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.