Spray-Flame Dynamics in a Rich Droplet Array

International audienceIn a recent numerical paper (Nicoli et al. Combust. Sci. Technol. vol. 186, pp. 103-119; 2014) [1], a model of isobaric flame propagation in lean sprays has been proposed. The initial state of the monodisperse mists was schematized by a system of individual alkane droplets initially located at the nodes of a face-centered 2D-lattice, surrounded by a saturated mixture of alkane and air. In the present study, the previous model is complemented with an original chemical scheme that allows us to study the combustion of rich alkane/air mixtures

Lean flame dynamics through a 2D lattice of alkane droplets in air

International audienceFlame propagation along a 1-D array or through a 2D-lattice of fuel droplets has long been suggested to schematize spray-flames spreading in a two-phase premixture. The present numerical work considers the fresh aerosol as a system of individual alkane droplets initially located at the nodes of a face-centered 2D-lattice, surrounded by a variable mixture of alkane and air, in which the droplets can move. The main parameters of the study are s, the lattice path, and phi_ L , the liquid loading, which are both varied, whereas phi_T , the overall equivalence ratio, is maintained lean ( phi_T = 0.85). Main results are as follows: (a) For a large lattice path (or when the droplets are large enough), spreading occurs in two stages: a short time of combustion followed by a long time lag of vaporization and a classical triple flame (with a very short rich wing) spreads around the droplets; (b) spray-flame speed decreases as liquid loading increases; (c) an elementary model invoking both propagation stages allows us to interpret flame speed as a function of the sole parameter s × phi_ L ; (d) when the lattice path shortens, the spray-flame exhibits a pattern that continuously goes from this situation to the plane flame front

Rich Spray-Flame Propagating through a 2D-Lattice of Alkane Droplets in Air

International audienceIn a recent numerical paper (Nicoli et al. Combust. Sci. Technol. vol. 186, pp. 103-119; 2014) [1], a model of isobaric flame propagation in lean sprays has been proposed. The initial state of the monodisperse mists was schematized by a system of individual alkane droplets initially located at the nodes of a face-centered 2D-lattice, surrounded by a saturated mixture of alkane and air. In the present study, the previous model is complemented with an original chemical scheme that allows us to study the combustion of rich alkane/air mixtures

A resonant response of self-pulsating spray-flame submitted to acoustic wave

International audienceRecently, experiments and theoretical investigations have shown that spray flame can exhibit oscillatory regimes for standard set of parameters. Theoretical and numerical investigations on flame propagation in two-phase premixtures have put forward an intrinsic (and robust) mechanism based on the interaction between the locus where droplets vaporize and the reaction zone. This mechanism invokes neither droplet inertia (very small droplets are studied) nor differential diffusive effects (pulsations take place for unity Lewis number, too). Self-oscillations of spray-flame occur as in a supercritical Hopf bifurcation, controlled by Zeldovich number (Ze, the reduced activation energy), the onset threshold being on the order of (Ze)c ≈ 10. The issue addressed in this contribution is whether acoustic wave and self-pulsating spray-flame can interact. This study was carried out in the open-loop context: a spray-flame was submitted to small amplitude fluctuations of pressure; the gain toward acoustics was found as depending on Zeldovich number because energy transfer is found magnified in the case of a close-frequency fit between acoustic resonator and natural spray-flame oscillations. Moreover, energy transfer is found as of resonant type

Etude en boucle ouverte de l’action d’une onde acoustique sur une flamme de brouillard

Lorsque l'échelle de réaction-diffusion est plus grande que la distance inter-goutte, le brouillard est homogénéïsable. Sa combustion peut donner lieu à des flammes pulsantes. La possible résonance de cet oscillateur est étudiéee quand la flamme est soumise à une onde acoustique. Le critère de Rayleigh prédit l’apparition d’une instabilité thermo-acoustique. Si la fréquence s'approche de la fréquence intrinsèque de pulsation, le transfert d’énergie est trouvé maximal

