GB2082688A

GB2082688A - Axial fluid flow machines

Info

Publication number: GB2082688A
Application number: GB8125503A
Authority: GB
Original assignee: Wilhelm Gebhardt GmbH
Current assignee: Wilhelm Gebhardt GmbH
Priority date: 1980-08-22
Filing date: 1981-08-20
Publication date: 1982-03-10
Also published as: DE3031686A1; JPS5773894A; SE457277B; FR2488950B1; DE3031686C2; FR2488950A1; SE8104956L; GB2082688B

Abstract

The machine e.g. a fan, has a cylindrical housing (2) around a rotor (1), and a guide wheel (3) downstream of the rotor (1). Upstream or downstream of the latter there is a further auxiliary rotor (4), co-axial with rotor (1), and having blades of a smaller radial size. In the part of the hub of rotor (1) where flow separation is likely, for decreasing the deflection of the relative flow and for this reason for decreasing the blade grid loading, the supply of energy is decreased in the profile sections in question by decreasing the change in the circumferential velocity component of the absolute flow between the inlet and outlet of rotor (1). The auxiliary rotor (4) supplies further energy in these profile sections for keeping substantially constant the design- dependent energy amount in all blade sections. In a modification boundary layer aspiration is used in the region of potential flow separation. Rings (5,9) may be provided. <IMAGE>

Description

SPECIFICATION Improvements in axial fluid flow machines The present invention is with respect to a process for the operation of an axial fluid flow machine with a rotor having a hub and placed in a housing which may, for example, be cylindrical.

The invention is furthermore with respect to an axial fluid flow machine for undertaking the new process and having a rotor placed in a housing which may, for example, be cylindrical.

It is known that in the case of axial fluid flow machines such as axial fans, axial compressors and axial pumps when run with heavy choking under the normal operation range, that is to say at a low throughput rate, there is an instability in the characteristic curve starting in the characteristic to the left of the "breakaway (or separation) point".

This discontinuity in the characteristic is caused by separation when the turning relative flow is separated in two typical parts of the axial rotor, that is to say in the separate spaces between the blades, that is to say instationary, turning separation, which in writings on the question is normally named "rotating stall" and on the turning rotor hub, that is to say a separation which in writings on the question is named "hub eddying".

The separation in the separate spaces between the blades is responsible for the blade spaces in which it takes place at any given point of time are almost completely stopped up because of the pulsating flow with strong eddying effect, while the other spaces have a normal flow therethrough, the zone in question being turned at a given circumferential speed in relation to the turning blading. Because the "hub eddying" comes into existence, the normal flow is limited in crosssection and the eddy range becomes greater with an increase in the volumetric flow, the point of separation or breakaway moving upstream. Such signs of separation or breakaway are responsible for the shortcoming that on the one hand the rotor blades are likely to be damaged because they are not strong enough and on the other hand a high noise level will be produced which is, as such, undesired.

Because of such changes in the point of operation, which may not be got under control, further dynamic loads are produced which may be responsible for fatigue and breakage caused by the vibrations, of the turning rotor blades and vibrations of the plant generally may be produced.

The noise level will be stepped up and because of the unstable properties in operation there will be changes in the noise level which are troublesome as far as they are specially unpleasing to the ear and by those in the art are named "pumping". A number of different suggestions have been made for stabilizing the flow. For example in German patent 697.816 the suggestion was to have a ring-like sheet metal guide, concentric to the axis of a fan on the pressure side of the rotor. On the other hand, the suggestion of Swiss patent 288,243 is to have a guide wheel upstream from the rotor of an axially pumping machine of the sort coming into question here and to have between the rotor and the guide wheel a coaxial cylindrical ring.The suggestion made in German Auslegeschrift specification 1.064.191 is to have a hub-ring or rings between the rotor and a downstream guide wheel, or to have such hub rings possibly after the downstream guide wheel as well. As part of a further suggestion in the prior art, a stabilizing face was to be placed upstream from the inlet of the rotor blading, see for example German Gebrauchsmuster patent 1.949.833.

