Analysis and prediction of the influence of high frequency combustion instabilities on heat transfer in a sub-scale LOX/H2 rocket thrust chamber

Abstract

The German Aerospace Centre (DLR) has in the past years been performing test runs with their experimental sub-scale combustion chambers to measure and investigate the effects of unstable combustion. Some of these investigations are focused on the heat flux to the chamber walls, changes in which can cause a significant shift in chamber wall strength and even melting, and can thus lead to structural failures. As a continuation of research regarding this phenomenon, the DLR combustion chamber version D (BKD) is used to investigate changes in heat flux, and its test data have been made available for this research. Using the test data, finding a relation between measured steady state heat flux and controlled chamber conditions became feasible, and changes in heat flux related to combustion instabilities can be identified. However, although these changes in heat flux are often observed and are described in literature as a known effect of combustion instabilities, there are currently no validated explanations as to why these effects take place. In an effort to provide this explanation, the results of a previously completed investigation regarding the DLR combustion chamber version H (BKH) are evaluated, as they allowed determination of a relation between combustion length and magnitudes of acoustic pressure oscillations. In this research, the results obtained for combustion length are converted to a model regarding varying heat flux profiles and modified as to implement them for BKD. Finally, the model predicted increases in heat flux during time periods when increases are observed experimentally, showing agreement with the experimental data. Besides this, a conversion in the model allowed reconstruction of a localised heat flux profile, which showed an increase in local heat flux near the injector face plate. This matches effects described in literature and observed in the experimental set-up after a test run with combustion instabilities. With this validation of the model, the theory developed and tested in this thesis work could prove a significant advancement in the understanding of effects of combustion chamber instabilities and their impact on the engine integrity and performance.