Experimental Analysis of Blade-Casing Contacts in a Centrifugal Compressor: Vibration and Thermal Aspects

Abstract

Due to an increasing need for efficiency of turbo-engines, rotor–stator clearances are being lowered. Therefore, new designs show higher probability for contacts between rotors and casings. When contacts occur, high dynamic excitation levels as well as high temperatures due to dissipative mechanical phenomena may be expected. While numerical investigations have been proposed in the past, experiments are of high interest to fully understand the underlying phenomena behind rotor–stator contact interactions. In order to assess this situation, and based on former work performed by part of the authors, a rotor–stator contact rig has been used to investigate the mechanical and thermal behavior of a centrifugal low-pressure helicopter engine compressor. This rig operates under vacuum conditions to significantly reduce influence of the air surrounding the studied components. A near-zero gap condition is set at rest, and then a rotational speed sweep allows to target the specific operating range of interest. Both structures are fitted with strain gauges (STGs), and a torquemeter is installed on the shaft to measure resistive phenomena on the bladed disk. A scanning laser Doppler vibrometer is aimed at the casing through a window to provide additional displacement measurements. Temperatures are measured by an array of thermocouples equally spaced around the casing, close to the expected contact area. Also, using temperature-sensitive markings, overall temperature mappings on the impeller are performed. During the tests, multiple contact phases have been identified through increased vibration and temperature levels, as well as torque and rotational speed variations. A comprehensive analysis of the dynamic and thermal phenomena occurring during these experimental tests is proposed in this paper. Dynamic measurements are analyzed in the time and frequency domains, and nodal diameter (ND) contents are evaluated as well through full spectrum analyses. As a result, major influences from synchronous excitations in the frequency range of interest but also of higher modal families are highlighted. Post-trial observations indicate severe contact conditions leading to very high temperatures, abradable coating removal, and material transfer between blade and casing.