WO1996008655A1

WO1996008655A1 - Low specific speed impeller

Info

Publication number: WO1996008655A1
Application number: PCT/EP1995/003606
Authority: WO
Inventors: Dan Adler
Original assignee: Dan Adler
Priority date: 1994-09-13
Filing date: 1995-09-13
Publication date: 1996-03-21
Also published as: AU3606895A; IL110951A0

Abstract

An impeller for a centrifugal compressor designed for high pressure at a relatively low discharge is characterized by a plurality of flow passages (2) of narrow free pitch positioned between vanes (1) that increase in thickness from a sharp inlet (3) towards the impeller periphery, while the width of the passages is sufficiently large to prevent high friction losses and blockage by thick boundary layers. The free pitch of the flow passages is either constant or decreases towards the outlet. The vanes (1) are either radial, backwards or forwards curved, while the compressor may be single-staged or multi-staged in accordance with the required performance.

Description

LOW SPECIFIC SPEED IMPELLER

The invention relates to turbo-comprssors for low discharge at high pressure, particularly to a novel design of impellers for turbo-compressors. BACKGROUND OF THE INVENTION

The invention relates to impellers for coicpressors adapted to convey a gas, at high Mach numbers, from one point to another and to raise its pressure, pareticularly to compressors designed for compressing a relatively small gas volume to a relatively high pressure.

In industrial and in laboratory applications the requirement of coinpressing a small amount of gas to a high-pressure level is generally solved by means of linear or rotating piston puraps which operate to everybody's satisfaction, but they are bulky, relatively expensive, operate at relatively low speed and demand frequent maintenance. Another solution offers itself in the form of multi-stage turbo-corapressores, but here again the high cost make their use preventive in many instances. On the other hand, it has been proposed to use high-speed single- stage compressores including impellers having a flow passage of very narrow width, but high friction losses and boundary layer blockage are prohibitive in these designs, and the present invention seeks to present a suitable solution of the problem of low discharge at high pressure ratio. The conventional centrifugal turbo-impeller is provided with a plurality of curved vanes of small thickness with the aim of presenting large fluid passsages. In addition, the inlet and outlet ends of the vanes or blades are made as sharp and narrow as possible in order to reduce inlet and outlet losses, as well as blockage, to a minimum. The thin outlet ends of the vanes are also supposed to provide a uniform fluid flow all around the impeller circumference, instead of a flow interrupted by wider vane ends. However, the actual flow in and around an impeller is not uniform in reality, as will be shown in the following:-

A typical example of a conventional impeller is iilastrated in Figure 1, wherein the impeller is provided with backward-curved vanes, the impeller rotating in the direction of the arrow F. On the left side, denoted by "1a", the outflow velocities v₁ to v₇ are shown in the traditional manner, and for simple calculations a median velocity is being chosen up to the present day. However, modern research has shown that the actual flow through the flow passages between vanes is not gradually decreasing from left to right as shown on the left side of the drawing,, but increases abruptly from low to high velocity at about a third of the outlet path between every two vanes, as shown on the right side of Figure 1, denoted by "1b". The actual flow diagram on the right shows clearly that in the area indicated by the letter "w" (for "wake") the flow has the nature of a thick boundary layer, mostly because of the concave vane curvature and the Coriolis effect. It is thoroughly turbulent and whirly with relative low radial velocity. The remaining two thirds of the flow area, denoted by the letter "j" (for jet), are distinguished by higher velocity and even higher turbulence than in the wake. In fact, the diagram 1b shows that in reality there is no even, axisymmetrical flow out of the impeller as is assumed for the sake of simply calculating the shape of an impeller and its vanes, but an alternate high and low velocity flow.

