Unsteady Flows

Swirling Flow Dynamics

Overview

Gas turbines are used every day, from power generation applications to jet engines. Modern day gas turbines face an increasing demand to reduce pollutant gas emissions, such as CO2 and NOx. To meet this challenge, many gas turbines use a lean premixed combustion process in which air and fuel are mixed upstream of the combustion chamber. Premixed systems are very effective at reducing emissions, but they present a problem during combustion: combustion instability. Combustion instability is caused by the coupling between combustor acoustics and flame heat release fluctuations. It can result in increased emissions, reduced hardware life, and catastrophic turbine failure. Flames in gas turbines are stabilized by use of a swirling flow field. A swirling flow field has many complex fluid-mechanical properties that directly influence combustion instability events. Namely, the velocity coupling mechanism that links the shear layer fluctuations of a swirling flow to combustor acoustic perturbations is key to understanding combustion instability in gas turbines.

The goal of this research is to explore the fundamental physics of swirling flows in order to understand the mechanisms by which velocity fluctuations are impacted by combustor acoustics. This knowledge will allow for the design of flow fields that mitigate and even prevent combustion instability events. Various experimental and analytical techniques are used, such as particle image velocimetry, pressure diagnostics, proper orthogonal decomposition, dynamic mode decomposition, and linear stability analysis.

Publications

Clees, S., Lewalle, J., Frederick, M., O’Connor, J., (2018) “Vortex core dynamics in a swirling jet near vortex breakdown,” AIAA SciTech. Kissimmee, FL.

Frederick, M., Manoharan, K., Dudash, J., Brubaker, B., Hemchandra, S., O’Connor, J., (2018) “Impact of Precessing Vortex Core Dynamics on Shear Layer Response in a Swirling Jet,” Journal of Engineering for Gas Turbines and Power, 140(6), 061503.

O’Connor, J. (2017) “Disturbance Field Decomposition in a Transversely Forced Swirl Flow and Flame.” Journal of Propulsion and Power, 33(3), p. 750-763.

Mathews, B., S. Hansford, J. O’Connor, (2016) “Impact of Swirling Flow Structure on Shear Layer Vorticity Fluctuation Mechanisms,” ASME Turbo Expo, Seoul, South Korea

O’Connor, J. (2015) “Visualization of Shear Layer Dynamics in a Transversely Forced Flow and FlameAIAA Journal of Propulsion and Power, 32(4), p. 1127-1136

Hansford, S., K. Manoharan, S. Hemchandra, J. O’Connor, (2015) “Impact of flow non-axisymmetry on swirling flow dynamics and receptivity to acoustics,” in ASME Turbo Expo, Montreal, Canada.

Manoharan, K., S. Hansford, J. O’Connor, S. Hemchandra, (2015) “Instability mechanism in a swirl flow combustor: Precession of vortex core and influence of density gradient,” in ASME Turbo Expo, Montreal, Canada

 

Interacting Flows

Overview

While the hydrodynamic stability characteristics of unit flows, like jets and wakes, have been studied extensively, the impact of flow interaction between unit flows on the hydrodynamic stability characteristics has not been explored. A limited number of studies have shown that flow interaction can change the stability characteristics of these flows. Recent work by the PI has investigated interacting wakes, where a single wake displays a global instability, referred to as the von Karman vortex street, as well as the Kelvin-Helmholtz shear layer instability. The interacting wakes, however, display very different dynamics, where the wake displays more intermittency and overall a more symmetric vortex shedding pattern. The three-wake system has several distinct features, including an extended region of K-H shear layer instability (as seen by the small-scale oscillations), and a more symmetric large-scale wake vortex shedding region downstream. Videos of the instantaneous velocity field also show that the three-wake system vortex shedding is more intermittent. These difference may indicate a change in the stability of the flow as a result of flow interaction. The goal of this work is to characterize the behavior of interacting flows, including wakes, in a quantitative manner.

Publications

Meehan, M., Tyagi, A., O’Connor, J., (2018) “Flow dynamics in a variable-spacing, three bluff-body flowfield,” Physics of Fluids, 30, 025105.

Dare, T., Berger, Z., Meehan, M., O’Connor, J., (2018) “Cluster-based reduced-order modeling to capture intermittent dynamics of interacting wakes,” AIAA SciTech. Kissimmee, FL.