Title

An Impellor for an Air Compressor and an Air Compressor

Technical Field

The present application relates to the field of air compressors, in particular relates to an impellor for an air compressor and an air compressor.

Background Art

An air compressor is a machine which converts mechanical energy of a prime mover into gaseous energy. As air supply devices, air compressors have already been applied in different walks of life. For example, electric air compressors are used as an air supply for braking in vehicles, an air supply of turbine boosters or an air supply of fuel cells.

An air compressor usually comprises a housing, an impellor and a driving device (for example, electric motor). The impellor is connected to the rotation shaft of the driving device and is driven by the rotation shaft to rotate at high speed to draw air into the housing, and in addition, the impellor enables air to be compressed as the impellor rotates. Since air pressures on the two sides of the rotating impellor are different, that is to say, the air pressure on the high-pressure side is greater than the air pressure on the low-pressure side, the impellor bears a thrust pointing from the high-pressure side to the low-pressure side in the axial direction of the impellor. Under the action of the thrust, the impellor tends to move in the direction of the thrust, and thus tends to drive the rotation shaft to move in the direction of the thrust. Such a movement is prevented usually by disposing a thrust bearing and enabling the thrust bearing to axially join a radial bulge radially extending from the rotation shaft. That is to say, the thrust is offset by enabling the thrust bearing to axially join the radial bulge. In this case, a great axial force acts on the thrust bearing. Such an axial force needs to be reduced. Otherwise, the thrust bearing will be at a high temperature, and thus the wear-out of the thrust bearing is sped up. As a result, the service life of the thrust bearing is shortened or even the air compressor breaks down. Therefore, the reduction of such an axial force must be considered for an air compressor.

In the prior art, various solutions for improving components of an air compressor to reduce the axial force have already been provided. For example, (1) back blades are provided on the opposite side of the impellor disc of the impellor to reduce the axial force, (2) a balance disc is provided to reduce the axial force, and (3) impellors are oppositely disposed back to back so that the thrusts on a plurality of groups of impellors can offset each other to reduce the axial force. However, these solutions will make the structure of an air compressor complicated and increase the size of the air compressor.

Summary of the Utility Model

In order to overcome at least one defect in the prior art, the present application provides an improved impellor for an air compressor and an air compressor comprising the above-mentioned impellor.

According to one aspect of the present application, an impellor for an air compressor is provided. The impellor comprises an impellor disc and a plurality of blades evenly arranged on the impellor disc in the circumferential direction of the impellor. Each blade has a front end, a rear end and a body extending between the front end and the rear end.

The bodies of at least some blades of the plurality of blades form oblique sections near the rear ends, the oblique sections are configured so that when the impellor rotates to compress air, air acts on the oblique sections of the at least some blades to exert a first resultant force on the at least some blades, and the first resultant force has an axial component F _CO mponent pointing from a low-pressure side to a high-pressure side in the axial direction of the impellor.

Preferably, the body of each blade of the plurality of blades forms the oblique section near the rear end. Preferably, the bodies of only some blades of the plurality of blades form the oblique sections near the rear ends and these blades are symmetrically arranged with respect to the axial direction of the impellor.

Preferably, the body of each blade of the plurality of blades comprises: a positive-pressure surface, the positive-pressure surface being configured to bear a positive pressure of air when the blade rotates, a negative-pressure surface, the negative-pressure surface being configured to bear a negative pressure of air when the blade rotates and the negative-pressure surface being disposed on the opposite side of the positive-pressure surface, a side surface, the side surface being configured to connect the positive-pressure surface and the negative-pressure surface, and a rear end surface, the rear end surface being configured to connect the positivepressure surface, the negative-pressure surface and the side surface at the rear end, wherein the turning portion of the body of the at least some blades near the rear end between the positive-pressure surface and the side surface is configured to form the oblique section.

Preferably, the axial width of the positive-pressure surface is less than the axial width of the negative-pressure surface on the oblique-section-formed segment of the body of the at least some blades.

