A Comparison of Rotordynamic-Coefficient Predictions for Annular Honeycomb Gas Seals Using Three Different Friction-Factor Models

Abstract

A two-control-volume bulk-flow model is used to predict rotordynamic coefficients for an annular, honeycomb-stator/smooth-rotor gas seal. The bulk-flow model uses Hirs’ turbulent-lubrication model, which requires a friction factor model to define the shear stresses at the rotor and stator wall. Rotordynamic coefficients predictions are compared for the following three variations of the Blasius pipe-friction model: (i) a basic model where the Reynolds number is a linear function of the local clearance, fs=ns Rems (ii) a model where the coefficient is a function of the local clearance, and (iii) a model where both the coefficient and exponent are functions of the local clearance. The latter models are based on data that shows the friction factor increasing with increasing clearances. Rotordynamic-coefficient predictions shows that the friction-factor-model choice is important in predicting the effective-damping coefficients at a lower frequency range (60∼70 Hz) where industrial centrifugal compressors and steam turbines tend to become unstable. At a higher frequency range, irrespective of the friction-factor model, the rotordynamic-coefficient predictions tend to coincide. Blasius-based Models which directly account for the observed increase in stator friction factors with increasing clearance predict significantly lower values for the destabilizing cross-coupled stiffness coefficients.

Friction Factor Behavior From Flat-Plate Tests of Smooth and Hole-Pattern Roughened Surfaces With Supply Pressures up to 84 bars

“…12) can have a major impact on predictions of the direct and cross-coupled stiffness coefficients. D'Souza and Childs [13] produced a similar prediction for compressible flow.…”

Multi-output calibration of a honeycomb seal via on-site surrogates

“…The turbomachinery literature and bulk-flow theory (Hirs, 1973) provides some insight into the relationship between these four properties. For example, D'Souza and Childs (2002) demonstrate that classical transfer for a honeycomb gas seal process can be expressed in a conventional linear motion/reaction-force model (1993); Kleynhans and Childs (1997); Delgado et al (2012).…”

On the modelling of aerodynamic force coefficients for over-shroud seals of turbine stages

“…Mutual relationship between pressure force and displacement of rotor against casing is usually described by means of the so called "seal dynamic coeffcients" [1][2][3][4][5][6][7][8][12][13][14] which can be incorporated into whole model of turbine set during its designing process. Three kinds of the dynamic coefficients can be distinguished: stiffness, damping nad inertia ones.…”