Diffuser for centrifugal compressor.

0001This invention relates to a diffuser for a centrifugal compressor of a type in which a plurality of stator blades are disposed on the outside of an impeller thereof, and which converts kinetic energy of fluid discharged from said impeller into pressure energy by operation of said stator blades.

0002Such a diffuser is suitable for use in a high speed centrifugal compressor in which high pressure ratio can be obtained by a single stage.

0003Generally, a centrifugal compressor generates a high speed air flow by the rotation of an impeller thereof. In a case of a high speed centrifugal compressor which is so designed that high pressure ratio can be obtained through a single stage, since the speed of air discharged from the impeller exceeds sonic velocity, a diffuser having stator blades is provided outside the impeller, this outside being corresponds to the down­stream of the impeller, for the purpose of converting kinetic energy of the fluid discharged from the impeller into pressure energy.

0004A plurality of stator blades forming the diffuser is provided in the periphery portion or outside of the impeller, and spaces between these stator blades form diffuser passages.

0005In the above-described type of compressor, when the rotational speed is relatively high and discharge (flow rate) is also relatively small, a separation flow region is generated in the negative pressure side of the stator blade, as a result of which, a problem arises such that the surge phenomenon is generated in which sufficient rise in pressure cannot be obtained. A diffuser for high speed centrifugal compressor which can overcome the above-described problem and in which the surging phenomenon is prevented even if in the high speed and small discharge state is disclosed in JP 57-159998, in which fluid passing through a diffuser is controlled by a rotatable sub-blade at the inlet portion of the diffuser for the purpose of controlling the fluid passing through the diffuser. Furthermore, in JP 53-119411, it is proposed that a blade-provided diffuser is formed in a double circular blade cascade and the length of the blade in the inner annular blade cascade is arranged to be no more than 0.9 times of the interval of the blades.

0006However, in the former case, since the diffuser passage formed between stator blades are drastically enlarged immediately behind the downstream of the sub-blade, a problem arises such that pressure loss is generated, and the choking flow is reduced, causing the performance of the diffuser to be deteriorated. Meanwhile, in the latter case, although the above-described problem of reduction in choking flow does not arise, but loss is generated due to a strong shear flow generated at the downstream of the blades of the inner circular blade cascade when the speed of the fluid at the inlet portion of the blades which form the inner circular blade cascade exceeds sound velocity. As a result, a problem arises such that the performance of the diffuser is deteriorated.

0007The object of the present invention is to provide a diffuser for a centrifugal compressor which can overcome the above-described problems and which exhibits wide operating range and high performance.

0008In order to achieve the above-described object, the diffuser of the generic kind comprises sub-blades whose length of chord is shorter than that of the stator blade and which are disposed near inner end of and between the plurality of stator blades, only one side surface of the sub-blade confronting the stator blade, and the sub-blade being situated at the positions intersecting a circle which have a center thereof at the center of a rotational shaft of the impeller and which passes through the inner end of the stator blade; and preferably intermediate blades which are disposed near the outer circumference or ends of and between the plurality of stator blades, and whose length of chord is shorter than that of the stator blade, each of which extends through the middle point of a perpendicular line drawn from an outer edge of the stator blade to the neighboring stator blade or to an extension of inside of the neighboring stator blade, whose outer edge reaches a circle which passes through an outer edge of the stator blade, the length of the intermediate blade disposed inside the middle point of the perpendicular line being no more than 20% of the overall length of the intermediate blade, and whole shape of the intermediate blade being formed in such a manner that if the intermediate blade is assumed to be rotationally displaced by a certain angle around the center of a rotational shaft of the impeller, the intermediate blade is included in a contour of said stator blade.

0009In the above-described structure, since only one side of the sub-blade confronts the stator blade forming the diffuser, this sub-blade does not enter the region in which the flow is strictly restricted by the neighboring stator blade. Therefore, the drastic or rapid enlarge­ment of the cross sectional area of the flow passage and reduction in the choking flow immediately behind the downstream of the sub-blade do not occur. Furthermore, since only one side of the sub-blade confronts the stator blade, the distance between the front edge of the region disposed between the stator blades and the rear edge of the sub-blade is relatively short. Therefore, the region which may generate the strong shear flow can be limited short, causing loss to be low.