Preferably, the oblique section extends from the rear end surface towards the front end along the body.

Preferably, the air compressor is a single-stage centrifugal compressor, the impellor is a semi-open impellor, the blades are 3-D twisted blades, the oblique sections of the bodies of the at least some blades all have the same shape and the same orientation, and the relationship of the axial component Fcomponent with the shape and the orientation of the oblique section is defined by the following formula: wherein Z is the number of the at least some blades, D is the diameter of the impellor, n is the design speed of rotation of the air compressor, q is the design flow rate of the air compressor, B is the length of the oblique section, L is the width of the oblique section, a is the included angle between the oblique section and the normal of the positive-pressure surface, and K is a compensation factor, which depends on the machining accuracy of the air compressor and the axial endplay during rotations of the impellor.

Preferably, an air film is formed on the oblique section when the impellor rotates to compress air, the air film has a front-end thickness hi and a rear-end thickness h - h tana = — ^! - - ho, the included angle a is defined by L , and the ratio of the frontend thickness hi to the rear-end thickness ho is greater than 1.

Preferably, the plurality of blades consist of long blades and short blades, each short blade is located between two adjacent long blades, and the long blades and the short blades take on one of the following forms: all the bodies of the long blades and the short blades form the oblique sections near the rear ends, only the bodies of the long blades form the oblique sections near the rear ends, or only the bodies of the short blades form the oblique sections near the rear ends.

Preferably, when all the bodies of the long blades and the short blades form the oblique sections near the rear ends, the length of the oblique sections of the long blades is greater than the length of the oblique sections of the short blades.

According to another aspect of the present application, an air compressor is provided, and the air compressor comprises the above-mentioned impellor.

When the impellor in the present application is applied to an air compressor, the axial force on the thrust bearing can be reduced to reduce the wear-out of the thrust bearing, thus prolonging the service life of the thrust bearing and guaranteeing the reliable running of the air compressor. Brief Description of the Drawings

The above-mentioned aspects and other aspects of the present application will be completely understood and known in combination with the drawings. It should be noted that the drawings are provided only for an exemplary purpose, but are not plotted in proportion. In the drawings,

Fig. 1 is a cutaway view of an air compressor to which the impellor according to a preferred embodiment of the present application is applied,

Fig. 2 is a schematic 3-D view of the impellor shown in Fig. 1,

Fig. 3 is another schematic 3-D view of the impellor shown in Fig. 1,

Fig. 4 is a schematic front view of the impellor shown in Fig. 1,

Fig. 5 is a schematic side view of the impellor shown in Fig. 1,

Fig. 6 is an enlarged view of the dashed line area A shown in Fig. 5,

Fig. 7 schematically compares the case in which the body of a blade does not form an oblique section at the rear end with the case in which the body of a blade forms an oblique section extending from the rear end surface, wherein the dashed line C represents the rough outline of the rear end of the body in the case in which the blade has no oblique section,

Fig. 8 is a schematic diagram for determining the relationship of the force on the oblique section with the shape and the orientation of the oblique section, and Figs. 9A and 9B respectively show the results of computational fluid dynamics (CFD) simulations performed for the pressure distributions of blades without/with oblique sections in one embodiment of the impellor shown in Figs. 2 to 6.

Description of reference numerals in the drawings:

I. Air compressor

3. Electric motor

5. Housing

7. Impellor pressure shell

8. Impellor end cover

9. Rotation shaft

II. First radial bearing

13. Second radial bearing

15. Radial bulge

17. First thrust bearing

19. Second thrust bearing 100. Impellor

101. Impellor disc

103. Blade

105. Rotation axis

107. Front end

109. Rear end

111. Body llla. Positive-pressure surface lllb. Negative-pressure surface lllc. Side surface llld. Rear end surface

113. Rotation direction

115. Low-pressure side 117. High-pressure side 119. Oblique section

Detailed Description of the Utility Model

The preferred embodiments of the present application are described in detail in combination with examples below. In the preferred embodiments of the present application, a single-stage centrifugal air compressor and the impellor are taken as examples to describe the present application. Those skilled in the art should understand that the exemplary embodiments are not intended to constitute any restriction of the present application. In addition, the features in the embodiments of the present application can be combined with each other as long as they do not conflict with each other. In different drawings, the same component is denoted by the same reference numeral and other components are omitted for the sake of simplicity. However, this does not mean the impellor and the air compressor in the present application cannot comprise other components. It should be understood that the sizes and scaling relations of different components, and the number of components in the drawings do not constitute any restriction of the present application.

