Mechanism Of Flame Extinction Research Articles

Burning and flame extinction patterns of PMMA under external heating and oxygen deficiency

The paper presents an experimental study investigating the impact of under-oxygenated conditions on the burning characteristics and flame extinction of horizontal transparent poly(methyl-methacrylate) (PMMA) slabs subjected to external heating. Small-sized solids are exposed to various constant levels of radiant heat flux in a nitrogen-diluted environment at ambient pressure. The study aims to understand the multivariable dependence of PMMA combustion on oxygen and heating levels. The analysis goes beyond the study of global and local variables to provide an understanding of the mechanisms involved, revealing a notable effect on burning characteristics and flame extinction mechanisms by blow-off. The results provide a detailed assessment of the surface energy balance and flame bulk properties. The results demonstrate good repeatability up to a certain oxygen limit, beyond which the flame combustion regime exhibits stochastic behaviour. The study of heat transfer mechanisms has revealed that flame radiation and external heating play a predominant role in the heating of horizontal solid fuels, In contrast, within the flame, convection is the main contributor to solid heating, rather than flame radiation. Solid characteristics show linear trends proportional to oxygen depletion, while gas phase characteristics show monotonic trends strongly influenced by the changing combustion regime. The surface temperature of the solid is highly sensitive to the test conditions that affect the energy balance. Dimensionless parameters are developed and data are correlated with existing literature. The analysis demonstrates that the linear or monotonic trends are maintained regardless of external heating. Finally, models are proposed and validated to predict both the extinction limit and the burning rate independently of irradiance and oxygen levels.

Blow-off of diffusion flame by moving air vortex ring

The blow-off is one of the typical flame extinction mechanisms, and such a principle has been widely used in firefighting when the water-based extinguisher is limited. This work explores the blow-off extinctions of different diffusion flames by air vortex ring. The vortex ring harnesses its kinetic energy within the fast-rotating vortex core, enabling the transmission of power over several meters to blow off a remote flame. The power required for vortex ring blow-off is found to be two to three orders of magnitude smaller than the power of the flame itself, demonstrating exceptional energy efficiency. It is observed that the poloidal flow (circulation) surrounding the vortex core can stretch the flame base to the critical state and then cause instantaneous extinction. To explain the vortex-induced blow-off limit, a critical Damköhler number that accounts for the competition between fuel gas flow and flame stretch was formulated. This work provides a fundamental understanding of the extinction mechanism by vortex ring, and it offers technical guidelines for using air as a flame extinguisher for remote firefighting within minimum energy input.

Effects of water vapor addition on downstream interaction in CO/O2 counterflow premixed flames

The effects of H2O addition on downstream interaction in counterflow premixed CO/O2 flames are investigated by varying the global strain rate (ag) and CO mole fractions (XCO,L, XCO,U) in the lower and upper nozzles, respectively. For interacting premixed CO/O2 flames, the flammable region is very narrow such that the flames cannot be sustained for ag > 11.75 s−1. When 1.0% vol H2O is added to O2/CO2 mixtures, the flammable region is appreciably extended. At low strain rate, the lean-lean and rich-rich extinction boundaries show strong and weak interactions similar to those observed previously in hydrocarbon fuels. The flammable lean-lean and rich-rich regions gradually shrink with the increase of ag. When XCO,U is small for asymmetric lean double flames at low strain rate, the extinction boundary exhibits weak interaction behavior, where the weaker flame is parasitic to the stronger flame by XCO,L. The stronger flame experiences heat loss to the weaker flame. As the strain increases, the reaction cannot be completed due to the reduction in the flow time. The thermal energy loss by incomplete reaction leads to the flame extinction. This effect changes the qualitative nature of extinction boundary as strain rate increases, resulting in the extinction boundary having only the strong interaction mode having near constant (XCO,L + XCO,U) and bending toward larger XCO,L, and eventually forming an island shape at higher strain rate. The local equilibrium temperature (LET) concept is introduced to explain these flame extinction mechanisms. Local temperature behaviors are well explained by investigating major reaction contributions to heat release rate. In all cases, LET decreases by the effect of preferential diffusion because of the Lewis number of deficient reactant being larger than unity. For asymmetric double flames, conductive heat transfer (CHT) from the stronger to weaker flame reduces the LET of the stronger flame. Flame extinction mechanism can be explained by introducing a loss ratio. For CO/O2 flames, the effects of incomplete reaction as well as preferential diffusion and CHT lead to flame extinction. For (CO/O2 + 1.0% H2O) flames with the increase of strain rate, thermal energy loss by incomplete reaction becomes appreciable, as compared with the effects of preferential diffusion and CHT.

