DE3517486A1 - AXIAL COMPRESSOR - Google Patents

Description

The invention relates to an axial compressor according to the preamble of claim 1 "

The invention is, for example, in the Rolls-Royce gas turbine engine GEM 4 applicable.

Outside of the design constraints of a compressor, the gas flow has around the individual rotor blade the tendency to dissolve into violent turbulence and the smooth flow pattern through the compressor stages is destroyed. The gas throughput through the compressor decreases and the stagnant gas develops into a rapidly rotating ring of high pressure gas over the blade tips of a blade ring or a group of compressor stages, if there is a complete breakdown of the flow in all Stages of the compressor impinges, so that in all compressor stages If the flow stagnates, the compressor starts to vibrate.

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The transition from flow stagnation to pumping oscillations can be done so quickly that it doesn't

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is noticed in time, but on the other hand it can also be so be weak, with only slight fluctuations, or poor acceleration or deceleration characteristics appear. A stronger compressor stagnation manifests itself in an increase in the turbine gas temperature and in an oscillation or "cough" of the compressor »A pump oscillation manifests itself in different ways in one stroke Strength from the engine compressor and in a rise in turbine gas temperature ;,

The values of flow rate and pressure ratio at which pumping vibrations begin are referred to as "Surge line". This surge line is a characteristic of every compressor speed / and one of the surge lines of all Line connecting compressor speeds, the surge limit curve (Fig. 1) gives the minimum stable air throughput which is available at the individual speeds. A compressor is always designed in such a way that another ■ more abundant Safety margin (area S) between the working line (flow rate and pressure ratio in normal Operation) and the surge limit curve (flow rate and pressure ratio, at which pumping vibrations occur) is given,

It is known that the individual must in a compressor Compressor stages are carefully coordinated with one another, as each individual compressor stage has its own air throughput characteristics Has. Therefore, it is extremely difficult to design a compressor so that it over a wider range of operating conditions like this is required in an aircraft engine, works satisfactorily.

To ensure efficient operation of the engine over a wide speed range and to maintain it of the above mentioned margin of safety

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it is necessary to use a system to control the compressor air flow. This system usually works with a tapping of compressor air from a middle compressor stage either into the outside air or, in the case of a bypass engine , into the bypass duct. The amount of air diverted from the compressor can be controlled by means of mechanically, electrically or hydraulically operated valves which are opened at low pressure ratios and closed when the engine is operating at higher pressure ratios. Examples of such valves are given in GB patents 1,223 490 and 1 104 007 and in the Rolls-Royce publication "The Jet Engine". When the valves are open, the air throughput through the high-pressure section of the compressor is reduced, while the air throughput through the compressor stages in front of it is increased. This prevents throttling of the first compressor stages due to a low axial flow velocity of the air and avoids the occurrence of pump oscillations.

In the ideal case, the amount of branch air from the compressor would have to go through the Verdi with increasing throughput tend to become progressively smaller in order to allow a corresponding safety margin between the working line and maintain the surge limit curve.

In practice, however, it is extremely difficult to bring about a progressive closing of the bleed valves, since the valves tend to be either fully open or fully closed To take position, whereby there is no essential intermediate position between these two extreme positions gives,

The sudden closing of the bleed valve has a strong influence on the working line of the compressor by

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the throughput decreases, the pressure ratio increases and the safety margin between the working line and the surge limit curve decreases considerably,

In this way, however, there is a sufficient safety margin between the working line and the surge limit curve remains, it is necessary to close the tap valve only at a higher flow rate than it actually is would be desirable, which leads to a considerable deterioration in compressor efficiency.

An alternative solution to the problem could be to design the bleed valve by designing the compressor housing to replace the one or more recesses in the inner wall surface of the housing in the area has the blade tips of a rotor blade stage. Typical designs of this type are in the US patents 4,239,452 and 4,238,170 and NASA Technical Notes TN D-65 38, and American Society of Mechanical publications 75-GT-95 and 75-GT-13 Engineers described.

Such designs have the effect of increasing the pressure ratio for a given throughput, at which pumping oscillations actually occur. The working line of the compressor can therefore be compared with the The surge limit curve can be set in such a way that the compressor is more efficient at high throughputs than a compressor without this measure can be operated, but still

30. a sufficient safety margin between the working line and the surge limit curve at low flow rates is retained. A substantial increase in the pressure ratio However, complicated slot geometries are required to achieve at which pumping vibrations occur, which are costly and difficult to manufacture and which can also result in a loss of performance.

The Rolls-Royce GEM 4 engine is a more powerful version of the GEM 2 engine with a throughput increased by about 15%, which is used in helicopters.

