The Design of Highly Loaded Axial Compressors

Increasing compressor pressure ratios (thereby gaining a benefit in cycle efficiency), or reducing the number of stages (to reduce weight, cost, etc.), will require an increase in pressure rise per stage. One method of increasing the pressure rise per stage is by increasing the stage-loading coefficient and it is this topic which forms the focus of the present paper. In the past, a great deal of effort has been expended in trying to design highly loaded blade rows. Most of this work has focused on optimizing a particular design rather than looking at the fundamental problems associated with high loading. This paper looks at the flow physics behind the problem, makes proposals for a new design strategy and explains sources of additional loss specific to highly loaded designs. Detailed experimental measurements of three highly loaded stages (Δh0/U2 ≈ 0.65) have been used to validate a CFD code. The calibrated CFD has then been used to show that as the stage loading is increased the flow in the stator passages breaks down first. This happens via a large corner separation which significantly impairs the stage efficiency. The stator can be relieved by increasing stage reaction, thus shifting the burden to the rotor. Fortunately, the CFD calculations show that the rotor is generally more tolerant of high loading than the stator. Thus, when stage loading is increased, it is necessary to increase the reaction to achieve the optimum efficiency. However, the design exercise using the calibrated CFD also shows that the stage efficiency is inevitably reduced as the stage loading is increased (in agreement with the experimental results). In the second part of the paper, the role profile loss plays in the reduction in efficiency at high stage loading is considered. A simple generic velocity distribution is developed from first principles to demonstrate the hitherto neglected importance of the pressure surface losses in highly loaded compressors.