However, none of these measure has been responsible for the desired outcome; although it seems that, by taking such steps, a certain stabilizing of the flow may be produced, undesired separation and breakaway still take place as noted. Lastly, in the suggestion in German patent 1.428.077, a ring-like opening was to be used for moving medium back to the space upstream from the rotor and later, in a further development, there was to be a stabilizing system in the form of a bypass duct with a return flow grating. Although with such systems signs of separation or breakaway were stabilized at given points, it was not possible for such separation to be stopped, quite in addition to the fact that the measures put forward in these patents are quite complex.

Furthermore, it would seem that the "hub eddying" was hardly abel to be decreased by the steps.

General Outline of the Invention One purpose of the present invention is that of putting an end to these shortcomings and, more specially, to make it clear how, with a relatively small and straightforward development in the case of axial machines for the highest possible volumetric numbers or coefficients, the highest possible pressure coefficients and the highest reaction degrees the signs of separation or breakaway taking place in the rotor hub and/or in the spaces between the blades may be put an end to with all their undesired effects.

For this purpose, as part of the invention, a new process is put forward, characterized in that on the one hand in the part of the hub, in which separation is likely, for decreasing the deflection of the relative flow and, for this reason, for decreasing the blade grid loading the supply of energy is decreased in the profile sections in question by decreasing the change in the circumferential components of the absolute flow Acu between the inlet and outlet of the rotor and on the other hand further energy is supplied in these profile sections upstream and/or downstream of the rotor for keeping generally constant the design-dependent energy amount in all blade sections.In this respect it is possible for example for the sytem to be so designed that for decreasing the change in the circumferential component of the absolute flow Acu between the inlet and outlet of the rotor in the profile sections where separation is likely, more specially in the hub part, the blade profile is designed for a smaller overall pressure increase than originally was the case in all sections, is being responsible for a less curved, changed blade profile. It is for example furthermore possible for the design to be such that for keeping constant the design-dependent energy amount, the overall pressure is increased upstream and/or downstream of the rotor.The axial fluid flow machine of the present invention for performing the new process which has a cylindrical housing placed round a rotor and a downstream guide wheel in the direction of flow after the rotor, is characterized in that upstream from the rotor there is a further or auxiliary rotor, coaxial to the said rotor, having blades of a smaller radial size. Thereby, provision can be made that the auxiliary rotor is surrounded by an outer ring forming a coaxial ring-like separating wall, the diameter of the outer ring being smaller than that of the housing.The system, however, may be so designed for example as well that downstream from the rotor there is, coaxial thereto, a further rotor whose blades are smaller radially and downstream from the further rotor there is a further guidewheel of generally the same size, and that at least the further rotor and, preferably furthermore, the guide wheel has a coaxial ring like separating wall in the form of an outer ring placed round it, whose diameter is smaller than that of the housing. In the case of an axial fluid flow machine of the present invention, which has a rotor placed within a housing which may, for example, be cylindrical, the design may furthermore be such that by way of a pocket aspiration of the boundary layer takes place in the part where aspiration would otherwise be likely, the aspiration line starting preferably in a zone in the outlet part of the rotor.Lastly, the design may be such that in the inlet part of the rotor the boundary layer is accelerated, for example by using a pressure line. In this case, the medium aspirated is used in a full-loop secondary circuit for accelerating the boundary layer.

It may be seen from this that the purpose of the inventions is that of putting an end to a discontinuity in the characteristic or, putting it better, moving the breakaway or separation point towards low volumetric flows so that, in this way, the stable operation range is increased in size.

