US 3868196 A
The high velocity leakage flow escaping tangentially through the annular clearance space between the impeller and rotating vaneless diffuser in a centrifugal compressor is employed to drive a turbine device attached to the diffuser back wall member, thereby self-powering the rotating diffuser and reducing the leakage flow. Either a turbine blade row sector with a partial arc of admission or a full admission turbine blade row can be used.
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Description (OCR text may contain errors)
United States Patent [191 Lown [ Feb. 25, 1975 CENTRIFUGAL COMPRESSOR WITH ROTATING VANELESS DIFFUSER POWERED BY LEAKAGE FLOW  Inventor: Harold Lown, Schenectady, N.Y.
 Assignee: General Electric Company,
22 Filed: Mar. 29, 1974 21 Appl. No.: 456,129
 U.S.Cl ..415/l46,415/2l1,4lS/219A  Int. Cl. F04d 29/44, F04d 29/46  Field of Search 415/146, 147, 207, 210,
 References Cited UNITED STATES PATENTS 2,490,066 12/1949 Kollsman ..4l5/l98 3,460,748 8/1969 Erwin 415/211 FOREIGN PATENTS OR APPLICATIONS 279 l/l9l0 Great Britain 415/147 France 415/219 A Switzerland 415/147 Primary ExaminerC. J. l-Iusar Assistant Examiner-Louis T. Casaregola Attorney, Agent, or Firm-Donald R. Campbell; Joseph T. Cohen; Jerome C. Squillaro  ABSTRACT The high velocity leakage flow escaping tangentially through the annular clearance space between the impeller and rotating vaneless diffuser in a centrifugal compressor is employed to drive a turbine device attached to the diffuser back wall member, thereby selfpowering the rotating diffuser and reducing the leakage flow. Either a turbine blade row sector with a partial arc of admission or a full admission turbine blade row can be used.
9 Claims, 5 Drawing Figures sum 1 OF 3 PATENTEU FEB2 5 I975 CENTRIFUGAL COMPRESSOR WITH ROTATING VANELESS DIFFUSER POWERED BY LEAKAGE FLOW BACKGROUND OF THE INVENTION This invention relates to centrifugal compressors of the type having a rotating vaneless diffuser, and more particularly to a centrifugal compressor utilizing the leakage flow to power the rotating diffuser for improved and more efficient performance.
In the place of the diverging vaned diffusers used in dynamic centrifugal compressors to convert fluid velocity to a static pressure, another technique is to employ a vaneless diffuser having two parallel walls defining an annular space in which high velocity fluid delivered by the impeller flows in a vortex or spiral path and increases in pressure as some function of the radius. Assuming that the vaneless diffuser walls are stationary, the fluid flow is subjected to high frictional losses, and this is especially disadvantageous in the case of high pressure ratio compressors. By rotating the vaneless diffuser walls independently of the impeller at a fraction of its speed, the relative velocity of the fluid is decreased with a resulting substantial reduction in the frictional losses. This reduction in vaneless diffuser frictional losses constitutes a significant improvement in compressor efficiency. The rotating diffuser, however, desirably operates at between one-third to onehalf of the impeller speed and requires an external source of power to overcome the bearing and outside wall frictional losses. An alternative drive technique involves a plurality of vanes extending axially between the parallel diffuser walls in the vaneless diffusion space toward the outer periphery. This imposes a barrier to the vortex flow, causing an additional pressure drop and reducing the compressor efficiency.
SUMMARY OF THE INVENTION In a centrifugal compressor with a rotating vaneless diffuser in which the impeller establishes a vortex flow in the diffuser as previously described, the improvement is made that the high velocity leakage flow escaping tangentially through the clearance space between the impeller and rotating diffuser is utilized to power a turbine or other suitable device operatively coupled to the diffuser to derive torque for causing it to rotate. Most commonly, the diffuser back wall member is extended inwardly and mounted for rotation with a relatively small axial clearance from the impeller. Preferably, the diffuser back wall member has an appropriately shaped recess in which the leakage flow powered turbine device is mounted, the recess further extending radially outwardly and inwardly of the turbine blade row to provide an entrance recess communicating to the clearance space for leakage flow and an exit recess eventually communicating through a labyrinth seal to the atmosphere. Two embodiments of impulse turbine powering devices are disclosed, one using a turbine blade row sector and arcuate entrance recess to obtain a partial arc of admission, and the second using a full turbine blade row with a circular entrance recess for complete admission of the leakage flow.
