Injection of working fluid into a centrifugal compressor in the reverse tangent direction has been invented as a way of preventing flow instabilities (stall and surge) or restoring stability when stall or surge has already commenced. If not suppressed, such instabilities interrupt the smooth flow of the working fluid and, in severe cases of surge, give rise to pressure and flow oscillations that can be strong enough to damage the compressor
and adjacent equipment.
The invention applies, in particular, to a centrifugal compressor, the diffuser of which contains vanes that divide the flow into channels oriented partly radially and partly tangentially. In reverse-tangent injection, a stream or jet of the working fluid (the fluid that is compressed) is injected into the vaneless annular region between the blades of the impeller and the vanes of the diffuser (see figure). As used here, "reverse" signifies that the injected flow opposes (and thereby reduces) the tangential component of the velocity of the impeller discharge. At the same time, the injected jet acts to increase the radial component of the velocity of the impeller discharge. The net effect is to turn the impeller discharge flow toward a more radial direction; in other words, to reduce the flow angle of fluid entering the vaned diffuser passage, thereby reducing diffusion ahead of the passage throat, reducing the pressure load and the incidence of flow on the leading edges of the vanes. The reduction of the flow angle also changes the dynamic coupling between the impeller and diffuser in such a way as to prevent the development of certain instability modes in the diffuser.
The number and distribution of reverse-tangent injectors can be tailored to match the expected stall/surge characteristics of the compressor and the space available for installation. Reverse-tangent injection can be implemented in any of three operating modes:
- Continuous operation, in which the working fluid is injected continuously;
- Open-loop operation, in which injection is initiated by on-off valves upon detection of compressor instability or conditions known to precede compressor instability and the injection is terminated when stable compressor operation returns; or
- Closed-loop or feedback-controlled operation, in which compressor stability is monitored and the number of active injectors and the injection rate are continually adjusted, using a dynamic control model and controlled valves.
In each of these operating modes, the injected working fluid can be supplied from an external source or from a downstream compressor stage where the total pressure is sufficient to produce the injected stream(s).
This work was done by Gary J. Skoch of the U.S. Army Research Laboratory for Glenn Research Center.
Inquiries concerning rights for the commercial use of this invention should be addressed to NASA Glenn Research Center, Commercial Technology Office, Attn: Steve Fedor, Mail Stop 4–8, 21000 Brookpark Road, Cleveland, Ohio 44135. Refer to LEW-17560-1.