Furthermore, the invention is responsible for lowering the noise level when the apparatus is choked back in operation. Lastly, a further purpose of the invention is that of producing, on purpose, a decrease in the hub ratio, that is to say of the parts of the rotor through which no flow takes place so that the speed profile at the outlet of the machine is made more even because the eddying core, which is formed at the point of change-over of the flow from the ring-like cross-section round the hub body and the full circle cross-section coming next to it at the outlet of the machine, takes the form of a pipe coming next, is decreased in size.For this reason, the purpose of the invention is not only to put an end to instability in the characteristic or decreasing it towards heavier choking, but furthermore, at the same time, to keep up the degree of reaction, to make possible greater volumetric coefficients and furthermore to make more even the velocity profile at the outlet of the machine.

for this purpose, the suggestion of the invention is for energy to be supplied in the part at which separation would otherwise be likely, such supply being undertaken by increasing the pressure upstream and/or dowstream from the rotor or by boundary layer aspiration from the part where separation would otherwise be likely, or by accelerating the boundary layer. A number of different useful effects are produced with the design of the invention: smaller size, a high degree of reaction, high volume coefficients, high pressure coefficients (answering to a high power density), increase in the stable operation range, that is to say moving the breakaway or separation point towards smaller volumetric flows, decreasing the noise level, producing a more even velocity profile at the outlet of the machine etc.

Further useful effects and details of the invention will be seen from the account, now to be given, of a number of different working examples of an axial fan of the present invention for undertaking the process of the invention.

Figure 1 is a view of a first working example of an axial fan of the invention in a side view, diagrammatically.

Figure 2 is a somewhat changed view of the system of figure 1, again from the side, again diagrammatically.

Figure 3 is a view of a second somewhat changed form of the invention, viewed on the same iines as in figures 1 and 2.

In the case of the axial fan to be seen in figure 1, the supply of energy takes place outside the rotor 1 , which is placed within a cylindrical housing 2, and which is placed upstream from the downstream rotor 3. The rotor 1 is placed downstream from a coaxial further rotor 4, whose blading, as may be seen, has a smaller radial size than the blading of the main rotor 1. This further rotor 4 may be placed within a coaxial ring-like separating wall 5 in the form of an outer ring, whose diameter is less than that of housing 2.

Such a separating wall 5 is convenient in many cases, however it is not necessary in each case. In this respect, the rotor 1 and the further rotor 4 are so designed in relation to each other that, at the outlet 6 from the rotor, there is a constant overall increase in pressure over the full height of the blading. The axial upstream rotor or further rotor 4 may, as is the case with the system of figure 1, be run in the opposite direction (arrow 7) to the direction of the rotor (arrow 8), although it may be run in the same direction as the rotor, the further rotor then having a guide wheel downstream from, it. The further rotor, which is placed in the hub part, may have its own separate driving motor, although the driving power may be taken from the driving system for the main rotor. Because of the small outer diameter of the further rotor, it may be run at a higher speed. The outer ring 5 of the further rotor may be stationary (fixed in position) or it may be turned with the further rotor. This ring may have an axial length, see the working example to be seen in figure 1, which is the same or less than the length of the further rotor, it being kept clear of the rotor, while, however, the outer ring of the further rotor may have an axial length which is greater than that of the further rotor and be overlapped with and into the rotor. Rotor I has an inbetween ring 9 turning with it, ring 9 running for the full axial length of the rotor or only over a part of this length and preferably running as far as a point outside the inlet part 10 of the rotor. Its diameter is greater than that of the outer ring 5 of the further rotor, it starting at a point near its back end.With the help of the outer ring 5, it will be seen that the incoming flow to the axial stage is cut up into a a main flow 11 and a further flow 12 which gets more energy. The inbetween ring 9 has the function of stopping any mixing of the main and further flows in the inlet part of the rotor or over its full axial length.