0010It is preferable for the inner sub-blade to be made as thin as possible, however, a certain thickness is required for keeping strength. Therefore, the number of the stator blades needs to be selected to secure sufficient cross sectional area of flow. In such a case, the performance may be lowered, because the flow passage near the outer periphery of the stator blade, or the interval of the stator blades is excessively enlarged. However, since intermediate blades are extended near the outer periphery of and between stator blades in such a manner that the intermediate blade extends through the middle point of a perpendi­cular line from the outer end of the stator blade to the blade surface of the neighboring blade, the intervals of the stator blades can be made proper value near the outer periphery of the stator blade. As a result of this, reduction in performance can be prevented.

0011In accordance with a further embodiment the diffuser of the generic kind comprises intermediate blades which are disposed between said plurality of stator blades, and each of which comprise inlet-side intermediate blade whose chord length and height are smaller than those of said stator blade. Preferably said intermediate blade includes a rectifier blade which is disposed at a downstream of said inlet-side intermediate blade, and a height of which is shorter than that of said inlet-side intermediate blade and conveniently an outlet-side intermediate blade is provided at the downstream of said rectifier blade.

0012As described above, according to the embodiments of the present invention, since the diffuser which converts kinetic or speed energy of the air flow discharged from an impeller of a centrifugal compressor or air fan into pressure energy and which is provided with a plurality of stator blades has inlet-side intermediate blades whose height is shorter than that of the stator blade, the operating flow rate range of the diffuser can be enlarged without deterioration in performance, so that the operating flow rate range of the centrifugal compressor or air fan can be significantly enlarged.

0013Embodiments of the present invention will now be described with reference to the drawings. <ul id="ul0001" list-style="none"><li>Fig. 1 is a sectional view of a diffuser for a centrifugal compressor according to an embodiment of the present invention;</li><li>Fig. 2 is a cross-sectional view taken along the line II-II in Fig. 1;</li><li>Figs. 3 to 6 are diagrams respectively illustrating the operation of the diffuser according to an embodiment of the present invention;</li><li>Figs. 7 to 9 are enlarged views respectively illustrating essential portions according to other embodiments of the present invention;</li><li>Fig. 10 is a cross-sectional view of a diffuser for a centrifugal compressor according to a still another embodiment of the present invention;</li><li>Fig. 11 is a perspective view of the diffuser portion according to the embodiment shown in Fig. 10; and</li><li>Figs. 12 to 15 are enlarged views respectively illustrating an essential portion according to other embodiments of the present invention.</li></ul>

0014Figs. 1 to 3 illustrate an embodiment of the present invention, wherein reference numeral 1 represents an impeller formed by blades 1A and core plate 1B. Reference numeral 2 represents a rotational shaft connected to a driving means or motor (omitted from illustration). Reference numeral 3 represents a casing, reference numeral 4 represents a suction pipe, reference numeral 5 represents a diffuser portion, and reference numeral 6 represents a scroll casing. The above-described diffuser portion 5 comprises: a blade-side diffuser casing 5A; a core-side diffuser casing 5B; a plurality of stator blades 7 disposed, as shown in Fig. 2, between the diffuser casings 5A and 5B; and intermediate blades 9 and sub-blades 8 provided for the blade-side diffuser casing 5A or for the core-side diffuser casing 5B in such a manner that the same project into the diffuser flow passage between the stator blades.

