Explosion risk study on liquefied propellants ejected from a space vehicle
Résumé
ArianeGroup is developing space vehicles using new Lox/LCH4 propulsion technology. Venting or draining out methane and oxygen from the tanks in flight at various altitudes concerns the safety management. This study aims at obtaining the interaction between the drained out propellants and the wake of the plane as well as the risk of explosion near the vehicle by numerical modeling completed with experimental approach. This paper will focus on methane venting and draining and the subsequent concentration zones. Firstly, a RANS (Reynolds Averaged Navier Stokes) turbulence model is chosen. The SST (Shear Stress Transport) k-ω model is well suited for aerodynamical calculations and is available in the CFD-ACE software. For the liquid draining, a cryogenic round jet in a gaseous crossflow has to be accounted for. However, the computational cost for a vaporizing liquid jet model is too high with respect to the goals of the study. A more time-friendly densified gas model has thus been developed. The thermal conductivity and the isochoric heat capacity have been modeled with a five-order polynomial in temperature. Both defuelling occur when the plane is propelled by two turbojets. The interaction between the exhaust of the turbojet and the methane cloud formed by the defueling is a first order risk. In order to be conservative with several types of engines, higher values of speed and exhaust temperature have been used and implemented as temperature and momentum sources at the rear of the engine. The methane flammable limits are kept between 5 and 15% of molar fraction in air. The gaseous methane in dangerous concentrations is located along the fuselage and is even expanding in the plane’s wake for the liquid draining. However, no significant mixing is noticed between the turbojet exhaust and the methane cloud within the flammable limits in the first calculations. Moreover, exhaust temperature is under the methane auto-ignition temperature of 873 K.