Search

Spontaneous ignition induced by high-pressure hydrogen leakage has primarily been discussed qualitatively, focusing on factors such as release pressure, downstream tube size, and the rupture of the burst disk. However, quantitative investigations and prediction models remain limited and unclear. In this paper, we introduce the relative molecular mass M and delve into its influence on the critical pressure threshold for spontaneous ignition of high-pressure hydrogen leaks. Additionally, we introduce the mass flow rate Q , as a metric to quantify the impact of opening area and shape on the ignition process. By integrating six key factors, including tube diameter D , tube length L , atmospheric pressure P a , burst disk opening time t , gas mass flow rate Q , and gas relative molecular mass M , four dimensionless factors affecting the critical leakage pressure were determined by using similarity analysis. The correlation between the dimensionless numbers was verified by correlation analysis and calibrated to the experiments of this paper and other authors. Finally, a critical criterion for spontaneous ignition from leakage in high-pressure hydrogen tubes was established to provide a basis for safety evaluation and prevention techniques for high-pressure hydrogen. [Display omitted] • The effect of the relative molecular mass of gases in the high-pressure region on the spontaneous ignition of high-pressure hydrogen leakage is discussed. • The dimensionless parameter affecting the critical pressure for spontaneous ignition of high-pressure hydrogen leakage is investigated. • The critical pressure criterion for the spontaneous ignition of high-pressure hydrogen leakage is established. [ABSTRACT FROM AUTHOR]

The issue of spontaneous ignition has become a bottleneck for hydrogen applications. This comprehensive review delves into the intricate mechanisms governing the spontaneous ignition of pressurized hydrogen released through tubes. Beginning with an exploration of fundamental concepts, including the properties of hydrogen and various theoretical frameworks, the study meticulously examines the development of diffusion ignition theory. Methodologically, both experimental and simulation approaches are scrutinized for their contributions to understanding this complex phenomenon. Moving deeper into the core of the investigation, the review dissects the spontaneous ignition mechanisms occurring inside pipes, unraveling the influence of different initial conditions as well as the impact of varying pipe structures. Subsequently, the review shifts its focus to the flame morphology at the pipe exit, analyzing combustion behavior, shock wave dynamics, flow structures, and the formation of jet flames, while exploring influential factors affecting combustion. Through a comprehensive synthesis of findings, this review provides valuable insights into the spontaneous ignition dynamics of pressurized hydrogen within tube systems, offering a foundation for future research and engineering applications in safety and energy sectors. • Introduced theories of self-ignition and development of diffusion ignition theory. • Summarized ignition mechanisms of pressurized hydrogen released inside pipes. • Summarized flame morphology at pipe exit based on vortex position. • Identified deficiencies in reviewed studies and areas needing further research. [ABSTRACT FROM AUTHOR]

The numerical simulation method is used to study the influence of the mixing ratio of methane/hydrogen mixture on the combustion characteristics and pollutant emission characteristics of the combustor. The results show that due to the action of combustion chemical reaction, there exists a certain difference in the structure of cold and hot flow fields. The flow velocity of the hot flow field increases, and the recirculation zone becomes larger. The hydrogen content has a significant impact on the structure and temperature distribution characteristics of the hot flow field, and a central recirculation zone is formed when the hydrogen content is less than 20%, which can maintain stable combustion. When the hydrogen content is greater than 40%, the central recirculation zone disappears, the external recirculation zone is extended, and the spontaneous combustion and flashback occur to varying degrees. As the inlet air temperature increases, the spontaneous ignition phenomenon becomes more obvious, and the inlet air temperature decreases, the flashback phenomenon becomes more obvious. NOx emissions increase with the increase of hydrogen content, CO emissions decrease with the increase of hydrogen content, and CO is concentrated in the combustion zone of the main combustion stage.

During air injection into an oil reservoir, an oxidation reaction generates some heat to raise the reservoir temperature. When the reservoir temperature reaches an ignition temperature, spontaneous ignition occurs. There is a time delay from the injection to ignition. There are mixed results regarding the feasibility of spontaneous ignition in real-field projects and in laboratory experiments. No analytical model is available in the literature to estimate the oxidation time required to reach spontaneous ignition with heat loss. This paper discusses the feasibility of spontaneous ignition from theoretical points and experimental and field project observations. An analytical model considering heat loss is proposed. Analytical models with and without heat loss investigate the factors that affect spontaneous ignition. Based on the discussion and investigations, we find that it is more difficult for spontaneous ignition to occur in laboratory experiments than in oil reservoirs; spontaneous ignition is strongly affected by the initial reservoir temperature, oil activity, and heat loss; spontaneous ignition is only possible when the initial reservoir temperature is high, the oil oxidation rate is high, and the heat loss is low. [ABSTRACT FROM AUTHOR]

Copyright of Journal of Shanghai Jiao Tong University (1006-2467) is the property of Journal of Shanghai Jiao Tong University Editorial Office and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

This paper presents an experimental investigation into shockwave and flame characteristics of compressed hydrogen released through various types of pressure relief devices (PRDs) for which data have not been previously reported. Burst disks and safety valves with different set pressure (P0) of 22–140 MPa were tested. Shockwave intensity/velocity spontaneous ignition/flame behavior was monitored by in-situ pressure/light sensors, respectively. Previous works have mostly focused on burst disks with low P0 (under 10 MPa), leaving safety valves and high-pressure burst disks uninvestigated. It was found that shockwave/spontaneous ignition behavior differs with PRD types. Spontaneous ignition occurs in all burst disk cases, along with an ignition/self-extinguishment/reignition process with relatively low P0, which has not been revealed previously. In contrast, none of the safety valves cause spontaneous ignition, resulting from the absence of shockwave due to lower overpressure values/rise rate during release. This suggests that shockwave formed by sudden release is the most dominant factor in spontaneous ignition. Also, the occurrence of self-extinguishment does not guarantee the absence of jet flame. This work provides a comprehensive database revealing shockwave and flame characteristics of hydrogen released through different PRDs, which offers basic data and theoretical support for the safety and risk assessment of high-pressure hydrogen facilities. [ABSTRACT FROM AUTHOR]

High-pressure pipeline storage presents a promising method for widespread and efficient hydrogen transfer. However, challenges arise in mitigating pressurized hydrogen leakage due to hydrogen embrittlement issues associated with conventional pipeline materials. Experimental findings indicate that pressurized hydrogen is prone to spontaneous combustion, even at relief pressures as low as approximately 2 MPa - well below the permissible pipeline pressure in most countries. Despite this, there remains a lack of consensus regarding the mechanism of spontaneous ignition from high-pressure hydrogen leakage, and current research in this area is deemed insufficient. This study aims to analyze and discuss the presumed mechanisms of spontaneous ignition comparatively, review the progress in the study of spontaneous ignition of hydrogen in high-pressure leakage based on diffusion ignition theory, and statistically compare and discuss the influences of significant factors existing in pipelines (e.g., macro size factors and internal structure) and/or pipe failures (e.g., rupture factors) on spontaneous ignition. It is hoped that this article will provide scholars involved in the development of hydrogen energy and the theories of spontaneous combustion with a systematic understanding of these phenomena.

