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Blade Tips - Clearance and Its Control PUBLIC ACCESS

Axial Flow Gas Turbines, by their Intrinsic Nature, Have Rotating Blades in Both Compressors and Turbines.

[+] Author Notes
Lee S. Langston

Professor Emeritus of Mechanical Engineering, University of Connecticut

Langston is a former editor of the ASME Journal of Engineering for Gas Turbines and Power and has served on the IGTI Board as both Chair and Treasurer.

Mechanical Engineering 135(08), 66-71 (Aug 01, 2013) (2 pages) Paper No: ME-13-AUG4; doi: 10.1115/1.2013-AUG-4

Abstract

This article focuses on studying blade tip clearance phenomena. It is important to realize that to be freely turning, a blade (or a cantilevered stator) must have a clearance gap between its tip and the engine casing (or hub). Such clearances introduce aerodynamic losses, decreasing gas turbine efficiency. Tip leakage losses in compressors can be significant and have been reviewed by the experts. During transient operations, gas turbine blade tip clearances will change based on blade/disk centrifugal loads and the different response times of engine parts to thermally induced expansions and contractions. Designers have perfected active clearance control (ACC) systems to deal with these transient conditions. ACC uses cool or hot gas path and fan air at appropriate times during transients to control the rate of expansion or contraction of internal parts adjacent to the gas path and outer casings. The research shows that continued enhancement of blade tip clearance management systems over a range of engine operating conditions has brought and will bring about gains in gas turbine efficiency.

Some may also have cantilevered stators abutting a rotating hub. It does not require great insight to realize that to be freely turning, a blade (or a cantilevered stator) must have a clearance gap between its tip and the engine casing (or hub).

Such clearances introduce aerodynamic losses, decreasing gas turbine efficiency. I recall one military jet engine program back in the 1960's, where efforts were made to “tighten up” compressor blade tip clearances, to reduce these losses. Unfortunately the tightening went to zero clearance, resulting in a “hard rub” between titanium high compressor blade tips and the titanium compressor case. The ignition temperature of titanium (2900 ̊F = 1593 ̊C) is lower than its melting temperature (3140 ̊F = 1723 ̊C), so the engine tip clearance tightening program ended up with a combusted compressor, reduced to a pile of white titanium dioxide powder.

Koff [1] has noted that in the mid 1980's, cubic boron nitride (CBN) blade tip coatings for compressors and turbines were developed to prevent blade tip wear during light rubbing. CBN application to blade tips allowed tip clearances to be reduced for increased efficiency without encountering the rub damage experienced during the foregoing 1960's episode.

Tip leakage flows for both compressors and turbine blades are complex, and depend on at least two critical parameters: The magnitude of tip clearance gap (e.g. as a percentage of blade span or chord) and the blade loading (local pressure difference between blade pressure and suction surfaces). In both compressors and turbine blades, a tip leakage suctionside vortex is formed which interacts with three dimensional flows within the blade-to-blade passage. Flow on the inner surface of the casing is dominated by viscous and pressure forces. I remember doing a flow visualization test on a large-scale stage-and-a-half turbine rig, watching droplets of ink injected upstream of the rotor on the inside surface of a plexiglas casing. The wall-bound droplets sped directly across the rotor in a purely axial direction, from leading edge to trailing edge, reacting to the favorable axial pressure gradient, and unaffected by the rotor blade motion.

Tip leakage losses in compressors can be significant and have been reviewed by Graf [2]. One study reviewed showed that for every 1% increase in tip clearance (based on chord) there was a 5% decrease in peak pressure rise across a compressor. Another reviewed study showed compressor efficiency penalties range from 1 to 2 points for every 1% increase in tip clearance (based on span).

An extensive review of turbine blade tip leakage effects has been given by Bunker [3]. Here, not only are turbine blade tip aerodynamic losses a concern, but also blade tip durability, especially in the highpressure turbine (HPT), which is subject to high gas temperatures. As Bunker points out, the degradation of blade tips constitutes about one-third of HPT failures, where failure is defined as the loss of the part from service inventory (unrepairable), or the accelerated degradation of the efficiency/ output in service.

Figure 1, taken from Bunker [3] shows the effect of blade tip clearance (as a percentage of blade span) on turbine stage efficiency, based on industrial experience. The plots shown are for various types of blade tip design, each of which is treated extensively in [3].

