-Some limitations of the classical Wall-Function approach for the near-wall boundary layer treatment in LES and URANS are presented for isothermal oscillating and pulsating channel flows. Despite their simple geometry, pulsating and oscillating flows are interesting unsteady flow test cases representative of many industrial components. A special attention will be focused on the unsteady wallshear stress prediction since it is also an indirect measure of the unsteady wall-heat transfer. A correct evaluation of the unsteady wall heat transfer is in fact critical, for example, in combustion chamber applications when flow unsteadiness due to flame instability occurs and in general in each unsteady flow situation where thermo-acoustic phenomena play an important role.

No abstract

Additive Manufacturing technology for innovative tilting pad thrust bearings can combine cleaner production process and performance improvement compared to traditional design. This new solution can also fit form and function of existing components in operation, opening to the possibility of a new kind of bearing upgrade service model. The new approach, allowing performance improvement, is based on conjugate heat transfer Computational Fluid Dynamic (CFD) simulation of the thrust bearing tilting pad sectors with oil supply definition. The solid pad has been simulated considering a series of micro-channels optimized to enhance pad temperature exchange with surrounding oil flow. Additive manufacturing has been used to realize the complex micro-channel geometry and the bearing has been tested back-to-back with a traditional design. This technology minimizes carbon footprint by reducing bearing size and associated oil flow consumption, at same/higher performance and reliability/availability. To contain compression system footprint, costs and increase efficiency the new rotor and bearing design has been pushed to higher load and speed. The new generation machines are facing new rotordynamic challenges and compressor OEMs are more and more moving towards rotor bearing integrated design and manufacturing. New manufacturing technologies like additive can help to face the new energy transition challenges and, in the near future, will play a key role. In the present work, a new manufacturing process has been developed leveraging the availability of in-house traditional bearing manufacturing line and new additive manufacturing technology labs. The material and design used allow for traditional babbitting and pad finishing whereas additive technology is opening new geometry boundaries. A first prototype has been tested back-to-back with the traditional design showing, at same load and speed, significant reduction of pad temperature up to 10 °C. The presented solution can fit form and function of existing components in operation opening to the possibility of a new kind of bearing upgrade service model able to enhance between maintenance bearing time or to reduce equipment footprint by increasing bearing specific load and reduce oil flow consumptions up to 20%-30%. The novelty of the present work was to validate enhanced CFD bearing simulations to unlock additive manufacturing potentials. This opens to the topological optimization of bearing geometry to enhance heat transfer and reduce bearing equipment footprint and oil consumptions. This is particularly suited for energy transition compressor applications.

The introduction of the tilting pad journal bearing (TPJB) technology has allowed the achievement of important goals regarding turbomachinery efficiency in terms of high peripheral speed, enhanced power density, higher efficiency, and tolerated loads. That kind of technology overcomes the typical dynamic instability problem that affects fixed geometry bearings but, under certain working conditions, can be subjected to thermal instability phenomena, which are particularly significant at high peripheral speeds. In this work, the authors propose an innovative iterative procedure to forecast the thermal instability onset by using two coupled models, a thermo-structural one and a fluid dynamic one. The first one calculates the vibrations and the deformations due both to the external forces and to the temperature distribution applied on the rotor. The fluid dynamic model calculates the temperature profile by using as inputs the characteristics of the rotor, of the bearing and of the orbits, obtained by the thermos-structural code. After a general description of the iterative procedure is given, details of each tool are provided. Code validation is presented by means of comparison with available experimental and numerical data. Finally, the results of the iterative procedure are shown to prove its potential in forecasting instability thresholds. The model has shown a good trade-off between accuracy and efficiency, which is very critical when dealing with the extended time windows characterizing thermal instabilities. This research activity is in cooperation with the industrial partner Baker Hughes, a GE company, which provided the experimental data obtained thorough a dedicated experimental campaign.

No abstract