In this document, "axial direction" refers to the extension direction of the rotation axis around which the impellor of the air compressor rotates, "radial direction" refers to the radial direction relative to the rotation axis, "circumferential direction" refers to the circumferential direction relative to the rotation axis, namely, the direction surrounding the rotation axis, and "normal direction" refers to a direction perpendicular to a surface, unless otherwise specified. In addition, in this document, "axial width" refers to a width in the axial direction and "radial width" refers to a width in the radial direction, unless otherwise specified.

In this document, "low-pressure side" refers to an axial side where the air pressure is low when the impellor of the air compressor rotates to compress air, and "high-pressure side" refers to an axial side where the air pressure is high when the impellor of the air compressor rotates to compress air, unless otherwise specified. In this document, "positive pressure" indicates a pressure above the normal pressure, and "negative pressure" indicates a pressure below the normal pressure, unless otherwise specified.

Fig. 1 is a cutaway view of an air compressor 1 to which the impellor 100 according to a preferred embodiment of the present application is applied. The air compressor 1 comprises a compressor housing and an electric motor 3 and an impellor 100 installed in the compressor housing. The compressor housing comprises a cylindrical casing 5 and an impellor casing connected to one end of the casing 5. The casing 5 is configured to accommodate the electric motor 3. The impellor casing consists of an impellor pressure shell 7 and an impellor end cover 8 and is configured to accommodate the impellor 100. When the air compressor 1 is assembled, the electric motor 3 is installed in the casing 5, with the rotation shaft 9 of the electric motor 3 extending into the impellor casing. The impellor 100 is installed on the rotation shaft 9 so that the impellor can be driven by the rotation shaft 9 to rotate. Specifically, the rotation shaft 9 is rotatably supported by the first radial bearing 11 and the second radial bearing 13 respectively disposed at the two ends of the casing 5, and fixes the impellor 100 at one end extending into the impellor casing to drive the impellor 100 to rotate. A space S is defined between the impellor end cover 8 and the casing 5, and the radial bulge 15 radially extends from the rotation shaft 9 into the space S. The first thrust bearing 17 and the second thrust bearing 19 are disposed in the space S and are respectively located on the two axial sides of the radial bulge 15 to axially position the rotation shaft 9. When the impeller 100 is driven by the rotation shaft 9 to rotate at high speed (for example, 100000 rpm), air is drawn into the impeller casing and is compressed as the impeller 100 rotates. This causes the air pressure on the high-pressure side 117 of the impeller 100 to be greater than the air pressure on the low- pressure side 115. Since the air pressures on the two sides of the impeller 100 are different, the impeller 100 bears a thrust Fthrust pointing from the high-pressure side 117 to the low-pressure side 115 in the axial direction. Under the action of the thrust Fthrust, the impeller 100 tends to move in the direction of the thrust Fthrust, and thus tends to drive the rotation shaft 9 to move in the direction of the thrust Fthrust. The first thrust bearing 17 axially joins the radial bulge 15 to offset the thrust Fthrust to prevent the rotation shaft 9 and the impeller 100 from moving. In this case, the axial force pointing from the high-pressure side 117 to the low- pressure side 115 in the axial direction acts on the first thrust bearing 17.

The configuration of the impeller 100 according to the preferred embodiments of the present application is designed to reduce the axial force on the first thrust bearing 17. Preferably the axial force is reduced to a value below the rated load of the first thrust bearing 17, and more preferably, the axial force is completely offset. A semi-open impellor is taken as an example to describe the exemplary configuration of the impellor 100 below.

