Numerical and experimental investigation of combustion instability in a dual inlet dump type ramjet combustor

One of the performance parameters of a ramjet combustion chamber is the pressure fluctuations within the chamber. In this study, a dual-inlet dump ramjet combustion chamber was numerically and experimentally examined for combustion instabilities. In numerical investigations, a square cross-section generic ramjet combustion chamber model available in the literature with corresponding test data was used. In experimental investigations, a similar model test rig was designed, manufactured, and tested under similar operating conditions. To conduct accurate numerical simulations, flamelet models based on detailed chemistry mechanisms were considered. It was observed that the Steady Laminar Flamelet (SLF) model, due to conditions close to stoichiometry, produced similar results as Flamelet Generated Manifold (FGM) and both models predict results very close to experimental data. Simulations were conducted using two turbulence modeling approaches. Richardson extrapolation was utilized to ensure the mesh convergence. M index, defined whether the computational mesh is fine enough for LES computations was used for that purpose. Large Eddy Simulation (LES) and Delayed Detached Eddy Simulation (DDES) analyses yield very similar results that reproduce the measured data to a very good degree of accuracy. Pressure fluctuations extracted from simulations exhibited similar oscillations at comparable frequencies to the sub-1 kHz region where test data existed in the literature. The authors noted that these oscillations originated from longitudinal modes. At higher frequencies, LES computed strong modes which are consistent with the theoretical transverse frequencies. DDES, on the other hand, damped the high frequency modes since it uses RANS, Reynolds-averaged Navier-Stokes, modelling at walls where transverse modes are travelling in between. The ANSYS Fluent solver was used to perform all CFD computations carried out within the study. Dynamic Mode Decomposition (DMD) analysis reveals weak effects over reaction field due to the transverse mode oscillations of the combustion chamber. Negligible fluctuations over reacting field are observed at transverse frequency of the air inlet ducts. Tests were conducted with a similar test setup, collecting data from high-frequency response pressure sensors and high-speed OH* chemiluminescence images from the combustion chamber. Data acquisition was successfully made at 25 kHz for pressure sensors and the camera. Results analyzed in the frequency domain are matched those from tests and simulations for the sub-1 kHz range found in the literature. In the region above 1 kHz where there was no test data in the literature, distinct peaks were observed consistent with the theoretically transverse mode of the combustion chamber and its harmonics. Unlike simulations, oscillations induced by transverse modes of air inlet ducts were weakly felt in pressure sensor measurements. Analysis based on OH* indicator images revealed dominant longitudinal modes in the front of the combustion chamber, with the most directional modes emerging as the flow progressed in the direction of flow. The study demonstrated that in dual-inlet ramjet combustion chambers, the transverse acoustic oscillations prevail. While the transverse modes originating from the combustion chamber affect reactions, those from the air inlet ducts do not influence combustion at all.