High-Resolution Velocimetry of a Dual-Mode Scramjet Flameholder

Cavity flameholders are a critical technology for hydrocarbon-fueled scramjets, lean premixed combustion in jet engines, and fundamental high-speed combustion studies. This dissertation aims to improve understanding of cavity flameholders by designing a small, immersed cavity flameholder for Direct Numerical Simulations (DNS), characterizing its inflow, and characterizing the turbulence-chemistry interactions at the cavity. This undertaking is part of a multi-disciplinary study on the effects of flow compressibility and heat release on turbulent flame structures and stabilization mechanisms.
First, an immersed, small-scale cavity flameholder for high-speed, premixed ethylene combustion is designed, additively manufactured, and tested at a Mach 5 enthalpy. Challenges related to transforming a direct-connect combustor flow path into a semi-free jet flow path for immersed models were encountered: primarily flow blockage and heat removal. The final design can sustain the ethylene flame for repeated cycles lasting up to an hour.

A critical element necessary for understanding the combustion physics of high-speed compressible flames is accurate and precise velocimetry. A methodology to successfully implement high-resolution two-components two-dimensional Particle Image Velocimetry (PIV) in the particularly challenging environment of a high-speed, high-enthalpy flow with free stream seeding is presented and demonstrated. A three-pronged approach consists of predicting the particle image size for a range of setups, enhancing the noisy raw signal with the innovative use of a logarithmic transform, and quantifying uncertainties. The methodology effectiveness is demonstrated with synthetic PIV and experiments conducted in a dual-mode scramjet combustor for which a velocimetry resolution of 355μm is achieved.

The high-speed inflow to the immersed model and a scaled-down cavity flameholder is subsequently characterized with high-resolution PIV. The effects of varying flow compressibility and heat release are analyzed. Key flow structures are identified, including stagnation points, boundary-layers, and pre-heat zones to inform numerical simulations. The new flowpath design is validated by the absence of flow spillage or separation at the cavity inflow plane.

Finally, combined PIV-PLIF measurements in a dual-mode scramjet combustor are conducted and constitute the first experimental database of its kind on a DNS-friendly dual-mode cavity flameholder. The knowledge of both the instantaneous flow velocities and the flame products locations enables deeper insights into the turbulence-chemistry interactions. Combustion in the cavity is unsteady and a hypothesis on a pulsed combustion cycle is suggested based on initial evidence of an oscillatory combustion process from 41 kHz chemiluminescence acquisitions. Coupling of this pulsed combustion regime with the isolator shock train through thermal choking is suggested. The described oscillations are of critical importance to accurately prescribe DNS boundary conditions and reliably operate scramjet engines.