Session: 23-11, Labyrinth Seals

Paper Number: 124505

124505 - Effect of Frequency Characteristics and Swirl Brakes on Fluid Destabilization Force Acting on Compressor Closed Impeller Leakage Path 

In turbomachinery, the trend toward higher performance, smaller size, and higher speed has led to a decrease in the rigidity of the rotor system and an increase in the pressure and flow velocity, which in turn has increased the fluid destabilization force and increased the risk of vibration problems.
In compressors and pumps, closed impellers with the impeller covered by a front shroud are often used, and labyrinth seals are generally installed in the leakage path to reduce leakage flow. The fluid destabilization forces generated at the front shroud and labyrinth seal components have been studied for many years, and it has been confirmed that an increase in the circumferential velocity (swirl) at the inlet causes instability. However, there are few measured data on the frequency characteristics of the fluid destabilization force in a system-level test combining a front shroud and a labyrinth seal.
In this paper, dynamic characteristic tests were conducted using air as the working fluid in a closed impeller single-stage test rig for the following purposes.
(1) To understand the dynamic characteristics, including frequency characteristics, of the fluid destabilization force acting on the closed impeller leakage path.
(2) Confirm the effectiveness of the swirl brakes installed in the front shroud to reduce the fluid destabilization force.
(3) Validate the CFD analysis method as a method for predicting the fluid destabilization force.
In the closed impeller single-stage dynamic characteristic test, the rotor was excited with a hydraulic shaker while the 381mm diameter impeller was rotating at a maximum of 11,000 rpm. The frequency characteristics of the fluid destabilization force were obtained over a practical frequency range by changing the direction and frequency of excitation. From the measurement results, it was confirmed that pressure fluctuations of the same order of magnitude as those of a labyrinth seal were generated in the front shroud, where the flow path height is greatly larger than labyrinth seal clearance. Since the front shroud has a larger pressure receiving area than the labyrinth seal, it was confirmed that the front shroud has a very large effect on the fluid destabilization force in the closed impeller leakage path.
As a device to reduce the fluid destabilization force in the closed impeller leakage path, this paper proposes the installation of the swirl brakes on the front shroud instead of at the labyrinth seal inlet. To validate the effectiveness of the swirl brakes, dynamic tests were conducted without and with the swirl brakes. Comparative test results showed that the swirl brakes can significantly reduce the fluid destabilization force in a closed impeller leakage path.
Transient CFD analysis was conducted as a method for predicting the fluid destabilization force, and the prediction accuracy of the CFD analysis method was confirmed by comparing the CFD analysis results with the measured fluid destabilization force. Although the CFD analysis prediction results were slightly larger than the measured fluid excitation force and evaluated conservatively, it was confirmed that the CFD analysis could approximately reproduce the measured results.
Furthermore, by visualizing a portion of the leakage path with glass, the areal distribution of pressure fluctuations acting on the closed impeller leakage path, including the front shroud, was measured using Pressure-Sensitive Paint (PSP). The visualization test results also confirmed that significant pressure fluctuations were generated not only at the labyrinth seal but also at the front shroud, thus confirming the validity of the CFD analysis method.

Presenting Author: Makoto Iwasaki Mitsubishi Heavy Industries, Ltd.

Presenting Author Biography: Makoto Iwasaki is a Research Engineer at the Vibration Research department of the Research & Innovation Center, Mitsubishi Heavy Industries, Ltd. (MHI)
He has B.S and M.S. degrees in Mechanical Engineering from Kyushu University in Japan, and has pursued research in rotordynamicsand FIV (Fluid Induced Vibration) for 12 years at MHI.

Authors:

Makoto Iwasaki Mitsubishi Heavy Industries, Ltd.
Shingo Nishida Mitsubishi Heavy Industries, Ltd.
Shuichi Yamashita Mitsubishi Heavy Industries, Ltd.
Takashi Oda Mitsubishi Heavy Industries Compressor, Ltd.