Figure 1 Effect of turbine blade tip clearance on stage efficiency, from Bunker[3]

Grahic Jump LocationFigure 1 Effect of turbine blade tip clearance on stage efficiency, from Bunker[3]

As an example, we will assume a near-zero clearance, 90% efficiency turbine stage for a Fig. 1 unshrouded flat tip design. From Fig. 1, if the tip clearance is increased to 1%, we see that the stage efficiency drops by 2 points, reducing stage efficiency to 88%. Similarly, a tip clearance of 2% drops about 4 points to yield a disappointing 86% stage efficiency. The two other curves in Fig. 1 yield smaller penalties for increased clearance, showing the benefits of tip treatments and shrouds.

During transient operations (e.g., start up, load variation, or a sudden trip condition) gas turbine blade tip clearances will change based on blade/disk centrifugal loads and the different response times of engine parts to thermally induced expansions and contractions. Designers have perfected active clearance control (ACC) systems to deal with these transient conditions. ACC uses cool or hot gas path and fan air at appropriate times during transients to control the rate of expansion or contraction of internal parts adjacent to the gas path and outer casings. Last year I visited a new combined cycle power plant in Irsching, Germany which has the new Siemens SGT5-8000H 375 MW gas turbine [4], currently the world's largest. Siemens has a unique blade tip clearance system, called hydraulic clearance optimization as shown in Fig. 2. With the daily load variation and the start-ups and shutdowns, rotating blade tip clearances can increase or decrease. To control the clearances, Siemen's HCO has hydraulic pistons that can shift the rotating gas turbine rotor along the axis of rotation.

Figure 2 Siemens hydraulic clearance optimization (HCO) system on the SGT5-8000H 375 MW gas turbine

Grahic Jump LocationFigure 2 Siemens hydraulic clearance optimization (HCO) system on the SGT5-8000H 375 MW gas turbine

The outer case of the gas path is conical in the compressor (apex downstream) and in the turbine (apex upstream). Thus an HCO rotor shift towards the gas turbine inlet decreases the blade tip clearance in the turbine but increases those in the compressor. However, since turbine power is twice the power to drive the compressor and the conical turbine case is four times steeper than that of the compressor, compressor tip losses are only one-eighth of the turbine's power and efficiency improvements. Figure 2 shows data illustrating a gratifying 5 MW increase in output, when HCO is applied.

This was a brief look at blade tip clearance phenomena. It does show that continued enhancement of blade tip clearance management systems over a range of engine operating conditions has brought and will bring about gains in gas turbine efficiency.

References

Koff, Bernard L., 2004, “Gas Turbine Technology Evolution - A Designer's Perspective”, AIAA Journal of Propulsion and Power, 20, No. 4, pp. 577– 596. [CrossRef]
Graf, Martin Bowyer, 1996, “Effects of Asymmetric Clearance on Compressor Stability”, Master of Science Thesis in Aeronautics and Astronautics, Massachusetts Institute of Technology, June, pp. 12-13.
Bunker, Ronald S., 2006, “Axial Turbine Blade Tips: Function, Design, and Durability”, AIAA Journal of Propulsion and Power, 22, No. 2, pp. 271– 285. [CrossRef]
Langston, Lee S., 2013, “Riding the Surge”, Mechanical Engineering Magazine, May, pp. 37– 41.
Copyright © 2013 by ASME
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References

Koff, Bernard L., 2004, “Gas Turbine Technology Evolution - A Designer's Perspective”, AIAA Journal of Propulsion and Power, 20, No. 4, pp. 577– 596. [CrossRef]
Graf, Martin Bowyer, 1996, “Effects of Asymmetric Clearance on Compressor Stability”, Master of Science Thesis in Aeronautics and Astronautics, Massachusetts Institute of Technology, June, pp. 12-13.
Bunker, Ronald S., 2006, “Axial Turbine Blade Tips: Function, Design, and Durability”, AIAA Journal of Propulsion and Power, 22, No. 2, pp. 271– 285. [CrossRef]
Langston, Lee S., 2013, “Riding the Surge”, Mechanical Engineering Magazine, May, pp. 37– 41.

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