As an ideal energy source, hydrogen is highly susceptible to spontaneous ignition once leaked, which is an urgent issue that needs to be addressed. Based on the shock tube model, this paper investigates flame propagation under various pressures, tube lengths, and diameters by employing the LES approach and a detailed hydrogen/air combustion mechanism. The results indicate that within the tube, the ignition kernels gradually evolve into tulip flames when specific conditions are satisfied. As pressure and tube length increase, the likelihood of forming a complete flame rises significantly; with the increase of tube diameter, the flame front is flatter and the flame intensity is more uniformly distributed. Furthermore, this paper develops a model to predict the formation of a complete flame: Pb/Pa=570.64(L/D)−0.6. Outside the tube, once the intact flame passes out of the tube and evolves into a jet flame, structures such as flame envelopes and jet vortices will appear. Higher release pressures make it more difficult for the flame to propagate steadily, whereas increasing tube length and diameter promotes combustion and sustains the flame outside the tube.

In the context of hydrogen-based energy storage systems, the safeguarding against spontaneous ignition during high-pressure hydrogen release is of paramount importance. This study delves into the thermal safety and management technologies pertinent to such systems by numerically investigating the effects of pipeline geometry on the risk of spontaneous ignition. Employing Large Eddy Simulation (LES) coupled with detailed chemical kinetics and a linear eddy model, the research assesses the impact of different pipe angles and burst pressures on ignition behavior. The simulations are validated against experimental data, ensuring the veracity of the findings. The results demonstrate a significant interplay between the ignition propensity and both the geometrical configuration of the pipeline and the pressure of hydrogen release. Notably, the emergence and interaction of transverse waves in pipe bends are revealed to amplify mixing processes, generating vortices that elevate the temperature and promote a conducive environment for chemical reactions leading to stable flame propagation. The ignition is shown to occur predominantly near the stoichiometric mixture ratio, suggesting a narrow ignition region. These insights are vital for enhancing the safety protocols and thermal management strategies of hydrogen-based energy storage systems, paving the way for safer and more efficient energy solutions. [ABSTRACT FROM AUTHOR]

The effect of an extension tube with a rectangular slit inlet on the opening status of the metal diaphragm, as well as spontaneous ignition of pressurized hydrogen are experimentally investigated. The results show that the opening ratio of the metal diaphragm exhibits a positive correlation with the increase of the slit area. It also shows an initial increase followed by a decrease as the failure pressure rises. Moreover, the shock overpressure and Mach number inside the tube with the slit entrance are significantly reduced compared to those within the circular inlet tube. The critical pressure for spontaneous ignition in the tube with narrow inlet is notably affected by the slit parameters, increasing progressively with decreasing slit area and slightly increasing with raising slit aspect ratio. These experimental results would serve as valuable references for further exploring the failure modes of hydrogen storage containers and mechanisms underlying the spontaneous ignition. • The opening ratio of partially ruptured burst disks is statistically measured. • The slit inlet attenuates the overpressure but does not modify its axial trend. • The abnormal rupture of diaphragm leads to the compression waves preceding shock. • The threshold exceeded opening ratio is not the main determinant of self-ignition. [ABSTRACT FROM AUTHOR]

Copyright of Fire Safety Science is the property of Fire Safety Science Editorial Office and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

It is necessary to investigate the impact of different obstacle shapes on high-pressure hydrogen spontaneous ignition due to the role obstacles play in promoting its generation. This paper first carried out model validation and found that the simulation could accurately reproduce the position and time of the initial flame detection, flame evolution, and pressure in the triangular-shaped obstacle tube as observed in the experiments. Subsequently, simulations were employed to explore the impact of three different obstacle shapes (triangular-shaped, arc-shaped, and square-shaped) on spontaneous ignition with the tube. The variation in obstacle shapes affects the initial flame detection position and the length of the flame, while it has minimal impact on the spontaneous ignition time. The square-shaped obstacles result in the closest initial flame detection position point to the burst disk and the longest flame in the tube, followed by the arc-shaped obstacles, and finally the triangular-shaped obstacles. • The effects of the obstacle shapes were numerically studied on the spontaneous ignition of high-pressure hydrogen. • The four aspects of the simulation results were compared and verified with the experimental data. • The distribution of the flammable mixture was investigated inside each obstructed tube. • The evolution of hydrogen molar fraction was investigated in the axial and radial directions. [ABSTRACT FROM AUTHOR]

Accidental release of high-pressure hydrogen can induce the self-ignition, and the spontaneously combusting flame can eventually induce a non-premixed jet flame or even explosion. In this study, the explosion overpressure characteristic of hydrogen jet flame outside the pipeline was investigated. Experiments were conducted on high-pressure hydrogen release pipelines with a diameter of 10 mm and pipeline lengths of 750, 1450 and 2200 mm, respectively, under different hydrogen release pressure. The impact of different initial conditions on the peak value of high-pressure hydrogen jet explosion was systematically investigated. The external pressure signals and flame images were completely collected. The results show that the external explosion overpressure decreased with the increase of axial distance away from the nozzle. When the length of the pipeline increased, the peak value of the overpressure was also gradually decreased. Meanwhile, as the high-pressure hydrogen burst pressure increased, the overpressure rose first and then decreased. With the increase of release pressure, when the hydrogen pressure exceeded 8 MPa, the hydrogen flow in the tube formed a blocked flow, so the increase of external explosion overpressure became slow. This topic can provide a reference for the safety development of the high-pressure hydrogen storage system. • External explosion overpressure of hydrogen jet flame were studied experimentally. • When the critical pressure (8 MPa) of hydrogen gas exceeded, the increase of explosion overpressure decreased. • As the length of the tube increased, the external explosion overpressure decreased. [ABSTRACT FROM AUTHOR]

The piloted and the spontaneous ignition of low and high molecular weight (LMW and HMW) polymethyl methacrylate are simulated using a one-dimensional condensed-gas phase model for constant heat fluxes in the range of 25–150 kW/m2. Purely thermal (nitrogen) and thermo-oxidative (air) decomposition is considered, described by a single and four-step kinetics for the low and high molecular weight polymer, respectively. Different optical properties are also examined. The same trends of the ignition time and other ignition parameters are always observed. Due to a more significant role of the chemical kinetics, the effects of the sample molecular weight and reaction atmosphere are higher at low heat fluxes. Times are shorter for the black HMW samples and thermo-oxidative kinetics. For piloted ignition, factors are around 2.8–1.6, whereas for thermal decomposition, they are 1.3–1.2. The corresponding figures are 1.8–1.3 and 1.3–1.1, in the same order, for the spontaneous ignition. Overall, the effects of the molecular weight are more important than those related to the reaction kinetics environment. These differences are confirmed by the comparison between predictions and measurements. [ABSTRACT FROM AUTHOR]

Abstract Coal spontaneous combustion is determined by a variety of factors. Testing can describe the changes incoal spontaneous combustion with various factors, however, the dependence of spontaneous combustion on various factors is unclear. Therefore, reliability theory was used to deduce the functional relationship of the dependence of coal spontaneous combustion on various factors, and construct a model algorithm for predicting the probability of occurrence of coal spontaneous combustion, which is adopted to evaluate and rank the degree of influence of various factors on coal spontaneous combustion. Effective prevention methods are proposed by strengthening the role of the most important factors. The results show that, by taking the duration of coal heating to 150°C as the measurement standard of coal spontaneous combustion, the duration of the initial stage of coal heating increases linearly with the increase of specific heat capacity, thermal conductivity, and moisture content of coal. With the increase of oxygen concentration, oxidation rate, and initial temperature of the coal, the duration of the initial stage of coal heating decreases exponentially. With the increase of gas flow seepage velocity in the coal body, the duration of the initial heating stage changes in a parabolic manner. At the same time, the probability of spontaneous combustion decreases exponentially with the increase of specific heat capacity and moisture content of the coal. The probability of coal spontaneous combustion increases linearly with the increase of coal thermal conductivity, oxygen concentration, gas seepage velocity, and rate of oxidation. The dependence of coal spontaneous combustion probability on different factors can be expressed as follows: coal temperature > gas seepage velocity > specific heat capacity > oxidation rate > oxygen concentration > moisture content > thermal conductivity.

