JPH04143499A - Diffuser of centrifugal fluid machine - Google Patents

Abstract

PURPOSE:To accomplish a diffuser having a high efficiency and wide operating range by furnishing a plurality of guide vanes (rib) on at least one of the two surfaces confronting each other in the position facing downstream from place around the vane rear edge of vane-equipped diffuser, wherein the wane height is smaller than half the flow-path height. CONSTITUTION:Ribs 3a, 3b are furnished on both surfaces, wherein the front edges are situated some nearer the bore than the wane rear edge of a vane- equipped diffuser 2. The main stream 7a is decelerated by the vanes 2 to a specified degree and proceeds in the same direction as the ribs 3a, 3b. A stream 7b, on the other hand, close to the wall surface is baffled forcedly by the ribs 3a, 3b to the main stream 7a direction. Thus the directions of the main stream 7a and the stream 7b are put identical to decrease the mixing loss, while the stream distribution in the flow-path height direction is made uniform by a rib-equipped diffuser 3, and effective recovery of pressure is assured. The height of the ribs 3a, 3b is smaller than half the flow-path height and no throat is provided by ribs, and the incoming angle of rib is put in alignment with the main stream direction, so that the rib-equipped diffuser will never narrow the operating range.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a centrifugal fluid machine, and relates to improving the performance of medium and high specific speed stages.

[Conventional technology]

Diffusers can be divided into those without blades and those with blades.
Generally, a compressor equipped with a vaned diffuser as shown in FIG. 7 has higher efficiency than a compressor equipped with a vaned diffuser as shown in FIG. 6, but its operating range is narrower.

As a diffuser with performance intermediate between these, a small chord ratio diffuser with a small chord node ratio (Reference ``Seno et al.
There is 45-396B (Sho 45Jj). As shown in FIG. 9, this small chord ratio diffuser has a smaller chord ratio than the vaned diffuser shown in FIG. Here, the chord ratio is expressed as the blade pitch ratio determined by the blade chord length, the average radius of the blade entrance/exit, and the number of blades, but in practice, if the ratio is smaller than 0.8, in most cases, As shown in , the angle α between the line connecting the leading and trailing edges of adjacent blades and the tangent to the leading edge is 90' or more. Therefore, such a small scale ratio vaned diffuser does not have a throat part like a vaned diffuser (or the throat area is extremely wide compared to a vaned diffuser), and chokes due to the diffuser may occur on the high flow rate side. It's fleeting. In addition, since the load on the blades is small, stalling is less likely to occur even when the angle of attack of the flow increases on the small flow rate side, and a wide operating range can be obtained. However, because the chord ratio is small, the flow that enters the diffuser cannot sufficiently flow along the blades, and the turning angle of the flow is smaller than that of a diffuser with blades. Therefore, the highest efficiency is that of a diffuser without blades. Although it is higher than a diffuser with blades, it is lower than a diffuser with blades.

In Japanese Utility Model Application Publication No. 56-97598, in a bladeless diffuser section downstream of a bladed diffuser, the flow path height is lowered immediately after the blade outlet.

In addition, as a diffuser suitable for a compressor equipped with a high specific speed impeller, guide vanes (ribs) with a blade height lower than half of the flow path height are provided on the flow path wall surface corresponding to the shroud side of the impeller. There is a diffuser with ribs (Patent No. 1347983).

JP-A-55-144896 discloses a diffuser suitable for a compressor equipped with an impeller with a relatively medium specific speed, in which a guide vane is used between the vanes of a vaned diffuser, the vane height of which is lower than the height of the flow path. The leading edge of the guide vane is located immediately after the impeller exit.

[Problem to be solved by the invention]

In the case of a small chord ratio diffuser, for example, as shown in FIG. 10, in particular, the low momentum fluid 7b near the wall surface is affected by the secondary flow inside the diffuser, as shown by the broken line in the figure. The flow angle is smaller than that of the main stream 7a.

