Main flow and bypass flow interaction modeling in low pressure turbines for aeronautical application

In recent years Aerodynamic performances of Aero Engines have reached extremely high levels and are undergoing quasi-asymptotic trends, in which traditional design and analysis methodologies are no more capable of bringing substantial improvements. From this perspective, complex CFD simulations are nowadays normally performed with the aim of increasing the level of detail of physical knowledge about flows established in the most peripheral and chaotic regions of the machine, thus bringing to the design engineers additional awareness about the phenomena occurring at the boundaries of the well-known flowpath. Since almost all the possible optimization and studies about the airfoil geometry inside the flowpath has already been completed, the attention is moving outside flowpath, towards tip portions of the rotor blades, the Rotor Tip Cavity. Low pressure turbines are highly technological turbomachinery equipment, based on a series of stationary and rotating blade rows producing the power needed by mechanical utilizer (booster and front fan engine modules). Since stator and rotor assemblies have to be coupled, axial and radial clearances in between subsequent blade rows are unavoidable. Axial clearances contribute to improve the safety of the engine (but concur in increasing its axial dimensions and weight, so an optimal balance has to be found) while radial clearance are needed to avoid metallic contact (thus enormous friction) between rotor and stator. Because pressure drop across every turbine blade is positive (downstream directed) fluid flow will be forced through all existing gaps, and a leakage effect will be established. Even though leakage is counteracted with several technological strategies (clearance minimization, abradable honeycombs, active clearance control) bypass flow represent a potential menace for the aerodynamic performance of the module, being a loss of work extracted from the fluid and also having a detrimental mixing interaction with the mainflow. For this reason, rotor tip cavity regions are acquiring growing attention in aeronautical turbomachinery world, and advanced CFD (Computational Fluid Dynamics) solvers capabilities of calculation have been extended to a complete simulation of cavity flow field, allowing a growing insight on physics of bypass flows and their impact on turbine performance. Present Thesis work is articulated as follows: 1 – Preface: A general overview about the company and the department in which present work has been carried out, introducing the corporate framework and my personal experience about the work environment. 2 – The company: A brief description of Avio Aero activities with particular focus on turbomachinery sector 3 – The aviation business: a growing market: civil aviation market future trends are discussed, with the aim of broadening the breath of following analysis, adding substantiation to technological choices and related investments. 4 – The Aero engine: basic working principles of aero engines are described, with particular focus on performance and architecture. A state-of-the-art engine is finally brought in example. 5 – Principles of Turbomachinery: basic principles and equations governing turbomachinery physics and are discussed. 6 – Low Pressure Turbine: key aspects of Low-Pressure Turbines are introduced, with particular focus on design strategies and tools applied 7 – Clearance: composition and design: it is provided a general overview of clearance composition, in-service behaviour and design principles. 8 – CFD simulation: CFD is introduced in its theoretical and operative aspects, and finally case study is extensively presented.

studio di interazione tra flusso di bypass da RTC e flusso principale in turbine di passa pressione per applicazioni aeronautiche