An additional advantage of the invention is the reduction of the leakage flow due to interposing the diffuser self-powering device in the leakage flow path. Applications include those where a high pressure ratio or very small diffuser passages in a centrifugal compressor are desired.
BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a simplified schematic view of a rotating vaneless diffuser type centrifugal compressor, with parts broken away or omitted and others shown in section to illustrate more clearly the relation of the main compress or components;
FIG. 2 is an axial cross-sectional view and partial full side view of a centrifugal compressor with a leakage flow powered rotating diffuser constructed according to a first embodiment of the invention with a partial arc of admission to a turbine blade row sector;
FIG. 3 is a plan view of the rotating diffuser back wall and attached turbine blade row sector used in the FIG. 2 compressor, further showing a ball bearing mount and the drive shaft in section;
FIG. 4 is similar to FIG. 2 and illustrates a second embodiment of the invention in which the leakage flow powered rotating diffuser is constructed for complete admission to a circular turbine blade row; and
FIG. 5 is similar to FIG. 3 including in particular a plan view of the rotating diffuser back wall and attached complete admission full turbine blade row powered by the leakage flow in the FIG. 4 compressor.
DESCRIPTION OF THE PREFERRED EMBODIMENTS Before preceding to the improvment made by the present invention, the operation of a rotating vaneless diffuser type centrifugal compressor will be discussed in general terms with reference to the simplified schematic diagram of the main compressor components shown in FIG. 1. A circular impeller 10 of conventional construction with a suitable number of equally spaced radial or forwardly curved blades 11 is rotated at a high speed in the clockwise direction. Low pressure air or other fluid taken in at the impeller eye is discharged at the periphery as a high velocity fluid flow having a radial component and a substantial or large tangential component which establishes a vortex flow in the diffuser. The rotating vaneless diffuser is indicated generally at 12 and is comprised by the annular, parallel, front and back wall members 13 and 14 defining between them (also see FIG. 2) an annular vaneless diffuser is indicated generally at 12 and is comprised by the annular, parallel, front and back wall members 13 and 14 defining between them (also see FIG. 2) an annular vaneless diffusion space. The vaneless diffuser 12 rotates in the clockwise direction at a fraction of the speed of the impeller 10, and at its preiphery the lower velocity, higher pressure, spiralling fluid flow discharges into the surrounding scroll 15. As in conventional, the circular scroll 15 has a gradually increasing cross section from the tongue to the outlet 16 such that there is a further increase in the static pressure and a decrease in the velocity of fluid flow. Since the impeller 10 and the diffuser 12 are rotating components moving at different speeds in the same direction, there is always an annular clearance space 17 between them and a resulting leakage flow to the atmosphere that reduces the efficiency of the compressor. At each point around the annular clearance space 17 a portion of the tangential component of the high velocity flow is impelled into the clearance space 17 and establishes the leakage flow. In addition to having considerable tangential velocity, the
static pressure of the leakage flow is approximately half of the static pressure rise of the compressor.
The two similar specific embodiments of the invention shows in FIGS. 2-5 are single stage, high pressure ratio (4:1 or higher), centrifugal compressors incorporating a selfpowered rotating vaneless diffuser utilizing the leakage flow to provide the torque. A derivative benefit associated with the leakage flow powered rotating diffuser is a reduction in the leakage flow. Typical applications are in aircraft and industrial gas turbines, cryogenic turbo-compressor systems, and for industrial centrifugal compressors and others where high pressure ratio or very small diffuser passages are desirable. Referring to FIGS. 2 and 3, a central drive shaft 20 rotates the impeller and by way of example has a key way connection to the back shroud structure 21 to which are secured the upstanding impeller blades 11. An inlet air passage is formed between a blunt nose structure 22 carried by the shaft 20 and a bellmouthed front shroud 23 secured to the scroll casing and overlying the impeller blades 11. A stationary rear housing 24 is fastened to the back of the scroll casing 15 and is supported on the shaft by a rear ball bearing mount. The housing 24 carries an inner, forwardly projecting cylindrical support 25 mounted at its forward end on the shaft 20 by a forward ball bearing structure 26. The stationary cylindrical support 25 is concentric with the drive shaft 20 and supports the diffuser 12 together with its mounting structure and the new leakage flow powering device for rotation about the axis of the machiine.