This tendency becomes more pronounced as the flow distribution flowing into the diffuser becomes non-uniform. This is because a diffuser with vanes having a small chord ratio can divert and decelerate the incoming average flow to some extent, but the effect of rectifying the flow is small.

In the case of a small chord ratio diffuser, the entrance/exit radius ratio is small, and the flow cannot be sufficiently slowed down in that part alone. Therefore, a bladeless diffuser exists downstream of the diffuser, and a constant speed is deceleration is performed. However, as mentioned above, if the flow direction differs between the main stream and the wall near the outlet of the small scale ratio diffuser, the mixing loss in the bladeless diffuser section downstream increases, and
Due to the non-uniformity of the flow distribution in the height direction of the flow channel, the effective flow channel area within the vaneless diffuser section becomes small, and a predetermined deceleration, ie, pressure recovery, cannot be obtained.

In Japanese Utility Model Application Publication No. 56-97598, the flow path height is reduced immediately after the outlet of the vaned diffuser.

The fluid near the wall is accelerated to reduce the difference in flow angle with the main flow, thereby reducing mixing loss. but,
This method is not very effective because the average flow velocity in the vaneless diffuser section increases, which increases friction loss and also increases the load on the downstream stator.

Although the ribbed diffuser (Patent No. 1,347,983) has a large flow rectifying effect, it does not forcibly decelerate by itself, so it approaches the ideal deceleration without blades, but cannot decelerate any further.

Furthermore, in the case of JP-A-55-144896, the ribs provided between the blades have the same curvature as the blades and are low in height, thereby preventing secondary flow within the flow path. Therefore, although high performance can be expected at the design point flow rate, where the blades and ribs have almost no angle of attack, a wide operating range cannot be obtained.

The purpose of the present invention is to first perform a predetermined deceleration using a vaned diffuser with a relatively small chord ratio (including, of course, a small chord ratio diffuser), and then reduce the loss in the downstream diffuser section, thereby making it ideal even there. The objective is to provide a diffuser with high efficiency and a wide operating range by achieving pressure recovery.

[Means to solve the problem]

The above purpose is to install a plurality of guide vanes (hereinafter referred to as ribs) with a vane height lower than half of the flow channel height toward the downstream side from near the trailing edge of the vane diffuser with a relatively small chord ratio. This can be achieved by providing it on one or both of the two surfaces.

[Effect]

The flow exiting the impeller is first decelerated to a predetermined degree by a vaned diffuser with a relatively small chord ratio on the upstream side, and then flows into the ribbed diffuser section. At this time, the ribs provided toward the downstream side from near the trailing edge of the blades of the vaned diffuser force the flow with a small flow angle near the wall surface to be directed toward the mainstream. By doing this, the momentum exchange between the main stream and the fluid flow with low momentum near the wall surface is performed smoothly, and the flow distribution in the height direction of the channel is made uniform with small loss.

The rib has a logarithmic spiral shape with a nearly constant flow angle, and its purpose is not to forcibly slow down the flow, but rather to reduce the flow in the height direction of the channel (flow velocity).

By making the distribution uniform (angle), the loss is small,
We are trying to approximate the flow of an ideal bladeless diffuser. In this way, in the diffuser of the present invention, the flow is forcibly decelerated in the first half of the vaned diffuser section, and the flow can be reasonably decelerated with small loss using the ribbed diffuser in the second half.

The height of the rib is less than half the height of the flow path, the rib does not create a throat, and the inlet angle of the rib is aligned with the direction of the main flow as described above, so the operating range is limited by the ribbed diffuser section. There are no words that can be narrowed down.