The design of an impeller configured for discharge of a small fluid volume to a relatively high pressure, while employing the narrow vanes as shown in Figure 1, results in a very narrow width of the flow passages with high friction losses and boundary layer blockage, and it is the foremost object of the present invention to prevent or at least reduce the "jet-wake phenomenon", and to provide impeller flow passages having cross sections of logical aspect ratio and thus to reduce the friction losses and blockage effects experienced by too narrow a width of the flow passages in axial direction. The "jet-wake" phenomenon in high-speed turbo compressors is increasing in its severity with the pressure ratio, i.e. with the exit Machnuraber.

SUMMARY POF THE INVENTION

According to the present invention the impoller for relatively low discharge at high pressure has been designed with regard of the fact a) that there is no uniform, axisymmetrical flow along the impeller circum ference, even if it is equipped with narrow, sharp-ending vanes, and b) that the width of the flow passage in axial direction cannot be reduced indefinitely, in view of the high friction losses and boundary layer blockage along the impeller sides. The non-uniform, unbalanced yield of the conventional impeller and unbalanced velocity diagram as shown in Figure 1b, typical for compressor impellers, has led the inventor to develop the novel design of reducing the free pitch of the flow passage between adjoining vanes by designing the vane thickness to a gradual increase in radial, outwardly extending direction towards a blunt peripheral end, while leaving the width of the impeller at a magnitude not conducive to relatively high friction losses and to blockage by thick boundary layers. The increased vane thickness effects a non-uniform velocity field similar to that of the conventional impellers, but it has the great advantage of allowing the above- mentioned larger width of the flow passages with the aforementioned advantages. A preferred embodiment of the impeller of the present invention designed for high pressure ratio at low discharge is characterized by a flow passage between adjoining vanes which is of substantially constant free pitch between inlet and outlet diameter, or of decreasing free pitch towards the outlet. The vanes in this impeller increase in thickness from a sharp inlet towards the periphery of the impeller, where they may occupy more than one third of the total circumferential length, i.e. the

peripheral width of the vane may be considerably larger than the outlet end of the flow passage between adjoining vanes. In this case the free pitch of the passage is constant or decreases on its way betwween inlet and out- let.The impeller vanes are preferably backward curved, but may be radial or forward curved as well. In order to obtain the required high pressure, it is proposed that the impeller has a relatively small diameter and that it is rotated at high revolutions. The impeller may be of any kind used in high-pressure compressors such as radial or mixed-flow, open or closed, provided the vanes are designed in accordance with the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a cross section of a conventional impeller, showing both theoretical and actual throughflow diagrams, Figure 2 is a cross section of an impeller with vanes of increasing thickness in outward direction forming backward curved flow passages of constant free pitch.

Figure 3 is a cross section of an impeller similar to that shown in Figure 2, with backward-curved flow passages of converging free pitch,

Figure 4 is a cross section of an impeller similar to that shown in Figure 2 , with flow passages of diverging free pitch,

Figure 5 is a meridional section of a closed, impeller designed for radial entry and radial outflow between the vanes, Figure 6 is a raeridional section of an open impeller designed for axial entry and radial outflow between vanes, and

Figure 7 is a meridional section of a closed impeller including mixed radial and axial flow passages.

DETAILED DESCRIPTION OF THE DRAWINGS

The standard or conventional impeller shown in Figure 1 has been duly discussed herein before and need not be explained in this section. The impeller shown in Figure 2 shows four vanes 1 extending between an inlet diameter d and an cutlet diameter D and forming four flow passages 2 therebetween. Each vane has a sharp inlet point 3 and gradually increases in thickness to a blunt end of maximum thickness 4 at the periphery of the impeller. The vanes are shown to have backward-curved envelopes 5 and 6 formed by large diameters or otherwise curved lines, so as to effect a smooth fluid flow. As can be clearly seen the flow passages 2 are of constant free pitch, thus not apt to create flew separation.

The impellers shown in Figures 3 and 4 are similar to that of Figure 2, except for the fact that in Figure 2 the free pitch of the flow passages decreases and in Figure 4 increases, by changing the vane thickness from a sharp inlet to a blunt outlet end. In the foregoing the flow passages are shewn to be backward curved, but it will be understood that i

llers with radial and forward-curved flow passages may be conceived in accordance with the reqirements of pressure and discharge.

