DE2423385B2 - Rotor for a supersonic centrifugal compressor - Google Patents

Description

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The invention relates to a rotor for a supersonic centrifugal compressor with an entry zone in which the rotor carries a small number of blades, and with an exit zone with a substantial radial Flow in which the rotor carries a higher number of blades

A common centrifugal compressor contains a rotor that is mounted on a shaft by a rotor and has blades provided disc is formed and is installed in a housing having a diffuser. To the Fluid entering through an axial opening of the housing becomes a through the vanes Speed is given, whereupon the pressure slows down towards the blades. And through Centrifugation is increased. Upon exiting the rotor, the fluid has a significant kinetic energy to be recovered in the form of pressure in the diffuser. Numerous types of diffuser systems are known, with the one at f> 5 the most used are the smooth diffuser and the winged diffuser and spiral.

In order for a centrifugal compressor to deliver a high degree of compression , the circumferential speed of the rotor must be high and of the order of 600 m / s for degrees of compression of st, B, of the order of 10 for air. Under these conditions, the gas flow emerging from the rotor has a supersonic absolute speed a centrifugal compressor with high specific power, ie a high ratio between the mass power and the end cross section of the rotor, and a high degree of compression, of z. B. 10 as in the previous case, the relative speed at the blade end at the entrance of the rotor is generally also a supersonic speed. For example, in a centrifugal compressor with a rotor in which the ratio between the circumference and the inlet radius is 1.5, the relative speed at the blade end at the inlet of the rotor is well above the speed of sound and is on the order of Mach 13. The simultaneous presence of an upper sonic flow in the inlet zones of the rotor near the blade ends and in the diffuser inevitably results in the presence of back pressure shock waves in these zones. The efficiency of the compressor depends to a large extent on the formation of the flow in these shock wave zones and on the stability of the same. Furthermore, in the known attempted, the speed of the fluid at the outlet of the rotor has a low homogeneity, both in the azimuthal sense and in the axial sense, which is detrimental to the efficiency of a smooth diffuser surrounding the rotor and in many cases forces you to work in to be provided for this governing body. Although these guide elements give satisfactory results, they have the disadvantage of complicating the compressor.

From CH-PS 5 06 714 a compressor with a front wheel forming an axial compressor is known, followed by a wheel attached to the same shaft, the blades of which are separated from those of the front wheel by a axial zone to stabilize the flow is separated. This increases the axial length and that Weight of the compressor, which is a serious disadvantage in aviation and making two Blade sets with different parameters leads to what complicates the machining.

From GB-PS 5 84 056 a compressor is known, the blades of which are continuously curved from the inlet to the outlet. So the gas is along the deflected azimuthally throughout the length of these blades and the radial deflection also takes place along the entire length of the rotor. A zone with purely radial flow can therefore be used in this known compressor do not occur.

From DE-OS 21 01 628 a supersonic centrifugal compressor is also known in which part of the Wall is formed by a fixed housing and part of the inner flange of the rotor. At a Such a construction, the speed is very different, so that with a smooth diffuser achievable improvement in efficiency is compensated again.

A centrifugal compressor with an auxiliary rotor is also known from CH-PS 2 69 886, in which the blades are only attached radially to a running disk. The entire azimuthal deflection of the gas is done by An auxiliary rotor hub, which is the inlet zone and the compression zone of the centrifugal compressor at the same time forms. This well-known centrifugal compressor is for sub-

The speed of sound is designed and the problems that occur with supersonic operation are not present here.

A limit performance centrifugal compressor is described from the book by Eckert / Schnell, but in which the relative speed at the impeller has a value below 1 Mach For higher throughputs it is at This known radial compressor proposed to connect an inlet guide device upstream of the impeller, in which a pre-swirl is generated in the direction of rotation, whereby the relative entry velocity is further reduced.

