Primer: The ABCs of blade-tip clearances

For an axial-flow gas turbine to operate without wear and tear caused by rubbing between rotating and stationary components, clearances between rotating blades and casing shroud, and between stationary vanes and hub, must be adequate—but not generous.

UConn Professor Emeritus Lee S Langston shows the general characteristics of leakage flow over a turbine blade with a flat tip in Fig 1. Leakage along the blade chord is caused by pressure-side axial flow migrating over the top of the airfoil to the lower-pressure suction side. A characteristic of both turbine and compressor blades is the formation of a suction-side vortex attributed to tip leakage; it interacts with the three-dimensional flow within each blade-to-blade passage. Tip leakage flow causes a loss in gas-turbine performance.

Tip Clearances 1-3A cross-sectional view at a given chord location of tip leakage for a turbine blade and a compressor blade is illustrated in Fig 2.  The general leakage-flow picture is basically the same for both, but differs in some details.  For example, the pressure difference between the pressure and suction sides of a turbine blade are higher than that of a compressor blade, so turbine tip leakage flows are stronger.  

Important to keep in mind is that the axial flow in a compressor stage is having work done on it, so its pressure is increasing while axial flow in a turbine stage is doing work and sees a decreasing pressure. As they rotate, the pressure side of a compressor blade leads, while the suction side leads for a turbine blade.

The two critical parameters for tip leakage are the blade loading (local pressure difference between pressure and suction surfaces, as well as that from leading edge to trailing edge) and the actual tip clearance (expressed as a percentage of blade span or blade chord). Loading is set by the blade designer and the tip clearance is the stepchild to be controlled.

Tightening tip clearances and keeping them under control is a constant worry for OEMs and operators. The author recalls one military jet-engine program back in the 1960s, where efforts were made to “tighten up” compressor-blade tip clearances, to reduce losses. Unfortunately, the tightening went to zero clearance, resulting in a “hard rub” between titanium high-pressure- compressor blade tips and the titanium case. Because the ignition temperature of titanium (2900F) is lower than its melting temperature (3140F) the engine tip-clearance tightening program ended up with a combusted compressor, reduced to a pile of white titanium dioxide powder.

In the mid-1980s, cubic boron nitride (CBN) blade tip coatings for compressors and turbines were developed to prevent tip wear during light rubbing. CBN application to blade tips (or to shrouds) allowed tip clearances to be reduced for increased efficiency without encountering the rub damage experienced during the foregoing 1960’s episode.

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

Tip clearance effects can be especially critical in late-model high-compression-ratio aeroderivative (aero) gas turbines with their low-aspect-ratio high-pressure compressor (HPC) blades—airfoils having comparable spans and chords. The low-aspect-ratio blading, and mechanical limitations on the actual magnitude of achievable clearances, could force new HPC designs to accept tip gaps from 1% to 4% (based on span).  The result would be reduced efficiency and stall margin.

Turbines. Efforts devoted to tip leakage control in turbines are extensive. The large pressure differences between the pressure and suction sides of, say, a single-stage aero HP turbine, promote detrimentally high tip-leakage flows. But, most important, these flows are at high temperature, contributing to shorter blade lifetimes. 

GE’s Ronald Bunker says blade-tip durability in aero HPTs is a critical area, accounting for about one-third of HPT failures. Here, “failure” is defined as the loss of the part from service inventory (unrepairable), or the accelerated degradation of efficiency/output in service.

The plot in Fig 3 shows the effect of blade tip clearance (as a percentage of blade span) on turbine efficiency for various blade-tip designs. Unshrouded flat tip refers to a simple untreated tip blade, such as shown in Fig 1. A shrouded cylindrical tip is one where a shroud (integral with the tip) is part of the blade, providing a bounding outer radius surface to the blade passage. A flared shrouded tip is the same, except that the mean radius of the shroud increases in the axial direction.

How to use the Fig 3 chart: Assume a near-zero-clearance, 90%-efficient turbine stage for the Fig 3 unshrouded flat-tip design. If tip clearance is increased to 1% (Point A), observe that the stage efficiency drops by 2 percentage points, reducing it to 88%.  Similarly, for a tip clearance of 2% (Point B), efficiency drops about 4 points to yield a disappointing 86%.

The squealer tip, most commonly used in aero HPTs and in the first turbine stage of frame engines for reducing leakage flow, has a short rim surrounding a shallow cavity to provide a simple two-tooth labyrinth seal. Fig 4 shows the tip leakage flow field for a squealer tip, as predicted by computer modeling.   

Tip Clearances 4

The illustrations at the right in Fig 4 show how Mitsubishi eliminated cap cracking and oxidation caused by turbulence in the squealer-tip area. The left-hand sketch describes the original squealer-tip design, the one at the right has a squealer only on one side of the airfoil. This allows hot gas to flow smoothly across the blade tip. Also, note that the height of the squealer for the redesigned tip is less than that for the original blade.

Dealing with transients. During transient operations (for example, 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 accommodate these transient conditions.  ACC uses cool or hot gas path at appropriate times during transients to control the rate of expansion or contraction of internal parts adjacent to the gas path and outer casings.

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