A Simple Inverse Cascade Design Method

Abstract

The design of profile sections is a key task in the axial turbomachinery design process. Both direct and inverse methods are in use for this purpose. Though it is relatively easy to fulfill mechanical and geometrical constraints with a direct method, where the complete geometry is specified, it can be difficult to obtain the desired distribution of velocity along the profile with this method. On the other hand, it can be difficult to get an acceptable geometry with an inverse method, where the velocity is prescribed and the geometry calculated. A simple inverse cascade design method, which combines the advantages of both the direct and the traditional inverse design method, is described. The theory behind this method directly shows how the prescribed loading distribution and the cascade design data, such as flow angles and blade thickness, influence the cascade geometry. Various practical aspects of the turbomachinery cascade design task and their relation to the described method are shown. The usefulness of the method is demonstrated through the recovery of the geometry of various cascades published in the literature. The method is valid for subsonic and shock free transonic turbine and compressor cascade design, but good results can also be obtained for turbine cascades with weak shocks, i.e. when shocks are not the dominating feature of the design.