The smooth-surfaced diffuser front wall member 13 and the back wall member 14 are rigidly fastened to one another in spaced parallelism by a plurality of axially extending struts 27, and as here illustrated rotate within an annular space formed between opposing recesses in the front shroud 23 and the radially directed extension of the scroll casing 15. The high pressure swirl fluid flow exiting from the rotating diffuser 12 is admitted tangentially into the scroll 15 through a stationary diffuser vane row 29 formed in the casing. lnwardly of the vaneless diffusion space, the back wall member 14 is formed with a forwardly opening annular recess or cavity 28in which is mounted, attached to the back wall member itself, a leakage flow powering device 30 for the rotating diffuser. In this case, the device 30 is a turbine blade row sector with a partial arc of admission for the leakage flow propelled through the annular clearance space 17 and flowing radially inwardly in the recess 28. As here shown, the inner end of the recess is bounded by an end plate 31 which extends rearwardly within an axially directed extension of the back wall member 14 and is bolted thereto to provide a rotative mounting structure indicated generally at 32. This structure also includes an inner plate spanning and fastened to the outer races of a pair of ball bearings 33 mounting the rotating diffuser 12 and associated leakage flow powering device 30 for free rotation about the stationary cylindrical support 25. A pair of labyrinth seals 34 carried respectively by the support 25 and shaft 20 impede the leakage flow exiting radially in the clearance space at the rear of the impeller back shroud 21 and passing to the atmosphere. Another labyrinth seal 34a carried by a forward extension of the housing 24 and facing the outer surface of mounting structure 32 impedes leakage flow originating at the periphery of rotating diffuser 12.
As shown in FIG. 3, the leakage flow powering device 30 comprises a turbine blade row sector including a plurality of turbine blades 35 of symmetrical design. typically extending through an arc of about 30 to 40. The turbine powering device in particular is a singlestage impulse turbine sector having a partial arc of admission. The impulse type turbine blades 35 are equally spaced and secured in upstanding relationship to a turbine blade disk sector 36 which in turn is attached or fixed to the back diffuser wall member 14 in the recess 28. If desired, the edges of recess 28 adjacent the turbine blade row can have a turbine blade contour as illustrated. To obtain the partial arc of admission, the central and outer portions of the recess 28 have an arcuate extent of about 30 to 40, or somewhat larger than this, while the inner portion can extend over a full 360. The arcuate outer portion 28a connects to the annular clearance space 17 and provides an arcuate entrance recess for the leakage flow, while the inner portion 28b of the recess 28 can extend over a full 360 since this is the exit recess for the radially established flow which subsequently passes through the impeller back shroud annulus to the atmosphere. Due to the radial extensions 28a and 28b of the recess 28 outwardly of and inwardly of the partial row or turbine blades 35, a radial fluid flow is established. The rate of velocity of this fluid flow is enhanced due to the limited arcuate extent of the central and outer portions of the recess 28, which function as a nozzle passage for the leakage fluid flow. A convenient way to manufacture this structure is to machine an annular recess in the back diffuser wall member 14, attach the turbine blade disk sector 36 with the turbine blades 35, erect baffle walls at the ends of the disk sector defining the limits of the entrance recess 28a, and fill the remainder to the radial level of the exit recess 28b with an appropriate filler material.
In operation, making reference to FIGS. l-3, the high velocity discharge fluid flow at the periphery of the rotating impeller 10 has a large tangential component, and a portion of the vortex flow passes through the annular clearance space 17 between the impeller 10 and rotating diffuser 12. This extracted leakage flow with high tangential velocity flows axially and radially into the entrance recess 28a which provides a partial arc of admission to the partial row of turbine blades 25. As the fluid has a high tangential velocity, it will impart a torque to the turbine blades, causing the turbine blade row sector to rotate which in turn causes rotation of the rotating diffuser l2 and its mounting structure. As the tengential leakage fluid flow imparts a force on the turbine blades 35, it is deflected radially inwardly by the configuration of the blades and discharges into the exit recess 28b. The flow of the leakage fluid is then radially inward through the clearance space rearwardly of the rotating impeller back shroud 21 and thereafter through the labyrinth seals 34 to the atmosphere. The additional back pressure imposed by the turbine blade row sector 35 lowers the pressure differential across the labyrinth seal and reduces the leakage flow. The invention, therefore, accomplishes two objectives in that it provides power for rotating the diffuser 12 without expending any additional flow energy from the compressor, and it reduces the compressor back shroud leakage flow.
