Colette Nicoli, Pierre Haldenwang, Bruno Denet. Premixed flame dynamics in presence of mist. Combustion Science and Technology, 2019, 191 (2), pp.197-207. ⟨10.1080/00102202.2018.1453728⟩. ⟨hal-01820207⟩ Plus de détails...
The injection of a water spray within an enclosure prone to explo- sion is reputed to reduce the risk. This strategy for safety improvement is at the root of numerous experiments that have concluded that pre- mixed flame can be extinguished by a sufficient amount of a water aerosol characterized by suitable droplet sizes. On the other hand, certain experiments seemingly indicate that flame speed promotion can be observed when particular water mists are injected within the premixture. To contribute to shed light upon these less than intuitive observa- tions, we propose to study the propagation of a nearly stoichiometric premixed flame within a 2D-lattice of water droplets. Main parameters of investigation are droplet size and droplet inter-distance (or equiva- lently, lattice spacing). When the droplet inter-distance is small, the results 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 folds subjected to Darrieus-Landau instability. The final discussion, which invokes a Sivashinsky-type model equation for DL instability, interprets such a speed promotion in presence of mist as a secondary non-linear enhancement of the flame surface.
Colette Nicoli, Pierre Haldenwang, Bruno Denet. Premixed flame dynamics in presence of mist. Combustion Science and Technology, 2019, 191 (2), pp.197-207. ⟨10.1080/00102202.2018.1453728⟩. ⟨hal-01820207⟩
Colette Nicoli, Pierre Haldenwang, Bruno Denet. Darrieus–Landau instability of premixed flames enhanced by fuel droplets. Combustion Theory and Modelling, 2017, 21 (4), pp.630 - 645. ⟨10.1080/13647830.2017.1279756⟩. ⟨hal-01678255⟩ Plus de détails...
Recent experiments on spray flames propagating in a Wilson cloud chamber have established that spray flames are much more sensitive to wrinkles or corrugations than single-phase flames. To propose certain elements of explanation, we numerically study the Darrieus–Landau (or hydrodynamic) instability (DL-instability) developing in premixtures that contain an array of fuel droplets. Two approaches are compared: numerical simulation starting from the general conservation laws in reactive media, and the numerical computation of Sivashinsky-type model equations for DL-instability. Both approaches provide us with results in deep agreement. It is first shown that the presence of droplets in fuel–air premixtures induces initial perturbations which are large enough to trigger the DL-instability. Second, the droplets are responsible for additional wrinkles when the DL-instability is developed. The latter wrinkles are of length scales shorter than those of the DL-instability, in such a way that the DL-unstable spray flames have a larger front surface and therefore propagate faster than the single-phase ones when subjected to the same instability
Colette Nicoli, Pierre Haldenwang, Bruno Denet. Darrieus–Landau instability of premixed flames enhanced by fuel droplets. Combustion Theory and Modelling, 2017, 21 (4), pp.630 - 645. ⟨10.1080/13647830.2017.1279756⟩. ⟨hal-01678255⟩
Romain Thimothée, Christian Chauveau, Fabien Halter, Colette Nicoli, Pierre Haldenwang, et al.. Microgravity experiments and numerical studies on ethanol/air spray flames. Comptes Rendus. Mécanique, 2017, 345 (2), pp.99 - 116. ⟨10.1016/j.crme.2016.10.013⟩. ⟨hal-01441677⟩ Plus de détails...
Spray flames are known to exhibit amazing features in comparison with single-phase flames. The weightless situation offers the conditions in which the spray characteristics can be well controlled before and during combustion. The article reports on a joint experimental/numerical work that concerns ethanol/air spray flames observed in a spherical chamber using the condensation technique of expansion cooling (based on the Wilson cloud chamber principle), under microgravity. We describe the experimental setup and give details on the creation of a homogeneous and nearly monosized aerosol. Different optical diagnostics are employed successfully to measure the relevant parameters of two-phase combustion. A classical shadowgraphy system is used to track the flame speed propagation and allow us to observe the flame front instability. The complete characterization of the aerosol is performed with a laser diffraction particle size analyser by measuring the droplet diameter and the droplet density number, just before ignition. A laser tomography device allows us to measure the temporal evolution of the droplet displacement during flame propagation, as well as to identify the presence of droplets in the burnt gases. The numerical modelling is briefly recalled. In particular, spray-flame propagation is schematized by the combustion spread in a 2-D lattice of fuel droplets surrounded by an initial gaseous mixture of fuel vapour and air. In its spherical expansion, the spray flame presents a corrugated front pattern, while the equivalent single-phase flame does not. From a numerical point of view, the same phenomena of wrinkles are also observed in the simulations. The front pattern pointed out by the numerical approach is identified as of Darrieus–Landau (DL) type. The droplets are found to trigger the instability. Then, we quantitatively compare experimental data with numerical predictions on spray-flame speed. The experimental results show that the spray-flame speed is of the same order of magnitude as that of the single-phase premixed flame. On the other hand, the numerical results exhibit the role played by the droplet radius in spray-flame propagation, and retrieve the experiments only when the droplets are small enough and when the Darrieus–Landau instability is triggered. A final discussion is developed to interpret the various patterns experimentally observed for the spray-flame front.
