Study of supercritical cryogenic spray

Propellant injection is a key issue for optimal rocket combustor performance due to its dominating effect on atomization, propellant mixing and combustion, as well as on thermal and chemical loads to the combustor walls. The injectors control by a major part efficiency and stability of combustion. Future launchers will use rocket propulsion systems burning CH4/LOx at supercritical conditions. Also the FLPP Programme, launched by ESA in February 2004, has emphasized that the trend in spacecraft propulsion systems are liquid rocket engines using methane and liquid oxygen. This kind of rocket has greater performances compared to solid rocket engines, so it can carry a bigger payload that are satellites in the case of FLLP. Despite this new trend there are no many experimental data or numerical studies available in literature using methane-oxygen combustion at these extreme conditions. Until now, only H2/LOx injection and combustion has been investigated deeply. For these reasons a current problem is to understand the injection, mixing and combustion in typical liquid rocket engines and combustion chambers conditions. An important help can be provided by numerical modeling, therefore a considerable research effort is developing in this direction. The aim of this investigation is the numerical study of the CH4/LOx injection, mixing and combustion in liquid rocket engines with shear coaxial injectors, at supercritical conditions. Above the critical pressure liquid and gaseous phases are no longer separated, some fluid properties are gas-like while others fluid properties are liquid-like. These extreme conditions cannot be predicted accurately by commercial CFD codes yet, because at such conditions the material properties can no longer be described as an ideal gas and real gas effects appearing in the mixing process are thermodynamically not treated correctly by the codes available today. In this paper a theoretical study has been done to clarify the great importance of real gas effects in the calculation of species properties at supercritical pressure, typical of liquid rocket engine. The comparison between properties calculated with ideal gas law and experimental data (from NIST) at 15 MPa, shows that the percentage differences near by the critical point is very high. This means that using ideal-gas properties in CFD calculations will give incorrect gas-dynamic fields, and this is true at both low and high temperatures. There are a lot of equation of state for real gas available in literature. In the present work numerical simulations have been done by using CFD code Fluent, implementing the Soave-Redlick-Kwong by external user routines . The test case is a methane-oxygen coaxial liquid rocket injector at supercritical conditions. The simulations have been done following a step-by-step procedure: at first cold flow simulations, without reactions and combustion, are performed. Then, starting from cold flow convergent solutions, reactions and combustion are activated, increasing problem complexity. EDM model has been used in combustion simulations.