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Multi-phase Combustion and Transport Processes Under the Influence of Acoustic Excitation

  • Author(s): Wegener, Jeffrey Lewis
  • Advisor(s): Karagozian, Ann R
  • et al.
Abstract

This experimental study examined the coupling of acoustics with reactive multiphase transport processes and shear flows. The first portion of this dissertation deals with combustion of various liquid fuels when under the influence of externally applied acoustic excitation. For this study, an apparatus at the Energy and Propulsion Research

Laboratory, UCLA, used a horizontal waveguide to create a standing acoustic wave, wherein burning fuel droplets were positioned near pressure nodes within the waveguide. Alcohol fuels (ethanol and methonal) as well as aviation fuel replacements (Fischer-Tropsch (FT) synfuel and an FT blend with JP-8) were studied here. During acoustic excitation, the flame surrounding the droplet was observed to be deflected in a manner consistent with the direction of a theoretical acoustic radiation force, analogous to a buoyancy force, acting on the burning system. Based on this degree of deflection, a method was developed for experimentally quantifying the acoustic acceleration and relating it to the theoretical acoustic acceleration. This technique employed phase-locked optical imaging of the flame in the ultraviolet band in order to capture hydroxyl radical (OH*) chemiluminescence as an indication of the flame structure and shape. The flame was observed to be deflected in a bulk manner, but also with micro-scale oscillations in time. The bulk or mean flame alteration was used to determine an experimental value of the acoustic acceleration for a range of different fuels and excitation conditions. This investigation showed experimentally determined acoustic accelerations which were quite consistent qualitatively with theory, but which were quantitatively inconsistent with theoretical predictions. Observed flame deformations were greatest for a droplet situated immediately next to a pressure node, in contrast to the theory, while milder flame deflections were observed for droplets positioned closer to a pressure antinode. These

observations were consistent among all fuels studied, qualitatively and with the same mean qualitative trends. Phase-locked OH* chemiluminescence imaging revealed significant differences in the amplitude of flame oscillation based on the applied frequency and droplet location. Low frequency acoustic excitation and proximity to the pressure node produced higher amplitude flame oscillations, suggesting an enhanced degree of acoustically-coupled combustion that could be responsible for qualitative differences between theory and experimental measurements of acoustic acceleration. The second portion of this dissertation utilized a similar, but more advanced facility

which was recently constructed at the Air Force Research Laboratory, Propulsion Directorate (RQR). These experiments explored the interaction between acoustics and nonreactive shear-coaxial jets under high chamber pressure, acoustically resonant conditions, using liquid nitrogen as the inner jet and gaseous helium as the outer jet. The

shear-coaxial jet was placed within the chamber, for which piezoelectric sirens could create a standing wave. The coaxial jet could thus be situated at either a pressure node or a pressure antinode location, and backlit high-speed imaging was used to resolve the naturally unstable mixing layer between the inner and outer jets. For jets with and without exposure to acoustic forcing, two different reduced basis methods were applied to the gray-scale pixel intensity data in order to extract instability frequencies and mode shapes from image sets; these included proper orthogonal decomposition (POD) and dynamic mode decomposition (DMD). A new POD-based method was used to quantify the susceptibility of coaxial jets to external acoustic forcing by comparing the pixel intensity variance induced by the acoustic mode to the total variance of pixel intensity caused by fluctuations in jet mixing. A novel forcing susceptibility diagram was then created for coaxial jet momentum flux ratios of 2 and 6 for both pressure node and pressure antinode locations. Measurements of the critical forcing amplitude were made to quantify the acoustic perturbation amplitudes required in order for the forced mode to overtake the natural mode as the most dominant instability in the jet, which is generally classified as "lock-in" to the applied mode. It was found that, for forcing frequencies greater than

the natural frequency of the jet, an increase in the forcing frequency caused the jet to be less susceptible to applied acoustic disturbances, thus requiring higher acoustic forcing to achieve "lock-in". This relationship held true for both pressure node and pressure

antinode conditions. The shear layer instability characteristics of unforced jets were also investigated, and a theoretical convection velocity which depends on inner and outer jet velocities and densities was validated for the range of experimental flow conditions used

in this study. An extensive description of the design of the experimental reactive facility is also offered, including preliminary results for oxygen-hydrogen coaxial jet flames acquired using high-speed OH* chemiluminescence imaging.

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