Vortex sheddings and Pressure Oscillations in Hybrid Rocket Combustion

Abstract

The similarity in internal flow of solid and hybrid rocket suggests that hybrid rocket combustion can be susceptible to instability due to vortex sheddings and their interaction. This study focuses on the evolution of interaction of vortex generated in pre-chamber with other types of vortex in the combustor and the change of combustion characteristics. Baseline and other results tested with disks show that there are five different frequency bands appeared in spectral domain. These include a frequency with thermal lag of solid fuel, vortex shedding due to obstacles such as forward, backward facing step and wall vortices near surface. The comparison of frequency behavior in the cases with disk 1 and 3 reveals that vortex shedding generated in pre-chamber can interact with other types of vortex shedding at a certain condition. The frequency of Helmholtz mode is one of candidates resulting to a resonance when it was excited by other types of oscillation even if this mode was not discernable in baseline test. This selective mechanism of resonance may explain the reason why non-linear combustion instability occurs in hybrid rocket combustion.

Low frequency instability in laboratory-scale hybrid rocket motors

“…Other frequency bands were also identified in the spectral domain with the frequency range of 450-550 Hz, 500-600 Hz and 600-650 Hz, respectively, though the amplitudes were negligibly small compared to those of low frequency oscillations. Details of each frequency range and the initiation mechanism are described in the literature [8].…”

Low Frequency Instability in Labscale Hybrid Rocket Motors

“…Other frequency bands were also observed in the spectral domain with a frequency range of 450~550Hz, 500~600Hz and 600~650Hz respectively, even the amplitudes are negligibly small compared to those of dominant low frequency oscillations. Details of each frequency range are described in the literature [8].…”