Centrifugal compressor

Abstract: A centrifugal compressor has a diffuser with stator blades (2) arranged therein for converting the kinetic energy of a fluid discharged from an impeller into a pressure energy. The stator blades (2) have leading edges inclined in the downstream direction while extending away from a side plate toward a core plate. Auxiliary blades (4) having a chord length shorter than that of each of the stator blades (2) have leading edges inclined in the downstream direction while extending away from the side plate toward the core plate and are arranged at positions radially inward of the stator blades (2) in such a manner that one of the surfaces of each of the auxiliary blades (4) faces a stator blade (2). The compressor is capable of operating without suffering from surging and rotating stall even in a low flow rate region.

Description: The invention relates to a centrifugal compressor comprising a diffuser provided with stator blades operative to convert the kinetic energy of a fluid discharged from an impeller into a pressure energy, wherein stator blades have leading edges inclined in the downstream direction while extending away from a side plate toward a core plate. Some centrifugal compressors include a plurality of stator blades arranged at equal intervals about a circumference of the diffuser disposed around the impeller. The spaces between the stator blades and the space between the side walls of the diffuser cooperate to provide a diffuser passage. In an operational region where the rotational speed is high and the flow rate is low, a separation area is generated on the negative-pressure surfaces of the stator blades, thereby failing to achieve a sufficient increase in pressure. This first results in the so-called rotating stall, in which the separation area rotates in the circumferential direction. When the flow rate is further decreased, a surging phenomenon occurs. For shifting the surging phenomenon to a low flow-rate side JP 1-247798 (A) discloses a centrifugal compressor (generic kind) in which auxiliary blades are arranged radially inwardly of the spaces between stator blades arranged at equal intervals on the diffuser and in which stator or auxiliary blades have leading edges inclined in the downstream direction while extending away from the side plate toward the core plate. However, such a centrifugal compressor, in which leading edges of stator or auxiliary blades are inclined in the downstream direction while extending away from the side plate toward the core plate, does not sufficiently prevent a surging phenomenon in a low flow rate region. Further, since the compressor has stator blades arranged at equal intervals throughout the circumference of the diffuser disposed around the impeller, sufficient consideration has not been paid to the prevention of rotation stall. EP-A-0 040 534 discloses a centrifugal compressor including a radial diffuser having opposed side walls defining a space between them for diffusing gas flow, and stator vanes extending from one side wall to the other across the said space to divide it into passageways. The vanes are arranged in two sets. The leading edges of the vanes of one set are swept back from a first side wall while the leading edges of vanes of the second set are swept back from the second side wall. The stator vanes of the first set are interposed between and alternate with those of the second set. With such a diffuser, it is possible to prohibit or minimise boundary layer flow build-up and recirculation into the compressor impeller, while at the same time avoiding shock waves normally accompanied with introduction of vanes into the transonic region. It is the object of the invention to provide a centrifugal compressor of the generic kind which operates without being subjected to surging and rotating stall even in a low flow-rate region. According to the invention, this object is achieved with the centrifugal compressor of the generic kind in that auxiliary blades, each having a chord length shorter than that of each stator blade, are provided, which auxiliary blades have leading edges inclined in the downstream direction while extending away from the side plate toward the core plate, and which auxiliary blades are arranged at positions radially inward of the stator blades in such a manner that one of the surfaces of each auxiliary blade faces a stator blade. The stator blades may be arranged uniformly or non-uniformly in the diffusor. Preferably said stator blades and said auxiliary blades are arranged in such a manner that said blades are sparse in the vicinity of a tongue portion of a scroll-shaped flow passage and dense in the part of said passage remote from said tongue portion. Said vicinity of said tongue portion is preferably an area including said tongue portion and corresponding to not more than one half of the entire circumference. According to the invention, both stator blades and auxiliary blades have leading edges adjacent the side plate which are disposed at positions closer to the impeller than corresponding positions in a conventional centrifugal compressor. As a result, streams flowing in directions approximating the tangential direction can be strongly led in the directions of the stator blades. This is effective to minimize or prevent the generation of counter current to thereby prevent surging even in the low flow rate region. Further, auxiliary blades are