In the case of the system of figure 1, the general teaching may be seen to be that the relative flow to the rotor at the hub of the rotor has to be deflected most strongly so that the blade grid loading is specially heavy in the hub part and, for this reason as well, danger of separation or breakaway specially likely as well in the case of the relative flow. The measures noted have, for this reason, the purpose of decreasing the supply of energy to those profile sections which are within the part of the hub where separation or breakaway is likely for putting an end, as far as possible, to the dangers noted.Because, however, on the other hand, the energy supplied (equal to the overall pressure increase Apt) is to be kept constant in all blade sections, in the working example of figure 1 the decreased energy supply in the hub part of the rotor is balanced by a further supply of energy upstream from the rotor blade inlet point so that, in this way, the energy supplied in each profile section is constant, generally on the lines of the formula: Apt = Xptt L + Apt, V+A N 11,,, N = const., wherein '\put L is the pressure increase produced in the rotor Apt, V is the pressure increase produced upstream from the rotor blading inlet and Apt, N is the pressure increase produced downstream from the rotor blading outlet, while Apt is the overall pressure increase, that is to say the supplied energy.

In the somewhat changed working example of figure 2, rotor 14 is fixed on hub 13 within a cylindrical housing 1 5 and downstream from it there is a downstream guide wheel 16, the direction of flow being marked by arrow 17. In this case, a coaxial further rotor 1 8 is placed downstream from rotor 14, the blading of rotor 1 8 being smaller radially than those of the main rotor.

Downstream from the further rotor 18, there is a further guide wheel 1 9 of about the same size.

Furthermore, there is a ring-like separating wall, coaxial with the different rotors, in the form of an outer ring 20, whose diameter is less than that of the housing and which is placed round the further rotor 1 8 and preferably, however, furthermore the further guide wheel 19. In this respect, the rotor and the further rotor are so designed in relation to each other that the flow at the outlet of the machine has a constant amount of energy supplied to it over the full ring cross-section. The further rotor 1 8 may be run in the same direction (arrow 21) as the rotor or the main rotor 14 (arrow 22); it may, however, as in the working example of figure 1, be run in the opposite direction to the main rotor.The further rotor may have, as in the working example of the invention noted earlier, its own separate driving motor, although it may furthermore have its driving power taken from the system for driving the rotor.

The outer ring 20 of the further rotor may be fixed in position or may be designed to be turned with the further rotor. As part of the design, the outer ring 20 of the further rotor have an axial length which is generally equal to the length of the further rotor and/or of the guide wheel, it being clear of the guide wheel placed downstream from the rotor, the design having to be such that this outer ring is opposite at least to the further rotor but is preferably placed round the further guide wheel without, however, overlapping into the guide wheel placed downstream from the rotor or the main rotor.'The outer ring of the further rotor may, however, be designed stretching as far as the guide wheel placed downstream from the rotor overlappingly so as to be causing a division of the same into an outer guide wheel and an inner guide wheel or in place of this it is possible to have a separate outer ring 23 for the main guide wheel, the outer ring 23 being separate from the outer ring 20. In all cases, the blading of the rotor will have a division in a main current making its way through the outer guide wheel (arrowed 24) and a further current 25 making its way through the further rotor, and in which within the further rotor the supply of further energy takes place.To this end as well, the design is furthermore such that the rotor or the main rotor 14 has an inbetween ring 26 turning with it and stretching over the full axial length of the rotor or only over part of this length and preferably running out from the outlet part of the rotor, its diameter being for example smaller than that of the outer ring used with the further rotor.

In the case of the working example to be seen in figure 3, it is a question of an axial fan having a rotor 30 within a cylindrical housing 31 and having a downstream guide wheel 32. In this case, medium is aspirated from the part, at which separation or breakaway is likely to take place, of the hub 33 as for example at 34 at the outlet 35 using a line 36 and furthermore using a pocket (not figured) which is joined up with the aspiration line 36. Aspiration takes place as marked by arrow 37.

In the case of a further working example, not presented in the present figures, the purpose of the invention may be put into effect using an axial fan which, as well, has a rotor placed within a cylindrical housing and, naturally, guide wheels of a known design, in the case of which medium under pressure is supplied by way of a pressure line into the inlet part of the rotor.

