Identifying feedback directions in the mechanisms driving self-sustained thermoacoustic instability in a single-element rocket combustor

TL;DR

This study uses symbolic transfer entropy to analyze feedback interactions among acoustic, heat release, and hydrodynamic variables in a rocket combustor, revealing mechanisms behind high-frequency thermoacoustic instability and potential mitigation strategies.

Contribution

It introduces a novel application of symbolic transfer entropy to quantify and rank feedback interactions during thermoacoustic instability in rocket engines.

Findings

Vorticity dynamics at the fuel collar influence heat release fluctuations.

Switches in dominant feedback mechanisms occur during TAI.

The method identifies variable influence and feedback directionality.

Abstract

The occurrence of high frequency (>1000 Hz) thermoacoustic instability (TAI) sustained by mutual feedback among the acoustic field, heat release rate oscillations, and hydrodynamic oscillations poses severe challenges to the operation and structural integrity of rocket engines. Hence, quantifying the differing levels of feedback between these variables can help uncover the underlying mechanisms behind such high frequency TAI, enabling redesign of combustors to mitigate TAI. However, so far, no concrete method exists to decipher the varying levels of mutual feedback during high-frequency TAI. In the present study, we holistically investigate the mutual influence based on the spatiotemporal directionality among acoustic pressure, heat release rate, hydrodynamic and thermal oscillations during TAI of a single-element rocket engine combustor. Using symbolic transfer entropy (STE), we identify the spatiotemporal direction of feedback interactions between those primary variables when acoustic waves significantly emerge during TAI. We unveil the influence of vorticity dynamics at the fuel collar (or the propellant splitter plate) as the primary stimulant over the heat release rate fluctuations to rapidly amplify the amplitude of the acoustic field. Further, depending on the quantification of the degree of the mutual information (i.e., the net direction of information), we identify the switches in dominating the thermoacoustic driving between the variables during TAI, each representing a distinct mechanism of a thermoacoustic state. Additionally, from this quantification, we analyze the relative dominance of the variables and rank-order the mutual feedback according to their impact on driving TAI.