Romain Thimothée, Christian Chauveau, Fabien Halter, Colette Nicoli, Pierre Haldenwang, et al.. Microgravity experiments and numerical studies on ethanol/air spray flames. Comptes Rendus. Mécanique, 2017, 345 (2), pp.99 - 116. ⟨10.1016/j.crme.2016.10.013⟩. ⟨hal-01441677⟩
Colette Nicoli, Pierre Haldenwang, Bruno Denet. Spray-Flame Dynamics in a Rich Droplet Array. Flow, Turbulence and Combustion, 2016, 96 (2), pp.377-389. ⟨10.1007/s10494-015-9675-4⟩. ⟨hal-01282878⟩ Plus de détails...
In 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.
Colette Nicoli, Pierre Haldenwang, Bruno Denet. Spray-Flame Dynamics in a Rich Droplet Array. Flow, Turbulence and Combustion, 2016, 96 (2), pp.377-389. ⟨10.1007/s10494-015-9675-4⟩. ⟨hal-01282878⟩
Colette Nicoli, Bruno Denet, Pierre Haldenwang. Rich Spray-Flame Propagating through a 2D-Lattice of Alkane Droplets in Air. Combustion and Flame, 2015, 162 (12), pp.4598-4611. ⟨10.1016/j.combustflame.2015.09.018⟩. ⟨hal-01255816⟩ Plus de détails...
In 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.
Colette Nicoli, Bruno Denet, Pierre Haldenwang. Rich Spray-Flame Propagating through a 2D-Lattice of Alkane Droplets in Air. Combustion and Flame, 2015, 162 (12), pp.4598-4611. ⟨10.1016/j.combustflame.2015.09.018⟩. ⟨hal-01255816⟩
Colette Nicoli, Bruno Denet, Pierre Haldenwang. Lean flame dynamics through a 2D lattice of alkane droplets in air. Combustion Science and Technology, 2014, 186 (2), pp.103-119. ⟨hal-00935131⟩ Plus de détails...
Flame 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.
Colette Nicoli, Bruno Denet, Pierre Haldenwang. Lean flame dynamics through a 2D lattice of alkane droplets in air. Combustion Science and Technology, 2014, 186 (2), pp.103-119. ⟨hal-00935131⟩
Colette Nicoli, Pierre Haldenwang. A resonant response of self-pulsating spray-flame submitted to acoustic wave. Combustion Science and Technology, 2010, 182 (4-6), pp.559-573. ⟨10.1080/00102200903465915⟩. ⟨hal-00907320⟩ Plus de détails...
Recently, 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.
Colette Nicoli, Pierre Haldenwang. A resonant response of self-pulsating spray-flame submitted to acoustic wave. Combustion Science and Technology, 2010, 182 (4-6), pp.559-573. ⟨10.1080/00102200903465915⟩. ⟨hal-00907320⟩
Colette Nicoli, Pierre Haldenwang, S. Suard. Effects of substituting fuel spray for fuel gas on flame stability in lean premixtures. Combustion and Flame, 2007, 149 (3), pp.295-313. ⟨10.1016/j.combustflame.2006.12.018⟩. ⟨hal-00907387⟩ Plus de détails...
We analyze flame propagation through a homogeneous three-component premixture composed of fuel gas, small fuel droplets, and air. This analytical study is carried out within the framework of a diffusional-thermal model with the simplifying assumption that both fuels--the fuel in the gaseous phase and the gaseous fuel evaporating from the droplets--have the same Lewis number. The parameter that expresses the degree of substitution of spray for gas is δ, the liquid loading, i.e., the ratio of liquid fuel mass fraction to overall fuel mass fraction in the fresh premixture. In this substitution of liquid fuel for gaseous fuel, the overall equivalence ratio is lean and is kept identical. We hence obtain a partially prevaporized spray, for which we analytically study the dynamics of the plane spray-flame front. The investigated model assumes the averaged distance between droplets to be small compared with the premixed flame thickness (i.e., small droplets and moderate pressure). Le, the Lewis number, Ze, the Zeldovich number, and δ are the main parameters of the study. Our stability analysis supplies the stability diagram in the plane {Le,δ} for various Ze values and shows that, for all Le, the plane front becomes unstable for high liquid loading. At large or moderate Lewis number, we show that the presence of droplets substantially diminishes the onset threshold of the oscillatory instability, making the appearance of oscillatory propagation easier. Oscillations can even occur for Le<1 when sufficient spray substitution is operated. The pulsation frequency occurring in this regime is a tunable function of δ. At low Lewis number, substitution of spray for gas leads to a more complex situation for which two branches can coexist: the first one still corresponding to the pulsating regime, the other one being related to the diffusive-thermal cellular instability.
