US20080170938A1

US20080170938A1 - Centrifugal compressor

Info

Publication number: US20080170938A1
Application number: US12/007,752
Authority: US
Inventors: Akinori Shinohara
Original assignee: Toyota Motor Corp
Current assignee: Toyota Motor Corp
Priority date: 2007-01-15
Filing date: 2008-01-15
Publication date: 2008-07-17
Also published as: JP4265656B2; JP2008169801A

Abstract

A technology for widening the operating range of a high-pressure ratio centrifugal compressor including a vaned diffuser is provided. The solidity of each vane in the vaned diffuser is set to the optimum value at which the region, in which burbles do not occur when the air flows into the diffuser, is wider than the region, in which burbles may occur when the air flows into the diffuser. In addition, the solidity is set in such a manner that the straight line, which extends outward from the inner peripheral edge of the vane at a right angle with respect to the chord of the vane, does not cross the adjacent vane. In this way, formation of a throat between the vanes is suppressed.

Description

INCORPORATION BY REFERENCE

The disclosure of Japanese Patent Application No. 2007-005858 filed on Jan. 15, 2007 including the specification, drawings and abstract is incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention
The invention relates generally to a high-pressure ratio centrifugal compressor that is applied to, for example, an engine for an aircraft, and, more specifically, to a high-pressure ratio centrifugal compressor including a vaned diffuser.
2. Description of the Related Art
A diffuser of a centrifugal compressor reduces the speed of the air, which has high speed and high kinetic energy at the outlet of a centrifugal impeller, without energy loss in order to recover the pressure of the air. Such diffusers are grouped into vaned diffusers and vaneless diffusers, and each type has both advantages and disadvantages.
Although the operating range of a vaneless diffuser is wide, the vaneless diffuser has a low diffuser efficiency and the pressure of the air at the outlet of the vaneless diffuser is low. Therefore, the diameter of the vaneless diffuser needs to be large in order to increase the pressure of the air at the outlet. This is because it is necessary to recover large energy loss caused by friction in the vaneless diffuser and the ratio of the inlet diameter to the outlet diameter of the vaneless diffuser is required to be high. Such large loss is caused because the airflow proceeds in the diffuser in a logarithmic spiral and therefore the total flow length is long. The ratio of the inlet diameter to the outlet diameter should be high in order to make the area ratio of the inlet to the outlet high.
On the other hand, a vaned diffuser has a high diffuser efficiency. When the same ratio of the inlet air pressure to the outlet air pressure is exhibited by the vaned diffuser and the vaneless diffuser, the vaned diffuser is smaller in diameter (size) than vaneless diffuser. However, the operating range of the vaned diffuser is narrower than that of the vaneless diffuser. The operating range of the vaned diffuser is further narrower in a high-pressure ratio compressor.
When the air flow rate is low in the vaned diffuser, if the inflow angle of the air flowing from the outlet of a centrifugal impeller offsets from the angle of a diffuser vane, the vane causes the burbles in the vaned diffuser. Accordingly, the air flow becomes unstable (i.e., surging occurs.) Meanwhile, a throat is formed between adjacent vanes of the diffuser. When the air flow speed at the throat reaches the sonic speed, even if the pressure is further increased, the flow rate does not increase any more (i.e., choking occurs).
Accordingly, when the air flow rate is considerably low in the vaned diffuser, even a small disturbance caused in a system makes the airflow unstable. Such unstable airflow may make it difficult to stably operate the centrifugal compressor. In addition, when the air flow rate abruptly decreases, for example, when the rotational speed of the centrifugal compressor is abruptly increased, the air flow may become unstable.
Japanese Patent Application Publication No. 2000-205186 (JP-A-2000-205186) describes the following technology that offers the advantages of both the vaned diffuser and the vaneless diffuser in order to suppress occurrence of above-described inconveniences. According to the technology, retractable vanes, which telescopically project from or retract into at least one of two diffuser walls that face each other, are provided. The retractable vanes are controlled to move from the retracted positions, at which the retractable vanes are housed inside the diffuser wall, to the contact positions, at which the retractable vanes contact the other diffuser wall, in accordance with the air flow rate.
Japanese Patent Application Publication No. 09-100799 (JP-A-09-100799) describes a technology in which retractable projections are provided at throat portions of the vaned diffuser. When the compressor is operated under regular condition, the projections are kept retracted. When the air flow rate is so low that surging may occur, the projections are projected so as to prevent occurrence of surging.
However, the structures according to the above-described technologies are complex, because these structures include movable portions. Such complex structures may make it difficult to reduce the cost, size, or weight of the centrifugal compressor.