Due to the high probability of hydrogen leaking from narrow cracks of the vessels, an experimental study is conducted to investigate the spontaneous ignition of high-pressure hydrogen released into a tube through inlets with different narrowness, including circular, square (length-width ratio χ = 1), and two slit shapes of χ = 2 and 3, with the same area. Pressure transducers and photoelectric sensors are used to detect shock wave dynamic variation and ignition occurrence. The results indicate that narrow inlets weaken the shock wave intensity and speed, and the weakening effect increases with narrowness. Moreover, spontaneous ignition is affected. The minimum burst pressure required for spontaneous ignition is higher in the cases with square and slit inlets. Most significantly, when χ is 3 , it is 2.5 times that of circular inlet. Meanwhile, for narrow inlets, the ignition flame intensity and speed are enhanced, and the inlet of χ = 2 has the greatest impact. • The influence of narrow releasing ports on self-ignition hydrogen is investigated. • Narrow ports weaken the shock wave intensity and speed. • It requires higher burst pressure for self-ignition under narrow ports. • The flame intensity is enhanced under narrow ports. • Different narrowness of the ports brings different effects. [ABSTRACT FROM AUTHOR]

Coal spontaneous combustion is determined by a variety of factors. Testing can describe the changes incoal spontaneous combustion with various factors, however, the dependence of spontaneous combustion on various factors is unclear. Therefore, reliability theory was used to deduce the functional relationship of the dependence of coal spontaneous combustion on various factors, and construct a model algorithm for predicting the probability of occurrence of coal spontaneous combustion, which is adopted to evaluate and rank the degree of influence of various factors on coal spontaneous combustion. Effective prevention methods are proposed by strengthening the role of the most important factors. The results show that, by taking the duration of coal heating to 150°C as the measurement standard of coal spontaneous combustion, the duration of the initial stage of coal heating increases linearly with the increase of specific heat capacity, thermal conductivity, and moisture content of coal. With the increase of oxygen concentration, oxidation rate, and initial temperature of the coal, the duration of the initial stage of coal heating decreases exponentially. With the increase of gas flow seepage velocity in the coal body, the duration of the initial heating stage changes in a parabolic manner. At the same time, the probability of spontaneous combustion decreases exponentially with the increase of specific heat capacity and moisture content of the coal. The probability of coal spontaneous combustion increases linearly with the increase of coal thermal conductivity, oxygen concentration, gas seepage velocity, and rate of oxidation. The dependence of coal spontaneous combustion probability on different factors can be expressed as follows: coal temperature > gas seepage velocity > specific heat capacity > oxidation rate > oxygen concentration > moisture content > thermal conductivity. [ABSTRACT FROM AUTHOR]

The present research is dedicated to investigating the fire hazard risks of molten metal particles from electrical wires or devices, in enclosed places (houses, factories, etc.). A mathematical model is proposed for the calculation of the temperature of the molten particles as well as the heat flux they can transfer to the attached surface and the temperature of the attached surface. It is concluded that the probability of fire ignition for a specific bed is a function of the molten particles' diameter and the ratio of the contact to total surface. The proposed model can be used for fire investigation analysis in indoor places due to potential electric faults. [ABSTRACT FROM AUTHOR]

This paper presents an experimental investigation into shockwave and flame characteristics of compressed hydrogen released through various types of pressure relief devices (PRDs) for which data have not been previously reported. Burst disks and safety valves with different set pressure (P0) of 22–140 MPa were tested. Shockwave intensity/velocity spontaneous ignition/flame behavior was monitored by in-situ pressure/light sensors, respectively. Previous works have mostly focused on burst disks with low P0 (under 10 MPa), leaving safety valves and high-pressure burst disks uninvestigated. It was found that shockwave/spontaneous ignition behavior differs with PRD types. Spontaneous ignition occurs in all burst disk cases, along with an ignition/self-extinguishment/reignition process with relatively low P0, which has not been revealed previously. In contrast, none of the safety valves cause spontaneous ignition, resulting from the absence of shockwave due to lower overpressure values/rise rate during release. This suggests that shockwave formed by sudden release is the most dominant factor in spontaneous ignition. Also, the occurrence of self-extinguishment does not guarantee the absence of jet flame. This work provides a comprehensive database revealing shockwave and flame characteristics of hydrogen released through different PRDs, which offers basic data and theoretical support for the safety and risk assessment of high-pressure hydrogen facilities.

Hydrogen is regarded as one of the promising fuels in the next decades. However, the spontaneous ignition of high-pressure hydrogen blocks its safe utilization. In this investigation, a visualization investigation is carried out to study on the characteristic and mechanism of spontaneous ignition condition of high-pressure hydrogen during its sudden release into a tube. A rectangular tube with transparent observation window is employed. The high-speed images of spontaneous ignition conditions and light signals emitted from ignition are obtained. Four spontaneous ignition conditions are observed: (1) no ignition; (2) failed ignition with one ignition spot; (3) failed ignition with two ignition spots and (4) successful ignition. The critical burst pressure for four ignition condition is determined. The jet fire (successful ignition) could be generated in the cases with a shorter distance from ignition spot to leading shock wave and contact surface due to the stronger shock wave affection and sufficient flammable mixture maintaining the chemical reaction and flame propagation. The second ignition is the precondition forming the successful ignition. The scaled ignition delay time and distance could be predicted by using the derived dimensionless shock pressure. • Four ignition conditions are captured during the release of high-pressure hydrogen. • Critical burst pressures of these four ignition conditions are proposed. • Mechanism of failed and successful ignition of high-pressure hydrogen release is reveled. • The scaled ignition delay time and distance could be predicted using the dimensionless shock pressure P∗. [ABSTRACT FROM AUTHOR]

Spontaneous ignition of electrical wire caused by the Joule heat generated in core due to overloading or a short circuit is common, and is significantly influenced by wind velocity, but has been very limited addressed yet. This paper, for the first time, presents an experimental investigation in spontaneous ignition of overloaded wire under transverse wind (perpendicular to wire). Polyethylene (PE)-insulated wires with a NiCr core diameter of 0.8 mm and the insulation layer thickness of 0.3 mm and 0.15 mm are employed as samples. It can be found that with the increase of transverse wind velocity, the ignition delay time demonstrates a non-monotonic tendency (firstly decreases then increases, and decreases again, finally increases) until ignition limit, where four regimes are clarified according to different controlling mechanisms. The ignition delay time declines with the increase of electrical current. One of the most noteworthy findings is that the ignition delay time declines as the insulation thickness increases, which shows an opposite trend with the results of piloted ignition for thermally thin materials. In order to distinguish the ignition and non-ignition zones, an ignitability map considering both the gas phase kinetic effects and the solid phase heat loss effect is established based on two non-dimensional parameters [Damköhler number (Da) and R loss ]. A theoretical ignition model combining the effects of pyrolysis time, mixing time and chemical time is established, which well interprets the variation trend of ignition delay time and verifies the significant role of the chemical time in spontaneous ignition. This work provides essential knowledge on spontaneous ignition of overloaded electrical wires under practical transverse wind. [ABSTRACT FROM AUTHOR]