As was previously mentioned, the leakage flow powering device 30, here an impulse turbine blade row sector with a partial arc of admission, rotates the parallel wall members 13 and 14 of the rotating diffuser at a speed approximately one-third to one-half of that of the impeller. In the vortex flow in the vaneless diffuser space, the change of tangential velocity of the fluid is proportional to the radius, and as the tangential velocity decreases the static pressure rises. since in the rotating diffuser the relative velocity of the vortex fluid flow with respect to the diffuser walls is decreased, there are reduced shear losses on the fluid itself and also reduced tangential frictional losses at the walls.
In the second embodiment of the invention shown in FIGS. 4 and 5, the leakage flow powering device 30' takes the form of a full admission impulse turbine blade row. The impulse type turbine blades 35' as well as the turbine blade disk 36' have a full 360 extent. Likewise, the entrance recess 28a connecting to the annular clearance space 17 for admitting the leakage flow extends over a full 360. Accordingly, there is complete admission of the leakage flow, which as before has a high tangential velocity and impinges on all of the turbine blades 35' to impart torque thereto and power the rotating diffuser 12. The axial length of the impulse turbine blades 35' is less than that of the turbine blades 35 in the other embodiment. Thus, the depth of the recess 28 in the diffuser back wall member 14 in FIG. 4 is less than that of the corresponding recess 28 in FIG. 2. This assures adequate radial velocity of the leakage flow to provide the necessary torque to overcome the bearing losses as well as the other losses in the rotating diffuser 12 itself. The operation is essentially the same with the exception of the complete admission of the leakage flow to the turbine blade row.
In summary, centrifugal compressors with a rotating vaneless diffuser are improved by the use of a leakage flow powering device for employing the energy of the leakage flow to derive torque for rotating the diffuser. Although other such powering devices may occur to those skilled in the art, a leakage flow powered impulse turbine arrangement incorporated into the rotating diffuser back wall member has been discussed, and in particular a turbine blade row sector with a corresponding partial arc of admission and a full admission turbine blade row. For a particular application the better arrangement of these two is determined experimentally. An added advantage is the reduction of leakage flow due to the resistance offered by the turbine blades.
While the invention has been particularly shown and described with reference to several embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention.
What is claimed is:
1. A centrifugal compressor comprising a rotating impeller, a concentric rotating vaneless diffuser comprising substantially parallel front and back wall members wherein said back wall member extends radially inward and is mounted for rotation separated from said impeller by a clearance space, and means for coupling high pressure fluid flow exiting from said diffuser to an outlet,
said impeller accelerating low pressure inlet fluid to a high velocity fluid with a substantial tangential wherein said recess extends radially outwardly and inwardly of said impulse turbine device to respectively provide an entrance recess and an exit recess in which a radial leakage flow is established.
4. A centrifugal compressor according to claim 3 wherein said entrance recess has a limited arcuate extent to provide a partial arc of admission, and said impulse turbine device is comprised by a turbine blade row sector of correspondingly limited arcuate extent.
5. A centrifugal compressor according to claim 3 wherein said entrance recess is circumferentially continuous, and said impulse turbine device is comprised by a full turbine blade row.
6. A centrifugal compressor comprising a central rotating impeller, an annular rotating diffuser mounted for rotation concentric to said impeller and comprising substantially parallel front and back wall members defining a vaneless diffusion space which is separated radially from said impeller by an annular clearance space, and means for coupling high pressure fluid flow exiting from said diffuser to an outlet,
said impeller accelerating low pressure inlet fluid to a high velocity fluid with a substantial tangential velocity which is discharged into said rotating diffuser to establish a vortex flow and partially escapes through said clearance space as leakage flow, and leakage flow powered turbine means operatively coupled to said rotating diffuser to derive torque from said leakage flow for causing rotation of said rotating diffuser and for reducing said leakage flow.
7. A centrifugal compressor according to claim 6 wherein said leakage flow powered turbine means comprises a turbine blade row sector of limited arcuate extent and associated means defining a partial arc of admission of said leakage flow.
8. A centrifugal compressor according to claim 6 wherein said leakage flow powered turbine means comprises a circumferentially continuous turbine blade row and associated means defining complete admission of said leakage flow.
9. A centrifugal compressor according to claim 6 wherein one of said rotating diffuser wall members is extended radially inwardly with a relatively narrow clearance from said impeller, and
said leakage flow powered turbine means comprises an impulse turbine blade row secured to said extended diffuser wall member in an appropriately shaped recess therein.