〔Example〕

The present invention will be explained in detail below. FIG. 1 shows the cross-sectional shape of the compressor. A flow 7 flowing in from a suction pipe 5 passes through an impeller 1, passes through a vaned diffuser section and a ribbed diffuser section, and is guided to a discharge casing 4. The rib has a leading edge slightly radially inward from the trailing edge of the blade of the vaned diffuser, and is provided on both surfaces. Figure 2 is the same as Figure 1.
-■ In the arrow view, the main stream 7a is decelerated by a predetermined speed by the blade 2,
That is, after the flow angle is turned, it flows into the ribbed diffuser section and flows in the same direction as the ribs.

On the other hand, if there were no ribs, the flow 7b near the wall would flow out of the vaned diffuser section at a smaller angle than the main flow 7a, as shown in FIG. be guided in a direction. By aligning the directions of the main stream 7a and the flow 7b near the wall surface in this way, mixing loss is reduced, and the flow distribution in the height direction of the channel is made uniform in the ribbed diffuser section, so that even in this section More effective pressure recovery is obtained.

Further, FIG. 3 shows an example in which ribs having different leading edge radial positions are arranged alternately in the circumferential direction, which corresponds to claim 2. In this example, the rib 3b close to the suction surface of the upstream blade 2a has a small leading edge radius, and the other rib 3b has a large radius. This is feather 2
Due to the development of the boundary layer and flow separation on the suction side of a,
Since low-momentum fluid tends to gather near the wall surface, by extending the front edge of the rib 3b in this area slightly toward the upstream side,
We are trying to rectify the flow more effectively.

FIG. 4 shows an embodiment in which the diffuser of the present invention is combined with an impeller having a high specific speed.

As shown in FIG. 4, an impeller with a high specific speed has a large flow path curvature on the shroud side, and the outlet flow of the impeller is biased toward the hub side. In other words, the meridional velocity is small on the shroud side and large on the hub side, and the flow angle is also small on the shroud side and large on the hub side. In the case of a flow that is non-uniform in the height direction of the flow channel, the low-momentum fluid is biased to one side, so even if the flow passes through a vaned diffuser with a small chord ratio, the state will not be very large. does not change.

Therefore, in such a case, it is more effective to attach the rib to the side corresponding to the shroud side. FIG. 5 shows an example in which ribs are provided on the wall surface opposite to that in FIG. 4.

〔Effect of the invention〕

According to the present invention, the flow is forcibly slowed down by the small bladed diffuser in the front half of the diffuser,
In the rear part of the diffuser, the ribbed diffuser can rectify the flow and bring it close to the ideal speed reduction of a vaneless diffuser with low loss, resulting in a highly efficient compressor without narrowing the operating range of the compressor. be able to.

[Brief explanation of the drawing]

Figures 1, 4, and 5 are half-sectional views of a compressor according to an embodiment of the present invention, Figure 2 is a view taken along the ■-■ arrow in Figure 1, and Figure 3 is the same as Figure 2. Main part of the diffuser part 1... impeller, 2...
...Diffuser with blades, 2a...Blades of diffuser with blades, 2b, 2c...Blades of double-row diffuser with blades, 3...Diffuser with ribs, 3a, 3b, 3c
...Rib, 4...Casing, 5...Suction pipe, 6
...Rotation axis, 7. Flow direction, 7a.. Main flow direction, 7b.. Flow direction near wall surface, 8.. Vane''
'I-''''! y: x-f・9゛° impeller 1 rotation 0・2
3□ Agent Patent Attorney Katsuo Ogawa 9.1 City Map 4 Construction 2-Diagram

Claims (1)

[Claims] 1. In a flow path composed of two opposing surfaces downstream of the impeller, on the upstream side there is a ratio of the blade pitch determined by the chord length of the blade, the average radius of the entrance and exit of the blade, and the number of blades, that is, In a centrifugal fluid machine equipped with a small chord ratio vaned diffuser having a chord ratio smaller than 0.8, one of two opposing surfaces downstream of the vicinity of the trailing edge of the small chord ratio vaned diffuser. on at least one side,
A diffuser for a centrifugal fluid machine, characterized in that a plurality of guide vanes each having a vane height lower than half the height of a flow path are provided.