To evaluate the effect of porous magnesium oxide (MgO-P) as a stabilizer for nitrocellulose (NC), we investigated the release behavior of nitrogen oxides (NOx) and the exothermic decomposition of NC mixed with MgO-P and other reference samples. The NOx measurements under 100-135 °C isothermal conditions showed that NC/MgO-P barely released NOx at the beginning of the measurement (especially at 100 °C), suggesting that the MgO-P can absorb NOx derived from NC into its pores on the particle surface. The NOx-reducing effect of some of the tested MgO-P additives was stronger than the effects of NC alone, its mixture with nonporous MgO, and even its mixture with a conventional stabilizer (acardite II (AKII)). The isothermal calorimetry results obtained at 135 °C in an O2 atmosphere showed that, irrespective of the difference in porous and nonporous morphology, the NC/MgO prolonged the induction time of the exothermic peak for NC decomposition to a greater extent than the NC/AKII. Among the investigated MgO samples, the one with the highest alkalinity resulted in the longest induction time of NC decomposition, indicating that MgO suppresses NC exothermic decomposition by neutralizing NC-derived acid, which can accelerate the decomposition. [ABSTRACT FROM AUTHOR]

Sudden releases of pressurised hydrogen may spontaneously ignite by the so-called “diffusion ignition” mechanism. Several experimental and numerical studies have been performed on spontaneous ignition for compressed hydrogen at ambient temperature. However, there is no knowledge of the phenomenon for compressed hydrogen at cryogenic temperatures. The study aims to close this knowledge gap by performing numerical experiments using a computational fluid dynamics model, validated previously against experiments at atmospheric temperatures, to assess the effect of temperature decrease from ambient 300 K to cryogenic 80 K. The ignition dynamics is analysed for a T-shaped channel system. The cryo-compressed hydrogen is initially separated from the air in the T-shaped channel system by a burst disk (diaphragm). The inertia of the burst disk is accounted for in the simulations. The numerical experiments were carried out to determine the hydrogen storage pressure limit leading to spontaneous ignition in the configuration under investigation. It is found that the pressure limit for spontaneous ignition of the cryo-compressed hydrogen at temperature 80 K is 9.4 MPa. This is more than 3 times larger than pressure limit for spontaneous ignition of 2.9 MPa in the same setup at ambient temperature of 300 K.

Abstract This study investigated the combustion kinetics and spontaneous ignition of sweet sorghum using thermogravimetric analysis and the Frank-Kamenetskii theory. The aim was to determine the proper operating conditions for a direct combustion reactor and predict the safe ambient temperature limits for given silo designs. Oxidative heating rates of 2, 5, and 10 °C/min were set up. Graphical observation shows that combustion was composed of two different stages representing the overlapping processes of pyrolysis and char oxidation, at 131–336 °C and 336–475 °C, respectively. Samples were found to ignite at 215 °C and were extinguished at 433 °C. Different heating rates shifted combustion characteristics to higher temperatures and increased reactivity for ignition and combustion indices up to 12 and 10 times higher. The Friedman method determined the apparent activation energies representing the combustion reaction by 132.91 kJ/mol. Regarding spontaneous ignition, the temperature safe limits were predicted to be 83–84 °C and 84–87 °C for cylindrical and box silos with diameter and height of 15 and 10 m, respectively. Calculations of silos were designed within the limits of certain dimension ratios. Graphical Abstract

Direct numerical simulations (DNS) with detailed chemical kinetics and chemical explosive mode analysis (CEMA) are performed to investigate the effect of operating pressure and hydrogen–methane blending on laminar wrinkled flames at reheat combustion conditions. Building upon previous three-dimensional DNS datasets of reheat hydrogen flames, the present work considers two-dimensional configurations that render computationally-feasible simulations of high-pressure combustion and significantly more complex hydrocarbon chemistry. A geometrically-simplified representation of the reheat combustor is adopted and the thermodynamic states considered in the simulations are carefully chosen to mimic gas turbine operation conditions, which are constrained to achieve a target flame position and flame temperature. The two-dimensional DNS results are first assessed against the available three-dimensional data and then analyzed to extract the main trends exhibited by the reheat combustion process with respect to the fraction of the fuel consumed by spontaneous ignition and flame propagation, for increasing pressures and hydrogen fractions. Due to significant differences in the characteristic features of the velocity and thermal boundary layers between the three-dimensional and the two-dimensional configurations, a different global flame shape is observed (V-shaped flame vs W-shape flame) together with a ∼ 10 % bias in the fraction of fuel consumed by spontaneous ignition. Crucially, results from the scaling studies reveal very clear trends with a significant decrease in the fuel consumption by spontaneous ignition for increasing pressures and hydrogen fractions, at the conditions investigated. Finally, this study highlights the important role that first-principle direct numerical simulations, even when conducted in computationally affordable two-dimensional simplified geometrical configurations, can play in obtaining detailed insights and quantitative trends useful for the characterization of complex reactive flows. Novelty and significance The reheat burners of the two-stage sequential gas turbine combustors are designed to stabilize flames primarily due to spontaneous ignition. However, prior numerical simulations revealed the presence of an additional assisted-ignition mode aided by recirculation zones. In this study, we used practically feasible two-dimensional direct numerical simulations of premixed hydrogen–air mixtures under lean conditions to quantify the roles of the two flame stabilization modes at different operating pressures. Achieving fundamental understanding of the effect of pressure and hydrogen fraction on flame stabilization is crucial to the development of two-stage sequential combustion and the present two-dimensional calculations provide important new insights. We show that at high pressures, both modes consume fuel to a similar extent. We also investigated the effect of methane blending on flame stabilization. This study also discusses how two-dimensional direct numerical simulations can be leveraged in the design optimization of reheat burners at low technology readiness levels. [ABSTRACT FROM AUTHOR]

[Display omitted] • Smoldering and ignition indicators were determined for open MSW landfill. • Elevated CO level showed presence of surface waste smoldering. • Elevated temperature level is major cause behind spontaneous landfill fire. The waste receiving capacity of most municipal solid waste (MSW) landfill sites in India is exhausted, resulting in the formation of larger waste heaps. In the majority of Indian cities, these old waste heaps are prone to frequent smoldering and ignition resulting into fires. In this study, the potential risk of spontaneous ignition of landfilled waste at landfill surface was analyzed based on the physico-chemical characteristics of waste, carbon monoxide (CO) levels, landfill surface temperature (LST). The leachate pollution index was also determined to analyze the leachate quality for three different seasons (monsoon, pre-monsoon and post-monsoon). The regression analysis was carried out to understand the thermal properties (smoldering temperature, smoldering time, ignition temperature etc.) of MSW. The results showed that old waste has a higher tendency to undergo ignition compared to fresh waste. It has also been observed that the lower MC of old waste samples in the range of 3.4% and 18.2% is the most likely cause of early smoldering (106.6–109.5 °C) and ignition (198.6–208.4 °C) of old waste. In pre-monsoon season, CO concentrations for sub-surface (10–30 cm depth) smoldering events (SSE) were observed to be between ∼ 150 to 200 ppm. This CO level substantially dropped to 10 ± 1 ppm in the post-monsoon season. The estimation of the leachate pollution index (LPI) showed an index score of 27.35, 30.47 and 10.71 for pre-monsoon, monsoon and post-monsoon seasons, respectively. The determination of CO levels, increased LST and physico-chemical properties of landfilled waste will greatly assist in the abatement of environmental pollution arising from landfill fires. [ABSTRACT FROM AUTHOR]