0015The characteristics of this embodiment lies in the sub-blades 8 and intermediate blades 9 disposed in the diffuser portion 5. The sub-blades 8 are disposed in such a manner that they intersect a circle 10 making the center of the rotational shaft 2 of the impeller 1 as its center and passing the inner end or edge (front end or edge) of the stator blade 7. Assuming that a perpendicular line drawn from the front end of a stator blade 7b, situated at a side of a center of radius of curvature of a neighboring stator blade 7a, down to the other stator blade 7a is represented by reference numeral 11, the sub-blade 8 is disposed not to intersect this perpendicular line 11. The intermediate blades 9 are disposed in such a manner that they pass the middle point of a perpendicular line 12 drawn from the rear end (outer end) of the stator blade 7a to the neighboring stator blade 7b and the rear end or edge of the intermediate blade 9 reaches a circle 13 which passes through the outer periphery or rear ends or edges of the stator blades 7. The length of the intermediate blade 9 projecting from the middle point of the perpendicular line 12 toward inside is no more than 20% of the overall length of this intermediate blade 9. The whole shape of the intermediate blade 9 is designed such that the blade 9 is included in the stator blade 7 if this intermediate blade 9 is assumed to be rotationally displaced by a certain angle around the center of the rotational shaft 2. Furthermore, the stator blades 7, the sub-blades 8 and the intermediate blades 9 are respectively restricted at their both ends by the confronting casings 5A and 5B in the diffuser portion, the resulted space forming the diffuser portion 5.

0016The operation of the diffuser of this embodiment will now be described. The kinetic energy of air flow A discharged from the impeller 1 is converted into pressure energy and the air is compressed at the time of its passing through the diffuser portion 5. In high speed centrifugal compressors, since the flow velocity of air flow A introduced into the diffuser portion 5 exceeds sound velocity, shock wave is generated, causing the flow velocity to be reduced to subsonic speed. Fig. 3 shows strong shock waves which are generated near the front ends of the blades 7 and 8, and which affect the flow, such shock waves being generated in a case where the Mach number of the air flow A approximates 1 (for example 1.1 or less. The angle ϑ defined by the air flow A and the stator blade 7 is changed in accordance with the flow rate of air compressed by the compressor. However, since the strong shock wave is generated at the front end of the blade, the state shown in Fig. 3 is not changed. A shock wave 15 generated near the stator blade 7 is only around the blade 7, but it does not strike or reach the other type of blades, that is, sub-blade 8 or stator blade 7. The reason for this will be described in detail. The shock wave 15a generated at the front end of the stator blade 7a does not strike or reach the sub-blade 8 and neighboring stator blade 7b since the shock wave 15a is extended substantially perpendicular to the stator blade 7a in a case where the Mach number of the air flow A approximates 1. Subsonic flow 17 passes through a region situated between the sub-blade 8 and the negative pressure side 16 of the stator blade 7a and a region in its downstream between a dashed line 18 and the negative pressure side 16. Since the shock waves are generated when a supersonic flow is decelerated to a subsonic flow, the shock wave 15b generated at the front end of the stator blade 7b confronting the stator blade 7a only reaches the dashed line 18, and it does not reach the negative pressure side 16 of the stator blade 7a. By provision of the sub-blade 8 in the manner as described above, the shock wave is prevented from reaching the negative pressure side 16, and the operating range can be enlarged by avoiding the occurrence of surging phenomenon. The reason for this will be described.

0017In general, pressure in the direction of air flow passing through the blade-provided diffuser rises according to the reduction in the flow rate of the compressor. If it exceeds a certain limit, the same generates a back run, causing the stop of normal compressing function. Thus, so-called surging phenomenon is generated, and the compressor cannot be operated normally. The limit causing the diffuser to generate the back run is varied in accordance with the shape of the stator blade or the like. The generation of the back run is likely to be easily generated by separation of the air flow from the surface of the stator blade or the wall surfaces facing both sides of the stator blade. In general, separation of the air flow from the negative pressure side of the stator blade is a major cause. In this state, if the shock wave has reached the negative pressure side, the boundary layer along the negative pressure side is likely to undergo, due to strong rise in pressure in front of and behind the shock wave, rapid increase in its thickness, partial separation, or large scale of separation depending on circumstances. Therefore, the separation of the air flow layer from the negative pressure side can be substantially prevented and the limit causing the back run can be shifted to lower flow rate range by preventing the shock wave from reaching the negative pressure side. Namely, occurrence of surging phenomenon due to the diffuser can be suppressed.

