In order to pursue the ultimate performance in a very limited volume, aeroengines (especially military ones) require more sophisticated materials and more sophisticated designs. The materials can meet the requirements of stable operation for hundreds to thousands of hours. That's it. The three-rotor (three-shaft) engine is produced by British Rolls-Royce, such as Rolls-Royce's previous RB211 series and the current Treada series.
France does not have the ability to develop advanced aero engines. Currently, there are only two countries and three companies that have the ability to develop advanced aero engines, namely Rolls-Royce of the United Kingdom, GE and Pratt & Whitney of the United States. Efficiency, thrust and ducting ratio, boost ratio, and temperature in front of the turbine all have a matching relationship. The higher the temperature in front of the turbine, the matching total boost ratio will increase. For civilian large bypass ratio engines, the bypass ratio will be increased as much as possible, and the matching fan ratio will be reduced. For military small bypass ratios, the temperature in front of the turbine will be increased as much as possible. The requirements are different from those for civilian use. It feels like the bottleneck of existing commonly used materials has been reached. The load-bearing temperature of nickel-based alloys increases relatively quickly from 700 to 1000°C, and it is very difficult to go up to 1100°C. 1400°C is the melting point range of nickel-based alloys, which is now 0.8Tm. For higher temperatures, only ceramic blades or composite blades can be expected.
Current aero engines include centrifugal and axial flow types
Ground gas turbines hope to achieve high efficiency, low cost, durability and long-term reliability (the temperature is relatively low and materials are required For longer periods of time (100,000 hours of stable operation), the volume requirements are relatively lower. The operating conditions of ground gas turbines are relatively stable (such as power stations), and materials can be used for a longer period of time; while the operating conditions of aerospace engines are more complex (takeoff, climb, cruise, violent maneuvers), resulting in faster material failure. To do well in these two fields requires decades of continuous investment and accumulation. If Germany and Japan want to develop advanced aviation engines, they will have to start from scratch on many things. After the war, Germany suffered a serious brain drain and its defense industry was suppressed. There is also the factor of insufficient demand. After all, Europe has to face the pressure of the Soviet Union. MD is very concerned about European defense. As long as the Europeans want it, they can always get fighter jets equipped with advanced aviation engines from the Americans. Although Germany has made great achievements in the field of gas turbines, the difference between aviation engines and gas turbines is still very large. Without sufficient driving force, several giants are unwilling to take this endless road of burning money.
Using its strong strength accumulated in the field of gas turbines, MTU has participated in many international cooperations in aerospace engines, most of which are responsible for the compressor and low-pressure turbine parts; the core engines are generally completed through cooperation between the United States and the United Kingdom. , this can be regarded as a specialization in the art industry. The EJ200 equipped with Typhoon seems to be RR responsible for the core machine, and the Germans are responsible for the compressor. Airbus' aviation engines are purchased from a few fixed sources, such as RR (trent series), GE and PW (GP series) or some jointly established companies (like IAE's V2500). Germany may still be mainly involved. Natural science and industry can accumulate and move forward step by step. The so-called future generations stand on the shoulders of giants. Next, the second-rate talents are engaged in business and trade, and the third-rate talents enter the IT industry. Among those engaged in technology, the only ones who can persist after recognizing the situation clearly are the fourth-rate ones. Finally, the on-site workers who implement scientific research into production are looked down upon by many people, but Aircraft has to produce, assemble, and debug with their hands.
This long picture is more intuitive
I personally think that what aviation engines pursue is to ensure long-term, stable and extreme performance under extremely harsh conditions (high temperature, high pressure and high stress). This high temperature has stumped many fields: the semiconductor industry has many technical difficulties, but problems at room temperature or around 100~200℃ can at least be studied through various common equipment (SEM, TEM, FIB, 3DAP, etc.), as well as experimental methods Sophisticated, even in situ studies are possible. In aerospace engineering, problems such as combustion problems in high-speed (or even supersonic) airflow, creep of materials at extremely high temperatures (1000°C), and in-situ research on phase change processes are difficult to achieve using existing means. .
