Session: 04-22: Combustion Dynamics - Experiments I

Paper Number: 80771

80771 - Hysteresis and Bi-Stability in Transversely Excited Swirling Flows 

In this work, we investigate the effect of transverse acoustic excitation on non-reacting swirling jets. The work is motivated by the azimuthal instabilities in annular gas turbine combustors which are one of the major challenges in aero-engines. Annular combustion chambers have a longer circumferential length in comparison to their length or height. As a result, thermoacoustic instabilities are driven by azimuthal acoustic modes.

We have designed and fabricated a multi-nozzle (up to three nozzles) linear array combustor to simulate the flow conditions of an annular combustor. The nozzle features a dual co-rotating radial swirler, with the swirl number of 0.3 and 0.7 for the primary and secondary swirler respectively, and with a flow split of 45% and 55% between the primary and secondary swirlers. Two compression drivers (power 400 W each) placed on either side of the combustion chamber are used to generate acoustic fields in the direction transverse to the flow. The configuration enables a velocity node or a velocity anti-node with variable amplitude at the position of the central nozzle. The chamber acoustics are modeled using an acoustic solver. Here we investigate a single nozzle configuration that is excited at 580 Hz (2^nd harmonic) creating a velocity antinode. Simultaneous 2D PIV and high-frequency pressure measurements are conducted to measure the time-averaged velocity field and the chamber acoustics, respectively. The time-averaged flow field of the quiescent swirling jet shows a typical low swirl number flow-field that features a narrow recirculation zone surrounded by a high-velocity annular jet. It is observed that once the swirling jet is exposed to transverse acoustic excitation, it instantaneously transitions to a wide-open conical type of vortex breakdown (CVB). In this condition, the flow moves radially outward and remains attached to the walls on either side of the nozzle, and is characterized by a recirculation zone with strong negative axial velocity. Our hypothesis is that the transverse acoustic excitation (velocity antinode) rapidly changes the local swirl number of the flow that causes it to instantaneously transition to a wide-open CVB. We will verify this through forced acoustic simulations to determine the swirl number at the nozzle imposed by the acoustic excitation.

To further understand the swirling jet transition to a wide-open CVB, we compared the time-averaged quiescent flow fields of nozzles by varying the inner geometric swirl number from 0.3 – 2.0 while maintaining the nominal geometric flow split between the two swirlers constant. When the transverse acoustic excitation is turned off, the swirling jet continues to be in the wide-open conical vortex breakdown state indicating a bistable flow state. This hysteresis phenomenon is observed for a range of swirling jets with flow Reynolds numbers varying from 4000 to 15000. We did not observe any bistable state for swirling jets with Reynolds number less than 2000 where the initial and final flow states are found to be identical. It is a known phenomenon that swirling jets transition to a conical mode of vortex breakdown upon transverse acoustic excitation. However, to the best of our knowledge, this is the first instance where we show that transverse acoustic excitation can lead to a bistable state in swirling flows. These bistable state transitions could severely disrupt the azimuthal uniformity of the combustor flow and thus, the azimuthal uniformity of the temperature field at the combustor exit.

Presenting Author: Ravi Gupta Indian Institute of Science, Bangalore

Presenting Author Biography: Ravi Gupta is a graduate student pursuing Ph.D. in the Department of Aerospace Engineering at the Indian Institute of Science (IISc), Bangalore, India. He did his undergraduate studies in the Department of Mechanical Engineering at the National Institute of Technology (NIT), Kurukshetra, India. His research involves combustion dynamics in aerospace propulsion systems.

Authors:

Ravi Gupta Indian Institute of Science, Bangalore
Rajat Gohiya Indian Institute of Science, Bangalore, India
Chandan Vempati Indian Institute of Science, Bangalore
Santosh Hemchandra Indian Institute of Science, Bangalore
Pratikash Panda Indian Institute of Science, Bangalore