A simplified chemical-kinetic cool-flame mechanism for n-alkanes has recently been developed and applied to the description of quasi-steady droplet combustion. Chemistry of this same general type can support premixed laminar cool-flame deflagrations. The present contribution derives the structure of the associated freely propagating cool premixed flame controlled by the low-temperature chemistry and develops formulas for calculating the corresponding laminar burning velocity. Application of activation-energy asymptotics reveals finite-rate chemistry, among essential intermediates (with concentrations so small that associated heat release is negligible), occurring throughout the preheat zone. There is leakage of both fuel and oxygen through the thin heat-release zone, with zones of consumption of an intermediate species on each side of the heat-release zone, thicker than that zone but still thin compared with the preheat-zone thickness. Predicted laminar burning velocities are compared with recently reported measurements for n-dodecane, performed in a newly designed high-pressure droplet ignition apparatus, resulting in reasonable agreement after account is taken of the process of insertion of the droplet into the furnace and of the velocity of the buoyant plume present in the experiment.

Particle-image velocimetry was employed to investigate the structure of the cold boundary layer surrounding a fire whirl generated in an experimental setup consisting of a pool fire enclosed by distant, inclined vertical vanes that deflect the entraining air. Results obtained for two different vane inclinations were compared with theoretical predictions previously derived using high-Reynolds-number asymptotics. The experiments confirmed the presence of a near-wall region characterized by pronounced inward radial flow, with its magnitude increasing with decreasing radial distance. Under the specific conditions examined, boundary-layer separation and reattachment were observed, giving rise to a long bubble of slow, recirculating flow along the wall. This feature has potential implications for future numerical modeling of fire-whirl structure and dynamics.Novelty and significanceAn accurate understanding of the flow surrounding fire whirls is necessary to support numerical investigations of their structure and dynamics. Particle-Image Velocimetry (PIV) is applied, for the first time, to obtain an experimental characterization of the cold boundary layer that develops around fire whirls. These novel measurements provide new insights into the structure and morphology of the boundary layer that are not captured by existing theories.

The hydrodynamic theory of flame propagation in closed vessels is extended here to configurations involving nonuniform initial distributions of temperature and composition, admitting arbitrary steady-planar-deflagration chemical kinetics not considered in previous publications. The analysis addresses planar, cylindrical, and spherical configurations in the high-P & eacute;clet-number limit in which a thin premixed flame separates fresh-mixture and burnt-gas regions that are free from diffusive transport in the first asymptotic approximation. The simplified approach reduces the problem to solving a system of ordinary differential equations in time for the Euler-equation descriptions of the hydrodynamics in the fresh and burnt gases, along with the motion of the deflagration separating them. Planar hydrogen-air flames with a detailed chemical-kinetic description are selected to illustrate the simplified computational procedure and to expose the influences of the nonuniform initial distributions admitted by the new general formulation. The good accuracy of the simplified approach is supported by favorable agreement with a full numerical integration of the problem as originally formulated, prior to imposition of the asymptotic simplifications. The new method may facilitate future investigations of stratified-charge and unexpected-accident scenarios.

Numerical simulations are conducted to study fundamental aspects of combustion stabilization in hydrogen-fueled gas turbines. The study focuses on laminar lifted flames at moderate Reynolds numbers in axisymmetric configurations, where a swirling hydrogen jet diluted with nitrogen is injected into stagnant, preheated, pre-compressed air. The conservation equations are formulated in the low-Mach-number approximation, employing a mixture-averaged model for molecular transport. Fuel oxidation is modeled using both detailed chemical kinetics and a previously derived explicit one-step reduced mechanism, which assumes steady-state behavior for chemical intermediates-a valid approximation under the high-pressure conditions typical of gas-turbine combustion chambers, and the accuracy of that approximation is ascertained. The investigation explores the interplay between vortex breakdown and flame dynamics, including liftoff and blowoff, as functions of the swirl and Damk & ouml;hler numbers. The results elucidate the required flow criteria for lifted-flame stabilization and demonstrate the predictive capability and computational cost reduction of the one-step chemistry in connection with hydrogen combustion at high pressures. A regime diagram in a plane of swirl number and Damk & ouml;hler number is derived, and conditions for the occurrence of steadily pulsating flames are established, along with indications of amplitudes and frequencies of those oscillations. While clearly not directly applicable to practical turbulent-flow conditions, the results can be useful in future analyses and design concepts for combustion chambers of hydrogen-fueled gas turbines. Novelty and significance statement This work presents, for the first time, results of computations of nitrogen-diluted hydrogen flame behavior for swirling fuel jets issuing into air that has been heated to temperatures expected at the entrance to gas-turbine combustion chambers. It is novel in that it compares predictions made using both detailed combustion chemistry and one-step systematically derived reduced chemistry. A significant finding is that the results obtained with the reduced chemistry are in general agreement with those of the detailed chemistry, thereby affording substantial reductions in computational cost. Another novel and significant result is the determination of injection and swirl gas-turbine conditions required for stable lifted flames to occur, rather than attached flames or blowoff. The existence and characteristics of pulsating oscillations also are established for the first time. These results will be useful in the design and analysis of hydrogen-fueled gas-turbine combustion chambers.

