Turbine Rotor Geometry

Introduction to a Turbine Rotor Geometry 

The turbine blades are used to transform the hot gases high energy into mechanical energy available on the shaft. The high-pressure turbine (HPT) blades are in a very hostile environment considering that they are in the gas-path  at the exit of the combustion chamber while rotating at high speed. One can therefore easily understand that these blades are exposed to large thermal and mechanical stresses.

The blades are fastened to the disc through the fixing . Generally, the gap between the blade root and the disc fixing is used to provide the blade with compressor bleed air for cooling purpose.

The turbine disc,  is a large rotating component surrounded by compressor bleed air. The main role of the disc is to mechanically support the blades and to transfer the mechanical energy produced by the blades to the shaft. A cover-plate might sometime be secured to the disc and used to direct the cooling air more efficiently inside the fixing to cool the blade .

Within PDDS, the geometry of each component of the turbine rotor is provided by specific design tools based on CAD parametrized models (Ouellet, Savaria, Roy, Garnier & Moustapha, 2016; Twahir, 2013). These tools allow a user to design a wide range of turbine rotor’s configurations including HPT rotors with cooled blade and cover-plates directing the cooling air, or low-pressure turbine (LPT) rotors with shrouds on top of uncooled airfoils .

Introduction to a Turbine Stator Geometry 

The static components above the blade tip that are of interest for this Ph.D. project are the housing (or casing) and the shroud segment (also called blade outer air seal). The assembly of these two components is referred to as “stator” in this work.

The shroud segments are the inner part of the stator. This “inner casing” is generally made up of a series of arc segments joined together to form a ring and attached to the “outer casing” by some hooks . These hooks allow a certain radial displacement due to thermal and mechanical loads (Hennecke, 1985). The cavity between the outer surface of the shroud segments and the housing is used to conduct compressor bleed air and to cool down the shroud by jet impingement cooling (Melcher & Kypuros, 2003).

Similar to the work done on the rotor, a stator design module was developed and used as a base for the static components thermal and stress analyses executed in this work (Savaria, Phutthavong, Moustapha & Garnier, 2017) .

Thermal Boundary Conditions Calculation 

In order to execute the necessary thermal analyses on the rotor and stator components, thermal boundary conditions at the surface of these components need to be calculated. These thermal boundary conditions consist of a combination of a heat transfer coefficient and a fluid (i.e. air or gas depending on the turbine’s zone considered) effective temperature, which allow evaluating the local heat flux.

Tip Clearance Importance 

The prediction of the tip clearance size variation is important in order to avoid rubs and to prevent blades wear so as to dramatically increase the engine service life. Indeed, the deterioration of the airfoil tip and/or of the shroud segment because of rubs produces an increase of the tip clearance. It follows that the engine has to increase the turbine inlet temperature to develop the same thrust. If the disc temperature reaches its upper limit, the engine must come off the wing for maintenance. Considering that, one can easily understand that a better prediction of the tip clearance variation can reduce the engine life cycle cost (LCC). It was demonstrated that improving the tip clearance in the HPT as much more impact on the LCC than the same improvement of clearance in the low pressure turbine or the high pressure compressor (Lattime & Steinetz, 2002).

Tip clearance also has a considerable impact on the turbine efficiency and thus on the engine SFC. Indeed, the bigger the tip clearance the more gas can leak from to pressure side of the blade’s airfoil toward the suction side without producing any work. Considering that work was required in order to bring that gas to its temperature and pressure, every percent of it that is not used to produce work is a loss for the whole engine.

Several tests were executed on a CF6-50C turbofan to evaluate the effect of the HPT tip clearance on the engine performance (Howard & Fasching, 1982). The authors performed these measures during both steady state and transient operations. The results showed that an increase of 0.305 mm in clearance produced a reduction of 0.7 % in turbine efficiency. In terms of the tip clearance effects on the whole engine efficiency, it was demonstrated that, once the gap between the rotor and the stator is larger than 1 % of the blade height (meaning a tip clearance of about 0.2 to 1 mm), an increase of 1 % in tip clearance produces a drop of about 1 % in efficiency (Hennecke, 1985). Finally, as explained previously the engine SFC is directly related to the HPT tip clearances. It was indeed reported that an increase of 0.254 mm in HPT tip clearance roughly produces a 1 % increase in SFC (Lattime & Steinetz, 2002).

Even if the values of the tip clearance impact on the engine efficiency or its SFC slightly vary from one reference to another, one can easily conclude that the prediction of the tip clearance size variation is mandatory in an integrated turbine design system considering the magnitude of its effect on the engine’s performances.