0018The larger or higher flow rate limit of the air flow is defined in accordance with the minimum cross-sectional area of the flow passage in the diffuser portion. Therefore, referring to Fig. 3, it is defined by the length of the perpendicular or normal line 11 drawn from the front end of the stator blade 7b to the negative pressure side 16 of the stator blade 7a. In this case, since the sub-blade 8 does not intersect the perpendicular line 11, avoiding the perpendicular line 11 to be shortened, it does not affect the larger flow rate limit. As described above, thanks to the provision of the sub-blade 8, the smaller or lower flow rate limit can be shifted to smaller flow rate range without any reduction in the larger flow rate limit.

0019The above-described effect of enlarging the flow rate range by means of the sub-blade 8 is improved when the sub-blade 8 satisfies the following condition without any deterioration in the performance of the diffuser: a first condition is; the rear end of the sub-blade 8 is situated at an upstream side of the perpendicular line 11. If the sub-blade 8 intersects the perpendicular line 11, the maximum flow rate, as described, decreases and the pressure loss is generated as well due to the rapid enlargement of the cross-sectional area of the flow passage. That is, since the passage situated at the downstream of the perpendicular line 11 is located between the stator blades 7a and 7b, the width of the passage is rapidly or drastically enlarged by the thickness h of the rear end if the rear end of the sub-blade 8 is situated at a position downstream of the perpendicular line 11. On the other hand, in the region within a distance p between the rear end of the sub-blade 8 and the perpendicular line 11, since the air flow 17 which has been reduced in its velocity to subsonic after passing between the sub-blade 8 and the stator blade 7b and the supersonic flow 19 at the upstream of the shock wave 15b are brought into contact with each other and mixed each other, large pressure loss is generated. Therefore, the distance p is required to be small enough. The distance p is required or preferred to be 50% or less of the distance m between the front end of the stator blade 7b and the perpendicular line 11.

0020The second condition is: the ratio r/q of the distance r at the outlet between the sub-blade 8 and the stator blade 7a with respect to the distance q at the inlet of the sub-blade 8 and the stator blade 7a is approx­imated to 1, for example, it being 1 to 1.1. If r/q is made outside of this range, the flow will be separated from the surface of the sub-blade 8, causing loss at the downstream of the sub-blade 8 or stator blade 7a, to be increased.

0021The third condition is: the ratio n/q of the length n of the portion where the sub-blade 8 and the stator blade 7 confronts each other with respect to the distance q between the front end of the stator blade 7a and the surface of the sub-blade 8 is required to be larger than 1 for the purpose of ensuring to make the air flow 17 subsonic.

0022According to this embodiment, since the sub-blade 8 does not protrude into the region where the flow is strictly restricted between the stator blades 7a and 7b, the rapid enlargement of the cross-sectional area of the flow passage and reduction in the choking flow rate at the immediately downstream of the sub-blade 8 do not occur. Furthermore, the distance between the outer end of the sub-blade 8 and the perpendicular line 11 is relatively short, and the region in which strong shear flow occurs is thereby short, reducing the pressure loss. Since the rear end of the sub-­blade 8 is situated between the front end of the stator blade 7a and the front end of the stator blade 7b, shock wave does not reach the surface of the stator blade 7a, reducing the fear of the air flow 17 to be separated from the surface of the stator blade 7a. As a result of this, the range of flow rate where the diffuser portion 5 can be normally operated can be enlarged.

0023Fig. 4 shows a preferred embodiment for use in a case where the Mach number of the air flow A introduced into the diffuser exceeds 1.1. In this embodiment, the front end of the sub-blade 8 is situated at the upstream of the front end of the stator blade 7a. When the Mach number of the air flow A increases, wave front of the shock waves 15 and 20 are bent or curved. Therefore, in order to prevent occurrence of collision of the shock wave 15a with the surface of the sub-blade 8, shock wave 20 is generated at the front end of the sub-blade 8 for the purpose of making the flow subsonic which passes through a passage 21 between the sub-blade 8 and the stator blade 7a.

0024The sub-blade 8 is, in the viewpoint of aerodynamics, preferable to be made as thin as possible, but it is required to be thick enough to have a reasonable strength structurally. That is, an appropriate length should be selected depending on the thickness (for example, 5 to 10 times of the thickness). In this state, the number of the stator blades needs to be selected to satisfy the above-mentioned relationship between the stator blade 7 and the sub-blade 8. In general, it should be decreased down to 80% or less with respect to the case where no sub-blade 8 is provided. As described above, by decreasing the number of the stator blades 7, the interval between stator blades 7 becomes too large near the outer periphery (outlet side), which may prevent the flow from passing along the surface of the stator blade 7 to result in the reduction of the performance.