Numerical computations employing detailed chemistry are used to characterize the different combustion modes emerging in mixing layers separating nitrogen-diluted counterflowing planar streams of hydrogen and oxygen. Attention is focused on high degrees of dilution, resulting in near-limit flames, with peak temperatures close to the crossover temperature. A bifurcation diagram is presented in a plane, having the stoichiometric mixture fraction and normalized strain rate as coordinates, that identifies six different combustion regimes involving four different flame types, namely, diffusion-flame sheets, advancing and retreating edge flames, multiple flame tubes, and single isolated flame tubes. Multiple-tube flame configurations vary from small, round, widely separated flame strings at high strain rates to wide, flat, densely packed flame strips, with narrow flame-free gaps between them, at lower strain rates, and they are steady and stable in various arrays over a continuum of tube-separation distances. The observed flame behavior exhibits hysteresis in a certain range of parameters, with the structure that is established depending on the ignition mechanism, as it also does at high strain rates, and a continuum of different stable steady-state flame configurations exists, each accessed from a different initial condition.

Numerical computations employing detailed chemistry are used to characterize the different combustion modes emerging in mixing layers separating nitrogen-diluted counterflowing planar streams of hydrogen and oxygen. Attention is focused on high degrees of dilution, resulting in near-limit flames, with peak temperatures close to the crossover temperature. A bifurcation diagram is presented in a plane, having the stoichiometric mixture fraction and normalized strain rate as coordinates, that identifies six different combustion regimes involving four different flame types, namely, diffusion-flame sheets, advancing and retreating edge flames, multiple flame tubes, and single isolated flame tubes. Multiple-tube flame configurations vary from small, round, widely separated flame strings at high strain rates to wide, flat, densely packed flame strips, with narrow flame-free gaps between them, at lower strain rates, and they are steady and stable in various arrays over a continuum of tube-separation distances. The observed flame behavior exhibits hysteresis in a certain range of parameters, with the structure that is established depending on the ignition mechanism, as it also does at high strain rates, and a continuum of different stable steady-state flame configurations exists, each accessed from a different initial condition.

Starting with a detailed-chemistry description involving 20 elementary steps for hydrogen oxidation and 40 elementary steps for ammonia oxidation, it is shown that systematic application of sensitivity analyses of premixed flames under typical gas-turbine combustion conditions reduces the description to 12 elementary steps for hydrogen oxidation, 4 of them being reversible, and an additional 19 steps for ammonia oxidation, 6 of them being reversible, yielding reasonable predictions for auto-ignition and deflagration processes. Subsequent introduction of steady-state approximations for chemical intermediates, afforded by the high-pressure conditions existing in gas-turbine combustion chambers, effectively reduces the fuel-oxidation description in systems utilizing H2-NH3 2-NH 3 fuel mixtures to two global steps for deflagrations, namely, 2H2 2 + O 2 2H2O 2 O and 4NH3 3 + 3O2 2 2N2 2 + 6H 2 O. Analytical expressions for the associated overall rates, involving the local temperature and the O2, 2 , H2, 2 , NH3, 3 , N2, 2 , and H2O 2 O concentrations, are derived through selective truncation of the steady-state expressions, resulting in a simplified chemistry description that can facilitate future numerical analyses based on direct-numerical and large-eddy simulations. Novelty and significance statement A new short mechanism involving only 31 elementary reactions between 16 reactive species has been derived for hydrogen-ammonia oxidation under conditions of pressure, temperature and dilution typically found in gas-turbine burners. Introduction of steady-state assumptions for all intermediate species leads to a two-step mechanism that is shown to predict burning rates with sufficient accuracy. The proposed mechanism can significantly reduce computational times in future direct-numerical and large-eddy simulations.