SUMMARY OF THE INVENTION

The invention provides a simply-structured high-pressure ratio centrifugal compressor including a vaned diffuser, which is able to operate in a wider operating range.
According to an aspect of the invention, the solidity of each vane in a vaned diffuser is optimized. In this way, the region, which is included in the region between the adjacent vanes and in which burbles do not occur when the air flows into the diffuser, is wider than the region, which is included in the region between the adjacent vanes and in which burbles may occur due to presence of the vane when the air flows into the diffuser.
More specifically, the aspect of the invention relates to a high-pressure ratio centrifugal compressor including the vaned diffuser, in which the solidity of each vane in the vaned diffuser is set in such a manner that the stable air flow region, which is included in the region between the adjacent vanes and in which burbles do not occur when the air flows into the vaned diffuser, is wider than the air swirl occurrence region, which is included in the region between the adjacent vanes and in which burbles may occur due to presence of the vane when the air flows into the vaned diffuser.
The solidity is a value which is obtained by dividing the chord length of the vane in the vaned diffuser by the pitch between the adjacent vanes. According to the aspect of the invention, the pitch between the vanes in the vaned diffuser is set to a sufficiently large value. In the case where the air flow rate is low, even when the inflow angle of the air discharged from the outlet of an impeller offsets from the angle of the vane with respect to line extending in the circumferential direction of the impeller and therefore burbles occur near the vane, a sufficiently large area of the region in the flow passage between the adjacent vanes, in which the air stably flows, is maintained to suppress the influence of the burbles on the entire air flow.
Thus, it is possible to suppress occurrence of surging, and to widen the operating range of the high-pressure ratio centrifugal compressor in the region in which the air flow rate is low.
In the aspect of the invention, the solidity of each vane in the vaned diffuser may be set in such a manner that a throat is not formed between the adjacent vanes.
The factor that restricts the operating range of the vaned diffuser together with the surging described above is diffuser choking that occurs when the flow rate of the air flowing into the diffuser increases. More specifically, when the flow speed of the air flowing into the diffuser increases and reaches the sonic speed at the region at which the flow passage area is smallest, the flow rate does not increase any more. This phenomenon restricts the operating range of the vaned diffuser in the region in which the air flow rate is high.
Therefore, according to the aspect of the invention, the region at which the flow passage area is smallest is not formed between the adjacent vanes in the vaned diffuser. In this way, even if the flow speed of the air increases and reaches the sonic speed, diffuser choking does not occur easily. Therefore, the operating range of the vaned diffuser in the region in which the air flow rate is high is not easily restricted.
More specifically, in the aspect of the invention, the solidity of each vane in the vaned diffuser may be set in such a manner that a straight line, which extends outward from the inner peripheral edge of the vane at a right angle with respect to the chord of the vane, does not cross the adjacent vane. Thus, it is possible to more reliably suppress formation of a throat between the adjacent vanes, and to suppress occurrence of diffuser choking.
In the aspect of the invention, the shape of the vane may be set in such a manner that the width of the passage formed between the adjacent vanes is substantially constant.
In the aspect of the invention, the solidity of each vane in the vaned diffuser may be set in such a manner that a surge margin is equal to or larger than 0.1.
As the surge margin increases, surging is less likely to occur during the operation of the high-pressure ratio centrifugal compressor. When the surge margin is equal to or larger than 0.1, the operating range of the vaned diffuser is sufficiently wide in the region in which the air flow rate is low. There is a high correlation between the solidity of each vane in the vaned diffuser and the surge margin. As the solidity decreases, the surge margin increases.
Accordingly, if the solidity of each vane in the vaned diffuser is set in such a manner that the surge margin is equal to or larger than 0.1, the sufficiently large operating range of the vaned diffuser is more reliably maintained in the region in which the air flow rate is low.
In the aspect of the invention, the solidity of each vane in the vaned diffuser may be set to a value equal to or lower than 1.5.
Next, the relationship between the solidity of the vane in the vaned diffuser and the surge margin will be described. In the region in which the solidity of the vane in the vaned diffuser is equal to or higher than approximately 2, the surge margin hardly changes even if the solidity changes. In contrast, in the region in which the solidity of the vane in the vaned diffuser is lower than approximately 2, the surge margin considerably increases if the solidity of the vane in the vaned diffuser increases.
Therefore, in the aspect of the invention, the region in which the solidity is equal to or lower than 1.5 is used. Thus, it is possible to reliably maintain a sufficiently large surge margin. In addition, the surge margin can be easily adjusted by appropriately setting the solidity.
The aspect of the invention may be applied to a high-pressure ratio centrifugal compressor in which the flow speed of the air flowing into the vened diffuser reaches the sonic speed. In the vaned diffuser used in such condition, diffuser choking easily occurs at the throat, which narrows the operating range of the high-pressure ratio centrifugal compressor in the region in which the air flow rate is high. Therefore, the effect of the aspect of the invention can be made more prominent by applying the aspect of the invention to such a high-pressure ratio centrifugal compressor.
In the aspect of the invention, the vaned diffuser may be used in an extraction engine. In the extraction engine, a change itself in the power output from the engine (namely, a change itself in the extracted gas amount) directly leads to a change in the flow rate of the air flowing into the vaned diffuser. Therefore, if the surge margin is not sufficiently large, it may be difficult to control the power output from the engine. Namely, it is especially necessary to maintain a sufficiently large surge margin in the extraction engine. Therefore, more significant effect can be obtained by applying the aspect of the invention to the extraction engine.
The above-described configurations can be combined with each other.
According to the aspect of the invention, it is possible to provide the simply-structured high-pressure ratio centrifugal compressor including the vaned diffuser, which is able to operate in the wider operating range.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and further objects, features and advantages of the invention will become apparent from the following description of example embodiments with reference to the accompanying drawings, wherein the same or corresponding portions will be denoted by the same reference numerals and wherein:

FIG. 1 is a view schematically showing the structure of a high-pressure ratio centrifugal compressor, especially, a portion near a diffuser according to embodiments of the invention;

FIG. 2 is a view schematically showing the structure of a vaned diffuser according to a first embodiment of the invention;

FIG. 3 is a view illustrating the burble occurrence regions in the first embodiment of the invention;

FIGS. 4A to 4C are graphs each illustrating the surge line and restriction on the operation of the high-pressure ratio centrifugal compressor according to the first embodiment of the invention;

FIG. 5 is a view showing the relationship between the burble occurrence regions and the stable air flow regions according to the first embodiment of the invention;

FIG. 6A is a graph illustrating the definition of the surge margin;

FIG. 6B is a graph illustrating the relationship between the solidity and the surge margin;

FIG. 7A is a view showing throats in a second embodiment of the invention;

FIG. 7B is a view illustrating the solidity at which throats are not formed; and

FIG. 8 is a graph illustrating the operating range of the high-pressure ratio centrifugal compressor, which is widened according to the embodiments of the invention.

DETAILED DESCRIPTION OF THE EXAMPLE EMBODIMENTS

Hereafter, embodiments of the invention will be described with reference to the accompanying drawings.
FIG. 1 is a view schematically showing the structure of a high-pressure ratio centrifugal compressor 1, especially, a portion near a diffuser 4 according to the embodiments of the invention. The high-pressure ratio centrifugal compressor 1 is applied to, for example, an extraction engine for an aircraft, and is used to compress air. The high-pressure ratio centrifugal compressor 1 includes an impeller 3 and the diffuser 4. The impeller 3 is rotatably provided near the center of a casing 2. The air flows into the impeller 3 in the axial direction of the high-pressure ratio centrifugal compressor 1, and flows out from the impeller 3 in the radial direction of the high-pressure ratio centrifugal compressor 1. The diffuser 4 is provided at a position outside of the impeller 3 in the radial direction of the high-pressure ratio centrifugal compressor 1. The diffuser 4 reduces the speed of the air flowing out of the impeller 3 to increase the pressure of the air, and sends the air to a scroll 2 a which is provided at a position outside of the diffuser 4 in the radial direction of the high-pressure ratio centrifugal compressor 1. The impeller 3 is provided coaxially with the high-pressure ratio centrifugal compressor 1. The impeller 3 is rotated by a turbine (not shown) via a turbine shaft 3 a. The diffuser 4 is formed as an annular passage, which is defined by diffuser walls 5 and 6 that face each other, which has a predetermined passage width, and which is coaxial with the high-pressure ratio centrifugal compressor 1. In addition, vanes 7 are provided in the diffuser 4 in order to efficiently reduce the speed of the air flowing into the diffuser 4 to recover the air pressure.
FIG. 2 is a view showing the vanes 7 provided in the diffuser 4. As shown in FIG. 2, multiple vanes 7 are provided on the diffuser wall 6 of the diffuser 4 at regular intervals in the circumferential direction of the impeller 3, and the vanes 7 are inclined θ degrees with respect to line extending in the circumferential direction of the impeller 3. The length of the vane 7 is referred to as a “chord length”, and the interval between the adjacent vanes 7 is referred to as a “pitch”. The value which is obtained by dividing the chord length by the pitch is referred to as a “solidity”. When the impeller 3 is rotated, the air is sucked into the impeller 3, energized by the impeller 3, and then discharged from the impeller 3. The air discharged from the impeller 3 flows into the diffuser 4. The diffuser 4 reduces the speed of the air to recover the air pressure.
In the high-pressure ratio centrifugal compressor 1 described above, when flow rate of the air discharged from the impeller 3 becomes lower and the inflow angle of the air flowing into diffuser 4 becomes smaller, the inflow angle offsets from the angle of the vane 7 with respect to line extending in the circumferential direction of the impeller 3, which may cause burbles from the front edge of the vane 7, as shown in FIG. 3. If the proportion of the area of a region 8 where the burbles occur (hereinafter, referred to as “burble occurrence region 8”) to the area of the air passage defined by the adjacent vanes 7 becomes high, the air flow becomes considerably unstable in the air passage. As a result, usual phenomena such as vibration occur, which may cause surging.
If surging occurs, it is difficult to continue the operation of the high-pressure ratio centrifugal compressor 1. Therefore, the operation of the high-pressure ratio centrifugal compressor 1, which may cause a decrease in the flow rate of the air flowing into the diffuser 4, is restricted. The restriction on the operation of the high-pressure ratio centrifugal compressor 1 will be described in detail with reference to FIG. 4.
