field of fire safety. The paper presents the results of experimental measurements of the minimum extinguishing concentration of powder mixtures that can be applied as effective explosion-suppressing barriers. Measurements of the minimum extinguishing concentration of the investigated powders were carried out using a laboratory method with their pulsed delivery to a microfire of class B using compressed air. In order to justify and assess possible errors of the mentioned laboratory method for measuring extinguishing efficiency, numerical 3D modeling of the interaction of a multiphase flow with a model combustion focus was performed. The analysis of the numerical modeling results has shown that, for the applied laboratory method, almost the entire portion of the investigated powder enters the combustion zone. Additionally, the numerical calculations indicate that under the specified experimental conditions, the particle size of the powder has no noticeable effect on their loss into the surrounding flame space. Thus, these results justified the use of the mentioned laboratory method for he comparative evaluation of fire-extinguishing powders with a wide range of dispersity. The application of this laboratory method for assessing the effectiveness of the fire-extinguishing powder allowed for the development of an optimal powder composition for explosion suppression, incorporating inert mineral particles as the main component and an additive of a chemically active potassium-containing combustion inhibitor.

Study on the thermal characteristics and heat-insulation ability of gel-stabilized foam used for preventing the spontaneous combustion of coal

Gel-stabilized foam serves a critical role in preventing spontaneous combustion of coal by impeding heat transfer from the coal-fire area to adjacent safe areas. To elucidate the thermal characteristics of gel-stabilized foam, the microstructure and heat-transfer characteristics were investigated. The results revealed that gel-stabilized foam was an accumulation system of many stable and closed bubbles, and there was a layer of gel structure on the bubble surface. This gel layer endows the bubbles with exceptional heat-insulation capability by effectively encapsulating air. To further analyze the relationship of heat-transfer characteristics and foam structure parameters, a theoretical thermal-conductivity model was established to revel the thermal characteristics of gel-stabilized foam. Moreover, the experimental tests were conducted to verify the accuracy of theoretical model, and found that the fitting errors were around 8 %, affirming the model's suitability for gel-stabilized foam. Notably, the theoretical model demonstrated that foam's thermal-conductivity was only monotonically related to foam expansion ratios in theory. In addition, the heat-insulation experiments indicated that the heat-transfer rate of foam depended not only on the thermal-conductivity but also on the foam's stability, both of which were affected by the expansion ratio. Hence, the effective heat-insulation time (EHT) was proposed to assess gel-stabilized foam's heat-insulation characteristics. The heat-insulation ability tests results showed that EHT values initially increased and then decreased with rising foam expansion ratios, primarily influenced by the volume of air encapsulated within the foam. Particularly noteworthy were the test results indicating that gel-stabilized foam with an expansion ratio of 10 times and a stacking thickness of at least 15 cm exhibited excellent heat-insulation capability against heat transfer from the spontaneous combustion zones. The research may provide a theoretical foundation and offer guidance for the prospective utilization of gel-stabilized foam in coal mines.

Experimental and Numerical Study on the Combustion Characteristics of Premixed Methane/Air in Porous Media Burner

ABSTRACT The porous media burner is considered as the promising alternative for residential water heaters. In this work, a two-layer porous media burner was designed and the maximum NOx emission was 48.7 mg/m3 under the firing rate of 666 kW/m2. A two-dimensional model was established; the effect of the pore parameters on the flame stability limit and NOx emission was investigated. The flashback, blow-off and flame instability were observed in the experimental and numerical investigations. Through the numerical optimization, the preferred pore parameters were 0.8 mm & 0.4 (preheating zone) and 1.6 mm & 0.7 (combustion zone); the predicted NOx emission was 32.9 mg/m3. The preferred porous media were produced and studied experimentally. Under the same condition, NOx emission was 28.7 mg/m3, which was less than 35 mg/m3 (the standard requirement, GB 18,111-2021) and reduced by 41% compared with that of the initial burner (48.7 mg/m3). The flash back limit was increased and the flame was stable at φ = 0.71.