As a result of the increased throughput, air is extracted from the compressor by means of a fluidic-controlled bleed valve branched off. The valve is designed so that it depends on a predetermined value of the engine torque works so that the engine works without pumping vibrations even at low power settings.

It has been shown that in helicopters which are used with a reduced overall weight, a bleed valve actuation can take place during take-off. For helicopters with two engines, if at one engine closes the bleed valve earlier than with the other engine, the resulting mismatch between the two engine powers cause confusion as to whether an engine or control ™ System malfunction or only bleeding valve actuation has occurred. This in turn causes doubts as to the question of whether the start must be aborted or not.

For example, the above problem can be overcome by lowering the pressure ratio, in which the bleed valve is operated in such a way that the bleed valve is completely during the start-up process remains closed. Unfortunately, for the reasons given above, it is impossible to adjust the pressure ratio at which the bleed valve is operated without the occurrence of pumping vibrations of the compressor to risk, which of course would mean a highly undesirable concession. Alternatively, the bleed valve be replaced by a compressor housing design of the type described above, in which case the working

line could be relocated relative to the surge limit curve in such a way that that you get the best performance from the compressor. For the reasons outlined above, however, this would be an extraordinary one complex housing design required before the working line could be laid out so that one could work at high throughput values the same performance characteristics as the working line of a compressor with a bleed valve receives. As stated above, complicated compressor housing designs are expensive and difficult to manufacture and, moreover, can lead to a loss of performance, all of these criteria being undesirable disadvantages.

It is clear from this that neither a blow-off valve nor a special housing design by itself a sufficiently flexible arrangement can lead to the problem mentioned without any disadvantageous consequences engine performance or costs.

The invention is therefore based on the object of a To find a solution to the problem, at xvelchem the disadvantages to be accepted remain minimal, but with a sufficient advantage with regard to the flexibility of the possible uses is achieved. ·

This object is achieved according to the invention by the arrangement specified in the characterizing part of claim 1 solved,

Advantageous embodiments of the invention are the subject the subclaims,

The solution according to the invention consists in that a much simpler design of the compressor housing used together with a bleed valve in such a way that the operating point of the bleeding valve to a lower one

given value of the engine retunoritents can to ensure that the bleed valve remains closed during start-up and consequently the occurrence of Adjustment errors of the engine power settings between engines in pairs is avoided. The simplified housing design makes a small Increase in the pressure ratio achieved at which pumping oscillations occur, especially with low throughput values, but this is sufficient to reach the working point of the bleeding valve without accepting the risk of to be able to relocate occurring pumping vibrations.

The invention is described in more detail below with reference to the accompanying drawings, for example. In the drawings shows:

Fig. 1 is a graphic representation of a

typical safety area between the work line and the surge limit curve of a Ver

poet,

FIG. 2 schematically shows a gas turbine engine with a compressor according to FIG Invention,

3 shows an enlarged schematic

Axial half-section of a compressor according to the invention,
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4 shows the working line and the surge limit curve for a GEM-2 engine without bleeding valve and without measures on the compressor housing with the throttle curve for the first

Compressor stage, and

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Fig. 5 is a graph similar to

Fig, 4 for a GEM-4 engine, there formed according to the invention is, the effects of which are provided according to the invention

Measures are also shown .

1 shows in a diagram in which the throughput is plotted on the abscissa and the pressure ratio is plotted on the ordinate, the course of the working line and the surge limit curve and the safety margin S (hatched) in between for a compressor.

Fig. 2 shows schematically a gas turbine engine 10, a low-pressure compressor, a high-pressure compressor 14, a combustion device 16, a high-pressure turbine 18, a Has low pressure turbine 20 and an exhaust nozzle 22. Of the Low-pressure compressor 12 and the low-pressure turbine 20 on the one hand and the high-pressure compressor 14 and the high- » pressure turbine 18 on the other hand are connected to one another and rotatable by means of coaxial shafts (not shown) stored. In the drawing, the housing 13 of the low pressure compressor is shown broken open,

Fig. 3 shows in schematic, but more detailed details axial half-section depicting the low-pressure compressor of the engine according to FIG. 2. The low-pressure compressor rotor 24 carries a first rotor blade ring according to FIG. 3 22 and a further rotor blade ring 32.

The inner wall 26 of the low pressure compressor housing is in their with the blade tips of the blade ring cooperating area with 5 Urafangsnuten 28 provided. These circumferential grooves can each have a depth of, for example, 1 mm and a width of 1 mm and are approximately perpendicular to the compressor axis and at the same mutual distances along the inner wall 26 of the housing.

In the area of the further rotor blade ring 32 there is one A tap line 30 is provided, which leads away from the inner wall 26 of the housing. This tap line contains a schematic Tapping valve 3-4 shown to control the air tapping from the compressor. A valve actuator 36 and a speed sensor 38 are in a manner known per se for opening and closing the bleeding valve 34 as a function provided by the speed of the compressor rotor 24.