Analysis of pulsating spray flames propagating in lean two-phase mixtures with unity Lewis number

International audiencePulsating (or oscillatory) spray flames have recently been observed in experiments on two-phase combustion. Numerical studies have pointed out that such front oscillations can be obtained even with very simple models of homogeneous two-phase mixtures, including elementary vaporization schemes. The paper presents an analytical approach within the simple framework of the thermal-diffusive model, which is complemented by a vaporization rate independent of gas temperature, as soon as the latter reaches a certain thermal threshold (θv in reduced form). The study involves the Damköhler number (Da), the ratio of chemical reaction rate to vaporization rate, and the Zeldovich number (Ze) as essential parameters. We use the standard asymptotic method based on matched expansions in terms of 1/Ze. Linear analysis of two-phase flame stability is performed by studying, in the absence of differential diffusive effects (unity Lewis number), the linear growth rate of 2-D perturbations added to steady plane solutions and characterized by wavenumber k in the direction transverse to spreading. A domain of existence is found for the pulsating regime. It corresponds to mixture characteristics often met in air-fuel two-phase systems: low boiling temperature (θv≪1), reaction rate not higher than vaporization rate (Da<1, i.e., small droplets), and activation temperature assumed to be high compared with flame temperature (Ze⩾10). Satisfactory comparison with numerical simulations confirms the validity of the analytical approach; in particular, positive growth rates have been found for planar perturbations (k=0) and for wrinkled fronts (k≠0). Finally, comparison between predicted frequencies and experimental measurements is discussed

Numerical study of flame propagation in a lattice of alkane droplets in air

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Combustion of gaseous co-flow jets

We present numerical results concerning the combustion that occurs in a three-plan jet system, which represents the two-dimensional version of a coflow gaseous injector of hydrogen and oxygen. The study focuses on the hydrodynamic effects--damped by combustion--that affect the high-speed jets at the entrance of a combustion chamber. The concerned parameters mainly involve the inlet flow velocities in a range where flame attachment occurs. The results confirm the classical idea according to which mixing-layer combustion damps shear-layer instabilities. Moreover, steady or unsteady solutions can be exhibited for the same set of parameters. For various ratios of density and inlet velocity (established between oxygen and hydrogen jets), we study the coflow dynamics (under combustion), which can be interpreted in terms of momentum flux ratio J. When increasing J, the dynamics become more and more complex, exhibiting large amplitude flapping, which produces the widening of time-averaged temperature field. For high J values, the dense oxygen jet is rapidly stripped and takes the same pattern as the liquid core observed in LOx injectors, with a dependence close to the Jm1/2 law measured for dense core length (albeit presently studied Reynolds numbers are one decade less)

Premixed flame dynamics in presence of mist

International audienceThe injection of a water spray within an enclosure prone to explo-sion is reputed to reduce the risk. This strategy for safety improvementis at the root of numerous experiments that have concluded that pre-mixed flame can be extinguished by a sufficient amount of a wateraerosol characterized by suitable droplet sizes. On the other hand,certain experiments seemingly indicate that flame speed promotioncan be observed when particular water mists are injected within thepremixture.To contribute to shed light upon these less than intuitive observa-tions, we propose to study the propagation of a nearly stoichiometricpremixed flame within a 2D-lattice of water droplets. Main parametersof investigation are droplet size and droplet inter-distance (or equiva-lently, lattice spacing). When the droplet inter-distance is small, theresults confirm that a sufficient amount of water quenches combustion.For larger droplet inter-distance, we observe a flame speed enhance-ment for suitable droplet size. Concomitantly, the flame front foldssubjected to Darrieus-Landau instability. The final discussion, whichinvokes a Sivashinsky-type model equation for DL instability, interpretssuch a speed promotion in presence of mist as a secondary non-linearenhancement of the flame surface

Numerical study of flame dynamics through a 2D lattice of alkane droplets in air : role of the chemical modelling

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