This application claims priority to German Patent Application DE 10 2006 055 869.3 filed Nov. 23, 2006, the entirety of which is incorporated by reference herein.
This invention relates to an airfoil design for rotor and stator blades of a turbomachine, more particularly of a gas-turbine engine, which is defined by a course of a skeleton line established by a skeleton line angle over a chord length, an airfoil height and a course of a leading edge as well as a blade tip ending at an air gap.
The airfoil of engine blades is, under the aspect of an aerodynamically optimum shape, composed by a stack of a plurality of individual profiles over the blade height creating a three-dimensional form, with the individual profile sections having a specific skeleton line and a specific material thickness on both sides of the skeleton line. The course of the skeleton line, which is a centerline in the respective profile section, is designed for minimum profile pressure loss and maximum working range in the respective blade area. However, the currently used CDA (controlled diffusion airfoil) blade profiles and their derivatives fail to meet these requirements in the area of the blade tip, i.e. in a gap-near blade area, since the currently used blade designs do not adequately take account of the aerodynamically negative influence of the gap between the blade tip and the machine casing or the hub, respectively. Flow around and over the blade tip leads to formation of swirls in this blade area which limit the stable operation of the machine, resulting in flow and performance losses which must be compensated by increasing the number of blades—with disadvantageous consequences on cost and weight.
A broad aspect of the present invention is to provide an airfoil design of rotor and stator blades of a turbomachine which minimizes the flow disturbances occurring close to the gap and leading to performance losses.
In essence, the present invention provides for blade profile sections, which in a gap-near area of up to 30 percent of the blade height starting at the blade tip have a specific course of the skeleton line defined by the skeleton line angle in relation to the chord length of the blade profile whereby a uniform pressure distribution is established along the blade section at the gap or near the gap and, thus, a stable gap swirl is obtained. Uniform distribution of load in the gap-near blade area reduces gap losses, as a result of which, performance and stability limits are increased or, with constant performance, the number of blades, and thus weight and ultimately cost, is reduced.
The dimensionless skeleton line angles for the inventively optimum course of the skeleton line, actually for blade profile sections falling within the aforementioned 30 percent range, lie in a certain skeleton line angle distribution range which is in a coordinate system established by the chord length (x axis, in percent) and the dimensionless skeleton line angle (y axis), with the upper and lower limiting curve of the skeleton line angle distribution being defined by the equations cited below.
The dimensionless skeleton line angle is established by the relation also cited.
Provided that the skeleton lines, or the respective skeleton line angles, in the gap-near blade profile sections lie within the limits established by the limiting curves, disturbances and losses caused by the gap are significantly reduced. The form of the skeleton lines according to the present invention is not limited to specific courses of leading edges of the blades.
The present invention is more fully described in the light of the accompanying drawings showing a preferred embodiment. In the drawings,
The skeleton line 4 in
α(l)=(αi(l)−BIA)/(BOA−BIA),
In an area which starts at the blade tip 6 and includes approx. 30 percent of the blade height “h” (
The skeleton line angle αoG for a multitude of values lx between 0 and 100 percent, i.e. lx1, lx2 etc of the chord length “l” is established for the upper limiting curve 7 from:
αoG=1.2893686702647×10−9×lx5−
3.17452341597451×10−7×lx4+
0.0000293283473623007×lx3−
0.00129356647808443×lx2+
0.0345950133223312×lx
and for the lower limiting curve 8 from:
αuG=3.97581923552676×10−11×lx6−
1.02257586096638×10−8×lx5+
9.81093271630595×10−7×lx4−
0.000042865320363461×lx3+
0.00082697833059342×lx2−
0.000113440630116202×lx.
An example of a rotor blade of a high-speed compressor is shown with respect to
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