This paper summarizes the first results from isolated droplet combustion experiments performed on the International Space Station (ISS). The long durations of microgravity provided in the ISS enable the measurement of droplet and flame histories over an unprecedented range of conditions. The first experiments were with heptane and methanol as fuels, initial droplet droplet diameters between 1.5 and 5.0 m m , ambient oxygen mole fractions between 0.1 and 0.4, ambient pressures between 0.7 and 3.0 a t m and ambient environments containing oxygen and nitrogen diluted with both carbon dioxide and helium. The experiments show both radiative and diffusive extinction. For both fuels, the flames exhibited pre-extinction flame oscillations during radiative extinction with a frequency of approximately 1 H z . The results revealed that as the ambient oxygen mole fraction was reduced, the diffusive-extinction droplet diameter increased and the radiative-extinction droplet diameter decreased. In between these two limiting extinction conditions, quasi-steady combustion was observed. Another important measurement that is related to spacecraft fire safety is the limiting oxygen index (LOI), the oxygen concentration below which quasi-steady combustion cannot be supported. This is also the ambient oxygen mole fraction for which the radiative and diffusive extinction diameters become equal. For oxygen/nitrogen mixtures, the LOI is 0.12 and 0.15 for methanol and heptane, respectively. The LOI increases to approximately 0.14 (0.14 O 2 /0.56 N 2 /0.30 C O 2 ) and 0.17 (0.17 O 2 /0.63 N 2 /0.20 C O 2 ) for methanol and heptane, respectively, for ambient environments that simulated dispersing an inert-gas suppressant (carbon dioxide) into a nominally air (1.0 a t m ) ambient environment. The LOI is approximately 0.14 and 0.15 for methanol and heptane, respectively, when helium is dispersed into air at 1 atm. The experiments also showed unique burning behavior for large heptane droplets. After the visible hot flame radiatively extinguished around a large heptane droplet, the droplet continued to burn with a cool flame. This phenomena was observed repeatably over a wide range of ambient conditions. These cool flames were invisible to the experiment imaging system but their behavior was inferred by the sustained quasi-steady burning after visible flame extinction. Verification of this new burning regime was established by both theoretical and numerical analysis of the experimental results. These innovative experiments have provided a wealth of new data for improving the understanding of droplet combustion and related aspects of fire safety, as well as offering important measurements that can be used to test sophisticated evolving computational models and theories of droplet combustion.

Two-stage autoignition of n-dodecane droplets with varying ambient oxygen concentrations in oxygen-nitrogen mixtures are investigated experimentally under microgravity conditions using high-speed shadowgraphy. The ambient pressure and temperature are held constant at 3 atm and 650 K, respectively, while the droplet initial diameter is fixed approximately at 1.2 mm. During the two-stage autoignition process, first a cool-flame front forms in the leaner regions farther away from the evaporating droplet, and it then propagates toward the fuel-rich region closer to the droplet surface, eventually encompassing the droplet. A hot-flame kernel is then initiated in the wake of the cool flame and very quickly expands, establishing a classical diffusion flame around the droplet. The first and second ignition delay times are measured from shadowgraphic images captured at 3000 frames per second. The first induction time is found to be insensitive to the ambient oxygen concentration, while the second induction time varies approximately as the negative 2 power of the oxygen mole fraction.

Melt-front instabilities during the combustion of a spinning polymethylmethacrylate disk in air are investigated. Mainly straight rivulet-type flow patterns were found, though under certain conditions saw-tooth patterns were observed. The measured wavelengths of the instabilities agree with earlier theoretical predictions of driven contact-line instabilities.

Experimental and computational investigations are carried out to elucidate the influence of ethanol addition on nheptane auto-ignition in counterflows. An axisymmetric stream of air, temperature gradually increased, is directed onto the surface of an evaporating pool of a liquid fuel. The air-stream temperature at auto-ignition is measured at various strain rates, defined as the axial gradient of the axial component of the flow velocity at the stagnation plane, for nheptane, ethanol, and various nheptane/ethanol mixtures. Critical conditions for auto- ignition are predicted employing the San Diego Mechanism for both fuels and the fuel mixtures, and the results are compared with the measurements. Measurements and predictions show that low-temperature chemistry plays a significant role in promoting auto-ignition of nheptane at low strain rates, but there is insufficient residence time at high strain rates for low-temperature chemistry to take place, so auto-ignition is promoted by high-temperature chemistry. Experimental and computational results show that addition of ethanol inhibits the low-temperature chemistry of nheptane. To identify the responsible elementary steps, computations are performed to identify those that dominate oxygen consumption and that contribute to the temperature rise in the reaction zone for nheptane and nheptane/ethanol mixtures at low strain rates. For nheptane oxygen is consumed primarily by the low-temperature steps that result in ketohydroperoxide; the temperature rise is produced by subsequent low-temperature-chemistry steps. For the mixtures, a key step that consumes O2 2 is O2 2 + CH3CHOH 3 CHOH = HO2 2 + CH3CHO, 3 CHO, and the heat release occurs through the classical high-temperature reaction mechanism. Thus, the inhibition of auto-ignition that is observed to occur when ethanol is added to nheptane arises from the competition for oxygen between this step and the low-temperature-chemistry addition of O2 2 to the heptyl radical and to the radical arising from the subsequent isomerization, for nheptane.