Research on the effect of water-cooling steel pipe on preventing spontaneous combustion of coal pile and its thermal migration behavior

During the storage and transportation process after mining, coal piles are placed in open environments, making them prone to self-heating and spontaneous combustion due to the nature of coal and factors like natural wind flow. In recent years, there have been frequent spontaneous combustion incidents involving coal piles, posing significant safety risks. To effectively prevent and control spontaneous combustion disasters in open-air coal storage piles, we propose a method involving the arrangement of water-cooling steel pipes within the coal piles. This method applies theories of coal spontaneous combustion mechanisms, porous media heat transfer, and non-isothermal pipeline heat transfer. The multi-physics coupling model of COMSOL numerical simulation software is used to analyze the spontaneous ignition process and prevention effect of open pit coal pile. In the model, the thin material transfer of porous media is taken as the oxygen concentration field, the heat transfer of porous media is taken as the temperature field, and the free and porous media flow is taken as the air seepage velocity field. The simulation results of the spontaneous combustion process in the coal pile indicate that the high-temperature zone of spontaneous combustion is situated within the range of 0.5 ~ 1.5 m inside the wind-facing surface and extends 0.5 m above the ground level. These findings serve as a basis for determining the optimal placement of water-cooling steel pipes within the coal pile. The simulation results of a single water-cooling steel pipe demonstrate a positive correlation between the cooling effect on the coal pile and the water cool flow, and a negative correlation with the water cool temperature. Additionally, the cooling radius of the water-cooling steel pipe is determined by the circumference of the pipe and remains unaffected by the water cool flow. Finally, simulations were conducted to evaluate the cooling effect of multiple rows of steel pipes, and optimal arrangement parameters were determined: a center distance between steel pipes of 1 m and a water cool flow rate of 1500 L/min. As a result, the onset of the self-heating period in the coal pile was delayed by 11 days, and the spontaneous combustion period was extended by 56 days. The arrangement of water-cooling steel pipes in the coal pile has demonstrated significant efficacy in preventing and controlling spontaneous combustion.

Enhancing the vaporization and secondary atomization of two-liquid droplets for fire suppression

Large-scale fires of various origins are most often contained and suppressed using fine water spray systems with a droplet size of 50–300 μm. Enhancing the extinguishing performance of these systems is an urgent task as this will reduce the amount of irreversible damage fires cause to forests, buildings, and compartments. Finer droplet size of the extinguishing liquid sprayed directly in the combustion zone prevents droplet entrainment, thus increasing the firefighting efficiency and reducing the extinguishing liquid consumption. However, too little is still known about alternative ways to intensify the vaporization and secondary atomization of fine water sprays in order to disrupt the chemical reactions responsible for combustion. Experimental findings on enhancing the secondary atomization and phase change of sprayed water with added vegetable oils for fire suppression are presented. Experiments on water droplets with sunflower, rapeseed, and tall oils were performed in the ethanol burner flame zone. The radii of two-liquid droplets were 0.85 ± 0.05 mm. The concentration of explosion-triggering additives to water was constant at 10 ± 2 vol.% in all the experiments. The addition of 10 vol.% rapeseed oil to water allows one to reduce the local gas temperature around the droplet up to 250 K in case of micro-explosion of an isolated droplet and up to 450 K in case of micro-explosion of a group of droplets. The experimental findings were processed to obtain approximations of the relationships established in the experiments: micro-explosion delay times, droplet lifetimes, and normalized evaporation surface area after breakup. A physical model is presented describing the processes involved in the experiments. An additional series of experiments involved suppressing a laboratory-scale wood crib fire using water and water/rapeseed oil emulsion to compare the fire suppression times and the extinguishing liquid consumption in both cases. The suppression times and the extinguishing liquid consumption were on average 15% lower when using a water/rapeseed oil emulsion compared with pure water. Practical recommendations are given on how to provide the intense disruption of extinguishing liquids with added vegetable oils to accelerate fire suppression. Two-liquid droplets exploding in the flame zone are established as a possible alternative to water capsules, extinguishing missiles, and cryogenic technologies for suppressing large-scale fires.

