Critical Ignition Conditions Research Articles

Simulation of Heat Transfer and Propagation Velocity for Different Heat Loss Conditions for Guided Propagation Fronts in Reactive Multilayer Foils

Engineering self‐propagating reactions in reactive material systems requires an understanding of critical ignition and propagation conditions. These conditions are governed by the material properties of the reactive materials responsible for net heat generation, as well as by external environmental conditions that primarily determine net heat loss. In this study, it is aimed to utilize a numerical model to investigate the critical conditions for reaction propagation based solely on the heat‐transfer equation, enabling thorough examination with significantly low computational effort. Comparing simulations with experiments demonstrates a high level of agreement in predicting reaction propagation. Additionally, this numerical model provides valuable information regarding heat distribution in substrate materials.

Critical conditions for ignition of boron particles in chlorine

This paper provides an analysis of ignition and extinction conditions of boron particles (with a diameter of up to 200 μm) in a nitrogen-chlorine gas mixture. The analytical description is carried out in a dimensionless form without using the Frank-Kamenetsky distribution. This makes it possible to more correctly describe extinction conditions of particle. An analytical approach is used to analyze critical conditions, which consists in founding dependencies in the form of parametric functions. It is shown that, depending on the parameters of the gas environment (temperature and proportion of chlorine), there are hysteresis areas, areas of ignition and extinction, and areas of the crisis-free chlorination mode of boron particles.
 The obtained dependencies are compared with experimental results. Their satisfactory agreement allows us to confirm the kinetic parameters of the chlorination reaction of boron particles, namely the activation energy E = 60 kJ/mol and the pre-exponential factor k0 = 4 m/s.

The role of smoldering in the ignition of Pinus palustris needles

The pyrolysis process and the critical ignition conditions of dead Pinus palustris needles were studied experimentally to understand the role of smoldering in the onset of flaming combustion. The results obtained show that methane (CH4), carbon monoxide (CO), carbon dioxide (CO2), and water vapor (H2O) are the main constituents of the pyrolysis gases generated. The predominance of these compounds was found to be independent of the external heat flux, while their concentrations were sensitive to it. The heat of combustion of the pyrolysis gas and the pyrolysis reaction rate increased with increasing external heat fluxes. Visual observations and parameter measurements during testing hinted at the significance of smoldering to flaming ignition under forced flow conditions. The contribution of smoldering to flaming ignition was quantitively analyzed by combining the measured parameters during testing with the characterized pyrolysis process. The quantification results suggested a significant contribution of smoldering combustion to the flaming ignition under low heat fluxes and high airflow rate conditions.

Critical ignition conditions for propane/air premixtures impinging on electric heated surfaces: An experimental study

To contribute to exact predictions of the ignition of flammable mixture by a hot surface and ensure safety against fire, this study aims to clarify the critical ignition condition of a propane/air premixture impinging on a hot surface with external heating. The ignition threshold of heat flux depends on the flow velocity, but that of temperature hardly depends on flow velocity and equivalence ratio. The ignition occurred at the position where the residence time of species was the longest. It corresponds to the position of the lowest heat transfer coefficient. The temperature boundary layer thickness played a significant role in the ignition process as only the heat flux from the top, bottom, and back surfaces of the heater contributed to ignition in the presence of forced convection. The prediction model of the ignition threshold of heat flux and temperature was derived through dimensional analysis. This model effectively explains the ignition characteristics, regardless of the differences in thermal convection mode, dimensions, and geometric arrangements of a heater, as confirmed by comparison with reference data. Additionally, the ignition threshold of temperature can be predicted if the external heating power is known, and it is useful to assess the fire risk.