Characterisation of the fire behaviour of wood: From pyrolysis to fire retardant mechanisms.

Wood is undeniably the most useful and readily available natural raw material. However, the susceptibility of wood products to fire is one of the crucial challenges faced in the wood industry. The fire behaviour of wood is a very complex phenomenon due to the different constituents and their independent reactions to fire. This article presents a thorough overview of the flammability stages of wood. It covers pyrolysis, thermal oxidative decomposition, ignition, combustion and heat release as well as flame extinction mechanisms. In the area of flame retardancy, conventional wood fire retardants, nanocomposites fire retardants and wood modification processes are investigated. Factors such as wood species, moisture content, density, experimental conditions such as external heat flux, heat exposure time, wood permeability and porosity are some of the deterministic parameters characterising the fire behaviour. This paper is a one-stop-shop for researchers analysing wood flammability due to the inclusion of all aspects pertaining to the burning of wood.

Experimental study of flame extinction with fuel diffusion combustion inside a wall-opening compartment under reduced ventilation conditions

The present study investigates experimentally the transient flame extinction behavior with fuel diffusion combustion inside a wall-opening compartments at reduced ventilation condition. The characteristic parameters to quantify the flame extinction mechanism include the Global Equivalence Ratio (GER) extinction time and critical gas temperature right before extinction. Their coupling effect on the dynamic process of flame extinction with fuel diffusion combustion inside compartments of various scales and opening sizes is quantified and formulated. It is found that: (1) flame extinction is easier to occur for smaller opening size or relatively larger compartment scale which could be well represented by two non-dimensional quantities i.e. the GER and non-dimensional heat loss; (2) the time to reach flame extinction is shorter as the fuel supply flow rate is higher while it is longer as the opening size is larger; (3) the gas temperature inside the compartment when flame extinction is reached first increases with fuel supply rate until a plateau is reached. A formula is established to describe the critical gas temperature right before extinction as a function of a newly defined non-dimensional integrated parameter including both fuel combustion heat release and heat loss factors in which the fuel supply rate opening ventilation compartment scale and transient extinction time scale are involved in general to reflect physically their interplay in controlling the flame extinction inside the compartment. The present study provides new observation quantification and formulation of the mechanism of flame extinction with fuel diffusion combustion inside a wall-opening compartment under reduced ventilation condition.

Static Stability and Combustion Characteristics of Oxy-Propane Flames in a Premixed Fuel-Flexible Swirl Combustor

This study investigates the stability and combustion features of premixed oxy-propane flames in a dry low emission (DLE) model combustor for clean fuel/oxidizer-flexible combustion applications in gas turbines. The stability of the oxy-propane flame is characterized by its blowout and flashback limits over ranges of equivalence ratios (ϕ) from 0.1 to 1.0 and oxygen fractions (OF: volumetric concentration of O2 in the O2/CO2 oxidizer) from 15 to 70%. For better understanding of the physics behind the flame extinction mechanisms, the stability limits were plotted within the ϕ–OF space against the contours of adiabatic flame temperature (Tad), inlet Reynolds number (Re), mixture mass flow rate (ṁmix), and combustor power density (PD). The flame speed (FS) was also estimated at selected operating conditions. It was observed that the FS is contingent to Tad only. A correlation of Tad–FS was developed for better characterization of premixed oxy-propane flames. It was also found that the blowout curve follows a constant Tad contour on the stability map, whereas the flashback curve does not. This implies that Tad is a more relevant controlling parameter choice of combustor stability near the blowout limit, whereas the reaction rate is a suitable indicator of the flame stability limit near flashback. The stability map of the present oxy-propane flame was compared with that of an oxy-methane flame on the same combustor. It was found that, at the same OF, the blowout and flashback limits of the propane flame occur at leaner ϕ, compared to those of methane flame, which was expected for a higher hydrocarbon fuel like propane. Flame shapes at different sets of operating conditions were investigated for inferring key information on the flame behavior and macrostructure while varying ϕ, OF, and hence, Tad. Similar flame shapes were obtained at the same Tad regardless of ϕ and OF values. Flames of similar Tad also demonstrated similar temperature distributions near the flame core. The findings confirm the dependence of the FS on Tad, irrespective of the combinations of ϕ and OF used to achieve any particular Tad.

