REST HF-7 and HF-9 Test Case: Lessons learned for comparing simulation results with test data of a representative LOX/H2 thrust chamber with high-frequency combustion instabilities
Creators: name:  W. ARMBRUSTER 
name:   J. HARDI 
name:   M. OSCHWALD 
Publisher: name: Proceedings of  the 9th European Conference for Aerospace Sciences. Lille, France, 27 June - 1 July, 2022
Year: 2022
Subjects: PROPULSION PHYSICS
LIQUID ROCKET ENGINE
COMBUSTION INSTABILITIES

description: Even though the phenomenon has been known since the Apollo program and have since then been under extensive research, high-frequency combustion instabilities still represent a major risk for the development of new liquid propellant rocket engines. Even though it is often thought that the consideration of combustion stability aspects is of minor importance for the propellant combination LOX/H2, high-frequency combustion instabilities have been reported for several unstable LOX/H2 rocket combustion chambers [1] [2] [3]. The DLR research combustion chamber BKD shows self-excited high-frequency combustion instabilities for highly representative conditions [3]. In the past decade the BKD chamber acoustics and stability characteristics have been investigated extensively [3] [4]. Therefore, the chamber acoustics are well understood and the instability driving mechanism has also been identified in the past. Based on the growing computational power, it is hoped that in the future numerical tools will be able to predict the stability behavior of new rocket engines during the development process. Several numerical stability prediction tools have been developed in the recent years. However, before these stability prediction methods can be applied by industry, these numerical tools need validation data of representative and well-instrumented rocket propulsion experiments. For that reason, several BKD operating conditions with varying stability characteristics have been formulated as a blind numerical test case within the French-German Rocket Engine Stability niTiative (REST) and since then has been known as the REST-HF7 test case. Several international research groups have modelled BKD and its stability behavior [5] [6] [7] [8]. Some significant progress of combustion stability modelling could be gained with the BKD test case. However, even though still being a sub-scale research combustion chamber, the simulations of BKD also revealed current limitations in both the experimental data as well as the numerical capabilities. Based on the experience gained, the HF-7 test case has therefore been updated and transformed into the REST test case HF-9, now including information of the flame dynamics in terms of line-of-sight intergrated OH* dynamics and a larger variety of operating conditions. Experimental studies on the combustion stability of BKD are also still ongoing. Within the REST community, it became obvious, that for both an improved experimental interpretation of the instability driving and the validation of the CFD-simulations, a 2D flame visualization of the flame dynamics is required. In the past years, new test runs with a modified BKD have therefore been conducted. From the new test campaigns, it was possible to derive mean flame images and the flame response for representative operating conditions. This paper will summarize the most important aspects of past experimental and numerical studies of the BKD experiment. The lessons learned will highlight what has been achieved by the combined experimental and numerical research activities, but will also describe limitations of both the modelling as well as the experimental data for the validation of the simulations. Finally, new additional experimental data will be presented, which could help to further validate the combustion modelling of BKD.