What Is a Gas Seal?

Leakage loss exists between the compressor rotor and the stator, such as the top of the rotor blade and the casing, between the inner ring of the rectifier and the rotor drum, and between the front and rear end surfaces of the rotor and the casing, which seriously affects the efficiency of the compressor . For this reason, in addition to the correct selection of the gap, a gas sealing device must be used.

Leakage loss exists between the compressor rotor and the stator, such as the top of the rotor blade and the casing, between the inner ring of the rectifier and the rotor drum, and between the front and rear end surfaces of the rotor and the casing, which seriously affects the efficiency of the compressor . For this reason, in addition to the correct selection of the gap, an air-tight device must be used ^[1]

According to the knowledge of aerodynamics, the amount of air leakage depends on the area of air leakage, the pressure difference between the two ends of the air leakage (which determines the air leakage speed), and the air density. Therefore, the effective measures to achieve the gas sealing effect are to reduce the area of air leakage and reduce the pressure difference ^[1]

Sealing devices can be divided into two types: contact seals and non-contact seals.

The ring seal is the most common type of contact seal. The advantage of the ring seal is that it effectively reduces the area of air leakage. There is friction between the tight ring and the ring. It is suitable for relatively low linear speed or Oil environment, such as the oil seal of the bearing case. The inter-seal of the compressor is mainly non-contact.

On the premise of ensuring the safe operation of the compressor, minimizing the gap between the blade tip of the working blade and the casing is one of the effective measures to reduce the area of air leakage. Choosing the smallest safety clearance correctly is a complex issue. It is closely related to the manufacturing accuracy of the parts, the rigidity of the rotor and the casing, and the coordination of the thermal expansion of each part during work. On some engines, in addition to using a double-layer casing to maintain the roundness of the inner casing, the following measures are also available:

(1) For JT9D74R and PW4000, after assembling the rotor of the high-pressure compressor, use a special grinding machine to grind the rotor blade tip as a whole, which can improve the balance of the rotor and ensure that the outer circle of the rotor is uniform;

(2) On some engines, the inner wall of the casing is coated with an abradable coating, and the rotor blade tips are covered with abrasion-resistant materials. When assembling, the rotor is used as a grinding wheel to grind a concentric drawing in the casing to maintain the blade tips. Uniform gap

(3) You can also choose a casing material with a small linear expansion coefficient. ^[1]

Toothed air sealing device is a common structural measure to reduce the back flow loss and air leakage loss between compressor stages. The tooth seal is a non-contact seal, which can reduce the leakage loss from reducing the leakage area and reducing the pressure difference.

This air sealing device is composed of several cavities formed by the teeth. During operation, the total pressure difference between the two sides of the air-sealing device has not changed, but the pressure difference across the leaked section (adjacent cavity) is reduced due to the segmentation of the teeth. At the same time, the gap can be kept as small as possible, because the teeth are of the blade type and the tooth tips are made thin. Once they collide with the static, it will not cause serious consequences. This reduces the pressure difference while reducing the area of air leakage, thereby effectively reducing the amount of air leakage ^[1]

(1) Leaks are always present in the cavities. In order to maintain the pressure in the low pressure region, the low pressure region must be continuously vented. For example, on the WP7 engine, in order to maintain a certain pressure in the back pressure reducing chamber of the compressor, the back pressure reducing chamber must pass through the throttle to the atmosphere.

(2) With the decrease of the gas after the air-sealing device, the air velocity at each gap increases, so the amount of air leakage also increases. When the pressure drops to a certain value, since the air velocity of the gap in the last stage reaches the speed of sound, if the air flow parameters in the high-pressure zone are unchanged, the air leakage volume reaches the maximum value at this time.

(3) Under a certain pressure difference, as the number of teeth increases, the pressure difference between adjacent cavities decreases, and thus the amount of air leakage decreases. However, for a certain size of the air sealing device, there is one of the most advantageous teeth, that is, too many teeth, because the cavity is too small, the throttling effect becomes worse, and the air leakage increases instead. Therefore, under the condition of a certain axial dimension, in order to increase the number of teeth, a multi-channel tooth seal is used. For example, in the WP7 engine compressor, in order to ensure the pressure of the rear decompression chamber, two sprocket teeth are provided on the web of the sixth-stage wheel disc. In order to further reduce air leakage, a graphite block is inserted at the second tooth, and the radial clearance at the tooth is reduced from 1.50 to 1.62 mm to 1.0 to 1.12 mm ^[1]

It is found that the aeroelastic force in the palatine cavity will show a negative damping force under certain conditions, which will cause the self-excited vibration of the rotor. Therefore, the teeth sealing device is one of the important reasons for the high-speed rotor vibration. There are two ways to solve this problem:

(1) According to the results of the study, a reasonable sealing structure or a labyrinth sealing ring is adopted to change the effect of aeroelastic force.

(2) A new type of sealing device is used in place of the tooth sealing device. Such as brush seals currently used on PW4000 engines, V2500 and F119 engines. The brush seal has a stator ring composed of a plurality of thin steel wire brushes, which are constantly in contact with the rotating shaft and rubbed, and the shaft is coated with a hard ceramic coating ^[1] .