Patent Publication Number: US-2009238677-A1

Title: Centrifugal compressor with air extraction and return at the casing

Description:
This application claims priority to German Patent Application DE102008004834.8 filed Jan. 17, 2008, the entirety of which is incorporated by reference herein. 
     The present invention relates to a centrifugal compressor with a rotor which is rotatably borne in a casing. 
     Centrifugal compressors are known from the state of the art, for example from specification U.S. Pat. No. 3,643,675. 
     While centrifugal compressors are capable of producing very high pressure ratios, they have the disadvantage that their efficiency is inferior to that of axial-flow compressors with the same pressure ratio. This is, among others, due to the fact that the rotor gap between rotor and casing ( FIG. 2 ) in relation to the blade height is very large as it must be capable, among others, of taking up the axial movement of the rotor to avoid contact between rotor and casing. The larger the rotor gap, the larger the gap losses and the lower the efficiency. Furthermore, the rotor gap limits the surge limit of the compressor, and the maximum obtainable pressure ratio of the compressor and the high-loss gap flow lead to blockage near the casing, inhomogeneity of the flow into the diffuser and an increase in diffuser losses. 
     For the further state of the art, reference is made to specifications DE 103 55 240 A1 and DE103 55241 A1. 
     As known from the state of the art, disadvantages arise for the rotor gap flow. 
     It is a broad aspect of the present invention to provide a centrifugal compressor of the type specified at the beginning which, while being simply designed and featuring high efficiency, can be manufactured easily and cost-effectively and avoids the disadvantages of the state of the art. 
     The essential aspects of the solution according to the present invention can be summarized as follows:
     1. Exhaustion (extraction) of the high-loss gap flow at the casing of a single centrifugal compressor rotor is accomplished at any point above the rotor or behind the rotor in the diffuser ( FIG. 3 ).   2. Injection of high-energy flow is accomplished before the rotor tip of the centrifugal compressor rotor to improve the axial velocity profile and the total pressure profile approaching the rotor ( FIG. 4 ).   3. Since the exhausted (extracted) flow is very energy-rich owing to the high pressure ratio of the centrifugal compressor rotor, and the static pressure drop between the tapping point (extraction point) and the injection point before the rotor tip is high, exhaustion (extraction) as per item 1 and injection as per item 2 can be coupled ( FIG. 5 ). This is accomplished by one or several lines. Here, flow is removed at the casing wall from the diffuser or at the casing wall from the passage of the same rotor and returned to re-introduce it before the tip of the same rotor.   4. Fluid removal (extraction) is, in accordance with the present invention, accomplished on a turbomachine which has only a rotor and a bladed or non-bladed diffuser and includes no stator. Moreover, the turbomachine is, in concrete terms, a centrifugal compressor, with fluid supply being accomplished on a rotor, more precisely by fluid return from the same rotor or the following diffuser, respectively.   5. Since the return of the energy-rich flow leads to a local temperature increase, line routing can be designed such that the temperature is decreased by heat exchange with cold air from the bypass duct using a heat exchanger, thereby preventing the efficiency and the mechanical integrity of the rotor from being affected by a temperature increase ( FIG. 6 ).   

    
    
     
       The present invention is more fully described in light of the accompanying drawings showing preferred embodiments. In the drawings, 
         FIG. 1  (Prior Art) is a schematic sketch of a known centrifugal compressor, 
         FIG. 2  (Prior Art) is a schematic sketch of a known centrifugal compressor rotor, 
         FIG. 3  shows a centrifugal compressor rotor with exhaustion (extraction) in accordance with the present invention, 
         FIG. 4  shows a centrifugal compressor rotor with injection in accordance with the present invention, 
         FIG. 5  shows a coupling of exhaustion (extraction) and injection in accordance with the present invention, 
         FIG. 6  shows a coupling of exhaustion and injection with intermediate cooling in accordance with the present invention, 
         FIG. 7  is a representation of exhaustion (extraction) behind the rotor (side view/view from top), 
         FIG. 8  shows exhaustion (extraction) above the rotor (side view/view from top), 
         FIG. 9  shows throttling in the return line in accordance with the present invention, 
         FIG. 10  shows injection before the rotor (side view/view from top) in accordance with the present invention, and 
         FIG. 11  shows an injection geometry for smooth and stepped casing wall in accordance with the present invention. 
     