In the case of all the working examples of the invention of axial fans as noted, the process may be seen to be undertaken in such a way that, on the one hand in the hub part, where separation or breakaway would otherwise be likely, of the rotor, in the profile sections within the hub part where separation would otherwise be likely, the energy supply is decreased for decreasing the deflection of the relative flow and, for this reason, as well of the blade grid loading and, for this reason furthermore the absolute velocity Acu is made smaller, and, on the other hand for balancing out the decrease in the supply of energy and for generally keeping constant the design-dependent energy amount in all blade sections upstream and/or downstream from the rotor energy is supplied again and, for this reason, APst is increased, for example by increasing the pressure at these points or taking off energy at the outlet of the blade or, lastly, guiding medium from the outlet of the blades back to the inlet.

Claims

1. A process for the operation of an axial fluid flow machine with a rotor having a hub and housing, as for example a cylindrical housing, characterized in that on the one hand in the part of the hub, where separation is likely, for decreasing the deflection of the relative flow and for this reason for decreasing the blade grid loading, the supply of energy is decreased in the profile sections in question by decreasing the change in the circumferential component of the absolute flow Acu between the inlet and outlet of the rotor, and, on the other hand, further energy is supplied in these profile sections upstream and/or downstream of the rotor for keeping generally constant the design-dependent energy amount in all blade sections.

2. A process as claimed in claim 1, characterized in that for decreasing the change in the circumferential component of the absolute flow Acu between the inlet and outlet of the rotor in the profile sections where separation is likely, more specially in the hub part, the blade profile is designed for a smaller overall pressure increase than originally was the case in all sections, this being responsible for a less curved, changed blade profile.

3. A process as claimed in claim 1 or claim 2, characterized in that for keeping constant the design-dependent energy amount, the overall pressure is increased upstream and/or downstream of the rotor.

4. An axial fluid flow machine for performing the process of anyone of claims 1 to 3, having a cylindrical housing placed round a rotor whereby downstream of which rotor a guide wheel may be arranged in the direction of flow after the said rotor, characterized in that upstream from the rotor there is a further auxiliary rotor, coaxial to the said rotor, having blades of a smaller radial size.

5. An axial fluid flow machine as claimed in claim 4, characterized in that the auxiliary rotor is surrounded by an outer ring forming an coaxial ring-like separating wall, the diameter of the outer ring being smaller than that of the housing.

6. An axial fluid flow machine as claimed in claim 4 and 5, characterized in that the rotor and the auxiliary rotor are so designed in relation to g each other that at the outlet from the rotor a constant overall pressure increase is produced over the full blade height.

7. An axial fluid flow machine as claimed in any of claims 4 to 6, characterized in that the auxiliary rotor or further rotor rotates in the same rotational direction as the rotor.

8. An axial fluid flow machine as claimed in claim 7, characterized in that between the further rotor or auxiliary rotor there is arranged a further rotor or auxiliary rotor and there is arranged a further or auxiliary guide wheel or guide appliance.

9. An axial fluid flow machine as claimed in any of claims 4 to 6, characterized in that the further rotor rotates in the opposite direction to the rotor.

10. An axial fluid machine as claimed in anyone of claims 4 to 9, characterized in that the further or auxiliary rotor has its own separate driving motor.

11. An axial fluid flow machine as claimed in anyone of claims 4 to 9, characterized in that the driving power for the further rotor is taken from the driving system for the rotor.

12. An axial fluid flow machine as claimed in anyone of claims 4 to 11, characterized in that the circumferential speed of the further rotor is greater than that of the rotor.

13. An axial fluid flow machine as claimed in anyone of claims 4 to 12, characterized in that the outer ring of the further rotor is fixed in position or stationary.

14. An axial fluid flow machine as claimed in anyone of claims 4 to 12, characterized in that the outer ring of the further or auxiliary rotor rotates with the same.

1 5. An axial fluid machine as claimed in anyone of claims 4 to 14, characterized in that the outer ring of the further rotor has an axial length which is equal to or smaller than the length of the further rotor, it being positioned clear of the rotor.

