Experimental Investigation of Transverse Mode Nozzle Damping

Combustion instability remains one of the most stifling problems in the design of propulsive technologies. Instabilities may emerge as a result of a feedback mechanism between unsteady heat release and the acoustic modes of the combustion chamber, and these instabilities often have the potential to seriously damage, or even destroy, the vehicle experiencing these oscillations. To address underrepresentation in the literature involving transverse mode nozzle damping relative to longitudinal damping, an experimental campaign involving three nearly-identical nozzles was conducted at the University of Tennessee Space Institute. The goals of this experiment were to produce data corresponding to the attenuation of excited transverse modes within the nozzles, and to use this data to determine an ideal nozzle geometry for each mode. The geometry of the nozzles differed only at the convergent sections, and this identifying characteristic was used to label the nozzles as follows: an equal-radius-of-curvature (ER) nozzle, a conical nozzle, and a linear-velocity-profile (LVP) nozzle.Excitation was provided by a frequency-variable, pneumatic siren, and attenuation was facilitated by a spring activated, 3D printed gate-valve placed at the outer wall of the chambers. The pneumatic siren injected oscillatory flow from the sidewall of the chambers, and could reliably operate from 1500 to 2000 Hz. The decay of the modes were measured by six pressure transducers arranged in either a tangential array or a mixed tangential-longitudinal array, and an exponential decay rate, α, was computed from the data collected at various locations within the nozzle by applying a linear fit to the natural logarithm of the upper envelope of the pressure traces. This thesis presents data on the first tangential (1T) mode in all three nozzles. In general, it was observed that the ER nozzle provides the most 1T damping, followed by the conical nozzle, which is followed by the LVP nozzle. The methodology described herein is easily applicable to other modes and geometries.