Colette Nicoli, Pierre Haldenwang, S. Suard. Effects of substituting fuel spray for fuel gas on flame stability in lean premixtures. Combustion and Flame, 2007, 149 (3), pp.295-313. ⟨10.1016/j.combustflame.2006.12.018⟩. ⟨hal-00907387⟩
Colette Nicoli, Pierre Haldenwang, Bruno Denet. Flame holding downstream from a co-flow injector. Comptes Rendus Mécanique, 2006, 334, pp.408-413. ⟨10.1016/j.crme.2006.06.002⟩. ⟨hal-00091162⟩ Plus de détails...
We present numerical results on the flame attachment in the downstream vicinity of the co-flow injector lip that separates the reactive fluids at injection. Two stability diagrams show the domains where the flame is anchored, blown off, or extinguished, in terms of separating plate thickness and injection velocities of both fluids. Different anchoring modes—stagnation point counter-flow holding or edge flame anchorage—are described, depending particularly on the plate rim thickness.
Colette Nicoli, Pierre Haldenwang, Bruno Denet. Flame holding downstream from a co-flow injector. Comptes Rendus Mécanique, 2006, 334, pp.408-413. ⟨10.1016/j.crme.2006.06.002⟩. ⟨hal-00091162⟩
Colette Nicoli, Pierre Haldenwang, S. Suard. Analysis of pulsating spray flames propagating in lean two-phase mixtures with unity Lewis number. Combustion and Flame, 2005, 143 (3), pp.299-312. ⟨10.1016/j.combustflame.2005.06.008⟩. ⟨hal-00907398⟩ Plus de détails...
Pulsating (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.
Colette Nicoli, Pierre Haldenwang, S. Suard. Analysis of pulsating spray flames propagating in lean two-phase mixtures with unity Lewis number. Combustion and Flame, 2005, 143 (3), pp.299-312. ⟨10.1016/j.combustflame.2005.06.008⟩. ⟨hal-00907398⟩
S. Suard, Pierre Haldenwang, Colette Nicoli. Different spreading regimes of spray-flames. Comptes Rendus Mécanique, 2004, 332 (5-6), pp.387-396. ⟨10.1016/j.crme.2004.02.004⟩. ⟨hal-00907403⟩ Plus de détails...
We present a minimal model of spray combustion to investigate a flame front propagating through a fuel-lean mixture of fuel vapor, droplets and air. The model relies on a main control parameter, Da, named the Damkoehler number, which allows us to take into account a large variety of fuel sprays. Numerical results reveal, as a function of Da, a wide range of spray-flame structures, including the classical gaseous premixed flame, a specific regime controlled by vaporisation, and a pulsating mode of propagation. The latter appears when the vaporisation is smaller than (or equal to) the reaction time, and it occurs even with a unit Lewis number.
S. Suard, Pierre Haldenwang, Colette Nicoli. Different spreading regimes of spray-flames. Comptes Rendus Mécanique, 2004, 332 (5-6), pp.387-396. ⟨10.1016/j.crme.2004.02.004⟩. ⟨hal-00907403⟩
Colette Nicoli, Pierre Haldenwang, Bruno Denet. Combustion of gaseous co-flow jets. Combustion Science and Technology, 2003, 175, pp.1143 - 1163. ⟨10.1080/00102200302346⟩. ⟨hal-00091178⟩ Plus de détails...
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).
Colette Nicoli, Pierre Haldenwang, Bruno Denet. Combustion of gaseous co-flow jets. Combustion Science and Technology, 2003, 175, pp.1143 - 1163. ⟨10.1080/00102200302346⟩. ⟨hal-00091178⟩
Eric Serre, Sandrine Hugues, Emilia Crespo del Arco, Anthony Randriamampianina, Patrick Bontoux. Axisymmetric and three-dimensional instabilities in an Ekman boundary layer flow. International Journal of Heat and Fluid Flow, 2001, 22 (1), pp.82-93. ⟨hal-01023080⟩ Plus de détails...
Eric Serre, Sandrine Hugues, Emilia Crespo del Arco, Anthony Randriamampianina, Patrick Bontoux. Axisymmetric and three-dimensional instabilities in an Ekman boundary layer flow. International Journal of Heat and Fluid Flow, 2001, 22 (1), pp.82-93. ⟨hal-01023080⟩
Journal: International Journal of Heat and Fluid Flow
