Ignition Theory Research Articles

Theoretical Analysis of the Fire Boundaries of Lithium-Ion Battery Eruption Gases Caused by Thermal Runaway According to the Thermal Ignition Theory

According to the thermal ignition theory, the fire triangle (i.e. combustible, oxidizer, and ignition source) of lithium-ion battery (LIB) eruption gases, which are caused by thermal runaway, is key to causing LIB fires. The three fire boundaries corresponding to the fire triangle are the minimum battery eruption gases (BEG) concentration (cBEG, ignition), minimum oxygen concentration (cO2, ignition) and lowest temperature required for ignition. The purpose of this paper is to theoretically clarify the three fire boundaries of BEG. The BEG identification results of 29 thermal runaway tests in an inert atmosphere in open literature were summarized, and a BEG time-sequence diagram was drawn. According to Le Chatelier's mixing rule, the physical properties of gases and the thermal ignition theory, the three fire boundaries were theoretically analyzed for batteries with four types of cathode materials (i.e. LixCoO2 (LCO), LixFePO4 (LFP), LixNiyCozAl1-y-zO2 (NCA), and LixNiyCozMn1-y-zO2 (NMC)) at different state of charge (SOC) (0%~143%). The results showed that both of cBEG, ignition and cO2, ignition had different trends for different types of batteries with the increase in the SOC values at discharged states, but they were almost unchanged in the full and overcharged states. For the four types of batteries, from low to high, cBEG, ignition was NMC < LCO < NCA < LFP, and cO2, ignition was NCA < LFP < NMC < LCO. By controlling the SOC and/or selecting a reasonable battery type, cBEG, ignition and/or cO2, ignition could be changed, thereby changing the probability of battery fire. Batteries were prone to forced-ignition with forced-ignition sources regardless of the BEG temperature. When the eruption gases of a battery are at the eruption temperature without forced-ignition sources, the battery is prone to auto-ignition. However, the battery is hard to auto-ignited when the BEG temperature is cooled below the minimum self-ignition point (about 260 ℃) of the BEG components. The BEG ignition mode can be controlled by changing its temperature and ignition sources.These results can be used to guide battery pack design, thermal management system design and fire safety protection.

A methodology for stratified-charge preparation via low-reactivity fuel multi-injection strategy in intelligent charge compression ignition (ICCI) mode

For optimizing the role of low-reactivity fuel in dual-fuel combustion, a new methodology for stratified-charge preparation via a multi-injection strategy of low-reactivity fuel was introduced on the theory of intelligent charge compression ignition (ICCI). An experimental study was conducted on a modified single-cylinder engine with two common-rail systems fed by commercial gasoline and diesel. The split ratio and injection timing of the first and second gasoline injection were thoroughly studied on engine combustion and emissions. Compared with the gasoline single-injection strategy, the optimized in-cylinder stratified condition can be achieved by gasoline multi-injection strategies. In regards to engine performance, the indicated thermal efficiency can reach up to 52%, enabling an increase of combustion efficiency and low NOx emissions well below 0.4 g/kWh. Among the different gasoline split injection ratios, the combustion and emissions of the 3:1 ratio are most affected by the variation of the gasoline second injection timing. The split gasoline ratio of 2:1 improves combustion efficiency and burning temperature, slightly increasing NOx emissions and combustion noise. Earlier injection timing and higher proportions of the first gasoline injection can ensure that the low-reactive fuel in the cylinder is in a more homogeneous environment, resulting in higher efficiency and lower emissions.

