EP0042044B1 - Axial-flow compressor with displaced surge limit - Google Patents

Description

The invention relates to a single-stage or multi-stage axial compressor with axially one behind the other, combined into stages guide vane and rotor blade rings, and with one or more from the inner surface of the housing surrounding the vane rings protruding in the circumferential direction around the axis, which according to Art are stair-stepped.

Such a device is described in GB-A No. 866793. In one embodiment according to FIG. 1 of said patent specification, sleepers are arranged on the inner wall of the axial compressor, the penultimate sleeper lying directly at the level of a moving blade in the direction of flow.

In the case of an axial compressor stage, in addition to the pressure increases when flowing through the blades, there are also pressure differences in the radial direction in the gaps between the guide and rotor blades. When designing an axial compressor at the normal operating point, these radial pressure drops are taken into account, so that there is a balance between the radial pressure drop before the running edge and the radial pressure drop after the running edge in connection with the pressure increase when flowing through the blades.

With decreasing throughputs, especially with delay grids, the tendency for the flow to separate on the suction side of the blades increases. This tendency to tear off can be determined with gratings of any shape; it is independent of the movement of the grid when the flow is viewed relative to the grid. The tendency to detach is therefore present in the guide and rotor blades of axial compressors. In addition, detachment does not occur simultaneously on all the blades of a compressor stage, but initially only at individual or at several points on the rotor blade grille (cf. illustration, p. 478/479 of the book Eckert / Schnell _" Axial- und Radialkompressoren", Springer-Verlag 1961 ).

However, there is also the possibility that this phenomenon, once it has started, is immediately amplified and the entire flow system in the compressor is thereby out of equilibrium, ie the work of the compressor becomes unstable. This process is commonly referred to as pumping, the limit at which pumping occurs is the _" pumping limit".

With a threshold that begins immediately behind a blade edge, as in GB-A No. 866793, a shift in the surge limit cannot be achieved when the throughput changes.

DE-C No. 809842 also describes an axial compressor in which the changes in the internal diameter of the housing required in the direction of flow are carried out by stepwise narrowing. The thresholds formed by the constriction are present both on the inside of the housing and on the outside of the rotor. The threshold surfaces are each cylindrical or conical. The flow guidance is to be improved by the thresholds in such a way that the risk of the flow becoming detached is reduced on the outside of the impeller and on the inside of the stator when the axial speed is shifted. Such a shift in the axial speed is then - according to the cited patent specification - under certain circumstances to avoid difficulties with regard to the Mach number at high peripheral speeds. This indicates that the known device should make it possible to improve the flow, particularly when shifting to high speeds.

The drawing of the mentioned patent specification and the corresponding description show a part of the flow channel of an axial compressor. It can be seen from this that the flow channel narrows in the foot area in front of the subsequent guide and rotor blades in the direction of flow. For this purpose, thresholds are formed in the peripheral lines of both the housing and the red. This arrangement is not suitable either, the surge limit of the compressor, i.e. to prevent the flow from breaking off at the blades with smaller flow rates.

The task thus arises of improving the behavior of an axial compressor of the type mentioned in the introduction, in particular in low-speed ranges, i.e. in particular to prevent the flow from separating in order to shift the surge limit to lower delivery rates. The primary aim is not to improve the design of the compressor, but to enlarge the work area. As experience shows, it is necessary to increase the flow in the axial direction at the blade tip, e.g. the peripheral speed is reduced.

This object is achieved in an axial compressor of the type mentioned at the outset in that the thresholds are arranged in the flow direction directly in front of the rotor blades on the inner surface of the housing, so that the streamlines are compressed in the region of the rotor blade tips.

To avoid the stall, it is also known (DE-B No. 2351308) to arrange a profiled ring which is arranged in front of a blade ring and is concentric with the machine axis, the outer ring surface of which forms an annular channel converging in the flow direction with the housing wall. The proposed solution according to DE-B No. 2351308 is indeed feasible, but in practice means relatively high additional construction costs, since the ring can be difficult to install in the housing if the other structural requirements are taken into account. The efficiency losses due to the additional, friction-generating surface and the so-called flow lag are also not to be underestimated.

Compared to the last mentioned status of Technology therefore has the advantages of the construction according to the invention that it can be installed economically and at a reasonable cost and that, in theory, it causes only a slight loss in efficiency which could not be measured in practice.

