The use of computational structural models that include geometrical non-linearity in many application cases may require high reliability in prediction of displacements. Nevertheless, large differences up to 60% on maximum total displacement have been found among results of static large-deformation analyses performed by means of the major commercial software packages in a simple benchmark study with linear material properties. In order to investigate the causes of such disagreement, the present work compares different finite element formulations including well-established stress update schemes. The various formulations are tested, and results are compared in three test cases. Rodriguez stress update algorithms have shown the best performance among methods reported in literature. Finally, the cause of the large differences found in the predictions of commercial codes is identified. It is linked to the energetic inconsistency of some stress update methods in the simulation of extension/compression loading conditions. Such inaccuracy is reproduced analytically by formulating and integrating the corresponding inconsistent constitutive equations. The identified problem is very important for designers, as it affects almost all the static simulations, which are the most common type of large-deformation analyses and usually involve extension/compression loading.
Dynamic compressors operating region is mainly constrained by fluid-dynamic instabilities occurring at low mass flow rate conditions, such as surge and rotating stall. This work presents a vibro-acoustic experimental investigation on a centrifugal compressor of an automotive turbocharger aimed to identify and confirm some surge precursor values in correspondence of its inception conditions. The experimental campaign was carried out on a turbocharger equipped by a vaneless diffuser compressor exploited for the pressurization of an innovative solid oxide fuel cell (SOFC) emulator. The investigated turbocharger is coupled with a pressure vessel for a former emulation activity on a pressurized SOFC. In such kind of plants, the joint effect of large volume size downstream the compressor makes more complex the dynamic behavior of the whole system during transients, thus significantly increasing surge onset risk. The main goal is to define a suitable quantitative indicator to detect in advance surge inception by relying only on system vibro-acoustic response. Several transient from a compressor stable condition to surge instability onset were performed by progressively closing specific valves in the air line. When moving close to surge , vibro-acoustic signals were acquired at a high sampling rate to investigate blade passage phenomena which might interact with rotating stall inception. Meanwhile, pressures, temperatures and mass flow rates measured at a lower sampling rate to correlate compressor vibro-acoustic and performance behavior. Cyclostationary analysis was performed on vibro-acoustic acquired signals to provide innovative diagnostic and predictive solutions (precursors) for early surge detection.
Compressor response investigation in nearly unstable operating conditions, like rotating stall and incipient surge, is a challenging topic nowadays in the turbomachinery research field. Indeed, turbines connected with large-size volumes are affected by critical issues related to surge prevention, particularly during transient operations. Advanced signal-processing operations conducted on vibrational responses provide an insight into possible diagnostic and predictive solutions which can be derived from accelerometer measurements. Indeed, vibrational investigation is largely employed in rotating-machine diagnostics together with time-frequency analysis such as smoothed pseudo-Wigner Ville (SPWVD) time-frequency distribution (TFD) considered in this paper. It is characterized by excellent time and frequency resolutions and thus it is effectively employed in numerous applications in the condition monitoring of machinery. The aim and the innovation of this work regards SPWVD utilization to study turbomachinery behavior in detail in order to identify incipient surge conditions in the centrifugal compressor starting from operational vibrational responses measured at significant plant locations. To this aim, an experimental campaign has been conducted on a T100 microturbine connected with different volume sizes to collect significant data to be analyzed. The results show that SPWVD is able to successfully identify system evolution towards an unstable condition, by recognizing different levels and features of the particular kind of instability that is going to take place within the plant. Instability phenomena regarding rolling bearings have also been identified and their interaction with surge onset has been investigated for diagnostic purposes.
The aim of the present work is to design a test rig suited to investigate the dynamic interaction between rotor and hydrodynamic journal bearings in micro gas turbines (microGT), i.e. with reference to small bearings (diameter in the order of ten millimeters). Particularly, the device is capable of measuring the journal location. Therefore, the journal motion due to rotor vibrations can be displayed, in order to assess performance as well as stiffness and damping of the bearings. The new test rig is based on Bently Nevada Rotor Kit (RK), but substantial modifications are carried out. Indeed, the relative radial clearance of the original RK bearings is about 2/100, while it is in the order of 1/1000 in industrial bearings. Therefore, the same RK bearings are employed in the new test rig, but a new shaft has been designed in order to reduce the original clearance. The new shaft enables us to study the bearing behaviour for different clearances, as it is equipped with interchangeable journals. The experimental data yielded by the new test rig are compared with numerical results. These are obtained by means of a suitable finite element (FEM) code developed by our research group. It allows the Thermo Elasto-HydroDynamic (TEHD) analysis of the bearing in static and dynamic conditions. In the present paper, bearing static performances are analysed in order to assess the reliability of the journal location predictions by comparing numerical and experimental results. Such comparisons are presented for both large and small clearance bearings of original and modified RK, respectively. Good agreement is found only for the modified RK equipped with small clearance bearings (relative radial clearance equal to 8/1000). Nevertheless, rotor alignment is quite difficult with small clearance bearings and a completely new test rig is designed for future experiments.
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