For example, when the high-pressure ratio centrifugal compressor 1 is operated at an operation point on the operation line shown in FIG. 4A, if there is only a small margin between the operation line and the surge line, even a small disturbance causes the operation point of the high-pressure ratio centrifugal compressor 1 to cross the surge line into the surge region where the high-pressure ratio centrifugal compressor 1 may be operated unstably. As shown in FIG. 4B, when the rotational speed of the high-pressure ratio centrifugal compressor 1 is abruptly increased, the operation point of the high-pressure ratio centrifugal compressor 1 once moves to the lower flow rate region. Therefore, the operation point may cross the surge line into the surge region. In such a case, because the rotational speed of the high-pressure ratio centrifugal compressor 1 cannot be abruptly increased, it is difficult to use the high-pressure ratio centrifugal compressor 1 in an aircraft that needs be abruptly accelerated relatively frequently. FIG. 4C shows the case where the high-pressure ratio centrifugal compressor 1 is applied to an extraction engine. In this case, decreasing the power output from the engine directly leads to a decrease in the flow rate of the air flowing into the diffuser 4. Accordingly, if there is only a small margin between the operation point and the surge line, it may be difficult to control the power output from the engine.
Therefore, according to a first embodiment of the invention, the solidity of the vane 7 in the diffuser 4 is set in such a manner that a stable air flow region 9, which is the region other than the burble occurrence region 8 in the air flow passage defined by the adjacent vanes 7, is wider than the burble occurrence region 8. Thus, even if the air flow rate in the high-pressure ratio centrifugal compressor 1 decreases and the burbles occur, it is possible to maintain sufficiently wide area of the stable air flow region 9 in which the air flow is not turbulent. Therefore, it is possible to suppress occurrence of surging in the high-pressure ratio centrifugal compressor 1 as a whole. FIG. 5 shows the relationship between the burble occurrence region 8 and the stable air flow region 9 under this condition.
The relationship between the air flow rate and the area of the burble occurrence region 8, and the relationship between the air flow rate and the area of the stable air flow region 9 in the high-pressure ratio centrifugal compressor 1 may be determined through experiments or according to a simulation method. It is possible to calculate the area of the burble occurrence region 8 and the area of the stable air flow region 9 based on the geometric relationship shown in FIG. 5. In this case, at first, it is necessary to preliminarily determine the relationship between air flow rate and the inflow angle of the air flowing into the diffuser 4 in the high-pressure ratio centrifugal compressor 1.
If the area of the stable air flow region 9 becomes wider than the burble occurrence region 8, the surge margin is increased. Therefore, it is possible to estimate whether the stable air flow region 9 becomes wider than the burble occurrence region 8 by detecting the value of the surge margin with respect to the solidity. The case where the solidity is set on the assumption, which is made based on the value of the surge margin, that the stable air flow region 9 becomes wider than the burble occurrence region 8 will be described below.
FIG. 6A is a graph illustrating the definition of the surge margin, and FIG. 6B is a graph showing the relationship between the solidity and surge margin.
FIG. 6A is a graph illustrating the definition of the surge margin. The surge line, indicated by the solid line, is the boundary between the region in which surging may occur and the region in which surging does not occur. The operation line, indicated by the dashed line, is defined by the rating of the high-pressure ratio centrifugal compressor 1. The surge margin is calculated according to Equation 1 based on the air flow rate G₀determined based on the rating when the rotational speed of the high-pressure ratio centrifugal compressor 1 is constant, and the air flow rate Gsurge at which surging occurs at the same constant speed. The air flow rate G₀is the air flow rate at the point at which the operation line and the constant rotational speed curve intersect with each other. The air flow rate Gsurge is the air flow rate at the point at which the surge line and the constant rotational speed curve intersect with each other.
Surge margin=1−Gsurge/G ₀ Equation 1
