Evaluation of combustion stability of single and multi-protrusion inserted hybrid rocket motor

Abstract

The present study explores the influence of protrusions on the combustion stability of a hybrid rocket motor. The primary aim is to analyze single, double, and triple-protrusion configurations taking into account variables such as their location, material, and spacing between them. Protrusions used were made of Nylon (regress during combustion) and graphite (non-regress during combustion). For the base-case (without protrusion), significant pressure oscillations were observed, arising from the interaction between vortex shedding and fundamental longitudinal acoustic modes of the chamber at the fuel grain exit. A protrusion at 0.8X/L (X-protrusion insertion point from head-end, L-fuel grain length) disrupted the interaction between acoustic modes and vortex shedding, substantially dampened its amplitude (around 96% reduction). In case of double (0.2–0.8X/L, 0.5–0.8X/L) and triple-protrusions (0.2–0.5-0.8X/L), the distance between protrusions is playing a key role in inducing the pressure oscillations. When protrusions are kept close (0.5–0.8X/L and 0.2–0.5-0.8X/L), the excitation of third mode acoustics were observed. This was attributed to the continuous increase in protrusion height. If protrusion height exceeds 6–7 mm, the flow over the front protrusion separated and impinges on the subsequent protrusion. This causes pressure oscillations in double and triple-protrusion cases. These pressure oscillations were mitigated using regressable protrusions made of nylon, as they keep the height within the range of 6–7 mm. The results of this research provide valuable perspectives on the significance of protrusions in controlling combustion instability in hybrid rockets.