TABLE OF CONTENTS As the name implies, a centrifugal compressor achieves part of the compression process by causing the fluid to move outward in the centrifugal force field produced by the rotation of the impeller. This part of the pressure rise differs from the pressure rise in axial-flow compressor rotors and stators; instead of arising from the exchange of kinetic energy for thermal energy in a diffusion process, it arises from the change in potential energy of the fluid in the centrifugal force field of the rotor. It is therefore less limited by the problems of boundary-layer growth and separation in adverse pressure gradients. Probably for this reason, the centrifugal compressor was the first type to attain a range of pressure ratio and efficiency useful for turbojet engines. It was used in the Von Ohain engine (1939) and the Whittle engine (1941).
An impeller (rotor) for a centrifugal compressor is sketched in figure 5.38. The air enters through the "eye" near the axis, is turned to the radial direction, and is brought to a tangential velocity near that of the rotor by the time it reaches the rotor tip. The essential feature is that all the fluid leaves the rotor at the tip radius, rather than over a range of radii as in the axial-flow compressor. If the flow Mach number were small in the impeller passages and the air had the tangential velocity of the rotor at all radii as it flowed outward, the pressure gradient in the radial...