1 6. An axial fluid flow machine as claimed in anyone of claims 4 to 14, characterized in that the outer ring of the further rotor has a greater axial length than that of the further rotor and is placed overlapping the rotor in an inward direction.

17. An axial fluid flow machine as claimed in anyone of claims 4 to 16, characterized in that th rotor has an inbetween ring turning with it and stretching over the full axial length of the rotor or' over a part of such length and preferably running as far as a position outside the inlet part of the rotor, its diameter being greater than that of the outer ring of the further rotor, it starting near the back end of the last named.

1 8. An axial fluid flow machine for undertaking the process as claimed in anyone of claims 1 to 3, having a cylindrical housing with a rotor within it and, downstream from the rotor, a downstream guide wheel, characterized in that downstream from the rotor there is, coaxial thereto, a further rotor whose blades are smaller radially and downstream from the further rotor there is a further guide wheel of generally the same size, and in that at least the further rotor and preferably furthermore the further guide wheel has a coaxial ring-like separating wall in the form of an outer ring placed round it whose diameter is smaller than that of the housing.

1 9. An axial fluid flow machine as claimed in claim 18, characterized in that the rotor and the further rotor have such a relation to each other that the flow at the outlet of the machine has a constant energy level supplied thereto it over the fully ring cross-section.

20. An axial fluid flow machine as claimed in claim 1 8 or claim 19, characterized in that the further rotor is turned in the same direction as the rotor.

21. An axial fluid flow machine as claimed in claim 1 8 or claim 19, characterized in that the further rotor is run in the opposite direction to the rotor.

22. An axial fluid flow machine as claimed in anyone of claims 1 8 to 20, characterized in that the further rotor has its own separate driving motor.

23. An axial fluid flow machine as claimed in anyone of claims 1 8 to 20, characterized in that the driving power for the further rotor is taken from the driving system for the rotor.

24. An axial fluid flow machine as claimed in anyone of claims 18 to 23, characterized in that the outer ring of the further rotor is fixed in position.

25. An axial fluid flow machine as claimed in anyone of claims 1 8 to 23, characterized in that the outer ring of the further rotor rotates therewith.

26. An axial fluid flow machine as claimed in anyone of claims 18 to 25, characterized in that the outer ring of the further rotor has an axial length which is generally the same as the length of the further rotor and/or of the guide wheel, it being clear of the rotor placed downstream from the guide wheel.

27. An axial fluid flow machine as claimed in anyone of claims 1 8 to 24, characterized in that the outer ring of the further rotor is overlapped with the guide wheel placed downstream from the rotor for causing division of the same into an outer guide wheel and an inner guide wheel.

28. An axial fluid flow machine as claimed in anyone of claims 18 to 27, characterized in that the rotor has an inbetween ring turning therewith, the ring stretching over the full axial length of the rotor or a part of its length, it preferably running outwards from the outlet part of the rotor, its diameter being for example smaller than that of the outer ring of the further rotor.

29. An axial fluid flow machine for undertaking the process as claimed in anyone of claims 1 to 3, having a housing round a rotor, characterized in that, by way of a pocket, aspiration of the boundary layer takes place in the part where separation would otherwise be likely.

30. An axial fluid flow machine as claimed in claim 29, characterized in that the aspiration line has its start in a zone in the outlet part of the rotor.

31. An axial fluid flow machine for undertaking a process as claimed in anyone of claims 1 to 3, having a rotor within a housing, characterized in that in the inlet part of the rotor the boundary layer is accelerated, for example by using a pressure line.

32. An axial fluid flow machine as claimed in anyone of claims 29 to 31, characterized in that the aspirated medium is used in a full-loop secondary circuit for accelerating the boundary layer.

33. A process for the operation of an axial fluid flow machine substantially as hereinbefore described with reference to the accompanying drawings.

34. An axial fluid flow machine substantially as hereinbefore described with reference to the accompanying drawings.