0025With reference to Figs. 5 and 6, the operation or action of the intermediate blade 9 will now be explained. Fig. 5 shows a case where such intermediate blade is not provided, wherein the flow does not pass along the surface of the blade on the negative pressure side 21 near the rear end of the stator blade 7a, causing large separation region 22 to be generated. This generation of the large separation region causes the reduction in the substantial cross-sectional area. As a result of this, the velocity reduction in the diffuser is deteriorated and the kinetic energy is dissipated in the separation region, causing the performance of the diffuser to be deteriorated. Such a type of large separation can be prevented by reducing load (deceleration) on the negative pressure side 21 near the rear end. This will be described with reference to Fig. 6. The amount of deceleration on a negative pressure side 21 near the rear end of the stator blade 7a can be expressed, according to the one-dimensional flow theory, as h·sinβ/f-1 by using the circumferential distance h between a rear end 23 of the stator blade 7a and a rear end 24 of the stator blade 7b, the outlet angle β of the stator blade 7, and the length f of the perpendicular or normal line drawn from the rear end 23 of the stator blade 7a to its neighboring stator blade 7b. As this value becomes larger, the deceleration load becomes larger. The value h·sinβ/f-1 is determined in accordance with the shape and the number of the stator blades. As the number of the blades become large, it becomes small. Table 1 shows examples. In the case where the intermediate blades 9 are provided, the amount of deceleration near the rear end negative pressure side of the stator blade 7a can be expressed as g·sinβ/e-1 by the same reason as above. A deceleration load when the intermediate blade is provided is also shown on Table 1. As shown on Table 1, thanks to the provision of the intermediate blade 9, the deceleration of 23% can be made deceleration of 19% in a case where the number of the stator blades is 17. As a result, the amount of deceleration can be reduced by 20%, causing the occurrence of large separation near the rear end of the negative pressure side to be suppressed. <tables id="tabl0001" num="0001"><table frame="all"><title>Table 1</title><tgroup cols="4" colsep="1" rowsep="1"><colspec colnum="1" colname="col1" colwidth="39.37mm" /><colspec colnum="2" colname="col2" colwidth="39.37mm" /><colspec colnum="3" colname="col3" colwidth="39.37mm" /><colspec colnum="4" colname="col4" colwidth="39.37mm" /><thead valign="top"><row><entry namest="col1" nameend="col4" align="center">Effect of Intermediate blades</entry></row><row><entry namest="col1" nameend="col1" align="center">The number of stator blades</entry><entry namest="col2" nameend="col2" align="center">21</entry><entry namest="col3" nameend="col3" align="center">17</entry><entry namest="col4" nameend="col4" align="center">17(with intermediate blade)</entry></row></thead><tbody valign="top"><row><entry namest="col1" nameend="col1" align="left">h·sinβ/f-1</entry><entry namest="col2" nameend="col2" align="char" char=".">0.16</entry><entry namest="col3" nameend="col3" align="char" char=".">0.23</entry><entry namest="col4" nameend="col4" align="char" char=".">-</entry></row><row rowsep="1"><entry namest="col1" nameend="col1" align="left">g·sinβ/e-1</entry><entry namest="col2" nameend="col2" align="char" char=".">-</entry><entry namest="col3" nameend="col3" align="char" char=".">-</entry><entry namest="col4" nameend="col4" align="char" char=".">0.19</entry></row></tbody></tgroup></table></tables>

