Cryogenic Flows in Rocket Engines
By S. Zurbach and L. Ballester, Snecma, SAFRAN Group, Vernon, France
The development of the european launcher ariane was granted to CNES by delegation of the European Space Agency, and Snecma has been responsible for all of the Ariane cryogenic rocket engines. The Ariane 5 launcher and its powerful, expanded capacity version Ariane 5 ECA are thrusted by the cryogenic rocket engines Vulcain or Vulcain 2 and HM7.Vulcain 2 is an extension of the Vulcain cryogenic engine, operating in a gas generator cycle with two separate turbopumps. The combustion chamber is cooled by cold hydrogen flowing through regenerative circuits and the nozzle extension is cooled by cold hydrogen that flows through helicoidal tubes (a dump cooling system).
 Vulcain 2 at the test bench |
Rocket engines are exposed to severe mechanical and thermal loads, such as high vibration, a wide range of temperatures (from 20K to 3600K), and a wide range of pressures (from vacuum to 200 bar). Due to this extreme environment and the performance quality and reliability required, an understanding of the physical processes and complex technologies relevant to these operating regimes is needed. As the prime contractor for the Ariane cryogenic engines, Secom has been modeling cryogenic flows for the sub-systems of the rocket engines for several years. The specifics of cryogenic flows depend largely on the thermodynamic behavior of the propellants. For oxygen, hydrogen, and methane, a real gas approach is mandatory, since the ideal gas equation of state is no longer valid. For both injection and cooling,the propellants operate in subcritical and supercritical regimes.
 Oxygen density as a function of temperature and pressure |
 Thrust chamber with the helicoidal cooling system and a thermal map of the helicoidal tube (supercritical hydrogen + solid) |
Besides hot tests, validated simulation tools are being used more and more to ensure the reliability of space propulsion equipment. CFD allows new technology to be evaluated without performing high cost tests. It is also a tool for assessing development risks and production non-conformance. One important example of the potential of CFD simulation was illustrated during the Flight Recovery Program (FRP) [1]. The FRP was set up after the maiden flight of the Ariane 5 ECA in December 2002, during which the nozzle extension lost its mechanical integrity. During the FRP, a reinforced concept for the nozzle extension was defined, and the new design was produced and qualified within a very tight time schedule.
 Mean temperature field for a LOX/H2 rocket engine injector |
In addition to the nozzle cooling analysis, Snecma has developed specific combustion models to predict the combustion efficiency in a rocket chamber [2]. For cryogenic engines, the atomization of the reacting fluids is often performed by coaxial injectors. Liquid oxygen at 90K flows at low speed through a tube, which is surrounded by an annular high speed flow of gaseous or liquid hydrogen. In order to guarantee sufficient atomization of the liquid oxygen and efficient mixing of the combustion products, optimization of the injection plate and injectors is necessary. For this purpose, a balanced analysis of both experimental tests and FLUENT predictions has been performed. One challenging component of these calculations is the prediction of the turbulent combustion at very high pressures, in excess of 100 bar. Because the combustion chamber pressure is higher than the critical pressure of the injected propellants, the turbulent combustion regime is called transcritical. Despite these complexities, an accurate prediction of the reacting cryogenic flows has been obtained by the development and validation of specific models created within the framework of R&D programs at Snecma. These models, implemented in FLUENT through user-defined functions (UDFs), have been extensively compared to sub-scale measurements and applied to characterize full scale rocket gas generators and chambers.
 Characterization of a LOX / CH4 coaxial injector for two different operating points; the predicted (left) and visualized (right) mean flame length are compared |
For the forthcoming years, cryogenic flows will be computed for real gas mixtures. In the combustion chamber, oxygen and hydrogen are injected at very low temperatures so that the classical ideal gas equation of state is no longer valid to describe properly all of the thermodynamics, such as density and enthalpy. It is therefore mandatory to have a CFD solver with a real gas formulation for a single fluid (for the cooling tubes) but also for mixtures to predict the flow field in the main combustion chamber, where supercritical turbulent combustion of hydrogen and dense oxygen occurs. Important components for the success of this work include numerical solver robustness and stability, thermodynamic integration of the necessary models, and a time reduction for large eddy simulation (LES) calculations.
For the evolution of the Vulcain Engine, a new nozzle extension was developed by Volvo Aero in Sweden, under a contract from EADS-ST GmbH in Germany.EADS-ST is responsible for the Vulcain 2 Thrust Chamber, under a contract with Snecma in France, who is responsible for the Vulcain 2 Engine.
References:
- Ferrandon, O.; James, P.; Girard, P.; Terhardt, M.; Blasi, R.; Johnsson, R.; Damgaard,T.: Vulcain 2 Nozzle Extension: Integrated European Team and Advanced Computational Model to the Service of Nozzle Design; AIAA-2005-4535, July 10-13,2005.
- Vingert, L.; Zurbach, S.: LOX / Methane Studies for Fuel Rich Preburner; AIAA 2003-5063, July 20-23, 2003.
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