The aero engine is a highly complex and sophisticated thermodynamic machinery that powers aircraft, and is known as the "heart" of the aircraft. The major components of the engine (compressor, combustion chamber, turbine, etc.) determine the "good" and "bad" of the engine, that is, whether it has high performance indicators; and the mechanical system of the engine determines the "ok" and "no" of the engine, that is, whether it can work safely and reliably.
Modern aero engines are moving towards high thrust-to-weight ratio, low fuel consumption, high reliability and long life, and strict parameter indicators put forward nearly demanding performance requirements for each part of the engine. The engine needs to withstand the test of high speed, high temperature and high load, and at the same time must meet the requirements of compact structure, light weight, high reliability and long life. Therefore, unlike the mechanical system of the general power unit, the mechanical system of the aero engine concentrates a large number of very sharp contradictions: such as high load and light weight, high temperature environment and low heat resistance working medium, complex structure and small space, harsh conditions and high reliability and long life, which also brings a large number of technical problems. And compared with the study of major components, another significant feature of the mechanical system is that it has a wide variety of parts, and the materials, structural characteristics and working principles of these parts are different, resulting in the mechanical system is actually an extremely complex research field involving many disciplines and intersecting and comprehensive.
In fact, the proportion of mechanical system failures in engine failures has always been high. In the early 1980s, the RB211 engine of the British Royce Royer Company had three consecutive serious accidents thrown out of the fan components, the reason was that the oil supply of the front roller bearings of the fan was insufficient, the lubricating oil was not cooled well, causing bearing damage, causing rotor deflection, tight wear, increased sealing clearance and oil spillage; the TF34 engine of the US Air Force had 90 accidents in 1983, of which 26 accidents were caused by mechanical system problems, accounting for about 28. 89%; the engine lubrication system failure rate of the JT3D model also reached 37. 95%。 In China, due to the one-sided placement of mechanical systems in a subordinate position, the basic research and development of key technologies has been ignored, and mechanical system failures have occurred frequently. This also reflects the importance of mechanical systems in the development of aero engines from another perspective.
Based on the importance of mechanical systems and the characteristics of a wide variety of parts and components involving many disciplines, countries with advanced aero-engine technology have set up special research departments for aero-engine mechanical systems very early on, and established corresponding research teams, such as NASA's power transmission laboratory in the United States, Pratt & Whitney's Development Department Bearing Sealing Division, and the air and oil system research team of German MTU Aero Engine Company. They have also set up a number of related research programs, from basic research all the way to applied research, investing heavily in a lot of money and gaining a wealth of experience. Starting from the "Tenth Five-Year Plan" in the mainland, each engine research institute has set up independent research departments for mechanical systems, and in several engine technology research programs, the research of mechanical systems has been raised to an important position.
Mechanical System Design of Aero Engines (edited by Liu Zhenxia et al. Beijing:Science Press,2022.] 1) For the four parts of the attachment transmission system, bearing application design, oil lubrication system and engine seal in the mechanical system of the aero engine, it is divided into four parts for system introduction. It is hoped that through the elaboration of each article, readers can grasp the current situation of related technology research and understand its future development trend.
▍The aero engine accessory transmission system mainly includes the central transmission device, the transmission rod, the accessory transmission receiver and the coupling. The main function of the accessory drive system is to transmit the torque of the starter to the engine spindle through the starting chain when starting, so as to start the engine, and extract power from the engine spindle during normal operation to drive the engine and aircraft accessories. For high-performance aero engines, the drivetrain needs to meet the requirements of light weight, long life and high reliability. The operating conditions of the gear are more demanding, and its working load, linear speed, working temperature, etc. are high, so the transmission system design must be optimized for comprehensive system matching (such as structural layout optimization, transmission chain optimization and system dynamic analysis, etc.), and a reasonable structure (such as integrated accessory casing), materials, and processes must be adopted.
The first part of the book is the aero engine accessory drivetrain, a total of 11 chapters. It mainly introduces the type and layout of the transmission system, the basic requirements or guidelines of the transmission system design, the design of the gear and support structure and the attachment receiver housing structure, the gear strength and vibration design, the transmission shaft and spline connection design, the transmission receiver housing coupling and clutch design, the accessory transmission system test and verification project and the common faults of the transmission system, the future development trend of the accessory transmission system, etc., and comprehensively expounds the development process and development direction of the transmission system.
▍The main function of the bearing is to support the engine rotor and the transmission shaft, guide the rotating parts to rotate precisely, transmit the load of the rotating parts, and provide good positioning accuracy and sufficient support rigidity for the rotor. According to the different parts of the bearing in the engine, the aircraft engine bearing is divided into main shaft bearing, accessory transmission bearing, reducer bearing and so on. With the continuous improvement of the thrust-to-weight ratio/ work-to-weight ratio of aero engines and the demand for life, higher requirements are put forward for the performance, life and reliability of bearings. For aero engine bearings, in addition to the optimization and improvement of the bearing itself in terms of design, materials, manufacturing, etc., the bearing application design is to meet the needs of the engine by reasonably selecting the bearing configuration, type and size, designing the appropriate installation and lubrication, and improving the adaptability of the bearing and speed, load and environmental conditions.