Under the premise that there are huge difficulties in understanding physical processes and engineering practice, we must continue to advance the technological frontier. I think it can be called the most difficult.
To be precise, the external airflow driven by the fan generates more than 80% of the thrust of the entire machine. The aerodynamic load on a single blade exceeds 2 tons, and the centrifugal load during operation reaches more than 13 tons. The composite of GE90-115B Material + titanium alloy edged blades are even displayed in museums as handicrafts (I can’t remember the name of the specific museum). As a descendant of GE90, GEnx reduces the fan blades to 19 pieces, and the aerodynamic load borne by a single blade is even greater. Large (the specific value has not been checked).
At the lower Mach number stage, the turbofan engine is more efficient
The turbine is actually an energy conversion component, just like the vortex of a water turbine that converts the potential energy of the water flow into the energy of the generator rotor. Kinetic energy comes back to generate electricity. The aircraft turbine converts the heat energy generated by fuel combustion into the kinetic energy of turbine rotation, which then drives the fan and compressor to generate thrust. The lower the turbine temperature, the more heat energy of the fuel is lost and the lower the conversion efficiency, so there is nothing we can do. The bearing capacity of alloy blades against high temperatures is limited. Can we change our thinking and focus our material research on high-temperature resistant coatings? High-temperature coatings can achieve good results through special processing, which can reduce dependence on metal materials and instead A huge breakthrough in coating materials. At present, a possible alternative material in the future is ceramic matrix composite (CMC), which can operate at much higher temperatures than metal and does not even require coating, but there are still many problems that need to be solved. It is said that GE has conducted experiments, but I don’t know the results yet. This should still be a promising path.
Engine materials are not inseparable from iron, but iron is not easy to completely eliminate as an impurity, and now the iron content of domestic nickel-based high-temperature alloys can be reduced to 0.05% according to the national military standard. The actual product has lower iron content. And not all nickel-based high-temperature alloys do not contain iron. For example, the IN718 alloy, which is used most in engines, contains 18% iron because iron is cheap. Also, the main reason why the engine material is selected as nickel-based instead of iron-based is not the creep temperature of iron, but because iron undergoes allotropic transformation, while nickel does not. In addition, cobalt-based materials are better high-temperature structural materials, but cobalt is too expensive, so overall nickel-based materials are optimal. In order to ensure smooth air intake, an aero engine does not have a dense filter. At best, an inertial or centrifugal separator is installed at the inlet. Only gas turbines equipped on the ground, such as the AGT1500 gas turbine equipped by M1 Abrams, will be equipped with filters due to the needs of the use environment. However, every time the M1 engine is overhauled, many compressor blades will be found to be dented by unfiltered sand. The pit or edge is damaged.
The early fans were narrow-chord fans, forged from solid titanium alloy
Russia (the former Soviet Union) was very good at using systems engineering theory to integrate components that were not advanced enough. It has become a product with outstanding overall performance, and the most typical one is the former Soviet MiG-25 fighter. Compared with similar military products in Europe and the United States, Russian related products are easy to maintain and rugged. It cannot be said that sophisticated and precise ones are necessarily good. Each has its own advantages. The Soviet-German battlefield during World War II fully exposed the advantages and disadvantages of the two styles: German tanks (Tiger, Panther, etc.) were very well made and the manufacturing process was quite advanced at the time, but they had high maintenance requirements and low output. It cannot effectively exert its effectiveness in the harsh Soviet winter climate; on the other hand, Soviet tanks (such as the T-34) have a simple structure, which is conducive to large-scale manufacturing and easier to operate. Workers at the Stalingrad Tractor Factory are producing a T-34. After -34, I drove to the battlefield. As the war continued, the German army's equipment suffered serious battle damage and could not be replenished in time. However, Soviet equipment continued to flow into the battlefield, and in the end the German army was dragged down alive. ?