An improved understanding of cool diffusion flames could lead to improved engines. These flames are investigated here using a spherical porous burner with gaseous fuels in the microgravity environment of the International Space Station. Normal and inverse flames burning ethane, propane, and n-butane were explored with various fuel and oxygen concentrations, pressures, and flow rates. The diagnostics included an intensified video camera, radiometers, and thermocouples. Spherical cool diffusion flames burning gases were observed for the first time. However, these cool flames were not readily produced and were only obtained for normal n-butane flames at 2 bar with an ambient oxygen mole fraction of 0.39. The hot flames that spawned the cool flames were 2.6 times as large. An analytical model is presented that combines previous models for steady droplet burning and the partial-burning regime for cool diffusion flames. The results identify the importance of burner temperature on the behavior of these cool flames. They also indicate that the observed cool flames reside in rich regions near a mixture fraction of 0.53. & COPY; 2022 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

Tsuji burners, in which flames may be anchored in the forward stagnation region of a cylindrical porous fuel injector placed in a uniform air stream, are addressed here for moderately large Reynolds numbers. Attention is focused on conditions under which the fuel-injection velocity is not sufficiently small compared with the outer air velocity for the boundary layer to remain attached to the forward part of the cylinder surface. In the resulting flow, the flame is embedded in the thin mixing layer that forms at the surface separating the outer air stream from the fuel stream, both having, in general, different densities. The flow on the air side of the mixing layer is potential, while that on the fuel side usually is rotational because exit conditions for the fuel injection generate vorticity, for example, by imposing a requirement that the fuel must emerge normal to the cylinder surface, which is the condition analyzed herein. It is shown that introduction of a suitably density-weighted stream function reduces the problem to that of constant-density flow, with the density-square-root-weighted ratio of injection velocity to free-stream velocity Lambda emerging as the only controlling parameter. The numerical solution, involving determination of the vorticity distribution in the inviscid fuel flow through an iterative scheme, provides the structure of the flow, including the mixing-layer location and the inviscid-flow strain rate there. Numerical results are presented for values of Lambda ranging from small (Lambda << 1) to large (Lambda >> 1) injection velocities. The inviscid results in the limit of vanishingly small injection velocities (Lambda approaching zero) demonstrate that, unlike the prediction of the potential-flow solution, when the fuel-side flow is rotational the outer air velocity never approaches the classical solution corresponding to potential flow around a solid cylinder (Lambda = 0), a result affecting the interpretation of analyses of experiments involving flames stabilized on Tsuji burners as the boundary layer is blown off. In particular, with rotational fuel-side flow, the streamline separating the fuel and oxidizer regions lies farther from the cylinder surface, resulting in a larger near-quiescent wake and a lower strain rate along the separating streamline.

Unsteady axisymmetric numerical simulations are used to determine the transition to bubble and conical vortex breakdown in low-Mach-number laminar swirling Burke–Schumann flames, for which an ambient-temperature fuel jet in solid-body rotation emerges into quiescent air. A critical value of the swirl number S for the onset of the bubble (SB*) and the cone (SC*) is determined as the jet fuel-feed mass fraction YF,j is varied for fixed Re=800, assuming typical conditions for methane combustion with air. During the first transition from pre-breakdown to the bubble, the jet core is relatively unaffected by the flame in the surrounding shear layer, and SB*=1.36 is constant for all values of dilution. This transition to the jet-like bubble breakdown flame is found to be in agreement with theoretical predictions based on the criterion of failure of the slender quasi-cylindrical approximation. Variation in the critical swirl number SC*, characterizing the second transition from the bubble to the cone, is relatively small (1.80≤SC*≤1.83) in the range 0.1≤YF,j≤1, but the resulting flow and flame shape for conical breakdown is found to depend critically on YF,j. For realistic values of dilution (YF,j≥0.2), the bubble transitions to a steady compact cone at SC* with a flame sheet that passes around the recirculation region, maintaining a jet-like flame. In the extreme dilution case (YF,j=0.1), the reaction sheet occurs closer to the fuel jet axis, increasing the radial velocities through thermal expansion and accelerating the transition to the cone (lower SC*). The reduced viscosity associated with the lower adiabatic flame temperature leads to an enlarged unsteady conical breakdown with the flame sheet stabilized near the inlet.