Unexpected spontaneous combustion and shock waves can occur when high-pressure hydrogen leaks into pipelines, whilst irregular leakage ports affect their features. In this paper, shock waves and self-ignition are experimentally and numerically studied after the pressurized hydrogen is released through the partially open inlet. And the effects of tube length and release pressure are investigated. Pressure signals, light signals, and flame images are used to characterize the shockwave, self-ignition, and flame propagation. Results show that the shock-affected region can be formed near the partially open inlet. It is accompanied by complex wave structures, shock wave interactions, and shock wave focusing. The contact surface is distorted and deformed. The flow field parameters near the inlet change dramatically and are unevenly distributed, which affect the overpressure characteristics recorded by the pressure sensors. The initial intensity of the shock wave is lower than that in tubes with the fully open inlet at the early stage of the leakage. In addition, the partially open inlet influences the critical pressure at which spontaneous ignition occurs and the flame evolution inside or outside the tube. It has an inhibition effect on spontaneous ignition, but this inhibition effect weakens with increasing tube length. [ABSTRACT FROM AUTHOR]

Experiments on shock waves propagation, spontaneous ignition, and flame development during high-pressure hydrogen release through tubes with symmetrical obstacles (O 1-1) and asymmetrical obstacles (O 1-2) are conducted. The obstacle's side is triangular with a length of 4 mm, a height of 3.6 mm, and its width is 15 mm. In the experiments, a reflected shock wave generates and propagates both upstream and downstream when the leading shock wave encounters the obstacle. At the same burst pressure, the reflected shock wave intensity in tube O 1-1 is significantly greater than that in tube O 1-2. Moreover, the presence of obstacles in the tube can induce spontaneous ignition. The minimum burst pressures for spontaneous ignition for tubes O 1-1 and O 1-2 are 2.84 MPa and 3.28 MPa respectively, lower than that for the smooth tube. Furthermore, both the initial ignition position and ignition time are greatly advanced in obstruction tubes, mainly affected by obstacle positions and burst pressures. Finally, the flame separation process near the obstacle is observed. After passing the obstacle, the flames grow rapidly in radial and axial directions on the tube sidewalls. And at the same burst pressure, the flame convergence time in tube O 1-2 is usually longer than that in tube O 1-1. • The influence of obstacles arrangements on self-ignition hydrogen is investigated. • Reflected shock wave forms and propagates both upstream and downstream. • The presence of the obstacles can facilitate the occurrence of self-ignition. • Obstacles arrangements have great effects on ignition positions and ignition time. • The flame separation is observed inside both obstruction tubes. [ABSTRACT FROM AUTHOR]

The evaporation, autoignition and micro-explosion characteristics of RP-3 kerosene droplets under sub-atmospheric pressure (0.2–1.0 bar) and elevated temperature (473–1023 K) were experimentally investigated using high-speed camera technology. The results showed that the droplet evaporation rate increased monotonically with increasing temperature and pressure under 573–873 K and 0.2–1.0 bar. The decrease of temperature and pressure was obviously detrimental to the successful autoignition of droplets and increased the ignition delay time. Autoignitions at 0.2 bar were very difficult and required an ambient temperature of at least 973 K, which was about 150 K higher than the minimum ignition temperature at 1.0 bar. Sub-atmospheric pressure environment significantly inhibits the formation of soot particle clusters during the autoignition of droplet. Reducing pressure was also discovered to reduce the likelihood of micro-explosions at 673, 773 and 823 K but increase the bubble growth rate and droplet breakage intensity. Strong micro-explosions with droplet breakage time close to 1 ms were observed at 0.6 bar and 773/823 K, showing the characteristic of bubble inertia control growth. [ABSTRACT FROM AUTHOR]

In the process of hydrogen storage and transportation, if the high-pressure hydrogen in the storage tanks is released suddenly into the downstream pipelines, spontaneous ignition may occur, which can result in jet fire or explosion accidents. The obstacles such as the valves and welding slags inside the pipelines may affect the process and mechanism of spontaneous ignition of the leaked hydrogen. Therefore, a Computational Fluid Dynamics (CFD) model for the spontaneous ignition as a result of the high-pressure hydrogen leakage was developed and its reliability was validated by the experimental data. Subsequently, simulation analysis was performed for the spontaneous ignition process resulted from the high-pressure hydrogen leakage under the effect of obstacles at different locations in the pipelines. The results show that, the presence of obstacles has a significant influence on the shock wave propagation process in the pipelines, the variation of shock waves shows sensitivity to the obstacles, and the presence of obstacles in the pipelines will promote the flame propagation. In addition, the presence of obstacles increases the possibility of hydrogen spontaneous ignition in the pipelines, and the reflected shock waves generated by the obstacles will significantly change the spontaneous ignition mechanism of hydrogen. Generally, the research results could provide useful references for the design and safe operation of hydrogen pipelines.

Self-ignition may occur during hydrogen storage and transportation if high-pressure hydrogen is suddenly released into the downstream pipelines, and the presence of obstacles inside the pipeline may affect the ignition mechanism of high-pressure hydrogen. In this work, the effects of multiple obstacles inside the tube on the shock wave propagation and self-ignition during high-pressure hydrogen release are investigated by numerical simulation. The RNG k-ε turbulence model, EDC combustion model, and 19-step detailed hydrogen combustion mechanism are employed. After verifying the reliability of the model with experimental data, the self-ignition process of high-pressure hydrogen release into tubes with obstacles with different locations, spacings, shapes, and blockage ratios is numerically investigated. The results show that obstacles with different locations, spacings, shapes and blockage ratios will generate reflected shock waves with different sizes and propagation trends. The closer the location of obstacles to the burst disk, the smaller the spacing, and the larger the blockage ratio will cause the greater the pressure of the reflected shock wave it produces. Compared with the tubes with rectangular-shaped, semi-circular-shaped and triangular-shaped obstacles, self-ignition is preferred to occur in tube with triangular-shaped obstacles. [Display omitted] • Effects of multiple obstacles on hydrogen self-ignition is numerically studied. • The simulation results are compared and verified with the experimental data. • The effects of obstacle location, spacing, shape, and blockage ratio on the shock wave propagation are analyzed. • Reflected shock wave generated by obstacle will spread both upstream and downstream. [ABSTRACT FROM AUTHOR]

