Abstract:
A centrifugal compressor including a vaned diffuser provided with a plurality of stationary vanes disposed around an impeller for converting into kinetic energy of a discharged fluid into pressure. The diffuser further includes rotatable vanes unevenly distributed in the stationary vanes.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a centrifugal compressor with a vaned diffuser. 
     2. Description of the Prior Art 
     Japanese Unexamined Patent Publication No. 57-159998 discloses a centrifugal compressor with its operable range broadened by small rotatable vanes at the inlets of a vaned diffuser. Japanese Unexamined Patent Publication No. 58-124099 discloses another design for broadening the operable range of a centrifugal compressor wherein a vaned diffuser comprises a pair of circular rows of vanes with the inner row of vanes movable in a direction parallel to the rotation axis of the impeller. 
     However, the centrifugal compressor of the Japanese 57-15998 requires a rather complex and highly accurate mechanism since the compressor must be provided with small rotatable vanes as many as the diffuser stationary vanes. The compressor of the Japanese 58-124099 requires large-sized movable parts in order to provide the parallel-movement mechanism, although the mechanism can be simpler than that of Japanese 57-159998. 
     SUMMARY OF THE INVENTION 
     An object of this invention is to provide a centrifugal compressor including a vaned diffuser having a broadened operable range achieved by a minimal mechanism. 
     A centrifugal compressor according to the present invention a vaned diffuser provided with a plurality of stationary vanes disposed around an impeller to convert the kinetic energy of a fluid discharged from said impeller into pressure, wherein the diffuser includes rotatable vanes which are unevenly distributed between the other vanes. 
     In accordance with further features of the present invention, some of the plurality of the stationary vanes are arranged such that the leading edges thereof are positioned radially outwardly of those of the other stationary vane, and such stationary vanes are unevenly distributed between the other vanes. 
     According to the present invention, the plurality of the stationary vanes are arranged such that the intervals of some of the stationary vanes are larger than the intervals of the other vanes and the vanes are unevenly distributed. 
     In accordance with still further features of the present invention, the plurality of the stationary vanes are arranged such that the intervals of some of the stationary vanes are larger than the intervals of the other vanes and the intervals of vanes adjacent the large interval vanes are smaller than those of other vanes and such sets of the larger and smaller interval vanes are unevenly distributed. 
     According to the present invention, some of the plurality of the stationary vanes are arranged such that the angles thereof to the radial directions of the impeller are greater or smaller than those of the other stationary vanes, with the stationary vanes being unevenly distributed. 
     The operable range of a centrifugal compressor with a vaned diffuser is mostly determined by the characteristic of the vaned diffuser. The lower limit of the operable range is determined by the stalling point of the vaned diffuser, while the upper limit is determined by the choking point thereof. The lower and upper limits are greatly dependent on the cross-sectional areas of the passages in the diffuser. Thus, the passages have been modified to be able to change the cross-sectional areas, e.g., the diffuser&#39;s stationary vanes are designed to be rotatable. An important factor is the total cross-sectional area of all the passages, not an individual one, since, generally, the flows through a vaned diffuser are distributed unevenly in the circumferential direction even if the stationary vanes and the passages are uniformly designed. Therefore, the operable range of the vaned diffuser can be broadened by modifying a few, not necessarily all, stationary vanes or small vanes at the entrances of the passages to rotatable vanes to adjust the total of the passage areas. 
     Aerodynamically, the lower limit of the operable range of a vaned diffuser means a point where the flows through all the passages begin to stall. At a point where the flows are greater than those at the lower limit, stall occurs in a few passages and the stalled areas then propagate in the direction of the circumference. This phenomena is called rotating stall and causes terrible noises and vibration. Therefore, the point where the rotating stall occurs is a practical lower limit. Therefore, the practical lower limit of the operable range also can be lowered by preventing the rotating stall as well as by the above-described method that delays the stall occurrence in all the passages. 
     For an effective prevention of the rotating stall, some passages may be designed so that the stall occurs therein earlier than in other passages. When the flow stalls in those passages, the flow through those passages is less than the flow through the other passages, and thus the flow through the other passages increases correspondingly, so that an occurrence of a stall is unlikely. 