FIG. 6B is a graph showing the relationship between the solidity and surge margin. In the region where the solidity is equal to or lower than 2, the surge margin is efficiently increased by decreasing the solidity. In other words, if the solidity is equal to or lower than approximately 2, the stable air flow region 9 is estimated to be wider than the burble occurrence region 8. In the first embodiment of the invention, the solidity is set to 1.5. In this way, the surge margin is equal to or larger than approximately 0.1, which is sufficiently large.
As described above, in the first embodiment of the invention, the pitch between the vanes 7 of the diffuser 4 is made large so that the stable air flow region 9 is wider than the burble occurrence region 8. Accordingly, even if the air flow rate in the high-pressure ratio centrifugal compressor 1 becomes lower, it is possible to prevent occurrence of surging and to continue stable operation of the high-pressure ratio centrifugal compressor 1.
In addition, in the first embodiment of the invention, it is presumed that the stable air flow region 9 is wider than the burble occurrence region 8 when the rate of increase in the surge margin in accordance with a decrease in the solidity becomes abruptly higher. The solidity smaller than the solidity at the boundary is selected so that the surge margin abruptly increases in accordance with a decrease in the solidity. Accordingly, it is possible to more reliably and easily set the solidity at which the stable air flow region 9 is wider than the burble occurrence region 8.
The burble occurrence region 8 corresponds to the air swirl occurrence region according to the invention. The stable air flow region 9 corresponds to the stable air flow region according to the invention.
Next, a second embodiment of the invention will be described. According to the first embodiment of the invention, the surge margin is increased by increasing the solidity of the vaned diffuser. Thus, the operating range of the high-pressure ratio centrifugal compressor 1 is widened in a region in which the air flow rate is low. In contrast, according to the second embodiment of the invention, the operating range of the high-pressure ratio centrifugal compressor 1 is widened in a region in which the air flow rate is high. Note that, the schematic structure of the high-pressure ratio centrifugal compressor 1 according to the second embodiment of the invention is the same as that shown in FIG. 1.
The throat restricts the operating range of the high-pressure ratio centrifugal compressor 1 in the region in which the air flow rate is high. The throat is the region where flow passage area between adjacent vanes 7 is smallest. If the air flow speed at the throat reaches the sonic speed, even if the pressure is further increased, the flow rate does not increase any more. FIG. 7A shows an example of the position of the throat.
In the second embodiment of the invention, as shown in FIG. 7B, the solidity is set in such a manner that the straight line, which extends outward from the inner peripheral edge of the vane 7 at a right angle with respect to the chord of the vane 7, does not cross the adjacent vane 7. In this way, formation of a throat between the adjacent vanes 7 is suppressed. As a result, it is possible to widen the operating range, in a region in which the air flow rate is high.
In the second embodiment of the invention, the solidity may be determined in such a manner that both of the following two conditions are satisfied. One of the condition is the condition that the stable air flow region 9 is wider than the burble occurrence region 8, as in the first embodiment of the invention. The other condition is the condition that the straight line that extends outward from the inner peripheral edge of the vane 7 at a right with respect to the vane 7 does not cross the adjacent vane 7. In this way, it is possible to widen the operating range in both the region in which the air flow rate is low and the region in which the air flow rate is high. Thus, it is possible to obtain significantly wider operating range than the comparative technology. FIG. 8 shows an example of the operating range of the high-pressure ratio centrifugal compressor 1, which is widen in both the region in which the air flow rate is low and the region in which the air flow rate is high.
As described above, the solidity is determined in such a manner that the straight line, which extends outward from the inner peripheral edge of the vane 7 of the vaned diffuser 4 at a right angle with respect to the chord of the vane 7, does not cross the adjacent vane 7. Accordingly, it is possible to suppress formation of a throat between adjacent vanes 7. However, the method for suppressing formation of a throat between the adjacent vanes 7 is not limited to this. For example, the width of the vane may be set, for example, in such a manner that the width of the passage formed between the adjacent vanes is substantially constant. Alternatively, the width of the vane may be set in such a manner that the width of the passage formed between the adjacent vanes is gradually increased or decreased.

