Influence of heat transfer on high pressure flame structure and stabilization in liquid rocket engines

This research work deals with the problem of the flame stabilization in the context of high pressure liquid rocket engines. Flame stabilization in a rocket engine is a critical feature. An instability can lead to important damages of the engine or the destruction of the launcher and the satellite. The engines (Vulcain 2 and Vinci) of the Ariane 5, and the future Ariane 6, use the hydrogen/oxygen propellants. One characteristic of this couple is its high specific impulse. The launcher performance is linked to the ratio of the payload to the total mass of propellants. For volume reasons the propellants are stored at low temperature of the order of a few tens of Kelvin. When injected in the combustion chamber, their combustion releases a huge amount of heat leading to temperature of 3500K. In order to predict the heat transfer between the flame, the solid injector and the cold propellants the Large Eddy Simulation, which allows to capture the unsteady features of the flow, is used in association with a thermal solver for the injector. This approach is validated with a low pressure experiment conducted at Centrale Paris, then a basic 1D configuration allows to understand the phenomena of high pressure flame-wall interaction. Finally a configuration representative of a coaxial rocket engine injector allows to evaluate the structure and the stabilization mechanisms of a cryogenic flame, the heat flux and the temperature of the injector

Effect of pressure on Hydrogen/Oxygen coupled flame-wall interaction

The design and optimization of liquid-fuel rocket engines is a major scientific and technological challenge. One particularly critical issue is the heating of solid parts that are subjected to extremely high heat fluxes when exposed to the flame. This in turn changes the injector lip temperature, leading to possibly different flame behaviors and a fully coupled system. As the cham- ber pressure is usually much larger than the critical pressure of the mixture, supercritical flow behaviors add even more complexity to the thermal prob- lem. When simulating such phenomena, these thermodynamic conditions raise both modeling and numerical specific issues. In this paper, both sub- critical and supercritical Hydrogen/Oxygen one-dimensional, laminar flames interacting with solid walls are studied by use of conjugate heat transfer simulations, allowing to evaluate the wall heat flux and temperature, their impact on the flame as well as their sensitivity to high pressure and real gas thermodynamics up to 100 bar where real gas effects are important. At low pressure, results are found in good agreement with previous studies in terms of wall heat flux and quenching distance, and the wall stays close to isothermal. On the contrary, due to important changes of the fluid trans- port properties and the flame characteristics, the wall experiences significant heating at high pressure condition and the flame behavior is modified

Numerical Benchmark for High-Reynolds-Number Supercritical Flows with Large Density Gradients

Because of the extreme complexity of physical phenomena at high pressure, only limited data are available for solver validation at device-relevant conditions such as liquid rocket engines, gas turbines, or diesel engines. In the present study, a two-dimensional direct numerical simulation is used to establish a benchmark for supercritical flow at a high Reynolds number and high-density ratio at conditions typically encountered in liquid rocket engines. Emphasis has been placed on maintaining the flow characteristics of actual systems with simple boundary conditions, grid spacing, and geometry. Results from two different state-of-the-art codes, with markedly different numerical formalisms, are compared using this benchmark. The strong similarity between the two numerical predictions lends confidence to the physical accuracy of the results. The established database can be used for solver benchmarking and model development at conditions relevant to many propulsion and power systems

Influence des transferts thermiques sur la structure et la stabilisation de flamme à haute pression dans les moteurs fusées cryotechniques

Ce travail de recherche s’intéresse au problème de la stabilisation de flammes, et du transfert de chaleur résultant, dans les moteurs fusées cryogéniques. La stabilisation de flamme dans un moteur fusée est un phénomène critique, et toute instabilité peut conduire à des dégâts importants, voire à la destruction du lanceur et des satellites embarqués. Les moteurs (Vulcain 2 et Vinci) qui équipent Ariane 5, et la future Ariane 6, utilisent le couple hydrogène / oxygène, dont la grande impulsion spécifique permet, en minimisant la masse des ergols par rapport à la masse de la charge, une meilleure performance du lanceur. Pour réduire le volume de stockage, les ergols sont refroidis à des températures très basses de l’ordre de quelques dizaines de Kelvin. Ils alimentent une flamme dont la température maximale peut atteindre 3500K, générant de très forts gradients de température dans le fluide, et des flux de chaleur extrêmes dans les parties solides de l’injecteur. Pour prédire les flux de chaleur entre la flamme, l’injecteur et les ergols froids, l’approche de Simulation aux Grandes Echelles (SGE), pour reproduire l’écoulement réactif turbulent instationnaire, est couplée au calcul de thermique du solide dans l’injecteur. Cette approche est d’abord validée par comparaison à une expérience en conditions ambiantes, menée au Laboratoire EM2C (Paris). L’interaction flamme-paroi en présence de transfert de chaleur, qui est un mécanisme de base de la stabilisation de flamme, est ensuite étudiée pour différents niveaux de pression. Finalement une configuration représentative d’un injecteur coaxial de moteur fusée est simulée pour étudier la structure et les mécanismes de stabilisation de la flamme, ainsi que les flux de chaleur reçus par l’injecteur, en vue d’évaluer la fatigue thermique du système.This research work deals with the problem of the flame stabilization in the context of high pressure liquid rocket engines. Flame stabilization in a rocket engine is a critical feature. An instability can lead to important damages of the engine or the destruction of the launcher and the satellite. The engines (Vulcain 2 and Vinci) of the Ariane 5, and the future Ariane 6, use the hydrogen/oxygen propellants. One characteristic of this couple is its high specific impulse. The launcher performance is linked to the ratio of the payload to the total mass of propellants. For volume reasons the propellants are stored at low temperature of the order of a few tens of Kelvin. When injected in the combustion chamber, their combustion releases a huge amount of heat leading to temperature of 3500K. In order to predict the heat transfer between the flame, the solid injector and the cold propellants the Large Eddy Simulation, which allows to capture the unsteady features of the flow, is used in association with a thermal solver for the injector. This approach is validated with a low pressure experiment conducted at Centrale Paris, then a basic 1D configuration allows to understand the phenomena of high pressure flame-wall interaction. Finally a configuration representative of a coaxial rocket engine injector allows to evaluate the structure and the stabilization mechanisms of a cryogenic flame, the heat flux and the temperature of the injector