Sudden releases of pressurised hydrogen may spontaneously ignite by the so-called "diffusion ignition" mechanism. Several experimental and numerical studies have been performed on spontaneous ignition for compressed hydrogen at ambient temperature. However, there is no knowledge of the phenomenon for compressed hydrogen at cryogenic temperatures. The study aims to close this knowledge gap by performing numerical experiments using a computational fluid dynamics model, validated previously against experiments at atmospheric temperatures, to assess the effect of temperature decrease from ambient 300 K to cryogenic 80 K. The ignition dynamics is analysed for a T-shaped channel system. The cryo-compressed hydrogen is initially separated from the air in the T-shaped channel system by a burst disk (diaphragm). The inertia of the burst disk is accounted for in the simulations. The numerical experiments were carried out to determine the hydrogen storage pressure limit leading to spontaneous ignition in the configuration under investigation. It is found that the pressure limit for spontaneous ignition of the cryo-compressed hydrogen at temperature 80 K is 9.4 MPa. This is more than 3 times larger than pressure limit for spontaneous ignition of 2.9 MPa in the same setup at ambient temperature of 300 K. [ABSTRACT FROM AUTHOR]

Nitrocellulose (NC) is prone to spontaneous decomposition with exothermic heat release and thus is generally evaluated for stability during the storage process using standardized test methods such as the Abel test that measures the amount of nitrogen oxides (NOx) released from NC. In the present study, we evaluated the relative stability of various NC and NC-based propellants by monitoring the heat release behavior in an oxygen atmosphere via isothermal calorimetry. The results were compared with ones of the Abel test to evaluate the validity of the conventional stability test. The comparison revealed that there was no correlation between the stabilities predicted by the isothermal calorimetry and the results of the Abel test; some of the samples were evaluated as stable in the Abel test even if they released the decomposition heat easily, which means that the Abel test can occasionally overestimate the NC stability. In addition, the time change in the NOx release behavior based on the chemiluminescence method suggested that the Abel test mainly determined the desorption amount of NOx physically adsorbed on the surface of the samples, not that derived directly from the decomposition of NC. This possibly resulted in the incongruity with the results of the isothermal calorimetry, which instantly measures the heat released during decomposition. [ABSTRACT FROM AUTHOR]

Increasing hydrogen storage pressure brings high economic benefits and high risks. Pressurized hydrogen leakage spontaneous ignition experiment is an important means to reveal the mechanism of hydrogen leakage spontaneous ignition and improve the safety of hydrogen storage equipment. However, due to the extremely high cost and danger of ultra-high pressure, there is a serious lack of experimental data. In this paper, a pressure-ratio equivalent (PRE) method of experiments is proposed based on the theory of the shock tube problem. By keeping the hydrogen-air pressure ratio constant while reducing the absolute pressure of air and hydrogen, the difficulty of the experiment is greatly reduced. The effectiveness of the PRE method is evaluated theoretically and experimentally. The results show the PRE method retains the ignition characteristics of hydrogen leakage spontaneous ignition largely when the air pressure is within 0.05–0.1 MPa. It is found the pressure ratio of hydrogen to air dominates the leakage spontaneous ignition process. In the experiments of different air pressures, the shock Mach numbers are close to theoretical values. In addition, leakage spontaneous ignition of hydrogen mixed with 30% (vol.) CO is found in experiments using the PRE method, with pressure ratios of up to 250. This indicates that when the storage pressure is high enough, there is also a risk of spontaneous ignition of syngas from high-pressure leakage. The PRE method can widely broaden the pressure scope of experimental research on leakage spontaneous ignition, and it provides a new idea for obtaining the experimental data of gas high-pressure leakage spontaneous ignition. • Pressure scaling for auto-ignition at expanding high-pressure fuel jets is discussed. • A pressure-ratio equivalent method is proposed for high-pressure leakage auto-ignition experiments. • This pressure-ratio equivalent method is evaluated theoretically and experimentally. • High-pressure leakage auto-ignition characteristics of hydrogen are obtained through the proposed method. • The leakage spontaneous ignition of hydrogen mixed with 30% (vol.) CO is found. [ABSTRACT FROM AUTHOR]

This study investigated the combustion kinetics and spontaneous ignition of sweet sorghum using thermogravimetric analysis and the Frank-Kamenetskii theory. The aim was to determine the proper operating conditions for a direct combustion reactor and predict the safe ambient temperature limits for given silo designs. Oxidative heating rates of 2, 5, and 10 °C/min were set up. Graphical observation shows that combustion was composed of two different stages representing the overlapping processes of pyrolysis and char oxidation, at 131–336 °C and 336–475 °C, respectively. Samples were found to ignite at 215 °C and were extinguished at 433 °C. Different heating rates shifted combustion characteristics to higher temperatures and increased reactivity for ignition and combustion indices up to 12 and 10 times higher. The Friedman method determined the apparent activation energies representing the combustion reaction by 132.91 kJ/mol. Regarding spontaneous ignition, the temperature safe limits were predicted to be 83–84 °C and 84–87 °C for cylindrical and box silos with diameter and height of 15 and 10 m, respectively. Calculations of silos were designed within the limits of certain dimension ratios. [ABSTRACT FROM AUTHOR]

High altitude and low temperature is the common extreme environment for internal combustion engines. Under such operating conditions, heavy-duty diesel engines often suffer from serious abnormal combustion, such as knocking combustion, which results in piston crown breakdown and cylinder head erosion. Spray-wall impingement and pool fires are considered potential causes; however, the detailed mechanism remains poorly understood owing to the lack of research data. In this study, for the first time, the destructive abnormal combustion induced by diesel spray-wall impingement was identified using an optical rapid compression machine under engine-relevant conditions at high altitudes. Combining instantaneous pressure and temperature measurements with simultaneously recorded high-speed photography gives useful insights into understanding the detailed combustion processes. The experimental results show that depending on the extent of diesel spray-wall impingement, supersonic detonation-like reaction fronts featuring bright luminosity can be observed. The propagation of these reaction fronts in-cylinder results in severe pressure oscillations with an amplitude approaching hundreds of atmospheres, which is like the super-knock events in boosted direct-injection spark-ignition engines. Further parametric analysis indicates that the interplay between the diffusion combustion controlled by diesel spray and the premixed combustion dominated by attached film evaporation results in the formation of abnormal combustion. Destructive reaction fronts tend to occur at a prolonged ignition delay time, which facilitates the mixing between diesel evaporation and hot air. [ABSTRACT FROM AUTHOR]

This paper experimental investigated the shock wave propagation characteristics and selfrignition produced by the high-pressure hydrogen release in tile three-way tubes. Two Y-shaped tubes (60°, 120°) and one T shaped tube(180°) wereused in the experiments and the initial release pressure was 3-8 MPa. The pressure and photoelectric signals in tubes were recorded by the sensor. The results showed that the intensity of shock wave was enhanced or attenuated during the entire releasing process, but the dominant effect was distinct under different conditions and the two effects synergistically affected the occurrence possibility of self-ignition. The critical release pressure for selfrignition in the three-way tubes decreased with the increasing of the bifurcation angle, and the most difficult to occur the self-ignition was the 60° Y-tube in this study. In addition, quenching occurred in the 60° Y-tube when the initial release pressure was 6 MPa, because the temperature of the mixture dropped by the expansion effect. Furthermore, the intensity of the reflected shock wave was not strong enough to promote hydrogen rekindled. This experimental results have reference value for the safety of high-pressure hydrogen production, storage and transportation, and are helpful to understand the influence of bifurcation structure on self-ignition in energy application. [ABSTRACT FROM AUTHOR]

