Analysis of the Experimental and CFD-Based Theoretical Methods for Studying Rotordynamic Characteristics of Labyrinth Gas Seals

Abstract

This paper presents an analysis of the experimental and theoretical methods used to study rotordynamic characteristics of short staggered labyrinth gas seal. Two experimental identification procedures referred to as static and dynamic methods are presented. The static method allows determining direct and cross-coupled stiffness coefficients of the seal by integrating measured circumferential pressure distribution in cavities at various shaft eccentric positions. In the dynamic method, identification of stiffness and damping coefficients is based on the rotor excitation using a magnetic actuator and utilizes the effect of alternation of rotor vibrations due to aerodynamic forces acting in the seal. The experimental results obtained by the static and dynamic methods demonstrate an apparent discrepancy most of all in the direct stiffness coefficients. A CFD-based model of the seal is used to predict rotordynamic coefficients and to analyze the discrepancies between the static and dynamic measurements. The seal forces are calculated in two ways similar to the experimental procedures. The predictions are in good agreement with experimental results obtained by both measurement techniques. The effects of pressure differential, inlet swirl, shaft rotational speed, shaft eccentricity, and inflow cavity on seal stiffness and damping are presented. The discrepancies between different methods must be kept in mind while studying rotordynamic characteristics of seals.

Prediction of Rotordynamic Coefficients for Short Labyrinth Gas Seals Using Computational Fluid Dynamics

“…along the seal (see Refs. [1,8]). Seal regions with high eccentricity ratio Ô = els (where s is the cavity height) dominate the formation of the radial force, i.e., direct stiffness.…”

“…Secondly, experimental and theoretical analysis presented in Ref. [1] showed that the difference in global and local cross-coupled stiffness coefficients was not significant for the SSS seal with the radial clearance of 0.27 mm, only the direct stiffness demonstrated large discrepancy between two methods. The values of local and global damping coefficients were also in good agreement.…”

“…The default length of the downstream region L"'' = 0.03 m is taken from the analysis presented in Ref. [1] for the SSS seal with 0.27 mm clearance where the global direct stiffness coefficients were in good agreement with the measurements.…”

“…4. The predicted local coefficients (in previous publications they were called static coefficients) correspond to the direct and cross-coupled coefficients identified on the no-whirl test rig [1]. The experimental stiffness coefficients are calculated by integrating the measured pressure distribution along the circumference.…”

“…Under the assumption of small shaft eccentricity, a twodimensional simulation would suffice to predict mass flow rate through the seal accurately. However, the prediction of rotordynamic coefficients remains a demanding and challenging task for gas seals [1]. Especially, the calculation of damping entails great difficulties because of its small magnitude which is typical for short gas seals.…”