Colette Nicoli

Soret and Dilution Effects on Premixed Flames

Abstract The structure of a wrinkled premixed flame is analysed theoretically. By assuming the reactive mixture diluted in an inert gas and a weak cross-diffusion coupling between the heat and mass fluxes, the effect of the change by the reaction of the physical gas properties (thermal conductivity, specific heat, number of molecules) and Soret and Dufour diffusions have been investigated in the limit of high activation energies. It turns out that Soret mass diffusion cannot be neglected in the study of the flame front dynamics. In contrast, the change of physical properties as well as Dufour heat diffusion generally appears to play a minor role.

Supercritical burning of liquid oxygen (LOX) droplet with detailed chemistry

Abstract A numerical study of the supercritical combustion of a liquid oxygen (LOX) droplet in a stagnant environment of hot hydrogen is carried out with a detailed chemistry model. Special attention is devoted to ignition process and diffusion flame structure. Ignition consists typically of the propagation of a premixed flame which is initiated in the H 2 -rich hot side. Propagation takes place in a nonhomogeneous hot environment (say 1500 K) with a considerable velocity (typically 50 ms −1 ). Despite the high temperature of the ambiance, the medium ahead of the flame can be considered as frozen during the transit time. In addition, it is found that droplets with diameter less than 1 μm vaporize before burning. A quasi-steady-like diffusion flame is then established. In this regime we observe that the D 2 law is approximately valid. In contrast to the case of a single irreversible reaction, a full chemistry model leads to a very thick flame where chemical consumption and production cover a surrounding zone about four times the instantaneous droplet radius. Reversibility of the reactions plays a determinant role in the flame structure by inducing a large near-equilibrium zone which is separated from a frozen region by a thin nonequilibrium zone. The length scale of the latter region is found to behave as the square root of the instantaneous droplet radius and a detailed analysis shows that just two elementary reactions are involved in this zone. Furthermore, the influence of several parameters is considered; temperature and pressure in the combustion chamber have a weak influence on the burning time. Influence of initial droplet radius confirms that droplet combustion is a diffusion controlled process. Chamber composition is also considered. Finally, it is shown that a precise description of the transport properties in the dense phase is not required.

High pressure vaporization of LOX droplet crossing the critical conditions

Abstract Vaporization of liquid O2 droplet in quiescent high-temperature and high-pressure H2 gas is numerically investigated. Classical thermodynamic modeling of high pressure mixtures allows us to study the transition from subcritical to supercritical vaporization regime. It is observed that subcritical vaporization can be obtained up to pressures several times the oxygen critical pressure. Respective domain of both regimes is determined vs temperature and pressure. Border region corresponds to minimum value of droplet lifetime. This results from two cooperative phenomena: transient effect and thermodynamic property of mixtures. Sensitivity analysis additionally shows that state of art in dense fluid transport modeling yields results that should be considered accurate only as far as orders of magnitude are concerned.

Effect of heat losses on the limits of stability of premixed flames propagating downwards

Abstract A previously developed theory of linear stability analysis of nonadiabatic premixed flames that was based on a diffusive thermal model is extended by eliminating the hypothesis of negligible expansion of the gas. The effects of heat losses on the limit of stability of slow burning flames propagating downward are investigated in detail. The influence of the proximity of the flammability limits upon the stability limits is studied with a view to describing the cellular threshold for flames propagating in tubes.

Effect of variable heat loss intensities on the dynamics of a premixed flame front

The theoretical study concerning premixed flame fronts in nonuniform and unsteady flows was developed for an arbitrary gas expansion coefficient for small amplitudes of front corrugation and then extended to the finite amplitude case in the frame of an asymptotic analysis for large values of the Zeldovich number ..beta.. (..beta.. ..-->.. infinity) and in peturbation of the relative stretch flame intensity epsilon. As was anticipated by early semiphenomenological analyses, these theoretical studies point out that two effects, front curvature and flow inhomogeneities, influence the normal burning velocity U/sub n/. But to leading order in the power expansion in epsilon of the relative modification in normal burning velocity, these two effects appear through only one geometric scalar expressing the nondimensional flame stretch tau (d Insigma/dt) experienced by the curved fronts in a nonuniform flow.