As shown in Figs. 2 to 5, the impellor 100, a semi-open impellor, comprises an impellor disc 101 and a plurality of blades 103 arranged on the impellor disc 101. The impellor disc 101 roughly takes on the shape of a disc and defines the diameter of the impellor 100. The impellor disc 101 further defines the rotation axis 105 (see Fig. 5) of the impellor 100. The blades 103 are 3-D twisted blades and are evenly arranged on the impellor disc 101 in the circumferential direction of the impellor 100. The blades 103 and the impellor disc 101 may be made of any suitable material. The blades 103 may be integrated with the impellor disc 101 (for example, smoothing formation after casting, electric discharge machining, paraffin precision casting and numerical control machining) or may be formed separately and then connected (for example, by welding or insertion) to the impellor disc 101. The impeller 101 is configured so that it can be installed on the rotation shaft 9 of the electric motor 3 and it can be driven by the rotation shaft 9 to rotate around the rotation axis 105. Each blade 103 has a front end 107, a rear end 109 and a body 111 extending between the front end 107 and the rear end 109. The front end 107 delimits the blade inlet and the rear end 109 delimits the blade outlet. As shown in Fig. 5, the body 111 of a blade 103 comprises a positive-pressure surface Illa, configured to bear a positive pressure of air when the blade 103 rotates, a negative-pressure surface 111b, configured to bear a negative pressure of air when the blade 103 rotates and the negative-pressure surface 111b being disposed on the opposite side of the positive-pressure surface Illa, a side surface 111c, configured to connect the positive-pressure surface Illa and the negative-pressure surface 111b and define the thickness of the body 111, in other words, the side surface 111c being the surface on one side of the blade 103 in the thickness direction, and a rear end surface llld, configured to connect the positive-pressure surface Illa, the negative-pressure surface 111b and the side surface 111c at the rear end 109, in other words, the rear end surface llld being the end surface of the blade 103 at the rear end 109. In addition, as shown in Figs. 2 to 4, the front end 107 of the blade 103 is linear so that air can flow between adjacent blades 103. However, it should be understood that the front end 107 of the blade 103 may also take on any other suitable configuration.

The blade 103 is configured to do work on air when the impellor 100 rotates in the rotation direction 113 (indicated by the arrow in Fig. 5, usually a single rotation direction) around the rotation axis 105 so that air is compressed as the impellor 100 rotates. The bodies 111 of adjacent blades 103 define a flow passage between them, air enters the flow passage from between the front ends 107 of adjacent blades 103 and leaves the flow passage from between the rear ends 109 of adjacent blades 103. The blades 103 do work on air to increase the pressure and kinetic energy of air. This causes the air pressure on the low-pressure side 115 (see Fig. 5) of the impellor 100 to be less than the air pressure on the high- pressure side 117. When the blades 103 rotate, the positive-pressure surfaces Illa of the bodies 111 of the blades 103 bear the positive pressure of air and the negative-pressure surfaces 111b bear the negative pressure of air. The bodies 111 of at least some blades (all blades in Figs. 2 to 5) of the plurality of blades 103 form oblique sections 119 near the rear ends 109. The oblique sections 119 are configured so that when the impellor 100 rotates to compress air, air acts on the oblique sections 119 of the at least some blades to exert a first resultant force Fresuitant on the at least some blades, and the first resultant force Fresuitant has an axial component F _CO mponent pointing from a low-pressure side 115 to a high-pressure side 117 in the axial direction (namely, in the extension direction of the rotation axis 105) of the impellor 100. As mentioned above, the axial force on the first thrust bearing 17 is in the axial direction from the high-pressure side 117 to the low-pressure side 115. Therefore, the direction of the axial component Fcomponent is opposite to the direction of the axial force. The axial component F _com - ponent can reduce the axial force. Preferably, the axial force is reduced to a value below the rated load of the first thrust bearing 17, and more preferably, the axial force is completely offset. In other words, the axial component Fcomponent can at least partially offset the axial force on the first thrust bearing 17. In this way, the wear-out of the first thrust bearing 17 is reduced, thus prolonging the service life of the thrust bearing 17 and guaranteeing the reliable running of the air compressor 1.