4 shows a graphic representation similar to FIG. 1 with throughput plotted on the abscissa and pressure ratio plotted on the ordinate, the surge limit curve A and the working line B being shown for a GEM-2 engine that has neither a bleed valve nor a special housing design of the compressor. The further drawn line C represents the throttle curve of the first compressor _{rotor stage. As already explained above, the requirement f is to let} the working line B run as close as possible to the surge limit curve A, whereby, however, a sufficient safety margin D must remain. From Figs. 4 it can be seen that when the throughput through the compressor increases, the surge limit curve A and the working line B diverge, with the consequence of a reduction in the compressor efficiency -2 has a throughput increased by about 15%. The bleed valve 34 serves α3ζμ to let the working line B of the compressor run as close as possible to the surge limit curve A of the compressor «,

FIG. 5 is a graphic representation similar to FIG. 4, but for a GEM-4 engine which has both a bleed valve 34 as well as with the annular grooves 28 on the inner wall of the compressor housing. In this graphic The illustration is the surge limit curve with E, the working line with F and the throttle curve of the first compressor

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rotor stage designated with G. The Purap limit curve E and the working line F represent those characteristics that can be found without the annular grooves 28 on the inner wall of the housing and the bleed valve 34 result. Line H represents the new one Surge limit curve, which is due to the on the inside wall of the housing 26 provided 5 annular grooves 28 results. From the The action of the bleed valve 34 can also be seen in the remote course of the working line F. If throughput is reduced by the Compressor increases, the working line and surge limit curve initially diverge, as explained above. The bleed valve 34 is controlled so that it closes at about point F1 in Fig. 5, whereby the working line jumps to point F2 and thereby again runs closer to the surge limit curve. When the throughput through the compressor decreases, the bleeding valve 34 opens approximately at point F3, as a result of which the working line jumps back to point F4 and prevents the compressor from being exposed to pumping vibrations Area works.

Because of the step-like characteristics of the action of opening and closing a bleed valve, this is the case with conventional ones Compressor arrangements not possible, the response point of the bleed valve below a specified engine torque value without the risk of Having to accept the occurrence of pumping vibrations.

The grooves 28 in the housing inner wall have the effect of FIG. 5 that the pressure ratio at which pump oscillations occur for a given throughput, is higher, namely on the new surge limit curve H, This upward shift in the surge limit curve is sufficient in order to reduce the response point of the bleed valve 34 to a lower engine speed value allow, while still having a sufficient margin of safety remains between the offset working line and the surge limit curve. At this setting point is

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in the case of an engine system with two engines, it is ensured that the bleed valve on both engines remains closed during the start-up process and thus a mismatch between the engine performance of both Engines is avoided.

Another advantageous effect of the annular grooves 28 is the change in the throttle curve G of the first compressor rotor stage according to FIG _.: 5,

Another advantage of using the measures according to the invention is that there is a risk of blade breakage is reduced in the first compressor stage. Studies have shown that as a result of an inadequate Bleed valve throughput at or near zero power conditions stresses due to blade vibrations These stresses can be so great that cracks can arise in the blades, and this ultimately leads to blade fractures with extensive secondary damage to the engine can. One way to overcome this problem would be to increase the bleed valve throughput. this however, would have the undesirable effect of increasing the magnitude of a torque mismatch between pairs arranged engines as described above. The measures according to the invention lead to an increase in the Surge limit curve at zero tapping conditions that are sufficient is large, so that the vibration stresses on the blades can lead to an impairment of the blade service life leading border remain.

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Claims (1)

Claims 1, axial compressor with a flow channel outside limiting compressor housing and a circulating therein Compressor rotor with at least one rotor blade ring, characterized in that the inner wall (26) of the compressor housing (13) in their at least one rotor blade ring (22) interacting with the blade tips Area is provided with a number of recesses (28) and that downstream of the one or more recesses (28) an air tap valve (34) is arranged for branching off air from the compressor (12), 2, axial compressor according to claim 1 _f, characterized in that five recesses (28) in the form of circumferential grooves are provided in the area of the housing inner wall (26) which cooperates with the blade tips of a rotor blade ring (22), 3, axial compressor according to claim 2, characterized in that each circumferential groove (28) has a width of approximately 1 mm and a depth of 1 mm and that the circumferential grooves are each arranged approximately perpendicular to the compressor axis and at approximately the same mutual distances along the compressor axis, 4, axial-flow compressor according to one of claims 1 to 3, characterized in that _f an actuator (36) is provided which actuates the Luftanzapfventil (34) when the rotational speed of the compressor rotor (20) exceeds a predetermined speed limit value.