arranged together with stator blades at blade intervals varied to provide both a sparse area and a dense area so that the stator blades are non-uniformly arranged together with the auxiliary blades. As a result, a stall occurs first in certain part of the passage where the blade intervals create a sparse area rather than in other part of the passage with dense arrangement of blades. Thus, the rate of flow through the inter-blade passage where a stall has taken place decreases below the rate of flow through the inter-blade passage where a stall has not yet taken place to increase the rate of flow through the inter-blade passageway in the second part in the inter-blade passage where the stall has not yet taken place, to thereby minimize the occurrence of rotary stall. Thus, the centrifugal compressor according to the invention provides high efficiency and a wide operational range. Embodiments of the invention will be described with reference to the accompanying drawings, in which Fig. 1 is a sectional view of a first embodiment of a centrifugal compressor taken in the direction of the axis of rotation of the impeller, Fig. 2 is a partial longitudinal sectional view of a diffuser portion of the centrifugal compressor of Fig. 1, Fig. 3 is a perspective view of an arrangement of a stator blade and an auxiliary blade, Fig. 4 is a sectional view as Fig. 1 of a second embodiment of a centrifugal compressor, Fig. 5 is a sectional view as Fig. 1 of a third embodiment of a centrifugal compressor, Fig. 6 is a sectional view as Fig. 1 of a fourth embodiment of a centrifugal compressor without casing, Fig. 7 is a sectional view as Fig. 6 of a fifth embodiment of a centrifugal compressor, Fig. 8 is a sectional view as Fig. 6 of a sixth embodiment of a centrifugal compressor, and Fig. 9 is a sectional view as Fig. 6 of a seventh embodiment of a centrifugal compressor. Referring to Fig. 1 to 3, a space between an impeller 1 and a casing 5 defines the interior of a diffuser for converting the kinetic energy of a flow discharged from the impeller 1 into pressure. The diffuser is provided with a plurality of blades. Specifically, a stator blade 2 has a leading edge which is integral with a stator blade extension 3. The stator blade extension 3 has a height equal to or less than a height of the stator blade 2 and also has a leading edge inclined in the downstream direction while extending away from a side plate 7 toward a core plate 8. An auxiliary blade 4, having a chord length and a height less than a height of each stator blade 2, is disposed between an adjacent pair of stator blades 2 with only one of the surfaces of the auxiliary blade 4 facing one stator blade 2. Each auxiliary blade 4 has a leading edge inclined in the downstream direction while extending away from the side plate 7 toward the core plate 8. In a scroll-shaped flow passage (which may be a scroll having a helical flow-passage whose width gradually decreases, or collector having a constant-width helical flow-passage), the plurality of stator blades 2 and the plurality of auxiliary blades 4 are arranged in such a manner that the blades are arranged sparsely in the vicinity of a tongue portion 6 of the casing 5 (i.e., in an area approximately corresponding to not more than one half of the complete circumference) and that they are arranged densely in the area other than the vicinity of the tongue portion 6. In the case where the stator blades 2 are combined with auxiliary blades 4 disposed only in that part of the flow passage which is other than the part adjacent the tongue portion 6 (approximately corresponding to not more than one half of the complete circumference), the effect of preventing rotating stall is further increased. In a condition where the operating flow rate of the compressor is high, a fluid discharged from the impeller 1 flows at an angle approximately equal to the entrance angle provided by the stator blades 2, the stator blade extensions 3 and the auxiliary blades 4. As a result, the kinetic energy of the flow is efficiently converted into pressure in passageways between the stator blades 2. In this process, since only one of the surfaces of each auxiliary blade 4 faces the corresponding stator blade 2, the width of the passageway between two adjacent pair of stator blades 2 is not reduced by the auxiliary blade 4, thereby assuring efficient conversion of the kinetic energy of the flow into pressure. When the operational flow rate of the compressor has decreased, the fluid discharged from the impeller 1 does not flow at an angle approximately equal to the entrance angle provided by the blades 2, 3 and 4 but flows at an angle which deviates from the radial direction of the impeller 1. As a result, the flow has a strong tendency of not moving along the stator blades 2 having a strong action of converting the kinetic energy of the flow into pressure. This tendency is serious on the side of the side plate 7. If the flow does not move along the stator blades 2, a part of the flow forms a counter current, causing a stall, whereby the conversion of kinetic energy into pressure is made difficult. The stall may also cause the occurrence of abnormal phenomena such as rotating stall and surging, which would make the operation of the compressor difficult. According to the present invention, the stator blade extensions 3 lead flow components at the leading edges of the stator blades 2 and adjacent the side of