Ignition and Burning of Fibreboard Exposed to Transient Irradiation

Natural materials like wood are increasingly used in the construction industry, making the understanding of their ignition and burning behaviour in fires crucial. The state of the art of wood flammability is based mostly on studies at constant heating. However, accidental fires are better represented by transient heating. Here, we study the piloted ignition and burning of medium density fibreboard (MDF) under transient irradiation. Experiments are conducted in a Fire Propagation Apparatus under parabolic heat flux pulses with peak irradiation ranging from 30 to 40 kW/m2 and time-to-peak irradiation from 160 to 480 s. The experimental results reveal that the critical conditions for ignition of fibreboard vary over wide ranges: mass flux between 4.9 to 7.4 g/m2-s, surface temperature between 276 to 298°C, and heat flux between 29 to 40 kW/m2. Flameout conditions are studied as well, with observations of when it leads either to extinction or to smouldering combustion. We explored the experiments further with a one-dimensional pyrolysis model in Gpyro and show that predictions are accurate. Assuming a non-uniform density profile (a realistic assumption) improves the predictions in comparison to a uniform density profile by increasing the mass loss rate by 12%, decreasing the temperatures by 45%, and increasing the ignition time by 20 s. These results further support previous findings that a single critical condition for igntion or flameout established under constant irradiation does not hold under transient irradiation which indicates that ignition and extinction theories need improvements.

Critical Conditions and Induction Time of Ignition for a Methane–Air Mixture

Combustion and ignition processes are considered from the standpoint of the theory of thermal ignition and reaction kinetics. An analytical solution to the problem of thermal hotspot ignition is obtained to describe the ignition of reactive gas mixtures. As a result of this solution, the critical ignition conditions and induction time were determined. A solution to the problem of ignition of a methane–air mixture characteristic of mine working conditions.

Experimental and Mechanism Study on Crude Oil Spontaneous Ignition during the Air Injection Process

The spontaneous ignition behavior is considered as one of the most important factors to the success of air injection process (AIP) application. Although the theory of crude oil spontaneous ignition...

Theory of ignition and burn propagation in inertial fusion implosions

A detailed analytic model is presented here to investigate the physics of burn propagation in inertially confined plasmas. The onset of ignition and burn propagation occurs when alpha heating of the hot spot causes rapid ablation of shell mass into the hot spot. This allows large energy gains to be achieved since most of the fuel mass is located in the shell. Here, we first present a comprehensive review of previous analytic models that have been used to describe the physics of hot-spot evolution and ignition; we then show that a proper description of a propagating burn wave requires a comprehensive model of hot spot and shell evolution that includes proper mass conservation in the shell, fusion reactivity, and fuel depletion. The analytic theory is in good agreement with detailed radiation-hydrodynamic simulations that predict the onset of burn propagation as occurring when the yield enhancement caused by alpha heating is between 15- and 25-fold, fα ∼ 1.4, where fα = alpha energy deposited/hot-spot energy at bang time, and the hot-spot burnup fraction is approximately 2%. We show that the definition of ignition is not sensitive to the alpha-particle stopping power nor asymmetries provided that the absorbed fraction of alpha particles θα is correctly accounted for. Finally, we use the results of 2-D simulations to show that even when θα is small and unknown (as is true in hot spots with mid modes that have significant leakage of alpha particles into the surrounding cold bubbles), one can still relate the experimentally measureable parameter χα53 to the yield amplification and the burning-plasma parameter Qαhs = alpha energy deposited/total input work delivered to the hot spot.

Ignition of thin fuel by thermoplastic drips: An experimental study for the dripping ignition theory

Ignition by dripping flame is widely observed in wire fires and façade fires, but little research has quantified its fire hazard. This work studies the ignition of thin papers (0.07–0.32 mm) by burning polyethylene drips with four sizes (2.6–6.2 mg) and dripping frequencies (0.8–1.8 Hz). The probability of dripping ignition as a function of key dripping parameters is quantified to determine the ignition limits. As the paper thickness increases, more drips and longer time are required for ignition, similar to the classical pilot ignition of thin fuels. The attached flame acts as the piloted source; heating effects from hot drips and dripping flame are comparable; and ignition occurs to the paper rather than landed drips. Moreover, the dripping-ignition capability is controlled by the dripping mass rate, which is the product of the drip mass and the dripping frequency. For the dripping mass rate of about 4.5 mg/s, the equivalent heat flux is 15 ± 3 kW/m2. The dripping-ignition time is inversely proportional to the mass of drip and the square of the dripping frequency, different from the piloted ignition under irradiation. This work provides important information to quantify the fire hazard of dripping and explores the ignition mechanism in dripping fire.

