Supercritical Burning of Liquid Oxygen (LOX) Droplet with Detailed Chemistry

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,-rich hot side. Propagation takes place in a nonhomogeneous hot environment (say 1500 K) with a considerable velocity (typically 50 ms-’ ). 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 I pm vaporize before burning. A quasi-steady-like diffusion flame is then established. In this regime we observe that the D’ 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.