Turbine blades are an important component of the turbine section in a gas turbine engine. The high-speed rotating blades are responsible for drawing high-temperature and high-pressure air into the burner to maintain the engine's work.
What are Turbine Blades?
Blades play the most important role in an aero-engine, without which an aero-engine cannot produce thrust. Aeroengine blades are further divided into turbine blades, fan blades, and compressor blades. The turbine blades are further divided into high-pressure turbine blades, low-pressure turbine blades, and guide turbine blades. Compressor blades are divided into high-pressure compressor blades and low-pressure compressor blades. One of the factors that determine the thrust of the engine is the performance of the high-pressure turbine blades.
To ensure stable and long-term operation in extreme high-temperature and high-pressure environments, turbine blades are often forged with high-temperature alloys and cooled in different ways. Such as internal airflow cooling, boundary layer cooling, or thermal barrier coatings to protect the blades. Ensure reliability during operation. In steam turbine engines and gas turbine engines, blade metal fatigue is the leading cause of engine failure. Strong vibration or resonance may cause metal fatigue. Engineers often use frictional dampers to reduce damage to the blade from these factors.
What is a High-pressure Turbine Blade?
High-pressure turbine blades need to withstand high temperatures above 1500 degrees, which is equivalent to the centrifugal force and aerodynamic force, corrosion, load, etc. of 2,000 times the blade's weight. Therefore, high-pressure turbine blades are required to have the characteristics of high-temperature resistance, high strength, and corrosion resistance. To make the high-pressure turbine blades have the above performance, special materials are used in the manufacture.
CNC blade processing:
Commonly known as a forged turbine, it refers to the intake side blade made of 7075 forged aluminum alloy. The biggest advantage is that it has high strength and a thinner blade, and it can be made thinner and longer near the axis, so at the same speed, The lower can generate higher air volume, thereby generating greater power output. The efficiency of turbine blades processed by aluminum alloy CNC is better, but in fact, almost all turbines seen on the market use ordinary cast blades. The main problem lies in the consideration of cost and technology. The blades produced by casting are not only the cost is low, but the manufacturing speed is also fast, and mass production can be carried out. However, the turbines manufactured with such blades often meet the general mass production requirements, and the performance can only be said to be average, but it is also sufficient for general occasions.
To manufacture forged CNC blades, it is necessary to use five-axis CNC machines that cost nearly 10 million yuan. However, there are not many such machines in China, and most of them are used to manufacture aerospace or military supplies. In the past, it was controlled by the government because it was a necessary tool for manufacturing aerospace parts and military weapons. You must have a relevant industrial background to introduce five-axis CNC. The so-called forged CNC blades use forged aluminum materials that are stronger than ordinary cast aluminum alloys, and then use CNC to cut the aluminum materials into turbine intake side blades. However, this manufacturing method is not only expensive, but also technically difficult, because the angle of the turbine blade is quite complicated, and generally three-axis and four-axis CNC machines cannot be cut out, and more expensive ones must be used. Five-axis CNC can manufacture blades with complex shapes.
Aeroengine Turbine Blade Cooling:
- Why cooling?
If the temperature in front of the turbine is 1600K, and the temperature resistance of the turbine blade material is only 1200K, will such material work? The answer is yes, but with advanced cooling technology. Taking the civil turbofan engine as an example, the difference between the temperature before the turbine and the tolerance temperature of the blade material has increased to more than 500K. Cooling technology is crucial to aero-engines. Although the material has been limited, the performance of the turbine and the life of the turbine blades can be improved through advanced turbine cooling technology. The turbine in the early engines did not use cooling technology, but the temperature in front of the turbine was not very high at that time. The development of cooling technology came from a contradiction. The development of turbine blade materials lagged behind the performance of aero engines (temperature in front of the turbine). Although the gas temperature before the turbine cannot exceed the tolerance value of the material, the introduction of turbine cooling technology has completely changed this situation and promoted the development of turbine cooling technology.
- What are the cooling methods of turbine blades?
In the field of aero-engines, convective cooling, impingement cooling, film cooling, and divergent cooling have been developed successively. The purpose of cooling is to increase the temperature before the turbine to improve engine performance, make the temperature field in the blades evenly distributed, and reduce thermal stress.
- Convection cooling:
Convective cooling is one of the cooling methods widely used today. The cooling air passes through several special passages inside the blade, and through this convection, it exchanges heat with the inner wall of the blade, so that the temperature of the blade is reduced to achieve the cooling effect, and the cooling effect is 200°C to 250°C.
- Impact type:
Impingement cooling is spray cooling, which uses one or more cooling air jets to face the surface to be cooled to enhance the local heat transfer capacity, and is suitable for enhanced cooling in local high-temperature areas, such as spray cooling at the leading edge of the blade was adopted first. In principle, impingement cooling still belongs to convective cooling.
- Film cooling:
The cooling air enters the inner cavity of the blade from the end of the blade, and the air film cooling turbine blade is designed and manufactured with many small holes. The high-temperature gas is separated to achieve the purpose of cooling the turbine blades.
- Divergent cooling:
Divergent cooling (sweat cooling) is a type of turbo-cooling technology in which the cooling air permeates from the inner cavity of the blade through numerous micropores on the wall of the blade, just like sweating. It is a hollow blade made of high-temperature alloy porous laminates, and the high-pressure cooling air flows out of the inner cavity of the blade through the dense pores on the wall and flows to the outer surface of the blade. A complete and continuous cavity heat insulation layer is formed between the high-temperature gas and the surface of the blade. It can not only completely separate the surface of the blade from the gas but also absorb part of the heat on the surface of the blade. Using this cooling method, the blade can be the material temperature is close to the cooling air temperature.
The technical problems faced by this cooling method are that the porous material is easily blocked after oxidation, each layer needs to be porous, the holes are not easy to align, and the process is complicated. For every 100°C increase in the temperature before the turbine, the performance of the engine will increase by at least 10% under the condition that the engine size remains the same. This is why the temperature before the turbine becomes an important indicator to measure the quality of the engine.