Theoretical study on ignition of titanium alloy under high temperature friction condition

Combustion of titanium alloy is a typical catastrophic failure of modern aeroengine. The abnormal friction between compressor rotor and stator is the main ignition source. A thermal theory model with friction heat source of titanium alloy is established based on the theory of heterogeneous ignition. The corresponding equation of critical temperature and ignition delay time are derived. The difference between the frictional ignition model and the classic model is discussed. The concept of critical heat generation temperature is proposed. The difference from the heterogeneous ignition model, and the effects of friction coefficient, oxygen concentration, flow velocity, contact radius and flame retardant layer thickness on the ignition parameters are analyzed. The research result shows that when the instantaneous temperature of the contact surface is lower than the critical heat temperature, the heat generation process is dominated by frictional heat, and when the temperature is higher than the critical heat temperature, the heat generation process is dominated by chemical reaction heat, that reducing the coefficient of friction can dramatically increase the critical temperature, but the change of friction coefficient has very little effect on the ignition delay time which can be ignored, that the critical temperature decreases significantly with the increase of oxygen concentration and the decrease of flow velocity. When the oxygen concentration increases from 21% to 42% and the flow velocity decreases from 310 m/s to 50 m/s, the critical temperature decreases by about 213 K and 197 K, respectively. The relative error between the experimental result and the theoretical result is 8.3%, which verifies the reliability of the model. The contact area has an effect on friction heat generation, reaction heat generation, and surface heat dissipation, and has a great influence on the critical temperature. The critical temperature decreases exponentially with contact radius increasing. When the contact radius increases to 0.007 m, the ignition temperature of the titanium alloy and its flame retardant layer are 899 K and 988 K, respectively. The increase of the thickness of flame retardant layer can effectively improve the critical temperature and ignition delay time. The critical temperature of titanium alloy with flame retardant layer is increased by about 172 K, and the ignition delay time is increased by about 3 s.

Investigation of methanol ignition phenomena using a rapid compression machine

The ignition phenomena of stoichiometric methanol/oxygen/argon mixture are comprehensively investigated at p = 12–24 bar, T = 840–1000 K, using a rapid compression machine (RCM). A strong tendency of stochastic ignition followed by non-forcible flame propagation at a speed of ∼11 m/s is demonstrated. Such an event may be responsible for the pre-ignition or super-knock issue in methanol engines. Under engine relevant spark ignition conditions, different ignition regimes are observed in the end-gas, including thermal explosion, supersonic deflagration, and detonation, characterized by the Chapman–Jouguet velocity criterion. All modes originate from a similar, early auto-ignition ahead of the spark-triggered flame. The auto-ignition process is proved to be dominated by chemical kinetics, where the Livengood–Wu correlation is applicable. In addition, the transition mechanism of different ignition regimes is thoroughly validated against previous ignition theories. Basically, the detonation onset is closely related to the initial thermodynamic condition of the mixture. At a given temperature, the decrease of pressure induces the gradual substitution of detonation by supersonic deflagration and thermal explosion due to a smaller reactivity gradient in the end-gas. The ε−ξ diagram proposed by Bradley is adopted to interpret the transition mechanism of methanol, which turns out to be different from previous results of isooctane. A moderate burned mass fraction range of 0.35–0.45 is found when detonation is initiated.

Application of the thermal pyrolysis model to predict flame spread over continuous and discrete fire load

Classical thermal theory of piloted ignition is applied in CFD predictions of flame spread over the combustible surface and fire growth in a rack-storage facility. In the thermal model, flaming ignition and burning of a solid combustible is described in terms of the ignition temperature and the prescribed burning rate, thereby avoiding consideration of the finite-rate pyrolysis, reducing the number of the model parameters, and enabling development of the simplified analytical theory. The analytical theory formulated in this work is shown to replicate the experimental dynamics of either pyrolysis zone propagation or total heat release rate; simultaneous replication of both is not possible because of unrealistic assumption of the planar pyrolysis front. Thorough investigation of the thermal model performance in CFD simulations of upward propagation of the turbulent flame over the vertical combustible surface (in comparison with the predictions by the finite-rate pyrolysis model) reveals the drawbacks inherent in the thermal model. In contrast with the finite-rate model and the experimental data available, the thermal model predicts an abrupt increase of the burning rate in the infinitely thin front and assumes spatially independent burning rate in the pyrolysis zone. To overcome these drawbacks, a simple modification to the thermal model is introduced allowing for the steady burning rate to increase with the vertical distance.The abovementioned limitations of the thermal model are not expected to be crucial for an array of combustible items with the vertical size of about 1 m and below. As such, the thermal model offers a simple and robust approach to predict fire growth in rack storage facilities. FDS simulations have shown a good representation of the experimental growth of the heat release rate in a rack storage fire. The model parameters (ignition temperature, mass burning rate, burn-out time, and heat of gasification) must be properly selected for a given type of fuel load. Dynamics of fire growth predicted in this work with the thermal pyrolysis model is in agreement with that observed in the full-scale test and in the earlier published simulations with a comprehensive pyrolysis model.

