TABLE OF CONTENTS High-pressure-ratio multi-stage compressors have been developed by placing several stages, each composed of a rotor and a stator, in series on the same shaft. The requirements that the successive stages have the same rotative speed and pass the same mass flow lead to some difficult problems at off-design operating conditions. To see how these come about, note first that as the pressure builds up through each successive stage the flow area must be reduced if the axial flow velocity is to remain about the same fraction of blade speed throughout the several stages. The area reduction can be accomplished by increasing the hub radius, decreasing the tip radius, or both. Both have advantages and disadvantages. Decreasing the tip radius lowers the blade tangential Mach number, adding to the decrease due to rising temperature of the air, and so lowers the pressure ratio of the downstream stages. On the other hand, tip clearance losses can be excessive in the last stages if only the hub radius is tapered, because the blade heights become small. Stresses in the discs supporting the blades also increase.
In any case, the reduction in flow area from front to rear stages, if correct at the design point where the compressor has a large pressure ratio, will not be correct at lower speeds where the pressure ratio is lower. Relative to the center stages, the front stages will have too large a flow area and the rear stages too small a flow area; the...