Dissociation and ignition of methane hydrate when in contact with typical sources of fire hazard

With their unique properties and structure, gas hydrates are a highly promising energy resource. In this research, we have experimentally studied the methane hydrate dissociation behavior when exposed to various sources of fire hazard. The experiments involved the ignition schemes most commonly identified as causes of fires: an open flame, a massive heated surface, a short circuit, and a local source of fire hazard. We have for the first time explored the impact of the following combination of the key factors on the ignition behavior: gas hydrate dissociation rate, heat flux, coverage of the free surface of the sample, gas diffusion rate, CO2 hydrate dissociation time during the combined methane hydrate and CO2 hydrate dissociation, as well as the background of the thermal field evolution in the gas hydrate. The experimental findings have become a foundation for a model predicting the critical conditions of hydrate ignition.

CRITICAL CONDITIONS OF SELF-IGNITION AT BIFURCATION POINTS

The article summarizes the results of the main research in the field of self-ignition theory. For symmetrical vessels, the following are presented: critical conditions of spontaneous ignition in dimensional coordinates, temperature distribution in the vessel, pre-explosive heating of fuel and the critical size of the vessel at the time of explosion. It is noted that the self-ignition conditions for different vessels differ only by a digital multiplier, which indicates that the shape of the vessel does not affect the physic-chemical processes going on in the fuel at the time of the explosion. Each singular point, being a bifurcation point, determines a number that allows finding a single critical condition of self–ignition from the set of solutions to the heat equation. This condition, in the coordinates of its variables, represents a multidimensional surface separating the zone of stationary existence of a combustible system from the zone of “no return”, where a combustible system cannot exist.

Ignition characteristics of methane-air mixture at low initial temperature

In this paper, FLUENT software coupled with the chemical reaction mechanism is used to study the ignition characteristics of methane-air mixtures at low temperature. Variations of the main free radical concentrations, the critical conditions for a successful ignition and the chemical reaction rate of each elementary reaction for a failure ignition are obtained, respectively. Results indicate that the consumption of methane immediately affects the concentration distribution of carbon monoxide, which presents the opposite relationship with that of methane. Mass fractions of the intermediate products and H, O free radicals are low in the burnt zone and unburnt zone, whereas reach the maximum on the flame front. OH is not only the intermediate free radical, but also the combustion product remained by the intermediate reactions. At a low temperature of 150 K, the critical radius and temperature of methane ignition are 4.6 mm and 1180 K, respectively. When ignition radius is smaller than the critical value, the failed ignition of methane is caused by the termination of the elementary reactions: R8: H + O2 = OH + O, R9: OH + O = O2 + H, R12: O + H2O = 2OH and R13: 2OH = O + H2O. However, when ignition temperature is lower than the critical value, because of the failed initiation of elementary reaction R1: CH4 = CH3 + H, the combustion of methane is not occurred.

Critical conditions of forced ignition in Ni-Al, Ti-Al powder systems under solid-phase synthesis

A theoretical study of the critical conditions of forced solid-phase ignition in Ti-Al, Ni-Al systems was carried out. Based on the analysis of the non-stationary diffusion equation with moving boundaries, the critical powers of the heating source and the corresponding critical temperatures were determined during the formation of NiAl, TiAl3 intermetallic compounds using the state diagrams of these binary systems. The diffusion activation energies, frequency factors, particle sizes, and heat removal conditions were considered as variable parameters. The diagrams of critical parameters which relate the corresponding specific heating powers with the indicated values were constructed. It has been established that for the same values of the parameters and under identical synthesis conditions, the critical temperatures in these systems differ by more than one hundred degrees. The latter is associated with a significant difference in the width of the solubility region for the corresponding compounds of the composition NiAl, TiAl3. The performed analysis allows separating the modes of isothermal sintering and thermal explosion which makes it possible to control the thermal and diffusion modes of synthesis. The comparison of the calculation results with the experimental data showed their satisfactory agreement.