Experimental study on flame stability limits of lithium ion battery electrolyte solvents with organophosphorus compounds addition using a candle-like wick combustion system

To evaluate the fire-retardant effectiveness of organophosphorus compounds (OPC) added to Li-ion battery electrolyte solvents, the limiting oxygen concentration (LOC) method is used in conjunction with a wick combustion system, called as wick-LOC method. With the wick-LOC method, two modes of stabilized flame are found, namely, wake flame and full flame. When OPC is added to the electrolyte, two distinct branches of extinction processes occur according to the different flame modes near extinction with no transition from the full flame to the wake flame in the case of higher OPC addition. The flame stability limits are measured as a function of OPC addition for both flame modes. The wake flame is shown to be consistently more stable at low levels of OPC addition. However, once the OPC addition exceeds a critical amount, the full flame shows higher stability with a lower LOC than the wake flame. These phenomena in the two regimes are also found in other cases of high OPC addition (different type of OPC and electrolyte solvent). In the most stable flame mode, the regime switches from the wake flame to the full flame with increasing OPC addition, and they are defined correspondingly as “blow-off regime” and “quenching regime”. To explain the presence of these two regimes, the thermal balance effect is considered in the discussion of flame extinction mechanisms. The difference in flame volume near the extinction limit shows that the quenching mechanism dominates flame extinction under higher OPC addition. The thermal balance effect on flame stabilization or extinction can be the additional impact on the fire retardation abilities of OPC itself.

Mechanisms of flame extinction and lean blowout of bluff body stabilized flames

Lean flame blowout is investigated experimentally within a high-speed combustor to analyze the temporal extinction dynamics of turbulent premixed bluff body stabilized flames. The lean blowout process is induced through fuel flow reduction and captured temporally using simultaneous high-speed particle imaging velocimetry (PIV) and CH* chemiluminescence. The evolution of the flame structure, flow field, and the resulting flame strain rate are analyzed throughout extinction to distinguish the physical mechanisms of blowout. The flame–vortex dynamics are found to be the main driving mechanism of flame extinction; namely, a reduction of the flame-generated vorticity coupled with an increase in the downstream shear layer vorticity. The vorticity dynamics are linked to hydrodynamic instabilities that vary as a function of the decreasing equivalence ratio. A frequency analysis is performed to characterize the dynamic changes of the hydrodynamic instability modes during flame extinction. Various bluff body inflow velocity regimes are investigated to further characterize the extinction instability modes. Both equivalence ratio and flow-driven instabilities are captured through a universal definition of the Strouhal number for the reacting bluff body flow. Finally, a Karlovitz number-based criterion is developed to consistently predict the onset of global extinction for the different inflow velocity regimes.

Downstream interaction between SNG – Air premixed flames

Experimental and numerical studies were conducted to understand flame extinction in SNG (synthetic natural gas) – air premixed flames with a counterflow configuration. The capability of some detailed reaction mechanisms in predicting flame extinction limit was evaluated through comparing predicted lean extinction boundaries with experimental ones. The results showed that those were best-fitted to numerical ones when the mechanism of UC San Diego was used and predicted flame locations were reasonably in agreement with experimental ones. Flame stability map was presented with a functional dependency on fuel volume fractions from lower and upper nozzles by varying global strain rate in interacting SNG – air premixed flames. Flame characteristics were identified to symmetric and asymmetric lean (rich) – lean (rich) flames and triple flames. The mechanism of flame extinction in interacting SNG – air premixed flames was explained and discussed by emphasizing important roles of downstream chemical interaction (via H-consumption) and upstream thermal interaction (via conductive heat loss to the ambience in the side of the stronger flame). Important role of H-radical (shared in downstream region) on chemical interaction was also discussed.

