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We present in this short communication a modification to our previous ethanol reduced combustion chemistry (Millán-Merino, 2018) that eliminates nonphysical values of the species concentrations which we discovered in applying the mechanism to the combustion of an isolated ethanol droplet. This unsteady test is reported here to check the multipurpose character of the reduced mechanism for a problem that combines non-homogeneous autoignition, rich and lean premixed-flame propagation, and the development of a diffusion flame, as well as a the presence of a cold moving boundary at the droplet surface. During the computations, production and consumption rates of the α -hydroxyethyl (CH 3 CHOH) intermediary radical became unbalanced, invalidating its steady-state hypothesis, which was used during the derivation of the reduced scheme. This difficulty is removed here by taking CH 3 CHOH out of steady state, thereby augmenting slightly the reduced mechanism. [ABSTRACT FROM AUTHOR]

Experimental observations are presented concerning radiative extinction of large n-alkane droplets in diluent-substituted environments at moderately varied pressures in microgravity onboard the International Space Station. The fuels considered are n-heptane, n-octane, and n-decane with carbon dioxide, helium, and xenon used as inerts, replacing nitrogen as diluents at varying amounts. It is shown that a simple scaling analysis, based on the assumptions that radiative extinction occurs when the flame temperature drops to a critical value and that the radiative heat loss rate is a fraction of the heat-release rate at the flame, is able to correlate the measured droplet diameter at extinction as a function of its initial diameter and of the ambient gas-mixture properties. [ABSTRACT FROM AUTHOR]

Pool fires are known to undergo a bifurcation to a globally unstable puffing state driven by baroclinic and buoyant vorticity production. Although the supercritical puffing regime away from the bifurcation has been studied extensively in the literature, no detailed account has been given of the critical conditions for its onset, that being the purpose of the present paper. For the relevant canonical case of round liquid pools without swirl, aside from the inherent thermochemical and transport parameters associated with the fuel, pool-fire puffing is governed by a single dimensionless number, the Rayleigh number, which scales with the cube of the pool diameter. Consequently, for a fixed fuel and under fixed ambient conditions, there is a critical fuel pool diameter, associated with a critical value of the Rayleigh number, above which the flame starts puffing. A global linear stability analysis that accounts for the axisymmetry of the prevailing instability mode is developed here to describe the bifurcation. The mathematical formulation employs the limit of infinitely fast reaction, with account taken of the nonunity Lewis number and vaporization characteristics of typical liquid fuels. Predictions of critical puffing conditions, including critical diameters and puffing frequencies, are provided for methanol and for heptane pool fires, and the results are compared with results of new small-scale experiments under controlled laboratory conditions, reported here, yielding reasonably good agreement. [ABSTRACT FROM AUTHOR]

The acoustic response of strained nonpremixed flames is investigated using as a canonical model problem the planar counterflow configuration subject to small harmonic fluctuations of the stagnation-point pressure and strain rate. To focus on effects of modified transport rates the analysis employs the limit of infinitely fast reaction for a general non-unity Lewis number of the fuel, including values of interest in hydrogen-oxygen and hydrocarbon-oxygen systems. For both acoustic-pressure and acoustic-velocity response, differential–diffusion effects are shown to promote the fluctuations of the flame location and reactant consumption rates (while in the pressure response, distinct behaviors of the flame temperature were observed depending on stoichiometry). The results are used, together with the Rayleigh criterion, to investigate the frequency dependence of the amplification/attenuation rate relevant in computations of acoustic instabilities. The analysis predicts acoustic amplification for all frequencies in the pressure response, whereas a critical crossover frequency is identified in the strain response demarcating the transition from attenuation to amplification of acoustic energy. [ABSTRACT FROM AUTHOR]

Axisymmetric numerical simulations are used to assess the swirl-induced stabilization of low-Mach-number non-premixed jet flames at a moderate Reynolds number (R e = 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 R A ′ rotating with angular speed Ω ′. The analysis considers order-unity values of the two relevant controlling parameters, namely, the Damköhler number D N , defined as the square of the ratio of the stoichiometric methane-air flame-propagation velocity to the mean air-jet velocity U A ′ , and the swirl number S = Ω ′ R A ′ / U A ′. As the Damköhler number D N is decreased the attached edge flame lifts off from the injector rim. The resulting lifted triple flame migrates downstream on further decreasing D N until a critical blowoff value D N , b is reached. Results for fixed S = 1 exhibit lower values D N , 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 S B * of the swirl number, equal to S B * = 1.2 for D N = 0.35 , a recirculation zone abruptly forms upstream of the lifted triple flame, enhancing the mixing and facilitating flame stabilization closer to the injector. [ABSTRACT FROM AUTHOR]

