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Droplet Combustion and Non-Reactive Shear-Coaxial Jets with Transverse Acoustic Excitation

  • Author(s): Teshome, Sophonias
  • Advisor(s): Karagozian, Ann R
  • Smith, Owen I
  • et al.
Abstract

This experimental study focused on the coupling of transverse acoustic flow perturbations with two different fundamental phenomena that take place in combustion chambers: droplet combustion, and injection of non-reactive shear-coaxial jets.

The study on fuel droplet combustion characteristics examined the response and behavior of various burning fuel droplets during exposure to external acoustical perturbations. These liquid fuels included ethanol, methanol, aviation fuel (JP-8), liquid synthetic fuel derived from natural gas, and a blend of JP-8 and synfuel. The study examined combustion during acoustic excitation conditions in a closed waveguide in which the droplet was situated at or near a pressure node, where the droplet experienced the greatest effects of velocity perturbations. A two-speaker configuration provided the means to produce a fairly symmetric acoustic field in the waveguide. In the absence of acoustic excitation, values of the measured droplet burning rate constant, K, were generally consistent with available values for the different fuels explored. During acoustic excitation of a burning droplet situated in the vicinity of a pressure node, the flame orientation was consistent with the sign of an acoustic radiation force acting on the burning system, creating conditions where the flame deflection switched, depending on the relative location of the droplet with respective to the pressure node. The acceleration associated with the acoustic radiation force was estimated by measuring the degree of deflection that the flame underwent relative to an unforced flame. Although overall there were no significant variations in the measured K values with changing acoustic excitation, in some cases, locally increased K values were observed to be associated with larger measured acoustic accelerations. This study also examined the extinction characteristics and made preliminary estimations of the extinction strain rates of the different fuels.

The non-reactive flow study investigated the mixing behavior and characteristics of dynamic flow structures of shear-coaxial nitrogen jets under varying flow conditions, with and without the presence of pressure and velocity perturbations due to acoustic

forcing transverse to the flow direction. The role of injector geometry was examined using two shear-coaxial injectors with different outer to inner jet area ratios, and different inner jet post thickness to inner jet diameter ratios. Flow conditions under chamber pressures spanning high subcritical pressures (reduced pressure or chamber to critical pressure ratio, Pr, of 0.44) to nearcritical pressures (Pr = 1.05), with varying outer to inner jet momentum flux ratios (J = 0.1- 21), and maximum or minimum amplitude in the pressure perturbation at the jet axis location were considered. The inner and outer jet temperatures were independently controlled so that the inner and outer flows were in liquid and gaseous states at Pr = 0.44, respectively, and in transcritical and supercritical states at Pr = 1.05, respectively. Back-lighting the coaxial jets resulted in a silhouette of the dense inner jet, which appeared as a dark column. This distinguished it from the outer jet, and thus, enabled high speed images to capture its flow dynamics. Dark-core length pertains to the axial length of the unmixed portion of the inner flow; such measurements were used to indicate the extent of mixing under the different flow conditions and injector geometries. In general, for baseline flows at both Pr values, the dark-core length to inner jet diameter ratio decreased with increasing J and inner post thickness, and with decreasing area ratio. During a maximum pressure perturbation forcing condition, the ratio of forced to baseline flow dark-core length, which stayed constant, around unity, for J < 10 for the small area ratio, thick post injector flows, also underlined the influence of geometry on the mixing and response to external pressure disturbances. A basic application of proper orthogonal decomposition on the intensity fluctuation of the high-speed images enabled the extraction of the spatial and temporal characteristics of the dominant flow structures that existed in the flow field during exposure to acoustic forcing. With increasing J, the flow response to forcing depended on the injector geometry. A comparison of the spatio-temporal characteristics of the baseline flows and their corresponding acoustically forced flows revealed that for the J > 5 flows of the large area ratio, thin inner post injector, the baseline flow behavior was retained in the forced flow, thereby indicating a flow regime with strong instabilities and which was less sensitive to external pressure disturbances. On the other hand, the J > 5 flows of the small area ratio, thick inner post injector showed strong response at the forcing frequency.

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