Abstract Biomass may ignite due to biological oxidation and chemical oxidation. If this phenomenon (spontaneous ignition) is controlled, it would be possible to produce biochar at a lower cost without the need for an external heat resource. We investigated if self-heating could be controlled by using sawdust and bark chips. When sawdust and bark chips were used under controlled conditions, the bark chips temperature increased to the torrefaction temperature. The ash content of bark chips was ~ 2%d.b. higher than that of sawdust; consequently, the inorganic substances contained in the bark chips might affect the self-heating. Self-heating was suppressed when inorganic substances were removed by washing with water. Therefore, the inorganic substances in the biomass might have affected self-heating. The inorganic element contents of the bark chips were measured by inductively coupled plasma optical emission spectrometry before and after washing. The potassium content of the bark chips was reduced remarkably by washing, and there was a possible influence of potassium on self-heating. Finally, the effect of moisture content on self-heating was investigated to obtain stable reactivity. Thus, at a moisture content of 40%w.b., a steady self-heating behavior may be realized.

Accidental release of high-pressure hydrogen can result in self-ignition, non-premixed jet flame and high overpressure, which will create potential security risks to people, buildings and equipment. In this study, pressure dynamics and flame induced by the self-combustible hydrogen flowing into the unconfined space were experimentally studied. The entire process was characterized by pressure sensors and cameras. Results show that the velocity of the hydrogen jet increases first and then decreases after it flows out of the tube. Its overpressure decays rapidly and stabilizes quickly. The dramatic changes for the flow field parameters in the near-field region can cause the self-ignition region to extinguish first and then reignite. And the overpressure in the near-field region, caused by the self-ignition jet flowing out of the tube, is lower in the unconfined space than that in the semi-confined space. In addition, axial and radial variations for the jet flame are characterized by phased development, which can be divided into three stages based on features of flame morphology and temperature distribution. Among them, a typical tadpole-like flame is formed. Its head with large size shows asymmetry, develops downward deflection and eventually separates from the flame body which are all affected by asymmetric large-scale vortices. Besides, the re-ignition of the jet can be induced during the nitrogen purge process, which is much more dangerous in real accident scenarios. [ABSTRACT FROM AUTHOR]

Accidental release of high-pressure hydrogen can result in self-ignition, non-premixed jet flame and high overpressure, which will create potential security risks to people, buildings and equipment. In this study, pressure dynamics and flame induced by the self-combustible hydrogen flowing into the unconfined space were experimentally studied. The entire process was characterized by pressure sensors and cameras. Results show that the velocity of the hydrogen jet increases first and then decreases after it flows out of the tube. Its overpressure decays rapidly and stabilizes quickly. The dramatic changes for the flow field parameters in the near-field region can cause the self-ignition region to extinguish first and then reignite. And the overpressure in the near-field region, caused by the self-ignition jet flowing out of the tube, is lower in the unconfined space than that in the semi-confined space. In addition, axial and radial variations for the jet flame are characterized by phased development, which can be divided into three stages based on features of flame morphology and temperature distribution. Among them, a typical tadpole-like flame is formed. Its head with large size shows asymmetry, develops downward deflection and eventually separates from the flame body which are all affected by asymmetric large-scale vortices. Besides, the re-ignition of the jet can be induced during the nitrogen purge process, which is much more dangerous in real accident scenarios. [ABSTRACT FROM AUTHOR]

The tendency of spontaneous ignition of high-pressure hydrogen during its sudden release into a tube is one of the main threats to the safe application of hydrogen energy. A series of investigations have shown that the tube structure is a key factor affecting the spontaneous ignition of high-pressure hydrogen. In this paper, a numerical study is conducted to reveal the mechanism of spontaneous ignition of high-pressure hydrogen inside the tube with local contraction. Large Eddy Simulation, Renormalization Group, Eddy Dissipation Concept, 37-step detailed hydrogen combustion mechanism and 10-step like opening process of burst disk are employed. Three cases with burst pressures of 3.10, 4.90, and 8.45 MPa are simulated to compare against the pervious experimental study. The spontaneous conditions and positions agree well with the experimental results. The numerical results indicate that shock wave reflection takes place at the upstream vertical wall of contraction part. The interacted-shock-affected region is generated at the tube center because of the subsequent shock wave interaction. The forward reflected shock wave couples with normal shock wave and increases the pressure of leading shock wave. The sudden contraction of tube blocks the propagation of hydrogen jet and decreases the speed from supersonic flow to subsonic flow. More flammable mixture is generated inside the contraction part, as a results, the length of the flame is increased. Two mechanisms are proposed finally. • Mechanism of spontaneous ignition of high-pressure hydrogen in the tube with local contraction is numerically studied. • Shock wave is reflected at the upstream vertical wall and interacts with each other at the tube center. • A reflected-shock-affected region and an interacted-shock-affected region are formed. • The results well reproduce the spontaneous ignition conditions and positions against previous experimental study. [ABSTRACT FROM AUTHOR]

This paper presents a numerical study of spontaneous ignition of a bagasse cube basket inside a convective oven. The study considered a cubicbagasse massas a porous medium allowing oxygen from the surroundings to diffuse into and react with the interior of the bagasse matrix. A set of coupled partial differential equations of heat and mass transfer was solved numerically with a well-known CFD softwarepackage,ANSYS FLUENT. The simulated results were then compared with experimental measurements. In general, good agreement between the simulated and measured centre temperatures of a cubic bagasse basket was obtained. Simulation results demonstrated that as the cubic bagasse basket size and itsinitial moisture content increased the spontaneous ignition time increased. Alternatively, decreasing the porosity increasedthe spontaneous ignition time. The differences between2D and 3D simulation resultswerealso investigated. It was found that the simulated centre temperature in the3D simulation was slightly higher than that obtained from 2D calculations.

This paper studies the effects of triangle obstacles inside the tube on the ignition mechanism and subsequent flame development in a semi-confined space when high-pressure hydrogen sudden release. Smooth tube and obstructed tube with optical glasses are constructed where controlled high-pressure hydrogen are released. High-speed direct photography is used to image the flame evolution while pressure transducers are used to obtain pressure-time traces both inside the tube and exhaust chamber. These cases that hydrogen with various burst pressure jets into the smooth and obstructed tubes are tested. It is found that spontaneous ignition occurs in the boundary layer of the tube in the smooth tube, a complete fire spanning the cross-section is reached as the flame propagates within the mixing region. The appearance of obstacles is found to have a significant effect on the ignition mechanism and flame structure. A two-way dissemination reflected shock wave forms and three possible ignition region emerges surrounding the obstacles. The flame propagation restricted between the leading shock wave and hydrogen jet. The flame experiences a split that the front flame in the vicinity of the obstacle gradually dies out and the rear flame continues propagating within the mixing region. The flame evolution and pressure variation in the exhaust chamber show that obstacles inside the tube do not aggravate the disaster in the semi-confined chamber. [ABSTRACT FROM AUTHOR]

