Rotary kinetic fluid motors or pumps – Working fluid passage or distributing means associated with... – Plural rigidly related blade sets
Reexamination Certificate
1998-08-31
2001-03-20
Look, Edward K. (Department: 3745)
Rotary kinetic fluid motors or pumps
Working fluid passage or distributing means associated with...
Plural rigidly related blade sets
C415S208400, C415S224500
Reexamination Certificate
active
06203275
ABSTRACT:
TECHNICAL FIELD
This invention relates to a centrifugal compressor and a diffuser used therein, and more particularly to a centrifugal compressor and a centrifugal blower for handling a relatively small volume of gas and a diffuser used therein.
BACKGROUND ART
Diffusers for centrifugal compressors are broadly classified into vaneless diffusers and vaned diffusers. Among these, the vaned diffuser changes the direction of flow by vanes, and also decelerates the flow by the vanes, and therefore generally the efficiency in the vicinity of a design flow rate is higher as compared with the vaneless diffusers. However, the efficiency is lowered at the large flow rate and the small flow rate because of an increased loss by the vanes and the stall of the flow by the vanes, so that the range of the operation is narrow.
Therefore, there has been proposed a diffuser with a small chord-pitch ratio (hereinafter referred to as “low solidity vaned diffuser) disclosed in Japanese Patent Unexamined Publication No. 53-119411, in which the operation range is not so narrowed even with the use of a vaned diffuser, and the efficiency can be enhanced to a certain degree.
Japanese Utility Model Unexamined Publication No. 56-97598 discloses an example in which a channel height is abruptly narrowed at a vaneless portion provided downstream of a low solidity vaned diffuser, and the channel length of the vaneless portion is shortened so as to reduce a friction loss. Further, Japanese Patent Unexamined Publication No. 1-125599 discloses an example in which in a compressor stage of a relatively low specific speed, a channel height of a vaned portion of a vaned diffuser is decreased progressively in a downstream direction so as to enhance the efficiency by reducing a friction loss.
DISCLOSURE OF THE INVENTION
In the case of an impeller for a so-called low-specific speed compressor in which the specific speed, determined by a rotational speed, a flow rate and an adiabatic head of a compressor, is not more than about 250 (rpm,m
3
/min,m), a discharge flow angle of the impeller, that is, a flow angle at an inlet of a diffuser, is small, and an axial height of a flow passage is low. Therefore, when a vaneless diffuser is used as a diffuser, there is encountered a disadvantage that a friction loss increases. On the other hand, when a vaneless diffuser is used in a low specific speed compressor stage, a rotating stall occurs, in many cases, in a vaneless diffuser portion. Therefore, in a multistage compressor in which a working fluid becomes high in pressure, there is encountered a disadvantage that the fluid oscillation due to a rotating stall limits the operation range.
In order to prevent the narrowing of the operation range by this rotating stall, there have been proposed various methods of delaying the occurrence of a rotating stall when a vaneless diffuser is used. In one of these methods, height of flow passage is reduced at an inlet portion of a diffuser (to reduce the passage height at a certain section will hereinafter referred to as “to contract”), thereby shifting the flow rate at the onset point of stall to the low flow rate. However, the diffuser of such a construction is lower in efficiency than a vaned diffuser, and besides the flow passage height of the stationary channel, including the diffuser, is low, and therefore there has been encountered a disadvantage that a wetted perimeter area increases, so that a friction loss increases. And besides, it is difficult to positively prevent the rotating stall even with the use of this diffuser, and the reliability is low.
On the other hand, the efficiency can be enhanced by the use of a high solidity vaned diffuser (vaned diffuser with a large chord-pitch ratio). However, when this type of vaned diffuser is used, the surging occurs in a small flow rate because of stall of vanes themselves, and besides since the choking is caused in a large flow rate area, there is encountered a disadvantage that the operation range is narrowed, and therefore this is not practical.
It is known that a low solidity vaned diffuser with a small chord-pitch ratio is higher in efficiency than a vaneless diffuser, and that a wide operation range can be secured. However, for achieving a larger pressure recovery with this low solidity vaned diffuser, it is necessary to provide a vaneless portion downstream of the low solidity vaned diffuser. In the case of using a low solidity vaned diffuser, the passage height of a vaneless portion, provided downstream of vanes, has heretofore been made equal to the passage height of a vaned portion. The flow can not be sufficiently deflected with the low solidity vaned diffuser. Therefore, if an inlet angle of the fluid into the diffuser is small, a rotating stall may occur in the vaneless portion even when the flow is deflected to a certain degree in the vaned portion of the diffuser, and in this case the operation range is limited.
In the example disclosed in the above Japanese Utility Model Unexamined Publication No. 56-97598, taking it into consideration that the diffuser may be used in a compressor stage of a relatively high specific speed, the passage height is abruptly contracted downstream of the vanes of the diffuser, thereby reducing the axial height of the flow passage. In this known example, however, any consideration is given to the rotating stall, and the enlargement of the operation range of the compressor is not always sufficient. Namely, in this known example, the following two points, which are important for preventing the rotating stall, are not taken into consideration. The first point is the passage height ratio (contraction ratio) between the vaned portion and the vaneless portion. In the example disclosed in Japanese Utility Model Unexamined Publication No. 56-97598, this value is 0.6 to 0.9, but this is not sufficient from the viewpoint of prevention of the rotating stall.
The second point is the instability of the flow due to the unevenness of the flow downstream of the vanes of the diffuser. The flow downstream of the diffuser vane is not uniform in the peripheral direction because of wake of the vane, and particularly in the low specific speed compressor stage, the flow angle (angle measured in the peripheral direction) is small even after the flow passes past the vaned portion. This uneven flow distribution is hard to be made uniform since the flow at the vaneless portion is a decelerated flow. Further, since the gradient of the static pressure is large in the radial direction, the flow becomes unstable. Therefore, if the diffuser is contracted abruptly or discontinuously at this vaneless portion, the static pressure gradient in the radial direction becomes discontinuous, and also the flow is not made uniform in the peripheral direction to become unstable, and this is a reverse effect from the viewpoint of prevention of the rotating stall.
In the low specific speed compressor stage, the inlet angle of the fluid into the diffuser is small, and therefore when the flow is not sufficiently deflected by the vanes of the diffuser, or when the vanes of the diffuser are locally stalled, there is a possibility that this causes a rotating stall at a region downstream of the vaned portion of the diffuser.
Japanese Patent Unexamined Publication No. 1-125599 discloses an example in which the flow is deflected by the vanes of the diffuser, and also the channel is contracted, and a relatively large deflection of the flow is enabled at the vaned portion of the diffuser without imposing a large load on the vanes. In this known example, a flow angle at an inlet of the vaneless portion, provided downstream of the vaned portion of the diffuser, is large, so that there is achieved an advantage that the flow passage length of the vaneless portion can be reduced. However, the channel height of the whole of the vaneless portion becomes low, and the wetted perimeter area of fluid becomes large. Therefore, the influences of the both cancel each other, and the effect of reducing the friction loss in the vaneless portion can not be sufficiently
Eino Takashi
Kobayashi Hiromi
Nishida Hideo
Takahashi Kazuki
Antonelli Terry Stout & Kraus LLP
Hitachi Ltd
Look Edward K.
Woo Richard
LandOfFree
Centrifugal compressor and diffuser for centrifugal compressor does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Centrifugal compressor and diffuser for centrifugal compressor, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Centrifugal compressor and diffuser for centrifugal compressor will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2491291