Measuring and simulation of fluid forces in annular gaps – Generic experiments cover-ing the relevant parameter range for turbulent and laminar flow in pumps

The reliability and performance of turbomachines like feedwater pumps with a shaft power up to 5 MW is often limited by shaft vibrations. These vibrations are strongly influenced by the forces induced by the laminar or turbulent fluid flow in (i) annular seals, (ii) pistons, and (iii) journal bearings. In general, the annular gap flow is three dimensional: the circumferential flow driven by to viscous forces is superimposed by a pressure driven axial flow. In addition, this axial flow convects swirl into the annular gap.

So far, there is a severe lack of understanding this flow. I.e. the state of the art simulation methods fail in reliably predicting the induced fluid forces. To fill this knowledge gap two similar test rigs are designed, build, and now operated at the TU Darmstadt. The generic experiments cover the relevant parameter range for turbulent and laminar flow in pumps.

Essentially consisting of two radial magnetic bearings for force measurement, excitation and displacement of the rotor the test rigs allow an adjustable flow number (ratio of axial flow velocity to speed of the rotor) in the range of 0 to 1.6. The ratio of circumferential flow velocity to the speed of the rotor at the inlet of the annulus is also controllable up to 1.4. Additionally the modular design allows relative gap heights in the range of 1 to 10 per mill as well as relative eccentricities up to 0.95. In order to reduce the measurement uncertainties through the use of mechanical seals, the friction forces generated during rotor excitation are systematically measured and the forces, obtained by the magnetic bearings, are corrected.

The paper closes by comparing the experimental results to the TU Darmstadt simulation method CAPM and state of the art calculation methods like the turbulent Reynolds equation.