Lagrangian mechanisms of flame extinction for lean turbulent premixed flames

Flame extinction is a critical impediment invariably limiting the performance of modern turbulent combustion technology. Combustion systems operating at lean conditions are highly susceptible to dynamic flame stability induced by local flame extinction. This stimulates flame blow-out and inevitably termination of the combustion process. The present study focuses on understanding the driving mechanisms which lead to flame extinction. A Lagrangian flame-vortex model is developed and used to study the flame extinction mechanisms. The model dynamically simulates the turbulent reacting flow exploiting a Lagrangian vortex element scheme and detailed strained kinetics. This systematic modeling strategy effectively encapsulates the dynamics of premixed turbulent flames in terms of stability and extinction. Two extinction modes of flame blow-out are analyzed using the model, the first of which is induced by decreasing equivalence ratio primarily resulting in diminishing strain rate limit of the flame. The alternate mode is focused on inflow-velocity induced extinction which is caused principally by increased hydrodynamic strain in the flow-flame field. The mechanics of both modes are examined utilizing the model’s unique Lagrangian tracking capability for extinction-inducing fluid element clusters. This technique enables isolation and detailed analyses of flame and fluid properties leading to blow-out, demonstrating the crucial driving mechanisms of flame extinction.

Treatment of local extinction in CFD fire modeling

A simplified approach capturing the major flame extinction mechanisms has been formulated and calibrated against the measurement data for critical strain rates of laminar diffusion counter-flow flames with fuel and (or) oxidizer streams diluted by nitrogen. The model is based on the perfectly stirred reactor concept, thereby assuming rapid reactant mixing in the reaction zone, where reactants are delivered at stoichiometric proportions. Model simplicity is achieved by considering the single-step global reaction model. Advancing previous studies, we demonstrate that this model is able to replicate critical strain rates at extinctions of both counter- and co-flow flames in a range of experimental configurations including opposed gaseous streams and evaporating liquid pool with a normally impinging oxidizer stream, in the entire range of gaseous diluent concentration enabling flaming combustion. The model correctly predicts the minimum extinguishing concentrations of different inert diluents (argon, nitrogen, water vapor and carbon dioxide) used in practice of fire suppression. The algorithm is simple and computationally affordable enough to be incorporated in a CFD fire model. A worked example is demonstrated in simulations of flame suppression by sprinkler sprays at different water flow rates. The proposed algorithm can be used for any practical fuel, as soon as the global kinetic model of fuel oxidation is calibrated using the procedure developed in this work.

Downstream interaction between stretched premixed syngas–air flames

Effect of strain rate on flame extinction is numerically investigated in downstream interaction among lean (rich) and lean (rich) premixed as well as partially premixed (50% H2+50% CO)–air flames. The strain rate varies from 30 to 4671s−1 until interacting flames cannot be sustained anymore. Flame stability diagrams mapping lower and upper limit fuel concentrations for flame extinction as a function of strain rate are explored. It is shown that significant increase of strain rate makes even the upper rich extinction boundary be slanted due to strong chemical interaction. The lower extinction boundary can be also extended to rich fuel concentrations over the stoichiometric mixture condition when strain rate significantly increases. The lower and upper extinction boundaries, mainly caused by the conductive heat loss from the stronger flame to ambience, become narrower and narrower in increasing strain rate. The results also show that the extinction boundaries with positive flame speed are extended and then reduced in increasing strain rate, thereby leading to an island of extinction boundary and subsequently being changed into a point on the symmetric fuel concentration line. The detailed explanations on those flame extinction characteristics in the stability diagrams are made through analysis of the flame structures in increase of strain rate. The mechanism of flame extinction is also proposed and discussed in detail.

