Transcritical oxygen/transcritical or supercritical methane combustion
Résumé
Injection of liquid fluid initially at subcritical temperature into an environment in which the temperature and pressure exceed the thermodynamic critical conditions is an important phenomenon in many high performance devices like liquid propellant rocket engines. This is found, for example, in the Space Shuttle main engines or in the Ariane 5 Vulcain engine both operating with liquid oxygen (LOx) and gaseous hydrogen (GH(2)). This article is concerned with the less standard situation where both reactants are in a transcritical state. One case of current interest in propulsion, that of combustion of cryogenic oxygen and methane injected at high pressure, is investigated experimentally. A coaxial injector delivers oxygen at a temperature of 85 K and methane at 120 or 288 K. The pressure in the chamber takes values between 4.5 and 6 MPa. Emission images from excited state OH (A(2)Sigma, denoted OH*) and CH (A(2)Delta, denoted CH*) are recorded and averaged. The Abel transform is used to determine the mean flame structure from these average images. Data indicate that the flame is stabilized in the vicinity of the injector. When both propellants are transcritical, the flame features two conical regions of light emission, one spreading close to the liquid oxygen boundary and the other located further away from the axis near the liquid methane boundary. The outer flame boundary is also conical with a relatively large expansion angle. This flame structure notably differs from that observed when one of the propellants is injected in a suberitical or transcritical state while the other is gaseous. An analysis of the relevant characteristic times suggests that under transcritical conditions the rate of combustion is mainly controlled by turbulent energy transfer to the propellants. This determines the mass fluxes from the dense regions to the lighter gaseous streams governing the rate of conversion into products.