The present invention relates to an improved attachment air inlet configuration for highly loaded single crystal turbine blades.
High turbine blades in modern turbojet engines are usually made of cast alloys of nickel which are specially formulated to be solidified as a single crystal. These alloys have a crystal structure which has very directional properties. The modulus of elasticity can vary more than 2 to 1 depending on the direction. The highest is across the corners of the crystallographic cube, the lowest is parallel to the edges of the crystallographic cube. Other properties such as Poisson""s ratio vary dramatically as well.
These blades require considerable cooling air to survive because the gaspath temperatures are well above the melting point of the blade material. Cooling air must be supplied through the attachment area which is typically a firtree shape to retain the blade within the disk broach slots which have a mating firtree shape. As the size and weight of the airfoil increases, the crushing load of the retention forces apply high compressive forces across the air passages which must be resisted by compressive stress in the ribs which separate the individual air passages.
The highly directional properties of the single crystal alloy cause very high concentrated stresses in the ribs between the air passages. The concentrated stress at a point in a part made of a single crystal alloy may be described as follows:
Concentrated stress at a point=[P/A +/xe2x88x92 Mc/I] *Kt*kc where:
[P/A +/xe2x88x92 Mc/I]=nominal section stresses at a point;
Kt=local stress multiplier due to local geometry for equiax materials; and
Kc=local stress multiplier due to overall part geometry and crystallographic orientation relative to that geometry.
Conventional flow passages and rib geometry produce very high concentrated stresses in modern blades which have both high radial loads and high crushing loads on the attachment. These high stresses cause plastic compressive redistribution of stress which results in tensile stresses on parts of the compressive ribs and rib cracking. Conventional attachments prove to be very sensitive to Kc effects.
Accordingly, it is an object of the present invention to provide an improved attachment air inlet configuration having an attachment area with a core/rib configuration which reduces the concentrated stresses while maintaining required flow and pressure loss parameters in cooling passages.
It is a further object of the present invention to provide an improved attachment air inlet configuration which solves the rib stress problem without increasing the overall size and weight of the attachment and the supporting disk.
The foregoing objects are achieved by the attachment air inlet configuration of the present invention.
In accordance with the present invention, an attachment air inlet configuration for a turbine blade comprises an attachment having a root portion with a center plane and a plurality of inlets in the root portion of the attachment communicating with at least two flow passageways in the blade. Each of the inlets communicates with a feed cavity and receives a cooling fluid such as cooling air. Each of the inlets has a non-circular shape with a major axis, which major axis is substantially normal to a central axis of the root portion center plane.
Other details of the attachment air inlet configuration of the present invention, as well as other objects and advantages attendant thereto, are set forth in the following detailed description and the accompanying drawings in which like reference numerals depict like elements.