Restoring the gas diffusion field before the fire of the LiNi0.7Co0.2Mn0.1O2 lithium-ion battery thermal runaway

Although ternary lithium-ion batteries promise high energy density, their safety issues have aroused widespread social concern. The thermal runaway phenomenon of ternary lithium-ion batteries presents a violent fire or explosion induced by the sparks. However, the reason for the disaster of thermal runaway remains unclear. This study restores gas diffusion before a fire or explosion using computational fluid dynamics(CFD) in order to explain the germination of the disaster. The calculation results revealed that the diffusion field cane divided into three zones:(1) fuel-surplus zone; (2) fuel‑oxygen zone; (3) oxygen surplus zone. The combustible zone (fuel‑oxygen zone) quickly enlarges after the safety valve is opened and is located on the top of the safety valve, so the deflagration tended to happen on the top of the safety valve a few seconds after the safety valve opened. Besides, the study also established the relationship between venting parameters and time based on the Sigmoid function. The established relationship function could provide boundary conditions for the fire dynamics of lithium-ion battery thermal runaway. Overall, the method could not only be used to further explore the combustion mechanism of lithium-ion thermal runaway, but also provided insight into the safety design of the battery pack structures.

Design Of Tangentially Fired Pulverized Coal Burner Nozzle For Enhanced Erosion Resistance

Erosion-induced failure of pulverized coal burner nozzles (PCBN) presents a significant challenge in pulverized coal-based thermal power plants. Investigations have identified multiple causative factors leading to the accelerated degradation and consequent reduced lifespan of PCBN. These factors include the erosive impact of solid fuel particles carried at high velocities and angles, as well as the high-temperature conditions within the combustion zone. To mitigate erosion, strategies often revolve around enhancing material resistance or optimizing aerodynamic profiles to minimize drag forces. This study employs Computational Fluid Dynamics (CFD) via the CFX code to model and analyze the erosion dynamics within PCBN. By adjusting the geometry of the inner plates within the PCBN, simulations indicated a notable improvement in erosion resistance. The optimized design achieved this by expanding the flow area between the innermost bifurcated plates without altering their angle, leading to a more uniform flow distribution and favorable velocity and pressure gradients. These modifications have the potential not only to prolong nozzle life but also to contribute to more efficient combustion in tangentially fired furnaces.

Experimental investigation of the effects of inlet high-swirl air preheating and dilution on kerosene flame

This study investigated the effects of co-flow air swirling on the combustion of Kerosene, as relatively heavy fuel, under preheated and diluted air conditions. Various factors affecting the flame structure, such as preheating temperature, dilution, swirl number, and airflow rate were studied using the upgraded MILD (Moderate or Intense Low-Oxygen Dilution) combustion tester of Sharif University of Technology (SMSTR). This tester, which is symmetrically axial, provides co-flow air with the desired temperature, swirl number, and dilution for a liquid fuel spray burner. The flame characteristics was investigated through direct photography and chemiluminescence. The results show that the stability range significantly increases with the air swirling, while the flame length decreases. Enhancement of combustion quality and more complete combustion of fuel were observed with preheating. Preheating resulted in geometric changes in the appearance of the flame, specially reducing flame lift-off and the front view area in high airflow. A decrease in the air oxygen content causes a general change in the structure of the flame, resulting in a shorter and more lifted one compared to non-diluted condition. The chemiluminescence images of the flame also revealed weaker and more uniform emission pattern with the inlet high-swirl air preheating and dilution. By utilizing around 15 % oxygen content dilution, preheating at around 656 K, and a swirl number of 0.8, the combustion zone exhibited similarities to the MILD combustion mode, suggesting the achievement of MILD combustion. The high swirl concept allowed higher dilution limits (about 12 %) and reduced preheating requirements (approximately 11 %) in achieving the MILD combustion regime compared to the previous study conducted without swirl conditions. In contrast to the non-swirl mode flame, the flame in this study transitioned completely to a MILD-like condition.