     In order to cause an earlier stall in particular passages, some of the stationary vanes are arranged such that the leading edges thereof are positioned outwardly of those of the other stationary vanes, thereby providing wider entrances between the leading edges of the vanes. Due to the wider entrances, the load on those vanes increases, making the stall likely to occur. Also, the outwardly positioned leading edges cause the boundary layers on the side walls to become thicker than those at the leading edges of the other vanes, so that the thicker boundary layers more easily separate the flow from the vane surfaces. 
     Another effective way to cause the stall to occur earlier in particular passages is to arrange the stationary vanes such that some of the passages between the vanes are wider than the other passages. A wider passage causes a larger load on an adjacent vane, thereby more easily separating the flow easily from the vane surface. Further, passages narrower than the others may be provided next to the wider passages, to enhance the prevention of the rotating stall. A narrower passage causes a smaller load on an adjacent vane, thereby resulting in making the flow separation from the vane surface more difficult. 
     Additionally, it is also possible in accordance with the present invention is to position some of the stationary vanes angled closer to or remote from the radial directions of an associated impeller than the other stationary vanes. When a stationary vane is angled closer to the radial direction of the impeller, the flow easily separates form the side (negative pressure side) thereof following the rotational direction of the impeller. Thus, a stall occurs earlier in the passage adjacent the side of the vane than in other passages. The leading edge of the vane may be positioned outwardly of those of other vanes in order to achieve an earlier occurrence of stall for the reason stated above. When a vane is angled further away from the impeller radial direction, the passage adjacent to the side (high pressure side) of the vane opposing the rotational direction of the impeller experiences the stall earlier since the divergent angle of the passage is greater than that of each of the others. The prevention of the rotating stall is enhanced by the passage adjacent to the negative pressure side of the vane which experiences the stall later than the others since the divergent angle of the passage is smaller than that of the others. Also, the leading edge of a vane may be positioned not outwardly of those of other vanes to achieve a still later stall-occurence in the passages adjacent to the negative pressure side of the vane, by the effect opposite to the one caused by the outwardly positioned leading edges. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an end view of an impeller and a vaned diffuser of a first embodiment of the present invention, taken along a line I--I in FIG. 2; 
     FIG. 2 is a sectional view of the embodiment of the present invention taken in a plane including the axis of the shaft of the impeller; 
     FIGS. 3 and 4 illustrate the operations of the embodiment shown in FIGS. 1 and 2; 
     FIGS. 5 to 7 illustrate modifications of the first embodiment; 
     FIGS. 8 to 16 illustrate embodiments comprising mechanisms to prevent the propagation of stalled areas; 
     FIGS. 17 to 21 illustrate still other embodiments, wherein FIGS. 17 and 19 are views of the embodiments taken in the axial directions of the impellers and the vaned diffusers and FIGS. 18, 20 and 21 are enlarged views of some of the vanes of the diffusers. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     With reference to FIGS. 1 and 2, a plurality of fixed vanes 2, arranged around an impeller l, cooperate with a side plate 4 and a central plate 5 to define passages 6 for fluid 3 to which kinetic energy is imparted by the impeller 1. A part of the kinetic energy of the fluid 3 is converted into pressure when the fluid 3 passes through the passages 6. Adjustable vanes 7 are fixed to rotatable shafts 8 so as to be adjusted to the most suitable angle according to an operation condition of the compressor. Two adjustable vanes 7 are provided at diametrically opposed positions since when a non-axial symmetry of the flow becomes a substantial level, a considerably great radial composite force acts on the impeller 1. 
     With reference to FIGS. 3 and 4, the operation of the compressor of the present invention will be described. When the flow rate is substantially as much as the design value or more, the fluid flow from the impeller 1 goes towards each fixed vane 2 in the direction of a camber line 9 of the fixed vane 2 or in a direction shifted clockwise from the camber line 9 toward a radial line, as indicated by an arrow 3a. In this condition, the adjustable vanes 7 are each rotated to an angle shown in FIG. 3 to provide wider passages between the diffuser fixed vanes so as to prevent the performance from being lowered by choking. When the flow rate is less than the design value, the fluid flow goes towards each fixed vane 2 in a direction shifted counterclockwise from the camber line 9, as indicated by an arrow 3b. In this condition, the adjustable vanes 7 are each set to an angle shown in FIG. 4 to narrow the passages between the diffuser fixed vanes. The total of the cross-sectional areas of the passages 6 is reduced in this way and causes the fluid flow to the fixed vanes 2 to be shifted from the direction 3b towards the direction 3a, thereby to prevent the occurrence of stalling. Thus, the lower limit of the operable range can be lowered. 
