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Plasma Actuator Tip Flow Control OPEN ACCESS

[+] Author Notes
Lee S. Langston

Professor Emeritus, Mechanical Engineering, University of Connecticut

Mechanical Engineering 136(12), 52-53 (Dec 01, 2014) (2 pages) Paper No: ME-14-DEC4; doi: 10.1115/1.2014-Dec-4

Abstract

This GE study represents the first open literature report of plasma actuators actually used on gas turbine blading at representative engine flow conditions. The exact mechanisms of interaction between weakly ionized gas and neutral air are still under study; however, the collisional processes between them are responsible for the momentum transfer causing the plasma actuator flow. Tip clearance effects are especially critical in gas turbine high-pressure compressors, where they are a large source of aerodynamic loss and stall inducing blockage. In the Dusseldorf discussion on the GE paper, it has been reported that the electronics associated with dielectric barrier discharge (DBD) are continuously being improved and miniaturized. Voltages needed are being reduced, and there is an unexplored area involving the great operational flexibility offered by frequency control. Given the flexible and superior response time of modern electronics, it should be possible to adjust DBD operation in response to transient flow phenomena associated with blade passing frequency.

Axial flow turbomachinery tip leakage, be it at the tip of a rotating blade or at the root of a cantilevered stator represents an insidious source of aerodynamic loss in gas turbines, decreasing component efficiencies in both compressors and turbines.

I wrote about tip clearance and its control in an earlier article [1]. Tightening tip clearances and keeping them under control is a constant worry for OEMs and operators. Two critical parameters for tip leakage flows are the airfoil loading (local pressure difference between pressure and suction surfaces) and the actual tip clearance (either expressed as a percentage of span or chord). Loading is set by the designer and tip clearance is the stepchild to be controlled.

Tip clearance effects are especially critical in gas turbine high pressure compressors, where they are a large source of aerodynamic loss and stall inducing blockage. According to Saddoughi, et al [2], due to low aspect ratio blading and mechanical limitations on the actual magnitude of achievable clearances, new high compressor designs are forced to accept tip gaps from 1% to 4% (based on span). This has a significant impact in reducing high compressor efficiency and stall margins, in these very low aspect ratio blade passages.

In our June 16-20, 2014 TURBO EXPO in Dusseldorf, Aspi Wadia of GE Aviation, a co-author of [2], presented promising results from an experimental evaluation of the impact of high compressor tip clearance with and without plasma actuator flow control. The evaluation was run on the Law/Wennerstrom single-stage transonic compressor rig at Wright Patterson AFB in Ohio.

The plasma actuators were placed on the compressor's casing inner wall upstream of the rotor leading edge. The compressor performance was mapped from part-speed to high speed at three clearances with axial and skewed configurations of the plasma actuators at six different actuator frequency levels. The authors report a maximum stall margin improvement of 4%, with the large clearance configuration benefiting the most. They attribute the improvement by plasma actuators to a reduction in unsteadiness of the tip clearance vortex under near stall conditions. No impact was seen in compressor steady state performance. One can speculate, that had the actuators been placed on the casing inner wall directly in line with the rotor blade tips, perhaps the steady state performance also would have been improved, by reducing tip clearance flows.

This GE study represents the first open literature report of plasma actuators actually used on gas turbine blading at representative engine flow conditions. Past studies have dealt with wind tunnel tests on flat plates and plane cascades, usually with subsonic flows at lower Reynolds numbers.

Just what is a plasma actuator? There are a number of versions, but the most popular is the dielectric barrir discharge (DBD) plasma actuator. The DBD version was used in the GE study [2] reported on in Dusseldorf by Wadia, and is shown in sketch of Fig. 1.

Figure 1 Dielectric Barrier Discharge Plasma Actuator

Grahic Jump LocationFigure 1 Dielectric Barrier Discharge Plasma Actuator

Following an explanation given by Kotsonis, et al [3], DBD is based on the ionization of air using an ac voltage between two electrodes, as shown in Fig. 1. The electrodes are separated by a dielectric barrier layer (e.g., a Kapton polyimide film) which prohibits arc formation, but allows accumulation of ionized gas in the vicinity of the exposed electrode end.

The ionized gas is acted upon by a Lorentz force (the body force in Fig. 1), and will cause it to flow concurrently away from the edge of the exposed electrode. The exact mechanisms of interaction between weakly ionized gas and neutral air are still under study [3], but the collisional processes between them are responsible for the momentum transfer causing the plasma actuator flow.

Ideally, DBDs would be mounted on a compressor (or turbine) casing inner wall circumferentially and axially in the blade path. The induced Lorentz forces then oppose the pressure gradient forces which cause tip leakage. It is important to point out that the resultant Lorentz force is a function of both DBD applied voltage and the ac frequency, thus allowing two ways to control tip leakage. Given the flexible and superior response time of modern electronics, it should be possible to adjust DBD operation in response to transient flow phenomena associated with blade passing frequency.

In the Dusseldorf discussion on the GE paper [2], Wadia reported that the electronics associated with DBD are continuously being improved and miniaturized. Voltages needed are being reduced, and there is an unexplored area involving the great operational flexibility offered by frequency control. Readers should keep eyes open for future developments in this potentially important area of tip leakage control.

References

Langston, Lee S., 2013, “Blade Tips - Clearance and It’s Control”, Global Gas Turbine News, August, pp. 64,69.
Saddoughi, S., Bennett, G., Boespflug, M., Puterbaugh, S.L., and Wadia, A.R., 2014, “Experimental Investigation of Tip Clearance Flow in a Transonic Compressor with and without Plasma Actuators", GT2014-25294, Proc. ASME Turbo Expo 2014, June 16-20, Dusseldorf, Germany, pp. 1-14.
Kotsonis, M., Ghaemi, S., and Veldhuis, L., and Scarano, F., 2011, “Measurement of the Body Force Field of Plasma Actuators”, J Phys. D: Appl. Phys., vol. 44, 045204, pp. 1-11.
Copyright © 2014 by ASME
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References

Langston, Lee S., 2013, “Blade Tips - Clearance and It’s Control”, Global Gas Turbine News, August, pp. 64,69.
Saddoughi, S., Bennett, G., Boespflug, M., Puterbaugh, S.L., and Wadia, A.R., 2014, “Experimental Investigation of Tip Clearance Flow in a Transonic Compressor with and without Plasma Actuators", GT2014-25294, Proc. ASME Turbo Expo 2014, June 16-20, Dusseldorf, Germany, pp. 1-14.
Kotsonis, M., Ghaemi, S., and Veldhuis, L., and Scarano, F., 2011, “Measurement of the Body Force Field of Plasma Actuators”, J Phys. D: Appl. Phys., vol. 44, 045204, pp. 1-11.

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