Laser Diagnostics of Turbulent Flames in High Speed Flows
Grady, Nathan Ryan
High speed turbulent flames can be found in most aerospace propulsion devices. In order to understand the behavior and phenomenological nature of combustion in these devices time resolved, non-invasive, in situ laser diagnostic measurements are needed. In this dissertation, laser diagnostics are used to study freely propagating turbulent premixed flames, and non premixed flames in a model scramjet combustor. In addition, a new laser method is developed to measure velocity profiles in reacting regions without seed particles. Premixed turbulent flames have been studied extensively over burner stabilized flames. However, the flame propagation in these burners is usually affected by the burner itself resulting in a geometric dependent propagation. Therefore, a fundamental understanding of the interaction between turbulence and premixed flames is obscured. Alternatively, spherically propagating flames (also known as flame kernels) that do not have these geometric dependencies can be studied using flame bombs. While flame bomb studies have been very successful in elucidating the interaction between turbulence and premixed flames, they typically have limited optical access and a mean radial inflow which can inhibit flame propagation. Therefore, a new means of studying flame kernels is described using a turbulent wind tunnel where flame kernels are allowed to freely propagate downstream. The results obtained in this new device are compared with traditional flame bomb measurements. The reaction progress of a non-premixed combustion inside a cavity-piloted scramjet combustor was determined by measuring all major species and temperature using spontaneous UV Raman scattering. A 70% CH4/30% H2 fuel blend was used to approximate the reactivity of liquid jet fuels, and minimize the number of Raman spectral lines to ensure tractable data sets. Inside the cavity, H2 fuel quickly burnt off while the CH4 and CO persisted until the fuel path reached the cavity shear layer. Finally, a new molecular tagging velocimetry method of obtaining non-seeded velocity measurements inside a reaction zone is described. This method uses a two-photon process to dissociate H2O and create vibrationally excited OH photofragments which can then be differentiated from flame generated OH radicals that are predominately in the vibrational ground state to make single-shot velocity measurements.
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