In addition, compared with various solutions for reducing the axial force in the prior art (for example, solutions (1), (2) and (3) in the background art), such an impellor 100 is structurally simple, is easy to manufacture, and can reduce the axial force without increasing the size of an air compressor, thus facilitating the miniaturization of the air compressor. This makes it possible for the air compressor to which the impellor 100 according to the present application is applied to be especially applicable to the scenarios where the space is limited.

Although the body 111 of each blade 103 of the plurality of blades 103 forms an oblique section 119 near the rear end 109 in Figs. 2 to 5, it should be understood that in other embodiments the bodies 111 of only some blades 103 of the plurality of blades 103 form the oblique sections 119 near the rear ends 109 and these blades 103 are symmetrically arranged with respect to the axial direction of the impellor 100 to guarantee a uniform force on the whole impellor 100. The following describes in detail an exemplary configuration of oblique sections 119. As optimally shown in Figs. 5 and 6, the turning portion of the body 111 near the rear end 109 between the positive-pressure surface Illa and the side surface 111c is configured to form the oblique section 119. That is to say, the oblique section 119 extends between the positive-pressure surface Illa and the side surface 111c and connects the positive-pressure surface Illa and the side surface 111c. In the embodiments shown in Figs. 5 and 6, the oblique section 119 extends along the body 111 from the rear end llld towards the front end 107. Although the oblique section 119 does not extend to the negative-pressure surface 111b in Figs. 5 and 6, it should be understood that the oblique section 119 may also be configured to just intersect with the edge of the negativepressure surface 111b in other embodiments.

Furthermore, see Fig. 7. Fig. 7 schematically compares the case in which the body 111 of a blade 103 does not form an oblique section 119 at the rear end 109 of the body 111 with the case in which the body 111 forms an oblique section 119, wherein the dashed line C represents the rough outline of the rear end 109 of the body 111 in the case in which the body 111 has no oblique section 119. In Fig. 7, the arrow G represents the direction of motion of the rear end 109 of the body 111 when the impellor 100 rotates in the rotation direction 113 around the rotation axis 105, and the arrow E represents a direction pointing from the low- pressure side 115 to the high-pressure side 117 in the axial direction of the impellor 100. From Fig. 7, it can be seen that, compared with the case in which the body 111 has no oblique section 119, in the case in which the body 111 forms the above-mentioned oblique section 119, when the impellor 100 rotates in the rotation direction 113 around the rotation axis 105, air acts on the oblique section 119 so that the oblique section 119 receives a force f perpendicular to the oblique section 119, and the force f has a component fi in the direction indicated by the arrow E. That is to say, when the impellor 100 rotates in the rotation direction 113 around the rotation axis 105, air acts on the oblique section 119 to exert a force f on the oblique section 119, and the force f has an axial component fi pointing from the low-pressure side 115 to the high-pressure side 117 in the axial direction of the impellor 100. Therefore, when the impellor 100 rotates in the rotation direction 113 around the rotation axis 105 to compress air, air acts on all oblique sections 119 of the blades 103 of the impellor 100 to exert a first resultant force Fresuitant on the impeller 100, and the first resultant force F _re suitant has an axial component Fcomponent pointing from the low-pressure side 115 to the high-pressure side 117 in the axial direction of the impellor 100. As mentioned above, the axial component F _CO mponent can reduce the axial force on the first thrust bearing 17, preferably, the axial force is reduced to a value below the rated load of the first thrust bearing 17, and more preferably, the axial force is completely offset. In other words, the axial component Fcomponent can at least partially offset the axial force on the first thrust bearing 17.