The invention is now based on the object of designing the rotor of an upper sonic centrifugal compressor in such a way that that despite the high entry speed, small losses occur, i.e. the efficiency of the rotor is high

This is achieved according to the invention in that a rotor is designed such that

the long blades of the entry zone along the edge of the blade tip between A and C cause a deflection of the flow of about 7 °, with point C being the base of a plumb bob felled from point A of the adjacent long blades onto the edge of the blade tip of the blade,

the divergence D in the entrance zone A - C is about 7 °,

at the blade tip at A the relative supersonic speed prevails at the blade root at B subsonic speed,

a central compression zone is provided between the entry zone AB and the exit zone (at 10), in which the rotor has a higher number of blades than the entry zone,

the long blades extending over the entire length of the rotor have a thickness, those from the leading edge of the long shovel up to the leading edges of the central blades increases, and

the angle of attack of the leading edge of the blades of the rotor, d. H. the angle between that of the hub facing edge of the blade at the front edge with an axially parallel straight line 60 to 65 ° amounts to

As is known, the azimuthal deflection between two successive points is one and the same Streamlines defined by the change in the angle of attack between these two points. The angle of attack itself is defined as the angle which is from the tangent to the streamline at a given point with the meridian track of the tangential plane to the Flow surface is formed at this point.

Furthermore, the entrance zone is provided in order to stabilize the back pressure shock wave. In this same entrance zone, the deflection on the suction side of the blades and the divergence are expediently of the order of magnitude of 7 °. The Sehaufel number in the entrance zone is generally between six and twelve. Between twenty-four and thirty-two blades can be provided in the compression zone, this number increasing with the space requirement at the end of the rotor. In practice, for. For example, intermediate blades can be provided between the blades Gb ″, r the entire length of the rotor from the inlet, e.g. three or four for a main blade are and replacement should prevent phenomena on the suction side. Generally, there must be at least thirty-two blades in the exit zone. In numerous In cases it is sufficient to have an additional guide vane in the exit zone in the middle of the vanes

Provide limited channels of greater extent.

The compressor designed with the rotor according to the invention is in numerous technical fields applicable. It is used in particular in aviation as the input element of a turbojet engine bar. It is also applicable in the industrial field, especially when heavy gases have to be compressed and the achievement of a high degree of efficiency and / or a high specific power is an essential characteristic. This is particularly the case with Compressors for heavy gases, e.g. B. LJranhexafluorid, the case that is used for isotope separation.

An embodiment of the invention is explained below with reference to the drawing F i g. 1 is a schematic half section of a Compressor through a plane passing through the axis; Fig. 2 is a perspective view showing the structure of the rotor of the compressor;

Fig.3 is a simplified diagram showing handled the entrance of the blades of the rotor and the corresponding speed diagram shows

F i g. 1 shows a centrifugal compressor for an application in which the greatest possible reduction in the frontal space requirement of the rotor is not mandatory. In practice, such a compressor is particularly useful re used in fixed systems Because the diameter of the rotor is relatively large, the total number of blades can be considerably larger than in a compressor be for aviation, which must have a smaller radial space requirement.

-to the in F i g. 1 has the compressor shown as common, a rotor 1, which is driven by a drive shaft 2, and is provided with blades. In a housing 3, which surrounds the rotor and leaves an inlet opening 4, are around the rotor smooth diffuser 5 and a wing diffuser 6 are provided. The rotor 1, which is provided with blades, sucks this in compressing fluids, e.g. B. a heavy gas, through the inlet 4 and promotes it in the smooth Diffuser, from which it is in the wing diffuser 6 and

so possibly in a spiral casing surrounding it.

The path of the fluid from the inlet 4 to the diffuser 5 can be three consecutive Zones are understood as consisting.

The entry zone begins at the leading edge of the blades 7 (FIGS. 1, 2 and 3). As stated above, the flow has a considerable supersonic speed at the blade end at point A (the relative speed of the fluid compared to the Blades is, for example, Mach 13) and a considerable subsonic speed at the blade root at point B (e.g. Mach 0.9). As a result, in the steady state, ie in normal operation, a back pressure shock wave Sa, which is directed to one of the

hi front edge chest of drawers inclined shock wave connects. The shock wave 8a is said to lie in a zone in which it is imparted to the fluid by the blades Deflection is very little, and in what the flow

schwacli diverges to prevent peeling.

