Landroger
Good to see you here again Roger, I'll try my best to give you my take on rotating stall. (I worked very closely with Rolls Royce in the Concorde days, and everything I know about the process is thanks to them). Turbine engine combustion is a precise and delicate affair, particularly during start, and too much or too little fuel can cause severe problems. With rotating stall, the rotating cells of stalled air. if they manage to take 'hold' can cyclically choke the flow into the latter compressor stages (it's the cyclic nature of the cells that is the real problem, hence the 'rotating' stall term). The cells as they 'hit' the compressor blades (the cells are rotating at half shaft speed in the opposite direction of shaft rotation) can cause blade vibration and can also cause minor surges within the engine. The combustion fire literally can burn in the turbine section, but any distortion to the combustion process will result in local overheating, due to poor air/fuel mixing etc. In some engine types, damage can be also be caused to the HP compressor blades (due to vibration) but with the Olympus the main danger was to the turbine blades and stators. It's difficult to relate to any common analogy for this lot I'm afraid Roger.
Rotating stall was avoided in the Olympus by starting the engine with the primary nozzle driven wide open, and controlling two parameters; those being the opening rate of the fuel valve and the rate of rise of exhaust gas temperature. (During the start sequence, once ignition had occured the EGT rise was held to 6 degrees per second, right up until rotating stall clearance at 65% temperature corrected N2 ). So the engine accelerates without let or hinderance right through the danger zone, but was prevented from dipping below 65% temperature corrected N2, where the danger zone starts again. (Absolute minimal idle for the Olympus 593 was set at 61% N2).
I sincerely hope this blurb helps Roger, if not then feel free to ask again or PM me.
Regards
Dude
