The Woodruff School

SUMMARY

Combustion systems utilizing bluff bodies to stabilize the combustion processes can experience oscillatory heat release due to the alternate shedding of coherent, von K�rm�n vortices under certain operating conditions. This phenomenon needs to be understood in greater detail, since unsteady burning due to vortex shedding can lead to combustion instabilities and flame extinction in practical combustion systems. The primary objective of this study was to elucidate the influence of combustion process heat release upon the B�nard-von K�rm�n (BVK) instability in reacting bluff body wakes. For this purpose, spatial and temporal heat release distributions in bluff body-stabilized combustion of liquid Jet-A fuel with high-temperature, vitiated air were characterized over a wide range of operating conditions. Upon comparing the spatial and temporal heat release distributions, the fuel entrainment and subsequent heat release in the near-wake were found to strongly influence the onset and amplitude of the BVK instability. As the time-averaged heat release in the near-wake decreased, the amplitude of the BVK instability increased. This was attributed to the corresponding decrease in the local density gradient across the reacting shear layers, resulting in less damping of vorticity due to gas expansion. The experimental results were compared to the results of a parallel, linear stability analysis in order to further understand the influence of the near-wake combustion process heat release upon the wake instability characteristics. The results of this analysis support the postulate that oscillatory heat release due to BVK vortex shedding is the result of local absolute instability in the near-wake, which is eliminated only if the temperature rise across the reacting shear layers is sufficiently high. Furthermore, the results of this thesis demonstrate that non-uniform fuelling of the near-wake reaction zone increases the likelihood of the absolutely unstable, BVK instability due to the possibility of near-unity products-to-reactants density ratios in the near-wake if the reactants temperature is high.

SUBJECT:	Ph.D. Dissertation Defense

BY:	Caleb Cross

TIME:	Friday, August 19, 2011, 10:00 a.m.

PLACE:	Guggenheim, 442

TITLE:	Combustion Heat Release Effects on Asymmetric Vortex Shedding from Bluff Bodies

COMMITTEE:	Dr. Ben T Zinn, Chair (AE/ME) Dr. Timothy Lieuwen (AE/ME) Dr. Jerry Seitzman (AE) Dr. Marc Smith (ME) Dr. Eugene Lubarsky (AE)

SUMMARY Combustion systems utilizing bluff bodies to stabilize the combustion processes can experience oscillatory heat release due to the alternate shedding of coherent, von K�rm�n vortices under certain operating conditions. This phenomenon needs to be understood in greater detail, since unsteady burning due to vortex shedding can lead to combustion instabilities and flame extinction in practical combustion systems. The primary objective of this study was to elucidate the influence of combustion process heat release upon the B�nard-von K�rm�n (BVK) instability in reacting bluff body wakes. For this purpose, spatial and temporal heat release distributions in bluff body-stabilized combustion of liquid Jet-A fuel with high-temperature, vitiated air were characterized over a wide range of operating conditions. Upon comparing the spatial and temporal heat release distributions, the fuel entrainment and subsequent heat release in the near-wake were found to strongly influence the onset and amplitude of the BVK instability. As the time-averaged heat release in the near-wake decreased, the amplitude of the BVK instability increased. This was attributed to the corresponding decrease in the local density gradient across the reacting shear layers, resulting in less damping of vorticity due to gas expansion. The experimental results were compared to the results of a parallel, linear stability analysis in order to further understand the influence of the near-wake combustion process heat release upon the wake instability characteristics. The results of this analysis support the postulate that oscillatory heat release due to BVK vortex shedding is the result of local absolute instability in the near-wake, which is eliminated only if the temperature rise across the reacting shear layers is sufficiently high. Furthermore, the results of this thesis demonstrate that non-uniform fuelling of the near-wake reaction zone increases the likelihood of the absolutely unstable, BVK instability due to the possibility of near-unity products-to-reactants density ratios in the near-wake if the reactants temperature is high.