Therefore, the extreme pursuit of advanced equipment has become a misunderstanding for many people. How to maximize the effectiveness of existing equipment may be a key issue that needs to be solved. Flying cruise, power comes first. Aviation technology is related to the national military power. It is a collection of the most sophisticated technologies from various countries. The problems it faces are wide and complex, and the test cost is unimaginable. For example, the temperature of the combustion chamber of a turbojet engine The higher the performance, the better, but which material and how to process it can be used at such high temperatures has become an absolute barrier, because it is impossible to conduct exhaustive tests. The aircraft engine looks rough but is actually extremely sophisticated.
The work process of aviation engines and gas turbines is the Brayton cycle
The pace of developing new materials has never stopped, but it is indeed difficult to develop materials that meet the requirements in this environment.
There is also research on pulse detonation and scramjets, but in the transonic range, turbofans are indeed very advantageous. I hope there will be anti-gravity engines in the future. The internal flow air system is very important to maintain the stability of the engine's transient working conditions. If there is a slight error, it may cause local overheating of components or excessive clearance deviation of parts, which will affect performance and even lead to safety accidents. Titanium alloys are generally used in fan and compressor blades. The working temperature is relatively low, and titanium fire will not occur under normal circumstances. I have read a paper on titanium fire before. The main reason is that on the one hand, the severe friction and impact caused by the impact of foreign objects, etc. cause titanium fire in the titanium alloy blades of the compressor; on the other hand, the surge and other reasons cause high-temperature gas to escape from the combustion chamber. It rushes backward to the compressor, causing titanium fire in the blades.
In order to improve the performance of aero-engines, RR is engaged in three-rotor engines and PW is engaged in gear transmission. The purpose is to decouple the speed of the medium-pressure turbine or low-pressure turbine and the fan or medium-pressure compressor (traditional design , they are on an axis). Large bypass ratio engine fans require the blade tips to be as non-supersonic as possible, and the fan diameter is very large, so the fan speed cannot be too high, otherwise the efficiency will deteriorate. On the contrary, the low-pressure turbine increases the efficiency. The higher the speed, the higher the efficiency. Two grasshoppers are tied to a rope, and they can only compromise with each other. I am more concerned about the use and maintenance of aviation engine bearings. The current high-nitrogen alloy steel bearings (inner and outer rings) and silicon nitride (ceramic ball rolling elements) still cannot meet the actual operating temperature requirements of aircraft engines.
Then the lubrication system needs to be supplemented. First, it must meet the needs of high speed, high temperature, and high load (high torque) to form a good oil film. Secondly, the lubricating oil needs to be exchanged to take away the heat, and then cooled and then returned to the oil circuit (oil circuit). circulatory system). The axial flow type is more suitable for multi-stage arrangement and improves the pressure ratio, but there is a corresponding possibility of air backflow, so the concept of adjustable stationary blades and the concept of bleed valve are introduced to prevent surge. In addition, the speed of n1 n2 rotor Matching must also be accurately controlled, because n1 can be considered to be idling, while n2 has to drive other accessories to rotate, so the speed matching between rotors is also very difficult, not to mention Rb211 and its subsequent three-rotor series, so it is possible to produce three-rotors There are very few technology companies.
The compressor adopts a rotor + stator structure
But why is it necessary to have three rotors? Because the compression process of the three-rotor compressor is smoother than that of the two-rotor compressor, it is less prone to surge, which means that the compression ratio can be increased, thereby increasing the total gas pressure of the turbine and increasing the thrust. In other words, if the difficulty is not great, the rotor The more possible the better the engine will be from a certain point of view.
Aerospace engines have gone through three generations of piston, turbojet, and turbofan. The potential of turbofans has basically come to an end. We are on the same starting line as the West in the new generation of scramjet and detonation engines. Although we The basics will still be a little worse, but relying on the advantage of concentrating efforts on big things, it is still possible to compete with the United States and Britain in the next generation of engines.