This chapter provides an overview of $$\textrm{H}_{2}$$ ignition and safety-related questions, to be addressed in the development of future $$\textrm{H}_{2}$$ thermal engines. Basics of $$\textrm{H}_{2}$$ ignition phenomena are covered in the first part, including the well-known branched-chain oxidation reactions described by Semenov & Hinshelwood, as well as useful analytical derivations of induction delay times. The second part provides an overview of classical canonical limit problems, including the explosion-limit (p, T) diagram, the propagation limits of both deflagrations and detonations, and shock-induced or thermal-induced ignitions. The two remaining parts address two opposite but complementary questions: how to ignite a $$\textrm{H}_{2}$$ engine, and how to prevent hazardous $$\textrm{H}_{2}$$ ignition. In the former, a list of available technologies is offered, while in the latter, simplified models are presented to predict ignition hazards from cold-flow numerical simulations.

The authors are a team of fire whirl researchers who have been actively studying whirls and large-scale wildland fires by directly observing them through fire-fighting efforts and applying theory, scale modeling, and numerical simulations in fire research. This multidisciplinary research-background team previously conducted scale model experiments to reconstruct hazardous large-scale fires in the laboratory, then conducted numerical simulations and developed fundamental theories to translate these findings into a basic understanding of combustion science and fluid dynamics. This article, a mix of reviews of the state of art experiments, theories, numerical modeling and artificial intelligence, and two case studies, is intended to address some safety concerns and raise awareness of large-scale fire whirls and forest fires with knowledge of thermodynamics, chemical kinetics, fluid dynamics, design, and practical fire-fighting experience, offering gaps that should be filled and future research to be conducted in each field, and crucial new observations and insights on large-scale fire incidents. We believe, this timely topic is of interest not only to fire research community but also to general readers, as the frequency and intensity of large-scale forest fires and fire whirls have increased, possibly due to the continuing global warming trend and human-induced changes in fuels. Each section and case study was written by one or two individual researchers based on their field of expertise which allows them to critically review progress made in their section of large-scale fire-whirls and forest-fires. Crucial observations and insights on the historical Great-Kanto-Earthquake-generated Hifukusho-Ato Fire-whirl (HAFW) and the slow rotations observed during recent forest firefighting efforts are presented. The first case study occurred in downtown Tokyo on 1 September 1923, as a result of the Great-Kanto-Earthquake, which claimed over 38,000 deaths within 15 min. The second case study discusses large-scale slow rotations observed during recent forest fires, which might had been responsible for the injuries and deaths of experienced firefighters.

Axisymmetric numerical simulations are used to assess the swirl-induced stabilization of low-Mach-number non-premixed jet flames at a moderate Reynolds number (Re=200). Using a one-step model chemistry describing methane-air partially premixed combustion, we carry out a parametric investigation of the coupling between vortex breakdown and laminar flame liftoff/blowoff in a concentric jet configuration involving a central non-swirling methane jet surrounded by a swirling annular air jet issuing from a pipe with radius RA′ rotating with angular speed Ω′. The analysis considers order-unity values of the two relevant controlling parameters, namely, the Damköhler number DN, defined as the square of the ratio of the stoichiometric methane-air flame-propagation velocity to the mean air-jet velocity UA′, and the swirl number S=Ω′RA′/UA′. As the Damköhler number DN is decreased the attached edge flame lifts off from the injector rim. The resulting lifted triple flame migrates downstream on further decreasing DN until a critical blowoff value DN,b is reached. Results for fixed S=1 exhibit lower values DN,b than the corresponding simulations with fixed S=0. For a fixed Damköhler number, it is found that increasing S results in increased entrainment and reduced liftoff heights. At a critical value SB* of the swirl number, equal to SB*=1.2 for DN=0.35, a recirculation zone abruptly forms upstream of the lifted triple flame, enhancing the mixing and facilitating flame stabilization closer to the injector.