Three-dimensional Mechanisms in Compressor Flows

Abstract

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. In this thesis it is argued 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 results of this thesis are contained within four chapters. The first two investigate the role of the transverse pressure gradient in 3D compressor blades. The first presents a method for designing blades with variable transverse pressure gradient but constant spanwise loading distribution. The second shows the effect of the transverse pressure gradient on all of the flow mechanisms that govern a blade design’s incidence range and loss generation. Increasing the strength of the transverse pressure gradient is able to switch the failure mechanism from a sudden open corner separation to a progressive trailing edge separation. An increase in the strength of the transverse pressure gradient also increases the total loss generation. The increase in loss is caused by an increase in the mixing component of the profile loss downstream of the blade. The final two chapters investigate the combined roles of the transverse and streamwise pressure gradients. While the transverse pressure gradient can be increased with 3D stacking, the streamwise pressure gradient is increased with greater pitch-chord ratios. The first chapter presents the entire design spaces in terms of loss generation and incidence range of a 3D blade design. Trends across the design space are examined and a method for practically arriving at an optimal 3D design is presented. In the final results chapter the effect of the transverse pressure gradient on the uncertainty that is inherent in the design space is investigated. Two sources of uncertainty are investigated, the uncertainty caused by cumulative errors built up in CFD over upstream blade rows and the uncertainty in the accuracy of CFD predictions at high loadings. It is shown that as 3D stacking is reduced the uncertainty in predicting the compressor’s operating range is significantly raised. This is caused by the sensitivity of open corner separations to inlet flow condition, in CFD a small error in the inlet can cause the range to be over-predicted. It is also shown that as pitch-chord ratio is increased the uncertainty in predicting the loss and range using 3D CFD is also raised. In order to reduce the uncertainty of a 3D blade design it would be necessary to raise the level of 3D stacking and reduce the pitch-chord ratio. However, changing design in this manner is in the directly opposite direction to what is required to reduce the loss. In order to consolidate these two competing objectives it is recommended that experiments are required early on in the design process and that they have a rapid rate of testing.