Passive Control of Low-Frequency Instability in Hybrid Rocket Combustion

Wonjeong Hyun,Changjin Lee,Jina Kim,Heesang Chae

doi:10.3390/aerospace8080204

Abstract

The occurrence of low-frequency instability (LFI) appears to be related to multiple interactions among many complex physical processes, such as vortex shedding, boundary-layer oscillation, and additional combustion in the post-combustion chamber. In this study, two combustion tests were conducted to suppress LFI and to examine which physical processes its occurrence was most sensitive. In the first test, two fuel inserts were used to modify the formation of a boundary layer, vortex shedding at the end of the fuel, and vortex impingement. In the second test, the fuel insert located at the front end was replaced with swirl injection. The first test was aimed at controlling and suppressing the initiation of LFI using fuel inserts, through which a small step appeared gradually due to differences in the regression rates of the two materials, i.e., polymethyl methacrylate and high-density polyethylene. The test results confirmed that (i) there are physical connections among several processes, such as the thermoacoustic coupling between p′and q′ and the oscillations of the upstream boundary flow, and (ii) LFI suppression is possible by disrupting or eliminating the connections among these physical processes. The second test was also aimed to control LFI while minimizing the deviation in combustion performance using proper swirl injection along with a fuel insert. Even when replaced by swirl injection, LFI suppression was still possible and showed reasonable combustion performance without causing too much deviation from the baseline in terms of the oxygen-to-fuel ratio and the fuel regression rate.

Highlights

In hybrid rockets, the combustion pressure usually oscillates in the low-frequency band, typically less than 100 Hz
The occurrence of low-frequency instability (LFI) seems to be related to the interaction of many complicated physical processes such as vortex shedding, boundary layer (BL) oscillation, and additional combustion in the post-combustion chamber
The thermoacoustic coupling of the low-frequency band due to the vortex shedding and impingement and the disturbed BL flow can be thought to lead to the occurrence of LFI

Summary

Introduction

The combustion pressure usually oscillates in the low-frequency band, typically less than 100 Hz. The occurrence of LFI seems to be related to the interaction of many complicated physical processes such as vortex shedding, boundary layer (BL) oscillation, and additional combustion in the post-combustion chamber (hereinafter referred to as the post chamber). Karabeyoglu et al [1] suggested a comprehensive dynamic model for hybrid rocket combustion to understand the transient behavior of LFI initiation, they concluded that resonance between the thermal lag of solid fuel and the heat-transfer oscillations is a critical process. Karthikeyan and Shimada [2] successfully simulated LFI initiation in their model, in which external perturbations are activated using flow disturbances. Previous studies have reported that vortex shedding and impingement, as well as the additional combustion of unburned fuel trapped in the vortices, are interesting physical processes that take place in the post chamber and are thought to be directly related to triggering LFI. The details of the physical processes leading to LFI are yet to be fully understood

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