Claims

1. A high-pressure ratio centrifugal compressor, comprising:

a vaned diffuser wherein a solidity of each vane is set in such a manner that a stable air flow region, which is included in a region between the adjacent vanes and in which burbles do not occur when air flows into the vaned diffuser, is wider than an air swirl occurrence region, which is included in the region between the adjacent vanes and in which there is a possibility that burbles occur due to presence of the vane when the air flows into the vaned diffuser.

2. The high-pressure ratio centrifugal compressor according to claim 1, wherein the solidity of each vane in the vaned diffuser is set in such a manner that a throat is not formed between the adjacent vanes.

3. The high-pressure ratio centrifugal compressor according to claim 2, wherein the solidity of each vane in the vaned diffuser is set in such a manner that a straight line, which extends outward from an inner peripheral edge of the vane at a right angle with respect to a chord of the vane, does not cross the adjacent vane.

4. The high-pressure ratio centrifugal compressor according to any one of claims 1 to 3, wherein the solidity of each vane in the vaned diffuser is set in such a manner that a surge margin is equal to or larger than 0.1.

5. The high-pressure ratio centrifugal compressor according to any one of claims 1 to 3, wherein the solidity of each vane in the vaned diffuser is set to a value equal to or lower than 1.5.

6. The high-pressure ratio centrifugal compressor according to claim 2, wherein the high-pressure ratio centrifugal compressor operates properly in a state where a speed of the air flowing into the vaned diffuser reaches a sonic speed.

7. The high-pressure ratio centrifugal compressor according to claim 3, wherein the high-pressure ratio centrifugal compressor operates properly in a state where a speed of the air flowing into the vaned diffuser reaches a sonic sound.

8. The high-pressure ratio centrifugal compressor according to claim 1, wherein the vaned diffuser is used in an extraction engine.