Furthermore, it is known for shifting the surge limit to smaller quantities (DE-A No. 2446512), in a turbomachine, in particular in the case of a pump with radial blading and bladed outer stator, to arrange impurities in the flow cross section which is at risk of being torn off. To carry out this measure, an impurity influencing the flow in the form of one or more webs is provided in the inlet area of the stator. The device according to DE-A No. 2446512 is, however, unsuitable for the axial compressor on which the invention is based, in order to stabilize the surge limit in the lower delivery range, in particular because the detachment which first occurs on the rotor blade grille cannot be corrected. On the other hand, this principle cannot be transferred due to the completely different blade configuration of a compressor with axial blading.

An essential criterion for the design of the object according to the invention is that, under normal load, the sleepers to be installed do not narrow the cross section so much that normal operation is significantly impeded. Experience shows that each threshold should restrict the overall free cross-section of the housing only to a cross-sectional area between 80 to 99%, preferably between 95 to 99%, of the cross-sectional area existing before the threshold.

In order to lead the flow through the housing undisturbed and with as little loss as possible, it is recommended that the outer contour of the rising surface of the sleepers be curved in the form of a convex curve, which is composed of circular and / or parabolic arcs. A similar rounding can also be found in DE-PS No. 809842.

Usually, the inner contour of the compressor housing can be assigned a certain average taper, which is related to the compression to be achieved. It is proposed, when using the sleepers according to the invention, to place the upstream base points of the sleepers approximately on a conical jacket describing the mean taper of the inner surface of the housing. This situation can also be deviated from, wind tunnel and standing tests can give the expert advice for improvement.

It is also proposed to give the inner surfaces of the housing a cylinder jacket configuration between the individual sleepers. You can deviate from this configuration. An expanding or narrowing configuration, in particular a conical jacket configuration with an opening angle of 0.5 and 10 °, can be selected.

To shift the surge limit to smaller delivery rates, the fact that the axial velocity in the outer blade area is accelerated by the narrowing due to the sleepers is used. The effect is intensified by reducing the flow cross-section. The swirl of the flow is not changed by the constriction; the circumferential component remains constant.

Due to the increase in the axial speed in the area of the blade tips, there is more flow on the blade profiles on the suction side than the design without a stepwise narrowing with the thresholds according to the invention. You are relieved. The flow is stopped at lower throughput speeds.

It is further proposed that the defined high point of the thresholds, for example the average threshold height, be approximately on a conical jacket K _z describing the average conicity of the inner surface 11 of the housing.

Finally, it is advantageous if the height of the thresholds increases in sections in the direction of flow.

• Fig. 1 einen Querschnitt durch den oberen Teil eines Verdichters gemäss Erfindung;
• Fig. 2 in mehr schematischer Darstellung die Anordnung der Schwellen;
• Fig. 3 eine Gegenüberstellung der Strömungsgeschwindigkeiten bei einem Axialverdichter mit und ohne Schwellen;
• Fig. 4 das gemessene Kraftfeld bei Axialverdichtern mit und ohne Schwellen.

In the following an embodiment of the invention is explained with reference to the drawing. The figures show:

• 1 shows a cross section through the upper part of a compressor according to the invention.
• 2 shows the arrangement of the thresholds in a more schematic representation;
• 3 shows a comparison of the flow velocities in an axial compressor with and without thresholds;
• Fig. 4 shows the measured force field in axial compressors with and without thresholds.

The exemplary embodiments of multi-stage axial compressors shown in FIGS. 1 and 2 have guide vane and rotor blade rings arranged one behind the other in conventional and known manner and combined into stages 1, 2, 3,. The guide blades 4, 6 are connected downstream of the moving blades 5, 7. In front of the first rotor blade ring there is a leading ring 8 with pivotable blades for regulating the throughput. The blades 4 to 7 are also partially pivotable and connected to corresponding adjustment devices. While the outer wall of the rotor 10 can have a constant constriction in the flow direction, sleepers 12, 13, 14 are provided on the opposite housing inner wall 11, which protrude inward from the inner surface of the housing surrounding the blade rings and in the circumferential direction about the axis. The sleepers are stepped in the direction of flow in the manner of a staircase, constricting the free overall cross section of the housing from step to step by 1 to 5% of the cross-sectional area in front of the threshold.

Each threshold 12 to 14 is arranged immediately in front of a moving blade, in the present case designated with the numbers 15, 16, 17, the operation being in the area of the moving blade tips Streamlines are compressed. As can be seen in particular from FIG. 2, the outer contour of the sleepers, ie the rising surface 18, is rounded in a convex manner. The curvature curve is composed of circular and / or parabolic arcs. The curvature is intended to prevent flow interruptions, turbulence and similar disturbances in the flow. On the other hand, the narrowing of the overall cross section through the thresholds 12, 13, 14 is not so great that an excessive flow constriction occurs in normal operation, which is also undesirable.

