High-Spatial-Resolution Laser Diagnostics for a Dual-Mode Scramjet

Dual-mode scramjets present new possibilities for the aerospace industry: high-speed commercial flights, high-speed defense applications, and more cost-effective access to space. Cavity flameholders are typically employed in scramjet combustors to create a recirculation region that increases the local residence time and production of key radical species, such as hydroxyl (OH). These radicals mix with the main flow through a highly-dynamic shear layer separating the main flow and the cavity flow. Most current experimental and computational techniques for cavity-stabilized flames do not resolve all spatial scales. Direct Numerical Simulation (DNS) models provide full resolution from Kolmogorov scales to integral scales, but are extremely computationally expensive and require validation through experimental measurements. The University of Virginia Supersonic Combustion Facility (UVaSCF) is an optically-accessible ground test facility capable of simulating the conditions of a scramjet in Mach 5 flight and generating experimental data critical for establishing simulation boundary conditions. Previous studies have characterized a wall-mounted cavity flame holder of height 9 mm. A scaled-down strut-mounted cavity of height 3 mm was recently designed to provide a domain with a volume small enough to simulate completely using DNS.

This dissertation presents OH planar laser-induced fluorescence (PLIF) measurements obtained for both cavity configurations, with maximum resolution 40 × 40 × 25 µm, significantly exceeding the resolution of previous measurements of comparable flows. OH-PLIF images are processed to obtain the intermittency of the flame and the distribution of local values for flame surface density and flame front curvature. These metrics describe the location and shape of the flame brush and the distribution of flame front length scales. Simulated OH-PLIF images are produced from DNS results for direct comparison with experiments through flame structure metrics. Simulated OH-PLIF images show flame structure sizes comparable to those found experimental images and the flame is shown to spread into the main duct flow at approximately the same angle as in experiments. This agreement with experiments validates turbulence-chemistry interactions resolved by DNS.

In addition, this dissertation describes the construction of a new hybrid fs/ps rotational/vibrational coherent anti-Stokes Raman scattering (CARS) system. Broadband pulse generation will enable this system to simultaneously measure temperature and major combustion species, e.g. O2, N2, CO2, and C2H4. The spatial resolution of this system is quantified using a microscale jet of nonresonant gas. The effects of crossing angle and beam astigmatism on the spatial resolution are investigated in order to minimize the size of the CARS interrogation volume. This will minimize the effects of spatial averaging over the small length scales present in turbulent flames. A minimum interrogation volume length of 200 µm was achieved. This system will be used to gather temperature and species information in future scramjet flow configurations.