Furthermore, see Figs. 5 to 7. It can be seen that when the body 111 forms the above-mentioned oblique section 119, the axial width of the positive-pressure surface Illa is less than the axial width of the negative-pressure surface 111b on the oblique section 119 formed segment of the body 111. When the impellor 100 rotates in the rotation direction 113 around the rotation axis 105 to compress air, such a configuration makes it possible for the above-mentioned force f to always act on the oblique-section-formed segment.

The following describes the relationship of the size of the axial component Fcomponent with the shape and orientation of the oblique section 119 in the case in which the oblique section 119 extends from the rear end surface llld of the body 111.

The shape and orientation of the oblique section 119 are determined by the following three parameters: (1) length B in mm, which is the length of an arc which the oblique section 119 extends from the rear end surface llld of the body 111 towards the front end 107 along the body 111, (2) width L in mm, which is the distance the oblique section 119 extends between the positive-pressure surface Illa and the side surface 111c, and (3) included angle a between the oblique section 119 and the positive-pressure surface Illa.

When the impellor 100 rotates to compress air, an air film is formed on the oblique section 119. As schematically shown in Fig. 8, when the impellor 100 rotates and air acts on the oblique section 119, the axial component fi of the force f exerted by air on a single oblique section 119 is analysed according to the theory on air films, and the size of the axial component fi is determined by the following formula (1): L B f . - Pdxd i )

Jo Jo wherein P is a component in bars in the axial direction of the air pressure acting on the oblique section 119.

When the oblique sections 119 of the bodies 111 of the blades 103 have the same shape and the same orientation, the axial component F _CO mponent is equal to the product of the number Z of oblique sections (namely, the number of blades 103 having oblique sections 119 in the impellor 100) and the axial component fi of the force f on a single oblique section 119, namely,

The relationship of the axial component F _CO mponent with the shape and the orientation of the oblique section 119 is determined by the following formula (3): wherein n is the design (working) speed in rpm of the impellor 100, q is the design (working) flow rate in g/s of the air compressor 1, d is the diameter in mm of the impellor 100, and K is a compensation factor, depending on the machining accuracy of the air compressor 1 (especially the machining accuracy of the pressure shell 7 of the air compressor 1) and the axial endplay (caused by the axial run-out of the rotation shaft 9 at a steady speed) during rotations of the impellor 100. The value of K may be in the range of 0.5 to 1.5, in the range of 0.8 to 1.2, in the range of 0.9 to 1.1, or any value in these ranges or any other suitable value, and is 1 in an ideal state.

Based on the theory on air films and as schematically shown in Fig. 8, the frontend thickness hi of the air film on the oblique section 119 is equal to the gap between the front edge (namely, the intersecting edge between the oblique section 119 and the positive-pressure surface Illa) of the oblique section 119 and the inner wall of the impellor pressure shell 7, and the rear-end thickness ho is equal to the gap between the rear edge (namely, the intersecting edge between the oblique section 119 and the side surface 111c) of the oblique section 119 and the inner wall of the impellor pressure shell 7. It should be understood that in Fig. 8, the inner wall of the impellor pressure shell 7 is depicted as a plane only for the purpose of schematically explaining the principle. The inner wall of the impellor pressure shell 7 is actually an annular curved surface. As shown in Fig. 8, the relationship between the front-end thickness hi, the rear-end thickness ho, the width L of the oblique section 119 and the included angle a is determined by the following formula (4):

The ratio of the front-end thickness hi to the rear-end thickness ho is greater than 1, and preferably is 2.2.

From formula (3), it can be learned that when the size of the axial component Fcomponent is determined, the relationship between the shape and orientation of the oblique section 119 can be determined. As mentioned above, the expected size of the axial component Fcomponent can be equal to the expected decrement of the axial force on the first thrust bearing 17. The expected size of the axial component Fcomponent can be given by the designer, manufacturer or user of the air compressor 1. For example, during the design of an air compressor, after CFD simulations are performed according to the design requirements to determine the preliminary 3-D structural sizes of the blades (the above-mentioned oblique sections are not formed on the blades at this time), the number of blades and the axial force on the thrust bearing, the expected decrement of the axial force can be determined, and then the expected size of the axial component Fcomponent can be determined. For example once again, simulation calculations and measurements can be performed for the axial force on the thrust bearing in a manufactured air compressor to determine the expected decrement of the axial force, and then the expected size of the axial component Fcomponent can be determined. Then, the specific shape and orientation of the oblique section 119 can be determined according to the above-mentioned formula.

