Rotating stall and flow instability

Compressor Rotating Stall Inception

One of the most challenging problems in turbomachinery flow predictions is stall & surge, typically for compression systems (compressors/fans). There have been many research questions attempted by a large number of papers in the last 20 years or so,  ranging from the length scales/the driving mechanisms during stall inception (e.g. the short scale ‘spike’ vs the long scale ‘modal wave’), from the role played by rotor tip clearance to what would be an ‘acceptable’ annulus sector as a truncated sector domain of computational analyses, and the ‘best’ turbulence modelling.  Our early work was mainly motivated by a qualitative understanding of the possible driving mechanisms by asking ‘what can and should happen when using a CFD model with a lower level fidelity’. This early piece of work and its findings now turn out to be much more relevant, in particular in relation to tip clearance flow, which had been widely regarded to be a key driver in stall inception in the past.

An example of compressor rotating stall inception analysis is given below. The 2D URANS (no tip clearance) is compared to a well-known experimental test data from a rotor rig (3D with tip clearance, of course). These early results and comparisons indicate that tip clearance might not be as a crucial driver for stall inception as might be believed.      

 rotating stall1.png

Some further calculations for a rotor row in a 3D multi-passage domain (10 passages, He and Ismael, 1999) had predicted part span 3D stall inception. Again the tip clearance effect was not observed to be very crucial in affecting the inception process. Instead, a major contrast between a subsonic and transonic flows was observed. In a subsonic flow, stall inception was relatively easily detected, whilst in a transonic flow the rotor tended to go to surge without developing into an easily detectable rotating flow pattern first.

 

References:

- L. He, "Computational Study of Rotating Stall Inception in Axial Compressors",  AIAA Journal of Propulsion and Power, Vol.13, No.1, pp31-38,1997;

- L. He  and J. O. Ismael, "Computation of Blade Row Stall Inception in Transonic Flows",   Aeronautical Journal, Vol.103, No.1025, pp.317-324, July, 1999.

 

Steam Turbine Rotating Instability

Rotating stall and rotating instabilities have been long associated with compressors  and fans.  It may sound strange that a rotating  flow instability can happen in a turbine, but it does!

A steam turbine sometimes needs to operate at a very low mass flow condition. The situation becomes more and more common, when flexible operations of steam turbines are increasingly required, e.g. in  a combined gas/steam turbine power plant, or in a responsive mode to top up often un-regular renewable generators. Recently some new experimental study has been carried out at University of Stuttgart to reveal that rotating instability happens in a near-tip region of last stage long rotor blades. The CFD prediction at Oxford (Zhang et al, 2010) has produced a similar base flow pattern (below), and rotating instability (thought to be the 1st of the kind CFD prediction of turbine rotating instability). Once again, the results do not indicate that tip clearance is the driving factor.

 rotating instability1.png

 

References

-L.Y. Zhang, L. He, and H Stueer, “A Numerical Investigation of Rotating Instability in Steam Turbine Last Stage”, ASME Paper GT2011-36073, also ASME Journal of Turbomachinery,  Vol.135, 2013.

-L.Y. Zhang,  L. He, and  H Stueer, “3-D Time Domain Unsteady Computation of Rotating Instability In Steam Turbine Last Stage”,  ASME Paper GT2012-69045.