In practice, despite the increase in the thickness of the blades from the leading edge, there remains a divergence due to the curvature of the blades, which must remain small, especially on the suction side. Above all, the shock wave 8a should be kept in the immediate vicinity of the front edge of the pole head on the inside (ie in the vicinity of the line A - C) by a suitable choice of the smallest cross section of the diffuser. It is known that a reduction in this cross-section causes the shock wave to advance towards the leading edge. Even if the shock wave practically coincides with AC there is no noticeable risk of pumping phenomena, since the relative flow at the blade root remains below the speed of sound.

In order to achieve a satisfactory degree of efficiency, the backpressure shock wave 8a should lie in a zone in which the deflection imparted to the fluid by the blades is very small. In this way, the excessive speeds on the suction side and thus the intensity of the impacts are kept small, and the occurrence of significant detachment phenomena is prevented. In practice it can be assumed that a deflection angle of about 7 ° between the leading edge A of the vane and the intersection point C between the vane and the normal established on the leading edge A of the next vane forms a value close to the most favorable value. A divergence of 7 ° is also acceptable.

Furthermore, only a small number of blades is required to achieve a high mass output 7 to be provided in the entry zone of the order of eight, so that the entire Dike of the blades smallest passage cross section for the fluid is not reduced too much. Furthermore, these are blades appropriately narrow and have a high hub ratio in the inlet cross-section. B. a Select the ratio between the radius at the head and at the blade root from 2 to 2.2 in terms of size. The thickness of the blade is z. B. in a special case treated below Umm at the blade root and 0.75 m at the shovel head. The angle of attack of the leading edge of the blade is z. B. at the head 60 "to 65 ° to the axis.

It should also be noted that the use of a small number of narrow blades allows with good accuracy the limit power which the rotor can deliver from the range shown in FIG. 3 to determine the speed triangle shown. The conveying speed or meridian _{speed C n} can namely be calculated by forming the speed triangle in which the rotational speed ω _{Γ is} known as well as the direction of the fluid moving in relation to the blades, which practically flows in the inlet section A-C in the direction of the blades

The fluid emerging from the entry zone enters the deflection zone in which the blades the flow in the azimuthal, d. H. tangential sense to deflect by an angle, which is generally of the order of about 58 ° if the Total deflection at the shovel "Opf is of the order of 65 ° and if, as stated above, the The deflection in the entry zone does not exceed 7 °

In order to obtain a flow with a strong deflection, its homogeneity at the exit of the rotor

where the blades are located radially, the measures provided according to the invention are satisfactory met. The following explanations provide guidelines for the design of the blades.

First of all, the shape given to the blades in the compression zone is to be selected so that they are the Give the flow a double deflection, namely in the azimuthal (circumferential) and in the meridian direction. Of the The radius of curvature of the center line 9a (FIG. 1) is given a defined by the following conditions Shape: the radius of curvature of the mean meridian line 9a is to be dimensioned so that the Pressure gradient on this line is normally zero. Starting from this line, the skeleton of the Blade defined by a straight line, which is on the axis of the rotor and on the central streamline 9a supported, remaining perpendicular to the mean flow surface, which is a surface of revolution, because the meridian plane rotates along the blade.

The corresponding equations can be written as follows:

Λ η

R = -

COSi

Clv

= 0

In addition to these equations, information about the mean meridian line is added (line whose distance from j5 of the axis at this point has a value such that it divides the flow into two sections of the same cross-section). These details are as follows:

- the law of change of the meridian velocity which is a function of the curved abscissa

4n of the meridian line is practically linear;

- the law of change of the azimuthal deflection as a function of the curved abscissa along the meridian line, which is an exponential law in which <x is between 1.5 and 2.