Furthermore, see Figs. 9A and 9B. Figs. 9A and 9B respectively show the results of CFD simulations performed for the pressure distributions of blades 103 of the impellor 100 without/with oblique sections 119 in one embodiment of the impellor 100 shown in Figs. 2 to 6. In the present embodiment, the inlet temperature and the inlet pressure of the air compressor 1 are 25°C and 1 bar, respectively, the design flow rate of the air compressor 1 is 70 g/s, the design speed of the impel- lor 100 is 100000 rpm, and the diameter D of the impellor 100 is 70 mm. In the case in which the blades 103 have no oblique section 119, the axial force on the first thrust bearing 17 is 45.7 N.

The axial force is expected to be reduced by 10%. Therefore, in the above- mentioned formula, n=100000 rpm, q=70 g/s, and F _CO mponent=4.6 N. The thirteen blades 103 of the impellor 100 are all made to form an oblique section 119 having the same shape and orientation, that is to say, Z=13. In an ideal state, K=l.

After these parameters are substituted into formula (4), the relationship between the shape and the orientation of the oblique section 119 can be determined. In one solution, B=10.2 mm, L=0.5 mm, a=22°, hi=0.6 mm and ho=O.4 mm, and the blades 103 are made to form an oblique section 119. After CFD simulations are performed for the pressure distributions of the blades 103 (without/with oblique sections 119) of the impellor 100, the results shown in Figs. 9A and 9B and listed in Table 1 are obtained.

Table 1

From Table 1, it can be seen that in the case in which oblique sections 119 are formed, the axial force on the first thrust bearing 17 is reduced by 4.6 N, while the pressure ratio of the air compressor is reduced only by 0.02%, compared with the case in which no oblique section 119 is formed. Thus, it can be proven that the axial force on the first thrust bearing 17 can effectively be reduced by enabling the bodies 111 of the blades 103 to form oblique sections 119 near the rear end 109, and the performance of the air compressor 1 will not be significantly influenced.

Although all the blades in the embodiments shown in Figs. 2 to 5 have the same 3-D twisted configuration, it should be understood that all the blades may have different 3-D twisted configurations, for example, the combination of long blades and short blades, in other embodiments. Long blades and short blades are evenly arranged in the circumferential direction of the impellor and a short blade is located between two adjacent long blades. Long blades and short blades take on one of the following forms: (1) all the bodies of long blades and short blades form the oblique sections near the rear ends, (2) only the bodies of long blades form the oblique sections near the rear ends, or (3) only the bodies of short blades form the oblique sections near the rear ends. In addition, in consideration of the differences of the air flows borne by long blades and short blades, if all the bodies of long blades and short blades form the above-mentioned oblique sections, the length of the oblique sections of long blades may be greater than the length of the oblique sections of short blades. Furthermore, it should be understood that the blades may also have a 2-D configuration in other embodiments. As used in this document, the terms "first" and "second" are used to distinguish one element or segment from another element or segment, but these elements and segments should not be restricted by such terms. The present application have been described in detail above in combination with specific embodiments. Obviously, the above description and the embodiments shown in the drawings should all be interpreted as exemplary, but do not constitute any restriction of the present application. For example, a single-stage centrifugal air compressor is used in preferred embodiments to describe the present application, but the present application can be applied not only to single-stage centrifugal air compressors, but also to multi-stage centrifugal air compressors, axial-flow type air compressors or other fluid mechanics with blades rotating at high speed. Those skilled in the art can make various changes or modifications without departing from the spirit of the present application, and all these changes or modifications should fall within the scope of the present application.