Stagnation-point polydisperse spray flame ignition

A theory of stagnation-point flow polydisperse spray flame ignition by an isothermal hot surface is presented for the first time. The configuration investigated consists of a mixture of fuel droplets and air flowing against an isothermal hot surface (such as a hot ignition probe). The polydisperse spray of droplets is modelled using the sectional approach. A single global chemical reaction is assumed for the case when ignition occurs. The mathematical analysis makes use of a small parameter that is exploited for an asymptotic approach. An analytical criterion for ignition is derived which includes effects of the flow field, the reactants and all the fuel spray-related parameters, including the initial size distribution of the spray's droplets. Numerical calculations disclose how the actual droplet size distribution impacts on the critical stagnation point temperature necessary to promote ignition. Additionally, the analytical estimates are compared with predictions of a numerical finite difference code with very satisfactory agreement.

APPLICATION OF SINGULAR PERTURBATION IN THE ANALYSIS OF LAMINAR PREMIXED FLAMES

Singular perturbation is widely used to obtain the approximate solutions for mechanical problems. A typical problem is the boundary layer in fluid mechanics. Yung-Huai Kuo has developed the singular perturbation theory for the boundary layer over a plate. Similar to the boundary layer in fluid mechanics, the laminar premixed flame in combustion can also be analyzed by the singular perturbation method, which is usually called as the large-activation-energy asymptotics. The laminar premixed flame structure consists of the preheat zone, reaction zone, and equilibrium zone. Under the limit of the large activation energy, the chemical reaction rate is very sensitive to the temperature and thereby chemical reaction only occurs in the very thin reaction zone. The ratio between the reaction zone thickness and the preheat zone thickness is a small parameter, based on which the asymptotic analysis can be conducted for a laminar premixed flame. This paper reviews the application and progress of the large-activation-energy asymptotic analysis in the one-dimensional planar premixed flame and spherically propagating flame. First, the structure of the premixed flame and its different characteristic length scales are introduced. The length scale separation due to large activation energy is analyzed. The detailed derivation of the large-activation-energy asymptotic analysis of a planar premixed flame is presented. The analytical solutions for the preheat zone, reaction zone, and equilibrium zone are first sepratedly obtained and then matched aound the interfaces among these three zones. The effects of radiation heat loss on premixed planar flames are discussed. Then, the application of the large-activation-energy asymptotic analysis to the ignition and spherically propagating flame is introduced. It is pointed out that in order to accurately predict the critical ignition conditions, the theory should be able to describe both the ignition kernel development and the spherical flame propagation afterwards. The ignition and flame propagation theory considering chain reactions is discussed, and the trend of theoretical research on flame instability is described. Moreover, the effects of radiation heat loss on spherical flame propagation are discussed. Finally, the future research directions are prospected based on the current research progress, which includes multi-step chemistry, low-temperature cool flame, complicated flow and radiation reabsorption.