    
    
       FIGS. 1 and 2  show a schematic sketch of a centrifugal compressor ( FIG. 1 ) and a centrifugal compressor rotor ( FIG. 2 ) in accordance with the state of the art. Here, a rotor  2 , which has a rotor tip/rotor leading edge  3  and is approached by an axial flow  4 , is rotatably borne in a casing  1 . A diffuser  10 , which is adjacent to a rotor trailing edge  12 , is arranged downstream of the rotor  2 . The rotor has a rotor gap  11 . 
       FIGS. 3 to 11  show variants according to the present invention. Here,  FIG. 3  shows a centrifugal compressor rotor with exhaustion (extraction), with possible extraction positions/ports (exhaustion positions) being indicated by reference numeral  13 .  FIG. 4  shows a centrifugal compressor rotor with injection at possible injection positions/ports  14 .  FIG. 5  shows a coupling of exhaustion (extraction) and injection by a line  5  going from an extraction position/port  16  (tapping point) to an injection point/port  15 . 
       FIG. 6  shows a variant with coupling of exhaustion (extraction) and injection with intermediate cooling, with the line  5  being routed through a cold airflow  9  of a bypass duct  8 , thereby acting as a heat exchanger and being cooled. 
       FIG. 7  shows exhaustion (extraction) behind the rotor  2  with extraction position or tapping point  16 , respectively. The right-hand side of  FIG. 7  schematically shows exhaustion (extraction) by means of circumferential slots  6  or discrete holes (cutouts  7 ) which can be differently dimensioned and designed.  FIG. 8  shows exhaustion (extraction) above the rotor  2 , analogically to  FIG. 7 , with extraction positions /tapping points  16  which can be provided in the rotor passage as discrete holes (cutouts  7 ) which, again, can be differently dimensioned, geometrized and disposed. 
       FIG. 9  shows an example with throttling in the return line  5  by a controllable shutoff element (valve)  19 . Introduction of the fluid flow is via a throat area (throttle  20 ). 
       FIG. 10  shows examples of injection before the rotor  2  in which, again, circumferential slots  6  or discrete injection cutouts/holes  7  can be provided which can be differently disposed, dimensioned and geometrized. 
       FIG. 11  shows an injection geometry for both a smooth and stepped casing wall. The left-hand representation of  FIG. 11  shows a smooth casing wall  17  with injection of the fluid flow, while the right-hand representation of  FIG. 11  shows a stepped casing wall. 
     Summarizing, then: 
     Tapping (extraction) and injection of the flow according to the present invention can, in detail, take the forms described in  FIGS. 3 to 11 . 
     Tapping (extraction) of the flow is, in accordance with the present invention, accomplished on the rotor casing  1  at any axial position either above the rotor  2  or behind the rotor ( FIG. 3 ). Tapping (extraction) can, in accordance with the present invention, be accomplished behind the rotor  2  either from a circumferential slot  6  of any size and form or from discrete holes  7  of any shape, number and size in the casing  1  ( FIG. 7 ). 
     Tapping (extraction) above the rotor  2  can, in accordance with the present invention, be accomplished from discrete holes (cutouts  7 ) of any number, shape, size and position ( FIG. 8 ). 
     The air can, in accordance with the present invention, be guided from the tapping points (extraction points) to the injection points at any angle to the casing wall using suitable lines, pipes and/or hoses  5 . While a valve  19  can here be used for flow control, the mass flow is also controllable by means of a throttle  20  of a defined throat area ( FIG. 9 ). 
     Injection of the flow can, in accordance with the present invention, be accomplished on the casing  1  before the rotor tip  3 . Preferably, the distance to the rotor tip  3  shall here be kept as small as possible. 
     Injection can, in accordance with the present invention, be accomplished through a circumferential slot  6  or through discrete circumferential slots or discrete nozzles of any shape, number and size ( FIG. 10 ). 
     Here, the injection slots/nozzles shall be adapted such to the casing  1  that minimum segregation of the flow lines occurs and the injected flow hits the rotor gap  11  only. This is achievable either by adaptation of the geometry to the smooth casing wall  17 , in that the injection geometry is inclined at a shallow angle ( FIG. 11 ), or by a step in the casing wall  18  to which the injection geometry is connected at an, again shallow, exit angle ( FIG. 11 ). 
     The present invention is advantageous in that the efficiency and stability of the centrifugal compressor are increased to such an extent that they come close to those of an axial-flow compressor, making the centrifugal compressor according to the present invention particularly suitable for application in aircraft engines by trading upon its advantages of less cost and less complexity. 
     LIST OF REFERENCE NUMERALS 
     
         
           1  Casing 
           2  Rotor 
           3  Rotor tip/rotor leading edge 
           4  Axial flow 
           5  Line 
           6  Circumferential slots 
           7  Cutout 
           8  Bypass duct 
           9  Cold airflow 
           10  Diffuser 
           11  Rotor gap 
           12  Rotor trailing edge 
           13  Extraction position 
           14  Injection position 
           15  Injection point 
           16  Extraction position/tapping point 
           17  Smooth casing wall 
           18  Stepped casing wall 
           19  Controllable shutoff element (valve) 
           20  Throttle