0026As described above, since the intermediate blade 9 serves to prevent the occurrence or generation of the large separation near the rear end of the stator blade 7a, the intermediate blade 9 is arranged in such a manner, for the purpose of ensuring to restrict the flow near the rear end of the stator blade 7, that it intersects the perpendicular line drawn from the rear end 23 of the stator blade 7a to the neighboring stator blade 7b, and that the rear end 25 reaches the circle 13. If the length of the intermediate blade is too long, the area which comes contact with the flow is increased. Therefore, the length i of the intermediate blade 9 which is situated inner than the above-described perpendicular line is arranged to be within 20% of the overall length of the intermediate blade 9. Since the flow at the upstream of the perpendicular line involves relatively small un-uniformity, the intermediate blade 9 is made pass through the middle point of the perpendicular line to equally divide the flow so that the flow at the outlet of the diffuser is made uniform. As a result of this, occurrence of additional loss due to non-uniform flow can be prevented. Since the overall shape of the intermediate blade 9 is formed in such a manner that, if the intermediate blade 9 is virtually or assumed to be rotationally displaced around the center of the rotational shaft 2, it is included within the contour of the stator blade 7, the flow can pass through smoothly, causing occurrence of loss to be reduced.

0027Fig. 7 illustrates an embodiment in which the perpendicular line cannot be drawn from the rear end of the stator blade 7a onto the neighboring stator blade 7b, since the length of the chord of the stator blade 7 is too short. In this case, a perpendicular line 27 is used which is drawn to an extension 26 of the mean thickness line at the front end of the stator blade 7b. This extension line 26 may be formed by a straight line, but a logarithmic spiral passing through the front end of the stator blade 7b and forming an inlet angle ξ achieves the same effect.

0028Fig. 8 illustrates a still another embodiment of the present invention. In this invention, the sub-blade 8 is rotatably, by an angular extent δ, supported by a supporting shaft 29 which is disposed in parallel to the rotational shaft 2 of the impeller 1. In large flow rate operation mode, the length of a perpendicular line 28 drawn from the front end of the stator blade 7b to the sub-blade 8 is selected to be greater, while in small flow rate operation mode, the length of the perpendicular line 28 is selected to be smaller. As a result of this, the flow rate range can be further enlarged due to the throttling effect. In this embodiment, the supporting shaft 29 for the sub-­blade 8 may be manufally rotated. The supporting shaft 29 for the sub-blade 8 may be arranged to be automatically operated by an appropriate control unit for controlling an apparatus including the centrifugal compressor according to this embodiment. According to this embodiment, the flow rate range can be enlarged due to the above-described operation so that practical advantage can be further improved.

0029Fig. 9 illustrates a still another embodiment. In this embodiment, the intermediate blade 9 is rotatably, by an angular extent γ, supported by a supporting shaft 31 disposed in parallel to the rota­tional shaft 2 of the impeller 1. The sum of the length of a perpendicular line 32 drawn from the front end of the intermediate blade 9 to the neighboring stator blade 7a and the length of a perpendicular line 30 drawn from the rear end of the intermediate blade 9 to the neighboring blade 7b is made greater in a large flow rate mode, while the sum is made smaller in a small flow rate mode. The same effect as that in the embodiment shown in Fig. 8 is intended to be obtained in which the flow rate range is enlarged by the throtting effect. If this embodiment is employed in combination with rotation control of the sub-blade 8, a better effect can be obtained.

0030As described above, according to the embodi­ments of the present invention, since the diffuser for converting kinetic energy of the air flow discharged from an impeller of a centrifugal compressor into pressure energy comprises, in addition to a plurality of stator blades, sub-blades at positions intersecting a circle which passes through the front ends of the stator blades such that one side surface of each sub-blade confronts the associated stator blade, and intermediate blades passing through the middle point drawn from the rear end of the stator blade to the neighboring stator blade and reaching a circle which passes the rear ends of the stator blades, the operatable flow rate range of the diffuser can be enlarged without reduction in the performance. As a result of this, the operable flow rate range of a high speed centrifugal compressor can be significantly enlarged.

0031Fig. 10 shows a still another embodiment. This embodiment is characterized in that: intermediate blades 40 are provided at the blade-side diffuser casing 5A in such a manner that the intermediate blades 40 project into the diffuser flow passage between the stator blades 7. Each of the intermediate blade 40 comprises: as shown in Figs. 10 and 11, an inlet-side intermediate blade 40A having length of chord and height smaller than those of the stator blade; a rectifier blade 40B with the height shorter than that of the inlet-side intermediate blade 40A, and outlet-side intermediate blades 40C connected with the rectifier blade 40B and having the same or similar dimensions as those of the inlet-side intermediate blade 40A.