Physical aspects of fire and explosion hazard of combustible dust. Part 1.Ignition

Based on the fundamental combustible systems properties - distributed nonlinear energy sources and flow connection between the environment elements - an analysis of various combustion dust modes is presented. Depending on the ratio betweencharacteristic burning times (thermokinetic characteristics) and heat transfer (scale characteristics), limiting modes are considered as self-ignition, diffusion combustion and auto-wave combustion. Unlike gases, for dusts all the characteristics of ignition and combustion essentially depend on the internal system scales - particle sizes and distances between them. In presented part of the study, the critical ignition temperature dependences and the ignition delay time on all physicochemical dust parameters are analyzed. It is shown that these characteristics for various fuels can be described in a unified way, based on the laws of oxidation (linear and parabolic), with the systematizing parameterfbeing the ratio of the solid fuel reaction surface to the external heat exchange surface.
 Taking into account the parameter f, which is the ratio of the dust reaction surface to the external aerosuspensionheat exchange surface of the dust, the critical ignition conditions are obtained in the form of a criterion similar to the single particles ignition. It is shown that it can be used for dust with both a linear and a parabolic oxidation law. Different fuels react in the same way to a change in f, which does not contain fuel characteristics and reflects thedustheat accumulation capability. Thus, the ignition temperature is a linear function of 1/lnf, and the induction period is 1/f.
 The ignition temperature and ignition delay time are not physical or chemical, but are relativedust activity properties in relation to the their ignitionprocess. After all, they depend on the size of the reaction vessel.

Benchmarking Between COMSOL and GPYRO in Predicting Self-Heating Ignition of Lithium-Ion Batteries

Recent studies have shown that self-heating ignition is a possible cause of fires when Lithium-ion batteries (LIBs) are stacked in large numbers, for example, during storage. The understanding of this ignition type is limited, and most current studies are based on numerical modelling. The different modelling tools found in the literature differ in their assumptions, capabilities, and resources needed, and may provide significantly different predictions. This study presents a benchmarking between COMSOL Multiphysics, which is one of the most prevailing tools used in modelling thermal-electrochemical behaviour of LIBs, and Gpyro, which is widely used in modelling ignition of solid fuels. Four case studies are designed with increasing levels of complexity: (1) just chemical kinetics at the microscale, (2) just heat transfer at the mesoscale, (3) self-heating behaviour at the mesoscale for coupled chemical reactions and heat transfer of a single cell, and (4) four-cell ensemble for multiphysics at a larger scale. The results of scenarios #3 and #4 are also compared to experiments. The results show that although COMSOL and Gpyro have significant differences in their assumptions and resources needed, both tools can accurately predict the critical conditions for ignition for self-heating, which validates their use to study the safety of LIBs.

Experimental study of the ignition conditions for self-sustained smouldering in peat

Tackling peatland wildfires, the largest fires on Earth in terms of fuel consumption, is an emerging combustion topic in the context of climate change and environmental protection. The understanding of the basic mechanisms of ignition of peat to initiate self-sustained smouldering is essential in the development of mitigation technologies, but it is not well studied yet in the literature. In this research, laboratory experiments were conducted to improve the understanding of how peat conditions (moisture content and bulk density) and the ignition protocol influence the ignition probability. A new ignition protocol was developed by stopping the heat source when 10% mass of the sample is lost. This mass-based ignition protocol was found to be robust to initiate self-sustained smouldering in peat samples for a wide range of soil conditions. By using the new ignition protocol, experiments using peat samples of moisture content from 100% to 180% with a range of bulk densities were conducted to investigate how these properties together influence the critical ignition conditions. Results show that although the moisture content plays a major role in the ignition probability, bulk density is also important. Increasing peat density increases the mass of water in a unit volume. This increase of heat sink makes the ignition more difficult. These findings contribute to studying smouldering peat fires and improve the understanding of ignition.