Equation (1) defines the mean streamline in connection with the above information. In these equations, F _n denotes the force component normal to the blade, ie in the direction of the straight line Δ in FIG. 1. The expression C _n , ² / R corresponds to the force generated by the curvature of the flow surface. Finally, the third expression corresponds to FIG Centrifugal force, where r denotes the radius at the point under consideration and ε the angle between the tangent to the flow meridian line and the axis. The third equation, in which C _n denotes the meridian velocity and ν denotes the absolute tangential velocity, defines the radius of curvature R. It can be seen that this radius of curvature of the mean meridian line at the entrance and exit is infinite while ε at the exit is 90 ° and ν (tangential velocity) is practically zero at the entrance

For a given curved abscissa, the deflections established by the above mode of generation are am Head and at the foot of the shovel from the deflection lengthways the middle meridian line different. The coefficient «Should be chosen so small that, on the basis of the information given on the other hand,

such as hub ratio, angle of incidence, etc., to one Lowering reaches the head, which remains acceptable in terms of the mechanical stresses on the blade root. If the peripheral speed is at If the shovel head is too high, you can be forced to generate the skeleton by starting straight lines, which one are no longer perpendicular to the central meridian line.

The other measure to ensure satisfactory efficiency and good homogeneity of the speeds in the compression zone is to multiply the number of blades. To the blades 7, which occupy the entire length of the rotor, blades 8 are added, which bring the total number in the compression zone to a number between twenty-four and thirty-two. In the example shown, this result is achieved in that three intermediate blades 8 are provided between two successive blades 7. It should be noted that the blades 8 have a profile which is completely identical to that of the portion of the blades 7 arranged in the same ring. The thickness at the leading edge is greater than along AB because the flow is subsonic. Furthermore, in the case of the example considered above, in particular a thickness and a diameter of the leading edge of 1.5 mm at the blade head and of 3 mm at the foot of the blade 8 can be selected.

To simplify the manufacture of the rotor, it can consist of two parts. A first that The part forming the front wheel carries the sections of the blades 7 lying in front of the front edge of the blades 8. A rear section that forms the actual bathroom

carries the rest of the shovels. The final machining of the rotor can be done after the union of the wheel and of the front wheel.

Finally, the zone of practically radial flow at the exit of the knee points towards more practical Completion of the azimuthal deflection except for blades 7 and 8, which extend to the exit of the rotor extend, short blades 10 to guide the flow and to prevent peeling on the outside of the blades.

The inner wall and the outer wall of the in Fig. 1 shown rotor are calculated so that the meridian velocity in each section, which is normal to the flow area corresponding to 9a, according to a linear law as a function of The curved abscissa along the central meridian line changes, which leads to this radial zone being given converging surfaces 11 and 12. For example, if a If a convergence angle of 12 ° is chosen, this radial section has surfaces which correspond to the radial Direction form an angle of 6 °. Of course, an asymmetrical arrangement is also possible and required even in the case of a twin-fluid compressor.

The above rotor gives the flow at the outlet a very homogeneous distribution in the axial direction as well as in azimuthal direction. The axial homogeneity of the flow allows the use of a smooth Diffuser, which, however, has poor efficiency in the case of pronounced heterogeneity. Such a smooth diffuser has the advantage, however, of the absolute supersonic speed of the flow to reduce smoothly at the output of the rotor.

For this purpose 2 sheets of drawings

Claims (6)

Claim: Rotor for a top-mounted centrifugal compressor with an entry zone in which the rotor carries a small number of blades, and with an exit zone with a substantial radial flow in which the rotor carries a larger number of blades, characterized in that IO 1. the long blades (7) of the entry zone along the edge of the blade tip between (A) and (C) cause the flow to be deflected by about 7 °, with point (C) being the base of one of point (A) of the neighboring one long shovels (7 ') on the edge of the shovel tip of the shovel (7) felled Lotes List, 2. the divergence D in the entrance zone AC is about 7 °, _ 3. at the blade tip at A there is a relative upper sonic speed at the blade root at B subsonic speed, 4. a central compression zone is provided between the entry zone A-B and the exit zone (at 10), in which the rotor (1) has a higher number of blades (7, 8) compared to the entry zone, 5. the long blades (7) extending over the entire length of the rotor have a thickness have, di? from the leading edge of the long shovel (7) to the leading edge of the middle blades (8), and 6. the angle of attack of the leading edge of the blades (7) of the rotor, d. H. de! Angle between the edge of the blade (7) facing away from the hub at the front edge with an axially parallel one Straight line is 60 to 65 °