To investigate the effect of ethanol concentration on the cool flame characteristics of isolated binary fuel droplet, experiments and numerical simulations on n-decane/ethanol droplet were conducted, varying the volume fractions of ethanol. Ambient pressure was set to atmospheric pressure and the temperature was varied from 600 to 660 K. Under these conditions, although cool flame was observed for n-decane, it did not induce hot flame ignition. CCD camera and K-type thermocouple were used to measure the droplet diameter and cool flame temperature, respectively. Moreover, one dimensional numerical simulation was performed with the fully transient numerical model. In addition to the assumptions on species flux, temperature continuity, fugacity equilibrium was assumed to simulate the evaporation process of multicomponent droplet. After the n-decane droplet was inserted into the hot ambience, evaporation was suddenly promoted and the temperature near the droplet surface decreased due to the cooling effect of evaporation. After the ignition of cool flame, the thermocouple nearest to the droplet showed the highest temperature, which implies that large heat release occurred near the droplet. When ethanol was added to n-decane, the cool flame ignition delay became longer. This is probably because the vapor formation of n-decane was delayed due to the high volatility of ethanol. However, the cool flame temperature was not significantly varied by the volume fraction of ethanol. This is probably because n-decane and OC10H19OOH accumulating at the cool flame location was almost the same for all conditions. [ABSTRACT FROM AUTHOR]

The self-heating and spontaneous ignition process pose a fire risk for industrial biomass piles during storage. Most studies, from theoretical to numerical, pay more attention on the effect of pile size on self-heating and self-ignition, which in essence is due to chemical reactions. However, the effect of ambient humidity on the self-heating and spontaneous ignition process, which is due to physical process of water evaporation and vapor condensation, is not well understood. In fact, fire accidents and the related experimental studies have shown that the sudden increase of ambient humidity would cause the rapid increase of biomass pile temperature, leading to spontaneous ignition. In order to fill this knowledge gap, this work proposed a computational self-heating model, coupling heat and mass transfer process and both the microbial and chemical reactions. The processes of moisture evaporation, transportation and convective exchange of vapor were considered in this model to study the effect of humidity on self-heating process. The model was validated against the full-scale experiments of Zhanjiang Biomass Power Plant firstly. The numerical results show that the sudden increase of ambient humidity can lead to a quick increase of pile temperature due to the condensation process. Spontaneous ignition is highly dependent upon the heat generation evolution of the chemical reaction within the pile. The sudden increase of humidity could speed up the chemical reaction process, leading to a fire. This study helps understand the role of humidity on self-heating process during biomass storage. [ABSTRACT FROM AUTHOR]

High-pressure hydrogen storage is currently the mainstream way of hydrogen storage. However, the risk of spontaneous ignition and subsequent jet flame are serious obstacles to the wide application of hydrogen energy. To further reveal the interaction of shock waves, vortices and flame induced by the spontaneous ignition of high-pressure hydrogen release under the effect of release process and burst pressure, a numerical study is conducted by employing LES, RNG, EDC models and detailed hydrogen/air combustion mechanism. The qualitative and quantitative comparison of flow field and flame splitting process outside the tube between this numerical study and previous experimental results validate the numerical results. It is found that an initial vortex is formed at the edge of the jet. Then, several vortices are generated at the edge of the jet and ahead of the Mach disk. The number of vortices in the outer side of the barrel shock is larger when the opening time is shorter. After moving out of the tube, the flame front becomes hemisphere and the lateral edge has a hemispherical bulge which is induced by the initial vortex. The temperature is low and the hydrogen concentration is high in the high-speed region which is induced by the reflected shock wave, leading to the splitting of the flame into two parts: the first and the second half of the flame. However, the flame cannot be formed at the initial stage when the opening time increases to 150 μs and the flame area is larger than with smaller opening times. After splitting, under the affection of the vortices generated at the lateral side, the first half of the flame rotates backward and gradually merges with the second half of the flame, finally forming a complete hydrogen jet flame. [ABSTRACT FROM AUTHOR]

The evaporation, autoignition and micro-explosion characteristics of RP-3 kerosene droplets under sub-atmospheric pressure (0.2–1.0 bar) and elevated temperature (473–1023 K) were experimentally investigated using high-speed camera technology. The results showed that the droplet evaporation rate increased monotonically with increasing temperature and pressure under 573–873 K and 0.2–1.0 bar. The decrease of temperature and pressure was obviously detrimental to the successful autoignition of droplets and increased the ignition delay time. Autoignitions at 0.2 bar were very difficult and required an ambient temperature of at least 973 K, which was about 150 K higher than the minimum ignition temperature at 1.0 bar. Sub-atmospheric pressure environment significantly inhibits the formation of soot particle clusters during the autoignition of droplet. Reducing pressure was also discovered to reduce the likelihood of micro-explosions at 673, 773 and 823 K but increase the bubble growth rate and droplet breakage intensity. Strong micro-explosions with droplet breakage time close to 1 ms were observed at 0.6 bar and 773/823 K, showing the characteristic of bubble inertia control growth.

Despite many advantages of the utilization of biomass as a renewable energy source, certain bottlenecks during biomass plant operation can be identified. Transport and collection of biomass as well as non-uniform material characteristics are issues related to decreasing efficiency of logistics and fuel manipulation which can also cause economic problems with biomass collection, transport, and storage. Since biomass is an especially reactive fuel, this has raised concerns over its safe handling and utilization. Fires, and sometimes explosions, are a risk during all stages of fuel production as well as during handling and utilization of the product. This paper presents a novel method for assessing ignition risk and provides a ranking of relative risk of ignition of biomass fuels. Tests within this method include physical and chemical properties of biomass, thermal analysis measurements, and the calculation procedure steps which were made using characteristic temperatures from thermogravimetry recordings. The results of thermogravimetry analysis were used for determination of tangent slope of the mass loss rate curves in devolatilization zone at considered heating rates for all tested samples. Linear interpolation of the data obtained by tangent slope analysis and used heating rates may provide unique straight line for each sample in the ignition testing. Thermogravimetry index of spontaneous ignition (TGspi) is obtained for all samples based on newly established formula. By varying gradient of linear dependence of self-heating coefficient against reference temperatures, mass and heat transfer limitations for various biomasses were discussed. The proposed method is accurate as well as relatively simple and quick, enabling determination of data necessary for design and application of appropriate measures to reduce fire and explosion hazard related to operation of biomass.

A microgravity experiment is going to be held in 2022 by using sounding rocket to clarify a cool flame dynamics of multiple n-decane droplets such as ignition delay time, ignition location, cool flame propagation, and so on. To predict the cool flame dynamics near ignition limit, 2D axisymmetric unsteady numerical calculation was carried out. A single droplet, droplet pairs whose inter-droplet distances are 8 mm and 16 mm, and droplet array with inter-droplet distance of 8 mm are simulated. Ambient temperature, pressure, and initial droplet diameter are 550 K, 1.0 atm, and 1.0 mm respectively. To acquire the mixture reactivity for the identification of the cool flame ignition start point, 0D reaction calculation is also carried out. It is found that the highly reactive mixture distributes in the outside of the droplets where temperature is high and mixture fraction is low. Temperature rise caused by the cool flame generation starts from the outside of the droplet pair, and the cool flame surrounds the droplet pair with small inter-droplet distance. In the case of the droplet array, the cool flame is generated at the outside of the array. As tracing the highly reactive mixture region, it propagates inward with average propagation velocity of 7.3 cm/s, and the cool flame propagation velocity accelerated just before burned out. [ABSTRACT FROM AUTHOR]