PSI - Issue 50

Diana D. Popova et al. / Procedia Structural Integrity 50 (2023) 236–250 Popova, Popov, Samoylenko/ Structural Integrity Procedia 00 (2022) 000 – 000

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1. Introduction One of the ways to increase the gas turbine engines (GTE) efficiency is to reduce the tip clearances (TC) in the turbine. Therefore, for example, the decrease in the efficiency of a stage due to the presence of leakage through the TC can reach more than 2%, and the secondary losses can be up to 25-75% of the profile losses (Venediktov V.D. 2010). It is also known that for a turbofan engine with a high bypass ratio, a change in TC by 10 thousandths of an inch (≈ 0.25% in relative terms) corresponds to a change in turbine efficiency by about 1% and a change in gas temperature behind the turbine by 10 degrees (Kratz J., & Chapman J.W. 2018). The increase in losses in the blade ring leads to a significant increase in fuel consumption and a decrease in engine life. In addition, more than 85% of the deterioration in the fuel efficiency of the engine during its operation is a consequence of the TC increase and only 15% is the result of wear of the blade profiles (YAkovleva S.YU. 2016). Therefore, the modeling of processes in the TC and the study of their influence on the efficiency of the working process in the turbine is a rather important aspect in improving the gas turbine engine efficiency. In most papers (You Donghyun et al. 2002), (Yan S et al. 2022), where modeling of the gas flow in the TC region is considered, the assumption is made that the TC is uniform, that is, its value is the same along the inlet and outlet edges (in the axial and circumferential directions). Also, in works related to the determination of the TC value (YAkovleva S.YU. 2016), (Chapman J.W. 2016), (Kai Peng et al. 2013), the calculation is carried out according to one-dimensional models, as a result of which the TC value is also averaged and does not vary in the circumferential and axial directions. One-dimensional models allow you to make the calculation less time-consuming. When using two-dimensional and three-dimensional models, the calculation of non-stationary TC values for a typical flight cycle (TFC) involves hydraulic calculations by modes and non-stationary thermal calculations, followed by determination of the stress-strain state (SSS) (Bondarchuk P.V., Tisarev A.U. & Lavruchin M.V. 2012). It requires a significant amount of time and computational resources, since the TFC of modern engines of mainline aircraft have a duration of more than 1.5 hours and have about 20 operating modes. However, the determination of the actual shape of the TC with subsequent modeling of the gas flow in the region of the periphery of the rotor blade (RB) will allow us to estimate the real losses from gas overflow and the formation of secondary flows in the region of the RB end (Piskunov S.E., Popov D.A., & Samojlenko N.A. 2020), as well as to estimate the error from the use of RB models with a constant value in gas-dynamic calculations TC value on the butt. The purpose of this article is to simulate the gas flow in the region of the RB end face with a TC real shape, where its value will not be uniform over the entire end of the blade, and to compare this model with the case of a flow around with an ideal TC uniform. To do this, it is necessary to calculate the thermal state and displacements of the parts involved in the formation of the TC dimensional chain. For parts that are a body of revolution (discs, turbine housing), an axisymmetric two-dimensional calculation is sufficient, for example, for a disk rim, displacements in the circumferential direction are the same and change only along the turbine axis. It is necessary to perform a three-dimensional calculation for RB, since it is a rod element of a complex shape and displacements at the end can differ both in the circumferential and in the axial directions. The evaluation of efficiency is carried out generally in cruising mode, since it is the longest steady state. It is also possible to evaluate the efficiency of the turbine stage in the takeoff mode (Lapshin K.L. 2017), where the highest power and specific fuel consumption take place. The relevance of evaluating the efficiency in takeoff mode is also supported by the fact that the operation of modern active clearance control system goes beyond the limits of the longest cruising mode (Samojlenko N.A., Popova D.D & Popov D.A. 2021), however, due to the short duration of the mode, the TC maximum unevenness can change significantly over time. The TC real shape in takeoff mode cannot be assessed with a stationary calculation of the thermal state of the displacement of parts, since only the blades have an almost instantaneous heating rate, and the rotor and stator parts are more massive and do not have time to warm up completely in a short takeoff mode. Therefore, to assess the TC real shape in the "cruising" and "takeoff" modes, it is necessary to have non-stationary displacements of the rotor and stator, as well as stationary displacements of the blade in these modes. The determination of the blade displacements is preceded by a three-dimensional conjugate gas-dynamic calculation; to determine the displacements of the rotor and stator, a non-stationary thermal-hydraulic calculation is required.