A Comparative Study on the Mathematic Models for the Ignition of Titanium Alloy in Oxygen-Enriched Environment

Metallic materials are always suffered from the risk of combustion when serviced under some extreme conditions such as high temperature, oxygen-enriched enrichment, and high-speed friction. Although different mathematic models have been proposed but it is still a challenge for accurately describing the ignition conditions of metals under the extreme conditions, which is of great significance for the safety-use of materials. In this paper, the mathematic models based on Semenov and Frank-Kamenetskii theory were introduced into describing the effects of size, oxygen concentration, and oxygen pressure on the ignition temperature and critical oxygen pressure of TC17 alloy. The results showed that the critical oxygen pressure of TC17 alloy increased with the increase of size, which was fitted well with the Frank-Kamenetskii model. As a comparison, the critical oxygen pressure was size independent in the Semenov model, which was inconsistent with the experimental data. For the Frank-Kamenetskii model, the fitting results showed that the activation energy, reaction order, as well as the adsorption coefficient of TC17 alloy were determined to be 99.23 kJ/mol, 1.69, and 4.01 MPa−1.69 respectively. Based on above, the ignition temperature of TC17 samples with different sizes were predicted well by the Frank-Kamenetskii model with the relative error within 3.58%, which could be suitable for describing the critical ignition conditions of bulk metallic materials under complex environment.

Ignition of hexane-air mixtures by highly under-expanded hot jets

We report an experimental study of ignition of flammable mixtures by highly unexpanded, supersonic hot jets. The high-pressure, hot-gas reservoir supplying the jet is created by impacting a projectile on a plunger to rapidly compress and ignite a rich n-hexane/air mixture, resulting in a peak reservoir pressure of more than 20 MPa. A locking mechanism was used to prevent the plunger from rebounding and the jet was created by rupturing a diaphragm covering a nozzle with an exit diameter between 0.25 and 1 mm. The jet development and ignition processes in the main chamber filled with hexane-air mixture were visualized using high-speed schlieren and OH* chemiluminescence imaging. The ignition threshold was determined as a function of composition in the jet and main chamber, the nozzle diameter, and the initial pressure in the main chamber. Unlike the case of subsonic jets in which ignition occurs at the shear layer near the nozzle exit, ignition of combustion in the main chamber was found to take place downstream of the Mach disk terminating the supersonic expansion and within the turbulent mixing region created by the startup of the supersonic jet. The results are interpreted using a constant-pressure, well-stirred reactor model simulating the mixing between the hot jet and cold ambient gas. The critical conditions for ignition are determined by the competition between energy release due to chemical reactions initiated by the hot jet gas and cooling due to mixing with the cold chamber atmosphere. The critical value (maximum for which ignition occurs) of the mixing rate was computed using a detailed chemical reaction model and found to be a useful qualitative guide to our observations.

Study of Critical Conditions of Spark Ignition and Burning Rate of a Boron Powder Suspension in a Propane–Air Mixture

Study of Critical Conditions of Spark Ignition and Burning Rate of a Boron Powder Suspension in a Propane–Air Mixture

Community Reaction Network Reduction for Constructing a Coarse-Grained Representation of Combustion Reaction Mechanisms.

A community-reaction network reduction (CNR) approach is presented for mechanism reduction on the basis of a network-based community detection technique, a concept related to pre-equilibrium in chemical kinetics. In this method, the detailed combustion mechanism is first transformed into a weighted network, in which communities of species that have dense inner connections under the critical ignition conditions are identified. By analyzing the community partitions in different regions, we determine the effective functional groups and driving processes. Then, a skeletal model for the overall mechanism is deduced according to the network centrality data, including transition pathway identification and reaction-path flux. The CNR method is illustrated on the hydrogen autoignition system which has been extensively investigated, and a new reduced mechanism involving seven processes is proposed. Dynamics simulations employing the present CNR model show that the computed ignition time and distribution of major species on a wide range of temperature and pressure conditions are in accord with the experiments and results from other methods.