Effects of methane concentration on flame propagation mechanisms and dynamic characteristics of methane/coal dust explosions

The study explores the flame propagation and explosion dynamic behavior of gas/powder two-phase compound explosions, taking methane and coal dust mixture as the research object. The influence of different methane concentrations on the compound explosion characteristics was researched. The vertical pipeline, high-speed camera, ion current probe, and pressure sensor be used to test the propagation behaviors, microstructure characteristics, and explosion dynamics of compound explosion flames. The function of methane concentration on the propagation behaviors, combustion reaction zone thickness, flame temperature, propagation velocity, and explosion pressure of compound flame was revealed, and the propagation mechanisms of gas/powder compound explosion flames were explained. The experimental research discovered that the compound flame of methane/coal dust explosion presents a white luminous zone with uniform distribution and appears to have an approximate homogeneous combustion state. The explosion dynamics parameters all showed a trend of first increased and then decreased as the methane concentration increased, but the reaction zone thickness of the combustion first reduced and then increased. In addition, the dominant position of homogeneous or heterogeneous combustion in the combustion reaction zone will also switch with the change in methane concentration. The compound flame is similar to premixed gas combustion under optimal methane concentration, the combustion zone thickness (δ) and preheating zone thickness (β) are the smallest, and the compound explosion shows powerful performance. In addition, the flame propagation mechanisms of methane/coal dust compound explosion were elucidated based on the theory of heat.

Formulation determination and performance study of high-volume fly ash-based grouting fire extinguishing materials

Considering the continued advancement of China's industrial sector, a substantial quantity of circulating fluidized bed fly ash (CFBFA) and calcium carbide slag (CS) is generated throughout the production process. Considering the rich calcium content inherent in CFBFA (circulating fluidized bed fly ash) and the inherently alkaline nature of CS(carbide slag), the amalgamation of these two constituents leads to a multitude of hydration reactions, culminating in the synthesis of polymeric grouting materials. This study undertaked a comprehensive examination and regulation of three factors within the binary system: water-cement ratio, CS content, and CS particle size. Through the utilization of scanning electron microscopy (SEM), X-ray diffraction (XRD), and (TG) thermogravimetric analysis, the investigation aims to elucidate the influence mechanism of the composite gelling system comprising fly ash and calcium carbide slag on the compressive strength, setting time, and other pertinent indicators of grouting materials. The data reveals that when the water-cement ratio is set at 0.85, CS content at 30%, and CS ball milling time at 1 h, the resulting grouting material exhibits a fluidity of 279 mm, a water separation ratio of 3.13%, and achieves a strength of 15.6 MPa at 28 days. It boasts a stable and easily transportable slurry, which possesses a certain degree of cementitious performance, effectively sealing off any air leakage channels. Moreover, by subjecting CS to appropriate pre-treatment measures, a suitable quantity of active silicon and aluminum elements can be thoroughly dissolved within the CFBFA (circulating fluidized bed fly ash) slurry system, giving rise to a staggered distribution and dense hydration product structure. Considering the requirements for gangue mountain fire extinguishing slurry, a CFBFA-CS based grouting material formula has been carefully selected. Additionally, simulation grouting fire extinguishing experiments have demonstrated that the injection of the slurry is highly effective in reducing the temperature of the combustion test block, isolating oxygen, and lowering the temperature within the combustion zone. After 5 h, the temperature drops to below 100 °C, and no re-ignition phenomena are observed. The outcomes of this research can provide valuable scientific guidance for the development of high-performance fire extinguishing grouting materials utilizing fly ash and carbide slag.

Laminar flame speed measurement and combustion mechanism optimization for ethylene–air mixtures

AbstractEthylene plays a crucial role as an intermediate component in the cracking and combustion processes of large molecular alkane and olefins. In this article, the laminar flame speed of ethylene–air mixtures was measured using the heat flux method. The mechanism of ethylene was simplified by utilizing the error propagation directed relationship graph (DRGEP) and sensitivity analysis (SA), and the Arrhenius pre‐exponential factors for 20 selected reactions in the skeletal mechanism were optimized using the particle swarm optimization (PSO) algorithm. Finally, an ethylene optimization mechanism including 39 species and 85 reactions was obtained. The prediction results for flame speed, ignition delay time, and species concentration were compared with experimental data and other mechanisms, covering a wide range of temperatures (298–1725 K), pressures (1–22.8 atm), and equivalence ratios (0.5–2.0). The findings demonstrate that the optimization mechanism not only improves the prediction results of laminar flame speed in the rich combustion zone and low oxygen environment but also enhances the prediction accuracy of the ignition delay time at high pressure and in the lean combustion zone, as well as the prediction accuracy of C2H4 and H2O radicals. In conclusion, the optimized mechanism exhibits higher accuracy and broader applicability.