The second part of this book is designed for bearing applications, with 11 chapters, which only covers the design of bearing applications. Starting from the basic knowledge of commonly used bearings, the common bearing selection and configuration forms of aero engines, the preliminary design and analysis methods of bearings are introduced, especially the lubrication and cooling of bearings, bearing mounting, bearing integration, etc. are highlighted through application cases, and the main test items of bearings and bearing failure modes and preventive measures are also briefly introduced.
▍Lubricating oil system is an important part of the mechanical system of the aircraft engine, the main function is to supply lubricating oil to the engine, ensure the effective lubrication and cooling of mechanical parts such as bearings and gears, and take away the abrasive particles and dirt on the moving surface. For the vast majority of aero engines that are still based on mechanical transmission, under high temperature, high speed and high load conditions, the working environment of its spindle bearings, transmission gears and other parts is very harsh, and it must be equipped with a lubrication system to ensure the reliable operation of the engine. The lubricating oil system involves a wide variety of components, the installation is scattered, and its external pipelines and accessories are mostly distributed outside the engine receiver, and problems with any one component may affect the normal operation of the entire system. Western aviation developed countries have formulated a series of special research programs for engine lubrication systems, and many breakthrough results have been achieved, some of which have been applied to practice. With the development of aero engine technology in mainland China, the relevant investment is increasing, and the engine design and research institutions have accumulated rich engineering experience, and have continuously explored the basic frontier problems such as heat generation and two-phase heat transfer, and basically formed the design and analysis capabilities of the lubricating oil system.
The third part of the book is the aero engine oil lubrication system, a total of 10 chapters. Starting from the type of lubricating oil system, the functions, design requirements, design and analysis methods, verification tests, etc. of the three subsystems of the lubricating oil system (oil supply, oil return and ventilation system) and their main components are introduced, and the bearing cavity and its thermal analysis are specially elaborated on the most concentrated and prominent part of the "thermal problem" in the lubricating oil system.
▍The research focus of engine sealing technology is on engine flow path dynamic sealing, main bearing cavity dynamic sealing, accessory transmission shaft end dynamic sealing and engine static sealing design technology and test technology. Since the birth of the engine, the failure and accident caused by sealing problems have occurred from time to time, which has a significant impact on the performance, reliability, life and maintenance of the engine. With the development of engine technology, sealing technology has become an important factor affecting engine performance and life, high-performance sealing can improve engine efficiency and reduce pollutant emissions, for example, some advanced engine sealing technology can increase the engine thrust by 1% to 3%, and reduce the fuel consumption rate by 3% to 5%. The next generation of engines will gradually increase the pressure ratio and cycle temperature, so the engine sealing technology that can maintain high performance, high reliability and long life under the conditions of high speed, high ambient temperature and high pressure ratio will become one of the important technologies for whether the engine can achieve the design goals.
The fourth part of the book is engine seals, a total of 8 chapters. Firstly, the design concept of the engine sealing system is expounded from the aspect of system design, and then the design and failure of the typical static sealing device and the dynamic sealing device of the engine are introduced; the materials and test items related to the engine sealing are also introduced. The design of the engine sealing system is closely related to the overall structure of the engine, the engine air system, the engine oil system, the engine ventilation system, and the engine sealing technology also takes into account the coupling effects of multiple disciplines such as materials, fluids (gases, liquids), motion, tribology, thermodynamics, heat transfer, machining and aerodynamics.
The above four subsystems have different objects of concern, research priorities, and disciplines and specialties, and each has its own characteristics. Starting from the basic concept, the structural forms, design requirements, design methods, analysis processes, application design cases, test verification, development prospects and new technologies involved in each part of the mechanical system are comprehensively discussed, and the relevant design practices of typical engines are fully combined, and the content is comprehensive, accurate and simple. This book is aimed at engineering and technical personnel, senior undergraduates and graduate students, and can be used as a reference book for the transition from senior undergraduate and graduate students majoring in aero-engine to aero-engine engineering and technical personnel.
This article is excerpted from "Mechanical System Design of Aero Engines" (Edited by Liu Zhenxia et al. Beijing:Science Press,2022.] 1) A book "Preface", with deletions, and the title is added by the editor.
Book series recommended
"Two aircraft" special project: aero engine technology publishing project
The publishing project is mainly based on the main scientific research institutes under China Aero Engine Group Co., Ltd., and is jointly compiled with the participation of well-known scholars from universities, comprehensively sorting out and summarizing the excellent engineering, scientific research and teaching achievements in the field of aero engines in the "two machines" special project, focusing on engineering applications and taking into account basic research. The first phase includes the design series, the test series, the basic and comprehensive series, with the purpose of training aero-engine engineering and technical personnel, especially to meet the training needs of the transition stage from students to engineers, and to contribute to the training of outstanding engineers in the field of continental aero-engines.
(Editor of this article: Liu Sitan)
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