Experimental analysis of the pyrolysis of solids exposed to transient irradiation. Applications to ignition criteria

Ignition and flame spread theory is fundamental for evaluating the risk posed by a material during the early stages of a fire. This paper presents an experimental investigation aimed at understanding the parameters which govern the ignition of solids exposed to transient irradiation. Emphasis is placed on the conditions at ignition, including an energy balance to describe surface phenomena and the link between gas phase and solid phase processes. Experiments were performed in a fire calorimetry apparatus and incorporated a gas analysis system to study gas phase composition. Samples of Polyamide 6 (PA6) measuring 85 × 85 × 20 mm were used. Experiments were carried out to independently measure temperature in the solid phase and mass loss rate (MLR) over time. The MLR at ignition was calculated to be between 2.0 and 6.0 g/(m2 s) for all but 3 experiments, were outliers presented values of 7.9, 10.7 and 14.4 g/(m2 s). Temperature was recorded through the thickness of the solid, at depths of 4, 8, 12 and 16 mm from the surface. A regression analysis was used to calculate the surface temperature at ignition for all experiments, and it was found to vary between 270 and 325 °C for all but one experiment, were a temperature of 402 °C was recorded. The temperature distribution in the solid phase was used to estimate the net absorbed heat flux at the surface by applying Fourier’s law; with values ranging between 2.0 and 9.8 kW/m2. From the gas analysis, it was possible to assess the identity, mass flux and concentration of three dominant species produced before ignition: carbon monoxide, methane and hexane. These results are of value for the physical modelling of ignition and flame spread phenomena, allowing for more accurate criteria to describe the onset of ignition under a range of heating conditions.

Piloted ignition delay times on optically thin PMMA cylinders

The theory to predict ignition of solid fuels exposed to incident radiant heat fluxes has permitted to obtain simple correlations of the ignition delay time with the incident heat flux which are useful in practical engineering applications. However, the theory was developed under the assumption that radiation does not penetrate into the solid phase. In the case of semi-transparent solids, where the penetration of radiation plays an important role in the heating and subsequent ignition of the fuel, the predictions of the classical ignition theory are not applicable. A new theory for the piloted ignition of optically thin cylindrical fuels has been developed. The theory uses an integral method and an approximation of the radiative transfer equation within the solid to predict the heating of an inert solid. An exact and an approximate analytical solution are obtained. The predictions are compared with piloted ignition experiments of clear PMMA cylinders. The results indicate that for opticallythin media, the heating and ignition are not sensible to the thermal conductivity of the solid, they are highly dependent on the in-depth absorption coefficient. Using the approximate solution, the correlation 1/tig∝q˙inc″ was established. This correlation is adequate for engineering applications, and allows the estimation of effective properties of the solid fuel. The form of the correlation that was obtained is due to the integral method used in the solution of the heat equation, and does not imply that the semi-transparent solid behaves like a thermally thin material. The approximate solution presented in this article constitutes a useful tool for pencil-and-paper calculations and is an advancement in the understanding of solid-phase ignition processes.

Effects of asymmetry and hot-spot shape on ignition capsules

Asymmetric implosion of inertial confinement fusion capsules is known, both experimentally and computationally, to reduce thermonuclear performance. This work shows that low-mode asymmetries degrade performance as a result of a decrease in the hydrodynamic disassembly time of the hot-spot core, which scales with the minimum dimension of the hot spot. The asymmetric shape of a hot spot results in decreased temperatures and areal densities and allows more alpha particles to escape, relative to an ideal spherical implosion, thus reducing alpha-energy deposition in the hot spot. Here, we extend previous ignition theory to include the hot-spot shape and quantify the effects of implosion asymmetry on both the ignition criterion and the capsule performance. The ignition criterion becomes more stringent with increasing deformation of the hot spot. The new theoretical results are validated by comparison with existing experimental data obtained at the National Ignition Facility. The shape effects on thermonuclear performance are relatively more noticeable for capsules having self-heating and high yields. The degradation of thermonuclear burn can be as high as 45% for shots with a yield lower than 2×10^{15} and less than 30% for shots with a higher yield.