Mechanical Analysis of the Forces Involved in a Pilot-Scale Blast Furnace Raceway Formation by Means of CFD/DEM Simulations

The CFD/DEM approach was used for investigating the forces playing a role in a furnace raceway formation and stability. The configuration is an actual pilot-scale hot blast furnace filled only with coke particles. In such a system, the raceway was unstable, with successively a growing phase and a collapse. The CFD/DEM numerical simulations were coupled with a core-shrinking model to mimic coke particle combustion. However, the kinetic reactions and heat transfers were not numerically predicted. Instead, the characteristic combustion timescale of one coke particle was imposed, and the combustion zone was adjusted to match the global combustion measured in the pilot-scale experiment. The results showed that the standard contact model was not enough to resist the pressure exerted by the granular weight on the raceway. However, the addition of a cohesive force, through the Johnson-Kendall-Roberts (JKR) model, allowed the qualitative reproduction of the gas pressure fluctuations and the collapse cycles in accordance with the experiment. A sensitivity analysis of the flow rate showed that CFD/DEM is able to reproduce quantitatively the time between two collapses, as observed in the experiment. Predicted raceway size and shapes are also in agreement with the experimental observations in the range of investigated parameters.

Experimental study and kinetic analysis of NO emission characteristics in ammonia/coal co-firing process with different ammonia injection methods

Ammonia/coal co-firing in coal-fired power plants is an effective way to achieve CO2 reduction. In this paper, the ammonia/coal co-firing study was carried out in a 45 kW wall-temperature-controlled staged combustion downstream furnace at 1100 °C. Bituminous coal was used as the experimental object to analyze the effects of ammonia injected at the main combustion zone, reduction zone, with the burnout air, and at the burnout zone on NO release characteristics at different ammonia co-firing ratios. To ascertain the significant reaction of NO production in the ammonia/coal co-firing, chemical reaction kinetics calculations were performed. When co-firing ratios of 10∼30%, the NO emission concentration is lower by the ammonia injection position of the main combustion zone which is 275–346 mg/m3. When co-firing ratios reach to 40% and 50%, NO releases are lower by the ammonia injection position of burnout zone which is 511–574 mg/m3. In addition to exhibiting a greater capacity to decrease NO in environments with low oxygen concentrations and high NO concentrations, ammonia can also, under specific circumstances, have a significant capacity to decrease NO in environments with high oxygen concentrations. Based on the experimental results and chemical kinetic calculations, the NO release pattern and mechanism during ammonia-coal co-combustion with different ammonia injection methods under air staged combustion were investigated, which provides a technically feasible solution to reduce CO2 and NOx emissions from coal-fired power plants.

Dynamic laser ignition characteristics of solid fuel and oxygen for hybrid rocket system

The transient laser ignition process in a slab burner that is similar to hybrid rocket motors is investigated in this study. The coupling characteristics between solid fuel properties, pyrolysis rate, and laser power were analysed. The results show that the laser ignition process can be divided into four stages: preheating, pyrolysis, ignition, and combustion, with the preheating and pyrolysis stages primarily controlled by the solid heating rate and the mixing characteristics in the fuel-rich combustion zone. Meanwhile, the ignition and combustion stages were characterized by ignition kernel growth and flame front propagation. The ignition delay time and the establishment of steady-state combustion were significantly affected by the ignition energy and fuel properties, i.e., The ignition delay time increases exponentially with decreasing laser energy. An increase in oxidizer flow flux reduces the demand for mixing during ignition, but the corresponding increase in combustion pressure poses challenges to the ignition process. A two-dimensional transient numerical model was established based on optical-thermal coupling theory to further understand the dynamic pyrolysis process of solid fuel particles. The model was validated through fire experiments under different laser ignition energy conditions. The results indicate that the deviation of the maximum penetration depth and ignition delay time between the experiment and the simulation is within 7 %.