The impeller shown in meridional section in Figure 5 has been designed for a small discharge at high pressure ratio by employing the vanes and flow passages illustrated in either of the Figures 2, 3 or 4, permitting a flow passage of relative large width in axial direction. The impeller includes a hub 7 and a shroud 8 as well as the flow passages 2 and vanes 1 between shroud and hub, the inlet ends of the vanes being denoted by numeral 3 and the blunt outlet ends by numeral 4. The impeller has an axial entry 9 and a radial outlet 10, while the vanes and flow passages occupy the radial portion only.

The impeller shown in Figure 6 is similar in design to that shown in Figure 5, but for the difference of being open without a shroud. In this impeller the vanes start at the entry 9 in the form of sharp inlet ends 3 and extend to the impeller periphery in tne form of blunt ends 4. Except for the fact that it includes the thick blades and flow passages of narrow free pitch, it is in all other aspects identical with a conventional open impeller. The mixed-flow impeller of Figure 7 includes a hub 7' and a shroud 8' enclosing vanes and flow passages 1 and 2 respectively. The flow passages and vanes extend from the axial entry 9' to the axial-radial outlet 10' and are characterized by the narrow free pitch of the flow passages due to the vanes of increasing thickness in outward direction. An impeller designed for a small air conditioning unit and provided with vanes of increasing thickness, similar to that shown in Figure 3 of the drawings, has the following dimensions:-

Outlet diameter 60 mm.

Inlet diameter 25 mm.

Exit blade angle 65°.

Inlet blade angle 28°.

Number of passages 4.

Rotary Speed 85000 r.p.m.

Pressure ratio 1 ; 7.

Inlet Mach number 1.

Sum of passage width to impeller circumference: length ratio 0.4.

It will be iunderstood that for very high pressures multistage compressors may be designed while employing the impeller according to the present invention. It will be understood that at low discharge flow rates less impellers will be necessary than with impellers of conventional design, owing to the higher pressure ratio of each impeller combined with improved efficiency and lower friction losses experienced with the impellers of the present invention. It should also be understood that the ombodiments shown and described in the foregoing represent only examples of the various possibilities of designing an impeller for a predesignated discharge at a predesignated pressure, while employing the principle of narrow free pitch and of the corresponding large width of the flow passages.

Claims

C L A I M S : - 1. An impeller of a turbo-machxne such as a centrifugal turbo-compressor configured to increase the initial pressure of a gas to a high pressure at a substantially low discharge, comprises,

a rotatable body of circular circumference including a hub firmly attached to a rotating shaft of said turbomachine and a plurality of outwardly extending, spaced vanes integral with said hub, the space between each two vanes forming a flow passage extending from a fluid inlet closse to said shaft to a fluid outlet close to the circumference of said body,

said impeller being characterized by f low passages of narrow free pitch and formed between vanes of large width and of gradually increasing thickness between said fluid inlet and said fluid outlet.

2. The impeller of Claim 1, wherein said flow passages are of constant free pitch.

3. The impeller of Claim 1, wherein said flow passages are of converging free pitch.

4. The impeller of Claim 1, wherein said flow passages are of diverging free pitch.

5. The impeller of any of the preceding Claims, wnerein said flow passages are covered by a shroud forming a closed impeller.

6. The impeller of Claim 5, including an axial gas inlet and a radial outlet.

7. The impeller of Claim 5, including two substantially parallel sides wherein said gas inlet of small diameter is positioned on one side of said impeller and said gas outlet of larger diameter is positioned on the other side of said impeller, forming a mixed-flow impeller.

8. The centrifugal impeller as defined in any of the Claims 1 through 7 and substantially as hereinbefore described with reference to the accompanying drawings.