Competing Three-Dimensional Mechanisms in Compressor Flows

Three-dimensional design is central to all modern compressor design systems, but many of these methods still rely on a two-dimensional and sectional view of aerodynamics at their core. This paper argues that this view fundamentally limits design by not considering the effect, on separation and loss, of the pressure gradient on the surface of the blade perpendicular to the meridional direction, here known as the transverse pressure gradient. The first part of the paper details how altering the transverse pressure gradient, by changing a blade's 3D stacking, switches the way in which the blade aerodynamically “fails,” from a open corner separation to a trailing edge separation. It also shows how the transverse pressure gradient significantly changes the blade profile loss. In the second part, the effect of the transverse pressure gradient on the uncertainty inherent in the compressor design space is investigated. It is shown that as blade pitch–chord ratio is raised and the amount of 3D stacking is lowered, the uncertainty of predicting a compressor's operating range is significantly raised. By increasing 3D stacking and the strength of the transverse pressure gradient, it is shown that this uncertainty can be significantly reduced.