Laser Diagnostics of Compressible and High-intensity Premixed Methane-air Combustion Inside a Backward Facing Step Combustor Using High-repetition Rate CH-PLIF and PIV
A novel high repetition rate (10 kHz) planar laser-induced fluorescence (PLIF) diagnostic technique was used for the flame front study. Premixed methane-air combustion was studied at different flow turbulence conditions (Re = 15000, 30000 and 64000; 12CFM, 25CFM and 50 CFM; 10m/s, 20 m/s and 40 m/s) and fuel loading (Փ =0.6 to 1.2). The study used an optically accessible backward-facing step high-speed combustor. Methane-air mixture was supplied to the combustor at atmospheric pressure and standard room temperature (T =25°C). CX-CH PLIF was used to detect the actual flame front structures. The flame dynamics were then correlated to the flow dynamics at various turbulent length scales (LT) and velocity fluctuations (u'). The turbulent length scales were varied using turbulence-generating grids with blockage ratios (BR) from 46% to 63% and two-hole diameters (HD) of 1.5 mm and 3 mm. The velocity fluctuations (u') were altered using different flow momentum at the combustor inlet. The different flow momentum yields a different Reynolds number at the edge of the backward-facing step. The flow properties such as velocity vector fields, velocity fluctuations (u'), turbulent intensity (I), and vorticity (ω) were measured at the edge of the backward-facing step using a 10 kHz time-resolved particle image velocimetry (PIV). The flame and flow characteristics were used to locate the flame regime on Borghi-Peter’s diagram. The study of flame-flow correlation was done at different Damkohler number (Da), Karlovitz number (Ka), and mixture flow rates. The effects of Reynold’s Number (Re) and grids on the flame and flow dynamics were studied. CH-PLIF was tuned by changing the dye of the existing laser system. A wavelength of 314.415 nm was found to be the most effective wavelength for the detection of the CX-CH band. This wavelength results in better visualizing the flame front. The detection of the CH signal and extraction of flame fronts was very challenging. This is due to the experimental complexity with overlapping OH bands, laser wall scattering, and in-flame soot formation. Two different MATLAB based image processing codes were developed to detect the flame profiles and flame edges. It was observed that the flame turbulence and flame width (lateral expansion) increased as the flame moves downstream. It was also observed that there was a presence of unburned gases inside the burned gases and vice-versa. Additionally, an irregularly shaped flame wrinkling was noticed. However, wrinkle size decreased throughout the flame evolution. The flame wrinkling was higher with small-hole diameters grids (grid 1 and grid 3), whereas the flamelet breakdown, burnout, flame expansion, flame stretching, and turbulent flame speed were higher under larger diameter grids (grid 2 and grid 4). The combustor system is being upgraded for supersonic combustion conditions (Ma =2) using customed air heaters and De Laval Nozzle. The analytical calculations, design optimization, experimental methodology, and post data analysis schemes for supersonic combustion tests are also reported in this dissertation.
Hossain, Md Amzad, "Laser Diagnostics of Compressible and High-intensity Premixed Methane-air Combustion Inside a Backward Facing Step Combustor Using High-repetition Rate CH-PLIF and PIV" (2020). ETD Collection for University of Texas, El Paso. AAI27999951.