Thermal theory of aluminum particle ignition in continuum, free-molecular, and transition heat transfer regimes

Most studies on nano- and micro- sized aluminum particle ignition have been focused on the processes occurring inside particles. In the current paper, thermal ignition of an aluminum particle in the air is simulated with different heat transfer models: continuum, free-molecular, and Fuchs model. A single parabolic oxidation law is assumed in the particle size range from nano- to millimeter diameters. A particle is considered ignited when it reaches the oxide melting point. The criterion defining the limits of validity for each model is the ratio of continuum and free-molecular heat transfer rates. The dependence of ignition temperature Ti on the particle size is in qualitative agreement with the experimental trends: Ti can have values in the range of 700–1500 K for nanoparticles due to the dominating contribution of a free-molecular heat transfer, and sharp growth of Ti with the particle size in the range of 1–100 μm diameter is due to the transitional character of heat transfer. For small values of the accommodation coefficient, ignition may occur in the critical ignition mode with the thermal runaway. The results suggest the importance of non-continuous heat transfer and, in particular, energy accommodation in ignition of nano- and micro- sized particles.

Ignition and pusher adiabat

In the last five years, large amounts of high quality data on inertial confinement fusion (ICF) experiments were produced at the National Ignition Facility (NIF). From this data we have significantly advanced our scientific understanding of the physics of thermonuclear (TN) ignition and identified critical issues that must be addressed to achieve a burning hotspot, such as implosion energetics, pusher adiabat, tamping effects, and confinement time. In this paper we present a review of recently developed TN ignition and implosion scaling theory (Cheng et al 2013 Phys. Rev. E 88 041101; Cheng et al 2014 Phys. Plasmas 21 10270) that characterizes the thermodynamic properties of the hotspot and the ignition criteria for ICF. We compare our theoretical predictions with NIF data and find good agreement between theory and experiments. We demonstrate the fundamental effects of the pusher adiabat on the energy partition between the cold shell and the hot deuterium–tritium (DT) gas, and thus on the integrated performance of ICF capsules. Theoretical analysis of NIF experiments (Cheng et al 2015 Phys. Plasmas 22 082704; Melvin et al 2015 Phys. Plasmas 22 022708; Cheng et al 2016 Phys. Plasmas 23 120702) and physical explanations of the discrepancies between theory, data, and simulations are presented. It is shown that the true experimental adiabat of the cold DT fuel can be inferred from neutron image data of a capsule implosion. We show that the ablator mix and preheat in the cold fuel can be estimated from the experimentally inferred hotspot mix. Finally, possible paths forward to reach higher yields at NIF implied by the theory are discussed.

On the Theory of Ignition, Combustion, and Detonation of Micro- and Nanoparticles

A review of some investigations performed in the field of mechanics of reacting heterogeneous media with micro- and nanostructures at the Khristianovich Institute of Theoretical and Applied Mechanics of the Siberian Branch of the Russian Academy of Sciences is presented. Some new results are also reported.

Experimental and theoretical research on the fire safety of a building insulation material via the ignition process study

A series of experiments were carried out on extruded polystyrene (XPS) foam using a vertical thermal radiator. Measurements were taken for the ignition time, the pyrolysis mass and the temperature distribution. Two kinds of XPS foam with flame retardant levels of B1 and B2 were tested in the experiments and the incident heat fluxes were set as 20, 30, 35, 40, 50 and 60 kW/m2. For the XPS shrinks to a thin layer under the thermal radiator heating, thus the classical ignition theory may be improper and the new functions relationship between the ignition time and the radiant heat flux was derived in theory and experiment. What's more, the pyrolysis mass increases with the radiant heat flux, while the extent of increasing reduces as the radiant heat flux increases. And the result of temperature distribution indicated that under high incident heat flux the flame retardant treatment becomes less effective. The research results were useful for the theory and the experimental study on the fire characteristic of foamed polymer under vertical radiation condition.

Non-thermal solvate ignition of solid-state combustion at organic self-propagating high-temperature synthesis

The original mechanism of the solid-state combustion, ignited by local addition of microquantities of water to mixture of disperse piperazine and malonic acid (hydroignition), was proposed. It consists eliminating the diffusion barrier at dissolving the product crust previously formed on the reagent particles at them mixing and jumping mechanism of proton transfer (Grotthuss mechanism).The data obtained and their explanations seems unusual from the point of view of ignition theory which did not consider ignition as a non-thermal process at the expense of intrinsic resources of reacting solids.New method of ignition (hydroignition) can be successfully applied for effective one-pot click-synthesis of both organic and inorganic products in the self-propagating high temperature synthesis (SHS) mode and for designing of autonomous ignition systems of mixes.