New Aspects of the Pulse Combustion Process

Pulse combustion is an attractive yet still little-known form of combustion that can be successfully used in many industrial applications. Experimental studies show that the course of the combustion process in the valveless pulse combustion chamber is conditioned by the process of creating a well-mixed fuel–air mixture inside the chamber. In the paper, numerical calculations were carried out for selected operating conditions of the pulse chamber and compared with experimental results. This allowed for a better understanding and interpretation of the course of the pulsating combustion process itself. The role and importance of the rate of changes in the volume of the combustible mixture zone in the process of improving the efficiency of the combustion process were determined, and the reasons for changes in the pulsation frequency of the combustion process were also explained.

Determination of Coal Spontaneous Combustion Zone in Overlaying Multilayer Goaf and Its Control Technology of Thickened Slurry

ABSTRACT In order to study the law of coal spontaneous combustion and prevention methods in the overlying multilayer goaf, this study combined the situation of 378 working face and overlying multilayer goaf in Anyuan Coal Mine and determined the coal spontaneous combustion zone in the overlying multilayer goaf by overlaying the oxidation zones divided by wind velocity and oxygen concentration, and verified it. Subsequently, a new type of thickened slurry was prepared based on sodium alginate, carboxymethyl cellulose, loess, and water as the main raw materials, and a thickened slurry fire prevention technique suitable for overlying multilayer goaf was proposed. The research results indicated that the coal spontaneous combustion zone was located near the 3117 return air roadway and the 378 return air roadway in 378 working face, as well as the upper part of 3117 return air roadway in 372 (4) goaf. When the water-soil ratio was 8:1 and thickener concentration was 0.3%, the suspension performance of the thickened slurry was better. At this point, the corresponding yield stress was the smallest and the flow ability was better. Through the used of fixed-point grouting and regional sealing of a thickened slurry fire prevention technique, it was found that the CO concentration gradually decreased to zero in the 372 (4) goaf. The oxygen concentration significantly decreased and the pressure difference remained stable in the sealing wall in the return air roadway. The outlet water temperature were below 25°C all boreholes.

Improving the combustion of scramjet engines with struts using grooves and bumps

Grooves and bumps are arranged on both sides of a normal strut (NS) to fabricate vortex generators, and introduce streamwise vortices. Struts with sawtooth grooves (GS-1), rectangular bumps (BS-1), and trapezoidal and triangular bumps (BS-2) have been proposed. In this study, struts with V-shaped grooves (GS-2) and trapezoidal and triangular bumps (BS-3) were configured. The flow fields in the cold and hot states of scramjet combustors with NS, GS-1, GS-2, BS-1, BS-2, and BS-3 were simulated numerically using steady RANS equations and an SST k-ω turbulence model. The results show that proper arrangement of the grooves and bumps on the strut can improve the mixing and combustion performance. Compared with the NS, for GS-1, GS-2, BS-1, BS-2, and BS-3, the distance required to achieve 100% fuel mixing efficiency is shorter by 21.2%, 9.1%, 18.2%, 12.1%, and 15.2%, respectively, and to achieve 100% combustion efficiency, the distance is shorter by 42.7%, 46.9%, 47.9%, 53.1%, and 58.3%, respectively. Therefore, improvement in combustion performance does not positively correlate with improvement in the mixing performance. Large-scale heat and mass convection in streamwise vortices play a dominant role in mixing. The larger the proportion of the fuel area affected by the streamwise vortices, the more conducive it is to achieving a higher mixing performance, whereas the wall has a negative impact. However, the large-scale heat and mass convection in the streamwise vortices and small-scale heat and mass convection in the high-turbulence zones are closely related to the supply of oxygen and fuel required for the combustion zone. Large-scale streamwise vortices combined with thick high-turbulence zones are conducive to achieving a high combustion rate. When the recirculation zone has a larger combustion area and smaller wall effects, a higher combustion performance is easily achieved. Compared with the grooves, the stronger disturbance effect of the bumps on the fluid effectively increased the thickness of the high-turbulence zone, thus obtaining a higher combustion performance. However, the greater blocking area and new oblique shockwaves generated by the bumps induced a greater total pressure loss. Therefore, combining bumps and grooves can further optimize strut configurations.