     Various modifications to the adjustable vanes are illustrated in FIGS. 5 to 7. In FIG. 5, an entrance portion of a fixed vane is pivotable. In FIG. 6, an adjustable vane 7 is provided at an entrance portion of a passage between adjacent fixed vanes 2. In FIG. 7, adjustable vanes 7 are disposed in adjacent relationship to each other. The embodiments shown in FIGS. 5 and 6 have an advantage that the adjustable vanes 7 can be formed in small-dimensions, while the embodiment shown in FIG. 7 offers a broader adjustment for the cross-sectional area of the passage between the diffuser vanes 2. 
     In the embodiments show in FIGS. 8 to 15, a small number of the passages between the vanes are formed so that the flow therethrough stalls earlier than in other passages. In this way, the propagation of stalled passages is suppressed to prevent the occurrence of a rotating stall. 
     In the embodiments shown in FIGS. 8 and 9, fixed vanes 2b are provided so that the leading edges thereof are positioned outwardly of those of the other fixed vanes 2a in order that the stall occurs earlier at the passages adjacent to the fixed vanes 2b than at the other passages. In the embodiment of FIG. 9, the fixed vanes 2b are disposed in adjacent relationship to each other for more effective prevention of the rotating stall. 
     The embodiment shown in FIG. 10 includes a few fixed vanes 2b which are disposed such so that the passages 10b between these vanes are wider than the passages 10a between other vanes 2. The passages 10b experience the stall earlier than the other passages 10a. 
     In the embodiment shown in FIG. 11, passages 10c narrower than the passages 10a are provided on both sides of wider passages 10b for more effective prevention of the propagation of stalled areas and thus of the rotation stall. 
     The embodiment shown in FIG. 12 has a narrower passage 10c disposed only at the back side of a wider passage 10b as viewed in the rotational direction 11 of the impeller 1. 
     The embodiment in FIG. 13 has a narrower passage 10c only at the front side of a wider passage 10b. 
     The embodiment shown in FIG. 14 has a small number of fixed vanes 2d rotated towards the radial direction 12 of the impeller 1 so that an angle θd between each fixed vane 2d and the radial direction 12 is smaller than a corresponding angle θa of each of the other fixed vanes 2a. The stall occurs earlier in a passage 10d adjacent to the negative-pressure surface of each fixed vane 2d than in the other passages 10a. 
     The embodiment shown in FIG. 15 has a small number of fixed vanes 2e rotated away from the radial direction 12 so that an angle θe is larger than the angle θa. The flow stalls earlier in a passage 10e adjacent to the positive-pressure surface of each fixed vane 2e than in the other passages 10a. 
     In the embodiment illustrated in FIG. 16, fixed vanes 2f are provided so that the leading edges thereof are positioned outwardly of those of the other fixed vanes 2a and passages 10f and 10g adjacent to each vane 2f are each wider than each of the other passages 10a. Further, a passage 10h narrower, than each of the other passages 10a, is disposed on the back side of a passage 10g as viewed in the rotational direction 11 of the impeller 1. This embodiment has both of the advantages provided by the embodiments shown in FIGS. 8 and 12 with respect of the prevention of the stalled-area propagation. 
     The embodiment shown in FIGS. 17 and 18 has fixed vanes 2i provided with leading edges having different radial positions adjacent the side plate and the central plate in order to broaden operable range at its lower limit. This embodiment also includes adjustable vanes 7. The fixed vanes 2i, highly effective to lower the lower limit of the operable flow range, are combined with the adjustable vanes 7 in order to achieve a still lower limit of the operable range. 
     For lowering the lower limit of the operable range, the embodiments shown in FIGS. 19 to 21 has fixed vanes 2j and auxiliary vanes 13 each having a cord length smaller than that of each fixed vane 2j and a height equal to or less than that of the fixed vane 2j, as well as adjustable vanes 7. Each auxiliary vane 13 is disposed inwardly of the leading edges of the fixed vanes 2j so that only one side of each auxiliary vane 13 faces a fixed vane 2j. The auxiliary vanes 13 may have the leading edges sloped as shown in FIG. 21 to reduce the noise and increase the strength. In the embodiments in FIGS. 19 to 21, the combination of the fixed vanes 2j and the auxiliary vanes 13, highly effective to lower the lower limit of the operable range, is employed in further combination with the adjustable vanes 7 in order to still further lower the lower limit of the operable range.