Visualization of Destabilization Force of Labyrinth Gas Seal Using Fast-Responding Pressure Sensitive Paint
Authors: Nishida Shingo , Iwasaki Makoto , Omura Naoto , Nakakita Kazuyuki , Nakajima Tsutomu 
Published: September 2021

Abstract
Labyrinth seals are used in the clearance between rotor and stator such as steam turbines and centrifugal compressors in order to suppress loss of leakage. The leakage flow is induced in labyrinth seal by pressure difference between the inlet and outlet of the seal. This leakage flow produces destabilization fluid force on the rotor in association with the swirl flow. Destabilization fluid force can lead the rotor system unstable by its phase delay from subsynchronous whirling vibration of the rotor. Recently, this kind of problems have been reported sometimes with increasing power output and improving performance by decreasing leakage flow. In order to understand detailed characteristics of this destabilization fluid force, so many CFD calculations have been performed and unsteady pressure measurements have been conducted using rotational test rig. However, CFD calculation results often do not agree with test result especially for complex seal configurations. Although unsteady pressure was also measured by pressure sensors, measurement results were not enough for validation of CFD results and detailed understand of the fluid force mechanisms because of insufficient spatial resolution. In this study, unsteady pressure filed in labyrinth seal cavity of rotational test rig was visualized using fast-responding polymer-ceramic pressure sensitive paint (PC-PSP). The unsteady pressure distribution on the surface of seal cavity was measured by high-speed camera. For the experiment, a 500mm diameter rotor was used so that size of the labyrinth seal and flow pattern in the cavity can be close to the large-scale steam turbine. And the rotor was excited in a circular whirl orbit by an electromagnetic actuator. The stator side cavity wall was made by transparent material in order to obtain optical access. In the case of existing PSP measurements, only rotor side pressure distribution could be measured in this set up. However, measuring rotor side surface by PSP is extremely challenging because of its high peripheral speed. The back side illumination measurement technique has been developed and applied in this study. PSP is coated on the inner surface of transparent stator wall. Excitation light sources were delivered from outside of the transparent wall and highspeed camera was also installed outside. This procedure allows PSP measurement only by processing motionless image of the stator wall. Furthermore, composition of the PC-PSP has been optimized for this measurement because back illumination procedure decreases response speed of the PSP. As a result, high-accuracy, high-definition pressure distribution data was obtained. Pressure data measured by PSP were compared with pressure sensor data and CFD calculation results. Unsteady pressure levels measured by PSP agreed well with pressure data measured by pressure sensors. Unsteady pressure distributions measured by PSP agreed well with pressure distribution calculated by CFD. From these results, detailed characteristics of destabilization fluid force were understood. And these data can be used for brush-up of CFD calculation model and procedure.