On the Divergence Mechanisms Analysis of a Vibrating Blade subjected to Repetitive Rubs with an Abradable Coating

Aeronautical compressor fans currently operate with minimal blade-casing clearance. This therefore makes the occurrence of rub events very likely. Under specific circumstances, the blade undergoes excessive amplification of contact-induced oscillations, called hereafter divergence, which can be critical for the structural integrity of the engine. This paper proposes an investigation of the mechanisms responsible for the blade divergence. Experiments are conducted on a fully instrumented laboratory set-up, consisting of a single flat blade being moved towards a rotating cylinder to initiate interactions, while monitoring the vibrations and the evolution of wear on the abradable coating. Two synchronization mechanisms have been identified as facilitating the divergence: (i) the inherent set-up synchronization between the vibration modes related to the horizontal and vertical motion of the blade; (ii) the preferential blade-coating interactions in the vicinity of periodically distributed irregularities of the abradable coating which act as a source of excitation of the vibrations.

Simulation and Experimental Study on Rotor System Dynamic Analysis with the Blade-Coating Rubbing Faults

In the modern turbo-machinery, reducing the clearance between the blade tip and casing inner face is an effective method to improve the power performance, but the clearance reduction leads to increased risk of blade-casing rubbing. In this paper, a blade-coating rubbing force model which considered the abradable coating scraping is developed to simulate the rotor system dynamic characteristics at blade-casing rubbing faults with abradable coating. An experimental tester is established to simulate the rotor system blade-casing rubbing faults; the AlSi-ployphenyl ester abradable coating is prepared and introduced into the blade-casing experiment to verify the model. After the vibration and force analysis in simulation and experiment, the dynamic characteristics and the influence factors of blade-casing rubbing rotor system are studied.

Assessment of Geometric Nonlinearities Influence On NASA Rotor 37 Response to Blade Tip/Casing Rubbing Events

This paper uses a recently derived reduction procedure to study the contact interactions of an industrial blade undergoing large displacements. The reduction technique consists in projecting the dynamical problem onto a reduction basis composed of Craig-Bampton modes and a selection of their modal derivatives. The internal nonlinear forces due to large displacements are evaluated with the stiffness evaluation procedure and contact is numerically handled using Lagrange multipliers. The numerical strategy is applied on an open industrial compressor blade model based on the NASA rotor 37 blade in order to promote reproducibility of results. Two contact scenarios are investigated: one with direct contact between the blade and the casing and one with an abradable material deposited on the casing. The influence of geometric nonlinearities is assessed in both cases. In particular, contact interaction maps and abradable coating wear pattern maps are used to identify the main interactions that can be detrimental for the engine integrity.

Assessment of Geometric Nonlinearities Influence on NASA Rotor 37 Response to Blade Tip/Casing Rubbing Events

This paper uses a recently derived reduction procedure to study the contact interactions of an industrial blade undergoing large displacements. The reduction technique consists in projecting the dynamical problem onto a reduction basis composed of Craig-Bampton modes and a selection of their modal derivatives. The internal nonlinear forces due to large displacements are evaluated with the stiffness evaluation procedure and contact is numerically handled using Lagrange multipliers. The numerical strategy is applied on an open industrial compressor blade model based on the NASA rotor 37 blade in order to promote re-producibility of results. Two contact scenarios are investigated: one with direct contact between the blade and the casing and one with an abradable material deposited on the casing. The influence of geometric nonlinearities is assessed in both cases. In particular, contact interaction maps and abradable coating wear pattern maps are used to identify the main interactions that can be detrimental for the engine integrity.

Minimum clearance technology to improve performance of twin-screw refrigeration compressors by spraying coating on rotors

During the operating of twin-screw refrigeration compressors, the leakage has a significant influence on the performance of compressors. In this paper, a novel minimum clearance technology that applies the abradable coating to the rotor surface is proposed to lessen the leakage. In order to adapt to the application of the abradable coating, the deformation of rotors during operation is analyzed and a suitable clearance is designed based on the fluid-solid coupling analysis. Then, a suitable coating pre-treated by sandblasting is obtained according to the results of friction coefficient and abradability test. Finally, a prototype is tested and the total efficiency, volumetric efficiency, refrigerating capacity, and COP of the prototype are compared before and after spraying the coating. Test results show that the volumetric efficiency and refrigerating capacity increase significantly after spraying the abradable coating, and the input power decreases slightly. As a result, the COP increases by 6.71% and 7.89% at two nominal operating conditions.

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

Due to an increasing need for efficiency of turboengines, 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. 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 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.

Galvanic Corrosion Performance of an Al–BN Abradable Seal Coating System in Chloride Solution

In this study, we investigated the galvanic corrosion performance of an Aluminum–Boron Nitride (Al–BN) abradable seal coating system (with a Ni5Al bond layer and a 0Cr17Ni4Cu4Nb substrate) in chloride solution by electrochemical methods. The results indicated a three-stage process occurred during the anodic dissolution of the coupled coating system, consisting of a spontaneous pitting stage I under charge transfer control with a decreasing rate, a corrosion developing stage II under mass transfer control with an increasing rate, and a final steady stage III. Precipitation of Al(OH)3 restricts the oxygen transport process to the cathode and induces localized acidification of the occluded pores of the Al–BN layer, which was the mechanism that could explain the changes of corrosion performance during the three immersion stages of Al–BN coating system. The study suggests that galvanic corrosion of the porous multi-layer Al–BN abradable coating system is mostly influenced by its corrosion product deposition.

Implementation of a Rig Test for Rotor/Stator Interaction of Low-Pressure Compressor Blades and Comparison of Experimental Results With Numerical Model

To investigate the contact phenomenon between the blade tip and the abradable coated casing, a rig test was designed and built. This rig test fills the following constraints: simplification of the low-pressure compressor environment but realistic mechanical conditions, ability to test several designs in short time, at low cost and repeatability. The rig test gives the opportunity to investigate the behavior of different blade designs regarding the sought phenomenon, to refine and mature the phenomenon comprehension and to get data for the numerical tool validation. The numerical tool considers a 3D finite elements model of low-pressure compressor blades with a surrounding rigid casing combined with a specialized model to take into account the effects of the wear of the abradable coating on the blade dynamics. Numerical results are in good agreement with tests in terms of: critical angular speed, blade dynamics and wear pattern on the abradable coated casing.

Dynamic characteristics analysis of blade-casing rubbing faults with abradable coatings

In this paper, a dynamic model of a “0-2-1” rotor system with rubbing fault between blade and abradable coated casings is developed. The sub-model of rubbing force considers scraping work energy of coating, casing stiffness, and initial clearance between blade tip and casing. A rotor rig is established and samples of abradable coatings are introduced into the rubbing experiment. Vibration characteristics of the rotor system under blade-casing rubbing fault are analyzed. Effects of rotating speed and initial clearance on the rub force and the system vibration are studied. Results show that the vibration of rotor focuses on the fundamental and multiple fundamental frequencies due to the blade-casing rubbing with the abradable coating. The multiple fundamental frequencies, the 2 × and 3 × in particular, are greatly affected by the rotating speed. The fractional harmonic frequencies are strongly influenced by the initial clearance between the blade tip and casing. Besides, the rotating speed and the initial clearance between the blade tip and abradable coating on the casing also affect the amplitude and distribution of the rub force.