the side plate 7 toward the stator blades 2, while the auxiliary blades 4 lead the flow components in the intermediate portions of the stator blades 2 toward the stator blades 2. Thus, the stator blade extensions 3 and the auxiliary blades 4 cooperate with each other to strongly lead the flow components adjacent the leading edges of the stator blades 2 toward the stator blades 2. In this manner, the occurrence of stall is restrained. Consequently, abnormal phenomena such as rotating stall and surging cannot easily take place. Therefore, it is possible to enlarge the operational range of the compressor in the flow-rate region. In order to enlarge the operational range of the compressor in a lower flow-rate region, it is necessary to prevent rotating stall. Rotating stall is a phenomenon which takes place at a level of flow rate higher than the level involving surging. In a rotary stall phenomenon, a stall occurs at a part of the stator blades 2 and moves in the circumferential direction, generating great noise and vibration to make the operation of the compressor difficult. In a centrifugal compressor having a scroll-shaped flow passage, the pressure at the discharge port of the diffuser is not uniform in the circumferential direction when the flow rate is low. Specifically, the pressure is high in the vicinity of the tongue portion 6 and it is low at the opposing portion. Thus, the stator blades 2 in the vicinity of the tongue portion 6 have a higher risk of encountering a stall than the stator blades 2 in the opposing part. If the stator blade extensions 3 of the first-group of stator blades 2 adjacent the tongue portion 6 are removed, as in the embodiment shown in Fig. 1, the degree of the risk of the first group of stator blades 2 to encounter a stall is higher than that of the second group of stator blades 2 in the opposing part. With this construction, even when the first-group of stator blades 2 in the vicinity of the tongue portion 6 encounter a stall, the second-group stator blades 2 in the opposing part are free from a stall. As a result, the stall does not easily move in the circumferential direction, thereby restraining the occurrence of rotating stall. Even if a part of the stator blades 2 encounters a stall, the levels of noise and vibration generated do not substantially increase when there is no circumferential movement of the stall. Thus, the compressor is rendered operable at lower flow rate. In the embodiment of Fig. 4, a part of the auxiliary blades 4, arranged in facing relationship with some of the stator blades 2 in the vicinity of the tongue portion 6, is removed. In the embodiment of Fig. 5, a part of the stator blade extensions 3 as well as a part of the auxiliary blades 4 are removed. The embodiment of Fig. 5 provides a greater effect of preventing a rotary stall than the embodiments shown in Figs. 1 and 4. The embodiments of Figs. 6 through 8 are suitable when the flow passage downstream of the diffuser is symmetric with respect to the axis. In the embodiment of Fig. 6, a part of the stator blade extensions 3 for the stator blades 2 is removed throughout the entire circumference. In the embodiment of Fig. 7, a part of the auxiliary blades 4 for the stator blades 2 is removed throughout the entire circumference. In the embodiment of Fig. 8, a part of the stator blade extension 3 and a part of the auxiliary blades 4 are both removed throughout the entire circumference. Fig. 9 shows an embodiment suitable for preventing surging as well as for reducing radial thrust while securing symmetry of the flow passage with respect to the axis.

Claims: A centrifugal compressor comprising a diffuser provided with stator blades (2) operative to convert the kinetic energy of a fluid discharged from an impeller (1) into a pressure energy, wherein stator blades (2) have leading edges inclined in the downstream direction while extending away from a side plate (7) toward a core plate (8), characterized in that auxiliary blades (4) each having a chord length shorter than that of each of said stator blades (2) are provided which auxiliary blades (4) have leading edges inclined in the downstream direction while extending away from said side plate (7) toward said core plate (8), and which auxiliary blades (4) are arranged at positions radially inward of said stator blades (2) in such a manner that one of the surfaces of each said auxiliary blade (4) faces a stator blade (2). A centrifugal compressor according to claim 1, characterized in that stator blades (2) are uniformly arranged in said diffuser. A centrifugal compressor according to claim 1, characterized in that the stator blades are non-uniformly arranged in said diffuser. A centrifugal compressor according to claim 3, characterized in that said stator blades (2) and said auxiliary blades (4) are arranged in such a manner that said blades are sparse in the vicinity of a tongue portion (6) of a scroll-shaped flow-passage and dense in the part of said passage remote from said tongue portion (6). A centrifugal compressor according to claim 4, characterized in that said vicinity of said tongue portion (6) is an area including said tongue portion (6) and corresponding to not more than one half of the entire circumference.

Inventors: NAKAGAWA KOJI, TERASAKI MASATOSHI, NAKAGAWA, KOJI, TERASAKI, MASATOSHI
Applicants: HITACHI LTD, HITACHI, LTD.
Application year: 1992
Main classification: F04D29
IPC codes: F04D29