A previously derived one-step reduced chemical-kinetic mechanism, describing hydrogen flames under near-limit conditions involving peak temperature not far from the crossover temperature, is used in computations of hydrogen-air flamelets at elevated pressures typical of gas-turbine combustion. Besides freely propagating laminar deflagrations with compositions spanning the whole range of flammability conditions, the calculations address strained premixed and nonpremixed flames as well as partially premixed propagating fronts. The comparisons with results of detailed-chemistry computations reveal that, for most purposes, the one-step mechanism provides sufficiently accurate predictions of burning rates under all conditions of interest for gas-turbine combustion. The reduced-chemistry model, featuring an explicit analytic expression for the hydrogen oxidation rate in terms of the local temperature and the O 2 , H 2 , and H 2 O concentrations, can be easily implemented in numerical codes, thereby facilitating future numerical analyses based on direct-numerical and large-eddy simulations. [ABSTRACT FROM AUTHOR]

The laminar flow resulting from impingement of two steadily fed low-Mach-number gaseous jets issuing into a stagnant atmosphere from coaxial cylindrical ducts at moderately large Reynolds numbers, often used in combustion experiments, is studied through numerical integrations of the Navier–Stokes equations. In the Reynolds-number range addressed, 50–1000, the flow of the approaching jets is nearly inviscid, with viscous effects and mixing being restricted to the thin mixing layers surrounding the jets and to a thin layer located at the separating stream surface. The analysis of the main inviscid flow shows that only two parameters, based on the scales associated with the radius and the velocity profiles of the two feed streams, are needed to characterize the flow, namely, the ratio of the inter-jet separation distance to the duct radius and the ratio of momentum fluxes of the jets. The numerical results for uniform and Poiseuille velocity profiles provide, in particular, the value of the strain rate at the stagnation point for use in the analysis of experimental studies of counterflow premixed and diffusion flames. [ABSTRACT FROM AUTHOR]

Recent droplet-combustion experiments onboard the International Space Station (ISS) have revealed that large n-alkane droplets, following radiative extinction of the visible flame, can continue to burn quasi-steadily in a low-temperature regime, characterized by negative-temperature-coefficient (NTC) chemistry. In this study we report experimental observations of n-heptane, n-octane, and n-decane droplets of varying initial size burning in oxygen/nitrogen, oxygen/nitrogen/carbon dioxide, and oxygen/nitrogen/helium environments at pressures from 0.5 to 1.0 atm, with oxygen concentrations from 14% to 25% by volume. These large n-alkane droplets exhibited radiative extinction of the hot flame, followed by quasi-steady low-temperature burning, which terminated with diffusive extinction accompanied by the formation of a vapor cloud, while small droplets did not exhibit radiative extinction but instead burned to completion or disruptively extinguished. Results for droplet burning rates in both the hot-flame and cool-flame regimes, as well as droplet extinction diameters at the end of each stage, are presented. The cool-flame extinction diameters for all three n-alkanes are shown to follow a similar trend as functions of the oxygen concentration, predicted here from a simplified theoretical model that is based on the reaction-rate parameters for the oxygen molecule addition to the alkyl radical and for ketohydroperoxide decomposition. [ABSTRACT FROM AUTHOR]

We present results of fully compressible Navier–Stokes simulations of pulsating flame instabilities in two dimensions using single-step, first-order Arrhenius kinetics. Model parameters correspond to Zel’dovich and Lewis numbers of Ze =9.5 and Le =10, respectively, and flame Mach numbers M L between 4.62×10−3 and 2.31×10−2. The results show that the pulsating instability creates two types of transverse waves: deflagrations and detonations. Both types of waves can coexist for the same reactive system, but transverse detonations become more likely as M L increases. The transverse detonations observed for our model system are of the intermediate type with most of the chemical energy release accompanying the pressure rise. They propagate inside the large preheat zone of the pulsating flame and do not spread into cold material. The results expand our knowledge of possible new phenomena associated with pulsating instability. [ABSTRACT FROM AUTHOR]

Abstract: Experimental observations of anomalous combustion of n-heptane droplets burning in microgravity are reported. Following ignition, a relatively large n-heptane droplet first undergoes radiative extinction, that is, the visible flame ceases to exist because of radiant energy loss. But the droplet continues to experience vigorous vaporization for an extended period according to a quasi-steady droplet-burning law, ending in a secondary extinction at a finite droplet diameter, after which a vapor cloud rapidly appears surrounding the droplet. We hypothesize that the second-stage vaporization is sustained by low-temperature, soot-free, “cool-flame” chemical heat release. Measured droplet burning rates and extinction diameters are used to extract an effective heat release, overall activation energy, and pre-exponential factor for this low-temperature chemistry, and the values of the resulting parameters are found to be closer to those of “cool-flame” overall reaction-rate parameters, found in the literature, than to corresponding hot-flame parameters. [Copyright &y& Elsevier]