Effects of preferential diffusion on downstream interaction in premixed H2/CO syngas–air flames

Effects of strain rate and preferential diffusion of H2 on flame extinction are numerically explored in interacting premixed syngas–air flames with the fuel compositions of 50% H2 + 50% CO and 30% H2 + 70% CO. Flame stability diagrams mapping lower and upper limit fuel concentrations at flame extinction as a function of strain rate are examined. Increasing strain rate reduces the boundaries of both flammable lean and rich fuel concentrations and produces a flammable island and subsequently even a point, implying that there exists a limit strain rate over which interacting flame cannot be sustained anymore. Even if effective Lewis numbers are slightly larger than unity on the lean extinction boundaries, the shape of the lean extinction boundary is slanted even at low strain rate, i.e. ag = 30 s−1 and is more slanted in further increase of strain rate, implying that flame interaction on lean extinction boundary is strong and thus hydrogen (as a deficient reactant) Lewis number much less than unity plays an important role of flame interaction. It is also shown that effects of preferential diffusion of H2 cause flame interaction to be stronger on lean extinction boundaries and weaker on rich extinction boundaries. Detailed analyses are made through the comparison between flame structures with and without the restriction of the diffusivities of H2 and H in symmetric and asymmetric fuel compositions. The reduction of flammable fuel compositions in increase of strain rate suggests that the mechanism of flame extinction is significant conductive heat loss from the stronger flame to ambience.

Effect of flame stretch in downstream Interaction between premixed syngas-air flames

The effect of strain rate in downstream interactions between lean (rich) and lean (rich) premixed syngas flames with the fuel composition of 50% H 2 and 50% CO is numerically investigated by varying the strain rate in the range of 5∼500 s −1. The flame stability maps for several strain rates are presented and main concerns are focused on the downstream interactions on the lean and rich extinction boundaries. The fuel composition of 50% H 2 and 50% CO with effective Lewis numbers larger than unity for both lean and rich extinction boundaries is chosen for grasping the important role of hydrogen with the deficient reactant Lewis numbers much smaller than unity. The results show that the lean extinction boundaries have the slanted shape, thereby leading to strong interactions; meanwhile the rich extinction boundaries at appropriately low strain rates are of square, indicating weak interactions. However, at highly strained interacting rich flames, the rich extinction boundaries show a slanted shape, thereby leading to strong interactions even for Lewis numbers much larger than unity. In such situations, thermal and chemical interactions are explained in detail. It is found that, in interacting flames, the excessive heat loss of the stronger flame partly to the weaker flame and mostly to the ambience is the mechanism of flame extinction.

Behavior of Low Strain Rate Flame Disks in Counterflow Diffusion Flame

A study was conducted to clarify flame characteristics through the evaluation of critical mole fractions at flame extinction and edge-flame oscillation of low strain rate flames using the global strain rate, velocity ratio, and burner distance as experimental variables. The transition from a shrinking flame disk to a flame hole was verified through gradient measurements of the maximum flame temperature. Evidence of edge-flame oscillation in flame disks was also found using numerical simulations in zero and normal gravity. The main mechanisms of flame extinction and edge-flame oscillation were analyzed by comparing the energy fractions in the energy equation. For low strain rate flame disks, radial conduction heat loss rather than flame radiation was a significant contributor to flame extinction and even edge-flame oscillation. This was experimentally demonstrated by evaluating the critical mole fraction at flame extinction and edge-flame oscillation, as well as measurements of the flame temperature gradient along the flame disk surface. These results suggest that low strain rate flame responses are determined not only by one-dimensional flame responses, but also by multi-dimensional flame responses such as radial conduction heat loss. The results also show that extinction of low strain rate flames is more probably due to multi-dimensional heat losses than to radiative extinction.

