The effect of reaction on compressor performance
Reaction is the fundamental parameter by which the asymmetry of the velocity triangle of a stage is set. Little is understood about the effect that reaction has on either the efficiency or the operating range of a compressor. A particular difficulty in understanding the effect of reaction is that the rotor and stator have a natural asymmetry caused by the centrifugal effects in the rotor boundary layer, being much larger than those in the stator boundary layer.
In the thesis a novel approach has been taken: McKenzie’s ‘linear repeating stage’ concept is used to remove the centrifugal force effects. The centrifugal effects are then reintroduced as a body force. This allows the velocity triangle effect and centrifugal effect to be decoupled. The ability to accurately decouple these two asymmetries has led to a number of major findings.
The thesis shows the surprising result that, depending on how the solidity of the stage is set, 50% reaction can either result in the maximum, or the minimum, profile loss. When the solidity is set by the shape factor of the suction-surface boundary layer at the blade trailing-edge, and conventional levels of design work coefficient (Ψ=0.44) and flow coefficient (Φ=0.60) are set, the profile loss becomes independent of reaction.
When the centrifugal effects are removed, 50% reaction is shown to minimise endwall loss, maximise stage efficiency and maximise operating range. When the centrifugal effects are reintroduced, the compressor with the maximum design efficiency is found to rise in reaction by 5% (from 50% reaction to 55% reaction) and the compressor with the maximum operating range is found to rise in reaction by 15% (from 50% reaction to 65% reaction).
In a real multistage compressor there is often a requirement for axial flow at the inlet and exit the compressor. This naturally results in high reaction. In the central stages of the compressor, it is possible to maximise the stage efficiency by reducing the reaction to 55%. This is done by raising the interstage swirl through the first stage and dropping it through the last stage. It is shown that if a 10 stage compressor, which originally had a constant stage reaction of 75%, was rebladed so that the central 8 stages had 55% reaction, then the overall design efficiency would rise by 0.58%.