Towards the highest rates of compression, the operation of compressors is limited by the appearance of unsteady flows, responsible for a fall in performances and sometimes the destruction of the machine. Because the compression system's limit of stability is difficult to estimate, the compressor has to be over-dimensioned with a resulting increase in the machine's weight. Understanding these unsteady phenomena is a pre-condition for making lighter machines that accelerate quicker and pollute less.
The Turbomachinery team has been studying these unsteady phenomena in the framework of a thesis financed by Onera, the LMFA, Snecma, Turbomeca and EDF. The mechanisms that come into play in a subsonic compressor stage during the change to an unsteady state have been analyzed using numerical simulations (Quasi-3D and 3D). The case studied was the CME2 single stage axial compressor located at the Lemfi (Orsay, France). To check the calculations, the numerical results were compared with the measurements provided by Lemfi.
3D calculation of a Rotating Stall State
Figure 1 : instantaneous entropy field
(Three-cell rotating stall state)
[videos : cross-section and cylinder-section]
Rotating stall is a 3D phenomenon characterized by the presence of pockets of separated fluid that move at a slower speed than that of the rotor. This instability is responsible for serious vibrations that may lead to a breaking by fatigue of the machine's blades. Furthermore, the large losses caused by the presence of the cells may push the compressor towards a more violent unstable state: surge. Because of the unsteady 3D character of the instability, we are forced to use parallel computing techniques on a configuration of more than 30 million points (fig. 1). The simulation performed with the elsA software can be used to predict the appearance of this type of state and delimit, at least in part, the origins for the rotating stall.
Quasi-3D Calculation of a Surge State with Rotating Stall
Figure 3 : instantaneous entropy field
(one-cell modified surge state)
Depending on the machine's configuration, and in particular the size of the volume downstream of the compressor, the operating point may evolve towards rotating stall, surge or even to a state presenting the two instabilities: modified surge. Surging is a 1D instability which results in the appearance of a pressure wave that may lead to a total reversal of the direction of flow in the machine. This very high energy state generally leads to a rapid destruction of the compressor. A Quasi 3D code (Colibri code) applied to a configuration containing less than one million points can be used to simulate this type of state and evaluate its impact on the operation of the machine (fig. 3).
