You're invited to attend
"The Precessing Vortex Core Instability in Swirl Nozzles"
by
Santosh Hemchandra
Professor, Department of Aerospace Engineering
Indian Institute of Science, Bangalore, India
Thursday, June 22
10:00 a.m.
Food Processing Tech Auditorium 102 in the NARA complex
Refreshments will be served
Abstract
Swirled jets are technologically important flows that have wide ranging applications in gas turbine propulsion and power generation systems. Combustors in these devices use nozzles that impart swirl to the flow as it passes through. Sufficiently high values of swirl intensity cause the axial vortex in the flow to break down and create an axi-symmetric recirculation zone in the flow - commonly referred to as the ‘vortex breakdown bubble’ (VBB). The precessing vortex core (PVC) is a self-excited instability that occurs in this flow due to the precession of the VBB around the flow axis. The presence of PVCs is known to critically influence combustor performance with regard to pollutant emissions and thermoacoustic instability. This talk focuses on how the PVC arises and how its occurrence can be controlled by passive nozzle geometry changes.
First, I will summarize results from a theoretical and experimental of a swirled axi-symmetric jet at a Reynolds number, 𝑅𝑒 = 59,000 – the subject of several studies in Jacqueline O’Connor’s group at Penn. State. The experiments show the onset of vortex breakdown at a critical swirl number, 𝑆! = 0.61 accompanied by a simultaneous emergence of a stable limit cycle PVC oscillation. I will show a results from the analysis that confirm that the PVC is a consequence of vortex breakdown and that the limit cycle is driven by internal feedback in the region at the upstream end of the breakdown bubble [1]. This suggests that interventions in this region of the flow can control the PVC.
Next, I will show results from experiments and LES studies performed on a swirl nozzle with different diameter centrebodies. These results show that the PVC can be controlled by varying the diameter of a cylindrical nozzle centrebody. I will discuss the mechanism involving disruption of flow feedback by the centrebody wake or the centrebody itself that allows for this [2, 3]. New results from resolvent analysis that can explain intermittent re-emergence of the PVC in this flow configuration will be discussed.
In the third part of the presentation, I will show results from another study on PVC characteristics in the PRECCINSTA model GT combustor that was the subject of a recent experimental campaign at DLR Stuttgart. I will show that flame attachment to the centrebody due to H2 enrichment of the fuel again disrupts internal feedback and causes PVC suppression.
These results therefore suggest a broad guideline for PVC management by design in swirl nozzles and demonstrate computational tools that can provide quantitatively accurate predictions that can guide design optimization for this purpose.
About the Speaker:
I’m an associate professor in the Department of Aerospace Engineering, Indian Institute of Science (IISc), Bangalore, India. My research group’s focus is broadly on uncovering new physical insights into reacting and non-reacting flows that are of relevance to space and aircraft propulsion industry funded mainly by key industry OEMs in these areas. We work actively to transition these insights into design tools and practice to help mitigate fuel burn, emissions (soot, NOX) and other operability concerns in GT and rocket systems such as thermoacoustic instability and jet noise.
We use methods such as stability analysis, large eddy simulations, experimental measurements (collaboratively) and data driven analysis that are aimed broadly at understanding coherent unsteadiness mechanisms in combustor flows and in other related areas such as sources of jet noise and wall-bounded flows. More recently, we’ve been working actively to incorporate machine learning methods that can assimilate data from LES or experiments into simpler, less computationally expensive models that are easily used for engineering design practice. I earned my Ph. D. in 2009 from the Lieuwen group in the department of aerospace engineering at Georgia Tech. and have been on the faculty at IISc since 2012.