Study of Critical Conditions of Spark Ignition and Burning Rate of Boron Powder Particles in a Propane-Air Mixture

Study of Critical Conditions of Spark Ignition and Burning Rate of Boron Powder Particles in a Propane-Air Mixture

Effect of boron on HEM radiant ignition characteristics

The paper presents the ignition characteristics of high-energy materials (HEMs) containing ammonium perchlorate, butadiene rubber, and a mixture of Al/B nanopowders with different component ratios. Bimetallic systems based on aluminum with boron increase the reactivity and intensify the ignition of boron particles, which helps to decrease the critical ignition conditions of HEMs during heating. It is shown that the use of systems based on aluminum-boron reduces the delay time (by 17–52 %) and the ignition temperature of propellants in comparison with a HEM containing aluminum powder, and increases the activation energy of HEM during radiant heating.

Thermal explosion characteristics of a combustible gas containing fuel droplets

This paper investigated the critical ignition conditions of combustible gas containing liquid fuel droplets. The analysis is done based on the criteria of the thermal explosion theory. It includes analytical and numerical solutions of modeling equations of fuel droplets heating and evaporation by convection and radiation from the surrounding reactive hot gas. The exothermic reaction is usually modeled as a single-step reaction obeying an Arrhenius temperature dependence. The thermal conductivity of the fuel droplet is dependent on temperature. The analytical solution produced relations between the main critical characteristic parameters in all planes of the solution. The results obtained from investigating the effect of the characteristic parameters on the explosion behavior of gas and fuel droplets and the thermal radiation proved that both of them are significant. The study proved that the criticality definitions of the thermal explosion of a single-phase system can be used effectively and efficiently to determine the critical conditions of a multi-phase system. Finally, the application of the numerical solutions of the modeling equations was used to analyze the explosion characteristics of a diesel fuel spray system.

Critical Ignition Conditions of Wood by Cylindrical Firebrands

This study investigated the thermal conditions preceding ignition of three dense woody fuels often found on structures by firebrands, a major cause of home ignition during wildland-urban interface (WUI) fires. Piles of smoldering cylindrical firebrands, fabricated from wooden dowels, were deposited either on a flat inert surface instrumented with temperature and heat flux sensors or on a target fuel (marine-grade plywood, oriented-strand board, or cedar shingles) to investigate critical conditions at ignition. The former provided thermal data to characterize the time before and at ignition, while the latter provided smoldering and flaming ignition times. Tests were conducted in a small-scale wind tunnel. Larger firebrand piles produced higher temperatures at the center of the pile, thought to be due to re-radiation within the pile. Ignition was found to be dependent on target fuel density; flaming ignition was additionally found to be dependent on wind speed. Higher wind speeds increased the rate of oxidation and led to higher temperatures and heat fluxes measured on the test surface. The heat flux at ignition was determined by combining results of inert and ignition tests, showing that ignition occurred while transient heating from the firebrand pile was increasing. Ultimately, critical ignition conditions from firebrand pile exposure are needed to design appropriate fire safety standards and WUI fire modeling.

Experimental Research on Igniting the Aviation Kerosene by W-Zr Reactive Fragment

Taking W-Zr reactive fragment as the research object, the influence of fragment density, shape and size parameters on the armor-piercing ignition fuel tank was studied through the simulated target test of 12.7mm ballistic gun launch. Combined with high-speed camera, the armor-piercing ignition behavior of the reactive fragment and ignition mechanism were revealed. The results show that the ignition fuel tank with low and medium density activity fragments has better effect, and the ignition speed of the fragments with high density decreases about 26%. The cube shape fragments have the best ignition performance, followed by cylindrical fragments and spherical fragments. The ignition ability of 7mm and 8mm fragments is equivalent, while the ignition ability of 8mm fragments is slightly stronger. The ability of W-Zr fragment to ignite aviation kerosene is closely related to the content of active components and the degree of breakage when it hits the target plate. Under the action of high-speed impact on the target plate, the fragments ware quickly broken into fragments with a certain flying Angle and high temperature combustion. Under the coupling action of the impact kinetic energy of the fragments and the chemical reaction heat, the critical ignition condition of fuel is reached, and the fuel tank is ignited.