Hole-Pattern and Honeycomb Seal Rotordynamic Forces: Validation of CFD-Based Prediction Techniques

Abstract

This paper deals with modeling of hole-pattern and honeycomb seals. These are frequently used as balance piston seals in high pressure centrifugal compressor applications as they have the potential to facilitate superior rotordynamic damping characteristics while providing good leakage control. On the other hand it is also well-established that the rotordynamic performance of hole-pattern and honeycomb seals is very sensitive to convergence and divergence in the streamwise direction. The ISOTSEAL bulk-flow code has shown difficulties in predicting the rotordynamic coefficients for convergent seal geometries or in cases with negative preswirl. This has led to increased interest in CFD-based analysis of seal dynamics. CFD-based models generally have less assumptions and are applicable for complex geometries or operating ranges not covered by bulk-flow codes. The CFD-based Instationary Perturbation Model (IPM) is utilized for the analysis of the hole-pattern and honeycomb seals. The rotordynamic forces are obtained by means of a time-dependent perturbation of the rotor position with respect to the stator. A sequence of perturbation frequencies is utilized to obtain the frequency dependence of the rotordynamic seal force coefficients. A strong effort has been put into validating the CFD-based perturbation modeling techniques against published experimental seal test data and the paper describes selected validation cases. A constant-clearance hole-pattern seal and a convergent honeycomb seal are analyzed and the results are compared to experimental results. The frequency dependence of the rotordynamic stiffness and damping characteristics of the seals is very well-captured for both types of seals.Finally, the IPM method was applied to a convergent hole-pattern seal to investigate the effects of eccentricity on the rotordynamic coefficients. The results are consistent with available experimental data.

Design of robust AMB controllers for rotors subjected to varying and uncertain seal forces

“…The stiffness and damping coefficients are shown in Fig. 3 & 4 and are found using CFD, ISOTSEAL and experimental work originating from Turbolab (Nielsen et al, 2012;Dawson et al, 2002). The estimated model uncertainty between the CFD and experimental results is marked as the grey area in the figures, and this information is utilized when synthesizing robust controllers.…”

“…All Lauridsen and Santos, Mechanical Engineering Journal, Vol.4, No.5 (2017) (Nielsen et al, 2012) 8 coefficients are considered uncertain independent to each other, i.e. although, for example, the direct coupled stiffness K has the same nominal value in both vertical and horizontal directions they have independent uncertainties.…”

“…The coefficients are a function of the rotational speed and the excitation frequency. The seal used in the turboexpander application is a hole-pattern seal with coefficients taken from (Nielsen et al, 2012). The coefficients are given for a constant rotational speed of 20,200 RPM and with excitation frequencies varying from 20-300 Hz, and the specifications are stated in Fig.…”

“…A number of publications have been presented focusing on the mathematical description of seal dynamics using either CFD or empirically-based Bulkflow models, and it has been shown that for seals under well defined single phase conditions a reasonable match between theoretical and experimental results can be achieved (Nielsen et al, 2012). However, seal dynamics under multiphase conditions, i.e.…”

“…although, for example, the direct coupled stiffness K has the same nominal value in both vertical and horizontal directions they have independent uncertainties. Mass coefficients are neglected since the fluid is air (Nielsen et al, 2012). …”