Abstract: The ignition time of hydrogen–air diffusion flames is a quantity of utmost interest in a large number of applications, with implications regarding the viability of supersonic combustion and the safe operation of gas turbines. The underlying chemistry and the associated ignition history are very different depending on the initial temperature and pressure. This article addresses conditions that place the system above the so-called second explosion limit, as is typically the case in SCRAMJET operation, so that a branched-chain explosion characterizes the ignition process. The roles of local radical accumulation, molecular transport, and chemical reaction in nonpremixed ignition are clarified by considering the temporal evolution of an unstrained mixing layer formed between two semi-infinite spaces of hydrogen and air. The problem is formulated in terms of a radical-pool mass fraction, whose evolution in time is studied with a WKB expansion that exploits the disparity of chemical time scales present in the problem, leading to an explicit expression for the ignition time. The applicability of the analytical results for obtaining predictions of ignition distances in supersonic-combustion applications is also considered. [Copyright &y& Elsevier]

Abstract: This work explores the applicability of one-step irreversible Arrhenius kinetics with unity reaction order to the numerical description of partially premixed hydrocarbon combustion. Computations of planar premixed flames are used in the selection of the three model parameters: the heat of reaction q, the activation temperature , and the preexponential factor B. It is seen that changes in q with equivalence ratio ϕ need to be introduced in fuel-rich combustion to describe the effect of partial fuel oxidation on the amount of heat released, leading to a universal linear variation for for all hydrocarbons. The model also employs a variable activation temperature to mimic changes in the underlying chemistry in rich and very lean flames. The resulting chemistry description is able to reproduce propagation velocities of diluted and undiluted flames accurately over the whole flammability limit. Furthermore, computations of methane–air counterflow diffusion flames are used to test the proposed chemistry under nonpremixed conditions. The model not only predicts the critical strain rate at extinction accurately but also gives near-extinction flames with oxygen leakage, thereby overcoming known predictive limitations of one-step Arrhenius kinetics. [Copyright &y& Elsevier]

Laminar diluted hydrogen flames have been studied in a counterflow burner. Measurements were made of profiles of temperature and of [NO], which are compared with predictions from numerical computations. A constant strain rate of 100 s−1 was used in every experiment. The hydrogen stream was diluted with the inerts N2, CO2, and He. Besides diffusion flames, the fuel streams were partially premixed to equivalence ratios from 2.7 to 3.9. There is evidence that, for these degrees of premixing, attached to the exit of the fuel duct there is a rich premixed flame, whose hot products burn in a diffusion flame close to the stagnation plane. Differences between the influences of the various diluents are discussed in terms of differences in heat capacities, thermal-radiative properties, and effects on chemical reactions. Maximum measured flame temperatures varied from 1830 to 2170 K, and measured [NO] ranged from 3.6 to 21.5 ppm. Numerical predictions show good agreement with experiments for both temperatures and [NO] at the lower temperatures. At higher temperatures, the predicted [NO] exceeds its measured value, the difference being attributed to probe effects. [Copyright &y& Elsevier]

Direct Numerical Simulations (DNS) are performed to investigate the process of spontaneous ignition of hydrogen flames at laminar, turbulent, adiabatic and non-adiabatic conditions. Mixtures of hydrogen and vitiated air at temperatures representing gas-turbine reheat combustion are considered. Adiabatic spontaneous ignition processes are investigated first, providing a quantitative characterization of stable and unstable flames. Results indicate that, in hydrogen reheat combustion, compressibility effects play a key role in flame stability and that unstable ignition and combustion are consistently encountered for reactant temperatures close to the mixture's characteristic crossover temperature. Furthermore, it is also found that the characterization of the adiabatic processes is also valid in the presence of non-adiabaticity due to wall heat-loss. Finally, a quantitative characterization of the instantaneous fuel consumption rate within the reaction front is obtained and of its ability, at auto-ignitive conditions, to advance against the approaching turbulent flow of the reactants, for a range of different turbulence intensities, temperatures and pressure levels. [ABSTRACT FROM AUTHOR]

The dynamics of gaseous detonation is revisited on the basis of analytical studies. Problems of initiation, quenching, pulsation and cellular structures are addressed. The objective is to improve our physical understanding of the development, stability and structure of gaseous detonations. New insights that have been gained from analytical investigations are emphasized. Specific problems discussed are the direct initiation of detonations in spherical geometry, the spontaneous soft initiation and quenching of detonations in a temperature gradient, the stability threshold and dynamics of galloping detonations, and the multi-dimensional instability threshold and cellular structures of both overdriven and near-Chapman-Jouguet detonations. It will be seen that, although there have been many accomplishments, some outstanding questions remain.

The motivation for devoting a Theme Issue to explosions is discussed. As subsequent articles in the issue are written with the assumption that the reader has had a certain amount of previous exposure to the subject, some of the history and necessary background information are presented here. The topics on explosions that will be encountered in the remaining articles are previewed. Finally, several important future outstanding research problems, beyond those addressed in the following articles, are discussed, with the objective of complementing the coverage of explosions in this issue.