Internal Flow Characteristics and Low-Frequency Instability in Hybrid Rocket Combustion

Abstract

The present study addressed the role of internal flow dynamics in determining regression rate and low-frequency instability in hybrid rocket. The first part was mainly devoted to examine the internal flow characteristics. Numerical simulations using large eddy simulation (LES) identified the formation of small-scale vortices by the interaction of fuel evaporation with axial flow near-surface region. Contours of instantaneous axial velocity show that the flow structure has been significantly altered and remained the isolated roughness patterns as found in several experiments. Also, the insertion of a diaphragm has been simulated with the dynamic change of turbulent coherent vortices and small vortices in the boundary layer. Results showed that small-scale vortices were not observed until a large-scale vortex shed in the downstream. Proper orthogonal decomposition analysis showed that inserting a diaphragm redistributed flow energy into lower modes 2–5, increasing the local regression rate after a diaphragm.

The second part focused on the initiation of low-frequency combustion instability (LFI) of pressure fluctuations with peak frequency of 10–20 Hz. A series of test was designed to examine the initiation of LFI. To this end, the sensitivity to instability was evaluated with parameters including volume ratio between main and post chamber, oxidizer flow rate, and solid fuel type. Results showed that the initiation of LFI was strongly related with flow dynamic change by the modification of chamber configuration. Additional studies revealed that pressure oscillations by vortex shedding in the post chamber could be closely related with the occurrence of LFI.