0032The operation of this embodiment of the present invention will be described with reference to Figs. 10 and 11 as well as Fig. 1.

0033Fluid is given energy to become a high speed flow while it is passed through the suction pipe 4 and the impeller 1 by the rotation of the impeller 1 and is introduced into the diffuser portion 5. In this diffuser portion 5, fluid discharged from the impeller 1 is introduced into the scroll casing 6 with main part of its kinetic or speed energy being converted into pressure energy. The rest fluid kinetic or speed energy has been further converted into pressure energy in the scroll casing 6, and then it is discharged from a discharge port (omitted from the illustration).

0034Since the inlet-side intermediate blades 40A of the intermediate blades 40 guide the flow along the stator blades 7, the flow can be made pass along the stator blades 7 even if the flow rate is small or low. Therefore, possibility of occurring the surging phenomenon can be reduced, and the operating range of the impeller 1 can be enlarged. Since the inlet-side intermediate blade 40A acts as described above, the height of the blade is preferably arranged to be in the order of 50% of that of the stator blade 7. Since the inlet-side intermediate blades 40A are overhung from the blade-­side diffuser casing 5A, the protruded height is preferably short for the purpose of securing strength. Therefore, it is made shorter than the height of the stator blade 7. Since the inlet-side intermediate blade 40A narrows the flow passage, the inlet-side intermediate blade 40A acts as a resistance if the discharging flow rate of the compressor exceeds the designed value. Also from this viewpoint, the height of the blade is preferable to be as short as possible, it being preferable to be within 70% of the height of the stator blade 7.

0035A strong vortex flow 110 is, as shown in Fig. 11, generated at the end and the root portion of the inlet-side intermediate blade 40A, i.e. at the downstream end thereof. Energy of the vortex flow is converted into heat energy to cause energy loss. The vortex flow discharged from the downstream end of the root portion, in particular, disturbes the flow in the diffuser 5, causing large loss. The rectifier blades 40B provided at the downstream of the inlet intermediate blades 40A serve to suppress generation of the vortex flow at the root portions of the inlet intermediate blades 40A, thereby serving to reduce the loss. Since the rectifier blade 40B is provided for the purpose of preventing generation of vortex flow, the height of the same may be arranged to be shorter than that of the inlet intermediate blade 40A.

0036The flow near the outer peripheral between the stator blades 7 is, in general, directed not along nearer the stator blade 7, but along nearer the circumferential direction. Therefore, the outlet-side intermediate blades 40C are provided to guide the flow along the stator blades 7 so that the performance of the diffuser is improved. The height of the outlet-side intermediate blade 40C is preferably in the order of 50% of that of the stator blade 7. On the viewpoint of securing rigidity, it is preferable to be within 70%.

0037Fig. 12 illustrates a still another embodiment of the present invention. This embodiment is characterized in that: the rectifier blades 40B are provided not only at a first side or face supporting the intermediate blade 40 but also at a second side or face which confronts the first side. In this case, vortex flows generated at downstream end of the inlet intermediate blade 40A as well as at the root of the same can be prevented from generation. Therefore, a further improvement in performance can be achieved.

0038Fig. 13 illustrates a further embodiment of the present invention. This embodiment is characterized in that the intermediate blade 40 is provided at each of the stator blades in a confronting manner. The same effect as that obtained in the above-described embodiments can be obtained by this structure. In this case, the height of the intermediate blade 40 is half as that of the intermediate blade 40 employed in the embodiments shown in Figs. 10 and 12.

0039Fig. 14 illustrates a still another embodiment of the present invention. This invention is characterized in that: the structure of the embodiment shown in Fig. 10 is simplified, in which the outlet-side intermediate blade 40C is omitted. In this case, although slight decrease in performance cannot be avoided, the cost of the diffuser can be reduced.

0040Fig. 15 illustrates a still another embodiment of the present invention, in which the structure is further simplified, and the rectifier blade 40B and the outlet intermediate blade 40C are omitted, as a result, it being constituted by inlet-side intermediate blade 40A only.