It is also essential that the sleepers are arranged in the direction of flow directly in front of the impellers. This means that the surge limit is shifted to smaller total delivery volumes. 3 and 4 illustrate this fact.

On the left side of FIG. 3, the flow conditions of an axial compressor without a threshold arrangement are shown, but with a conically narrowing flow channel. The amount delivered is assumed to be 100% of the bucket height. The speeds C _{ax are} essentially the same over the blade _height . the flow velocity in the axial direction. If, on the other hand, the quantity is reduced significantly, for example to 60% of the nominal delivery quantity, the speed decreases measured over the bucket height. This can result in tearing, particularly in the upper blade area. If, on the other hand, the flow channel is equipped with thresholds B, there is an increase in the flow velocities in the upper region, recognizable by the nose, which results from the envelope of the velocity verifiers. It is therefore plausible that the surge limit can be shifted to smaller flow rates - calculated in relation to the nominal flow rate.

This plausible assumption is also confirmed by measurements. 4 shows various characteristic curves which were measured on an axial compressor with an output of approximately 400,000 m ³ / h. The flow channel with a tapered but smooth inner surface was used; on the other hand, a flow channel was used which had the sleepers according to the invention. The design point for both compressors was a pressure ratio of about 4.4 and a nominal delivery rate of about 375000 m ³ / h. The surge limit in the original form, designated Gi, was significantly less favorable than when using sleepers according to the invention. This resulted in a curve G ₂ . The characteristic curves a to f have been measured in each case at different guide vane positions, the percentages relating to the adjustment range of the guide vanes, that is to say defined by the paths of the vane adjustment mechanism. The nominal speed was around 3000 rpm.

As can also be seen from FIG. 1, the installation of sleepers was initially dispensed with in the entrance area. However, it is entirely possible to provide the entire housing with such thresholds, for example if the gases used have a high speed of sound, as is the case for helium, for example.

As indicated in FIG. 2 by the line K, the upstream base points of the sleepers 12 to 14 lie approximately on a conical jacket K, which describes the mean taper of the inner surface 11 of the housing. This shape is relatively easy to produce and brings a defined compression performance.

In order to allow an exact fit of the blades 15, 16, 17 within the housing, the blades are turned cylindrical or conical at their tips. This gives them exactly the same distance from the inner surface 11 of the housing, the gradation by the threshold permitting a defined flow gain which is the same for all individual blades.

Claims (7)

1. Single or multi-stage axial compressor with guide blade rings and rotor blade rings (4, 6; 15, 16, 17) arranged axially in series and associated with the stages (1, 2, 3) and having one or more ledges (12, 13, 14) which project inwardly from the inner surface (11) of the housing which surrounds the blade rings, which ledges extend around the axis in the circumferential direction and are arranged as a series of steps, characterised in that the ledges (12, 13, 14) are arranged in the direction of flow directly in front of the rotor blades (15, 16, 17) on the inner surface (11) of the housing, so that in the region of the rotor blade tips the flow lines are compressed.

2. Axial compressor according to Claim 1, characterised in that each ledge (12, 13, 14) constricts the free total cross-section of the housing to a cross-sectional area between 80 and 99%, preferably between 95 and 99%, of the cross-sectional area in front of the ledge.

3. Axial compressor according to Claim 1 or 2, characterised in that the outer contour of the ascending surface (18) of the ledges (12, 13, 14) is curved into the form of a convex line, which is composed of circular and/or parabolic curves.

4. Multi-stage axial compressor according to one of Claims 1 to 3, characterised in that the base points of the ledges facing outwards towards the flow lie close to a conical envelope (K, ) describing the median conicity of the inner surface (11) of the housing.

5. Multi-stage axial compressor according to one of Claims 1 to 3, characterised in that fixed high points of the ledges (12, 13, 14), for example the median ledge heights, lie close to a conical envelope (K_z) describing the median conicity of the inner surface (11) of the housing.

6. Axial compressor according to Claim 1, characterised in that the rotor blades (15, 16, 17) are bent over cylindrically or conically at their tips.

7. Axial compressor according to Claim 1, characterised in that the heights of the ledges (12, 13,14) increase stepwise in the direction of flow.

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