Laminar premixed methane/air flame extinction characteristics influenced by co-flow water mists

Based on the tubular burner, the burning velocities, flame stretch and inhibition rules influenced by co-flow water mists were studied using a high-speed schlieren system. Moreover, the variation rules of the flame critical extinction in our burner equipment were also obtained by analyzing the process and mechanism of flame extinction and inhibition. It is shown that the flame stretch is related to the fuel concentration, co-flow fluxes and water mist diameters. For droplets with a larger diameter, the smaller the co-flow fluxes, the more obvious the flame stretch. When the water mist loading rate is rather smaller, for fuel-rich premixed flame with Le>1, the flame with larger burning rate tends to backfire more easily. Under the same water mist conditions, for fuel-lean premixed flame with Le<1, the smaller the gas concentration, the easier the flame is extinct.

Optimal reactor dimensions for homogeneous combustion in small channels

This paper addresses the question of choosing the appropriate reactor length, wall thickness, and reactor opening size for self-sustained homogeneous combustion in parallel plate channels. First, the flame characteristics and stability of homogeneous combustion in micro-scale (<1 mm) and meso-scale (>1 mm) channels are studied, and the roles of heat recirculation and heat loss on the mechanisms of flame extinction and blowout are investigated. Based on these insights, the effects of reactor dimensions on flame stability are studied. Increasing the reactor length results in shrinking of the region of self-sustained combustion due to increased heat losses through the reactor solid structure. An optimum gap width in the range of ∼600–1200 μm (transition from micro- to meso-scales) provides the largest region of self-sustained combustion. Size effects are stronger for methane than for propane combustion.

Computational Assessment of Afterburning Cessation Mechanisms in Fuel-Rich Rocket Exhaust Plumes

Abstract : A computational study was conducted to identify fundamental physical processes governing the cessation or shutdown of the afterburning of fuel rich rocket exhaust with the atmosphere. Several mechanisms were examined which are: 1) a relaminarization phenomenon, 2) a Damkohler number effect and 3) a classical flame extinction mechanism. Analysis of the simulation results revealed the relaminarization mechanism to be implausible while the Damkohler number effect and the flame extinction mechanisms were found to be valid. The extinction mechanism was also found to dramatically alter the emission characteristics and enhance the shutdown behavior which has important implications with respect to radiative heat transfer to the body and missile defense systems. This is a significant finding because strain rate induced extinction is a previously unrecognized phenomena occurring in rocket exhaust plumes during afterburning cessation.

Kinetic modeling of spectra of flames with suppressants

The phase out of halons in response to the Montreal Protocol and the quest for alternative fire suppressants has focused attention on the current lack of understanding of mechanisms of flame extinction. Challenges to the free-radical scavenging mechanism in which chemical chains in combustion are interrupted have raised serious questions as to why the halons are effective. Low-resolution ultraviolet computer spectroscopy can be used to observe the OH radical and other simple molecules when acted upon by halon or alternative suppressants. However, the physical and chemical interpretation of such observations requires reaction rates leading to the OH radical and other observable molecules in 3-6 eV electronic states. Using a simple kinetics model, the authors illustrate where major reaction rates to excited states would be helpful to the interpretation of the interaction of suppressants with flames. Quantum chemists could provide a valuable service by providing such rates and helping to assess whether or not flame suppression by free-radical scavenging is a significant mechanism. 29 refs., 5 figs.

Dynamics of flow stretch on coal ignition mechanism

The dynamics of flow stretch on the mechanism of coal ignition has been investigated experimentally. The hydrodynamic and global stretch rates in this mechanism are proposed whereby the ignition position and flame configuration are correlated. There is a limit of stagnation point ignition, when the specific global stretch rate is 50 1/s for a constant flow temperature of 1000 K and oxygen concentration of 21%. When the global stretch rate is less than the specific global stretch rate, the coal ignites near the stagnation point of the particle and the shape of the developing flame is spherical. When the global stretch rate exceeds the specific global stretch rate, the coal ignites at the rear of the particle and the configuration resembles a Bunsen flame. The coal ignited in the middle zone of the particle at the transition state is interpreted by the definition of the hydrodynamic stretch rate. The effect of the ignition stretch factor on the ignition mechanism compared with the effect of the flame stretch factor on the mechanism of flame extinction is discussed in detail.