Abstract:
A method of cooling the flow in radial gaps formed between rotors and stators turbo machines is provided. The method includes the step of using water as a cooling fluid for the stator part adjacent to the radial gap. To this end, either at least one recess is formed in the interior of the stator part adjacent to the radial gap or at least one cavity is arranged at the stator part. The recess or the cavity is connected to both a feed line and a discharge line for the cooling fluid.

Description:
FIELD OF THE INVENTION 
     The invention relates to a method and an arrangement for the indirect cooling of the flow in radial gaps formed between rotors and stators of turbomachines. 
     BACKGROUND OF THE INVENTION 
     To seal off rotating systems, non-contact seals, in particular labyrinth seals, are widespread in turbomachine construction. In the separating gap, through which flow occurs, between rotating and stationary parts, high friction power occurs as a result of the forming flow boundary layers. This leads to heating of the fluid in the separating gap and thus also to the heating of the components surrounding the separating gap. The high material temperatures result in a reduction in the service life of the corresponding components. 
     DE 195 48 852 A1 discloses a radial compressor of simple construction without a sealing geometry formed in the separating gap. In this case, too, the friction heat produced as a result of flow shearing layers at the rear wall of the compressor impeller causes heating of the compressor impeller and thus reduces its service life. 
     EP 0 518 027 B1 discloses air cooling for radial compressors with a sealing geometry on the rear side of the compressor impeller. To this end, an additional annular space is formed between the individual sealing elements the casing wall side of the radial compressor. A cold gas which has a higher pressure than the pressure prevailing at the outlet of the compressor impeller is directed into this annular space. The air supplied acts as impingement cooling. In the process, it divides in the sealing region and flows mainly radially inward as well as outward. This is intended to additionally achieve a blocking effect against the flow of hot compressor air through the separating gap from the outlet of the compressor impeller. However, the air blown in in this way causes an increase in thrust and additional friction losses in the flow boundary layers. 
     In addition to this direct cooling, DE 196 52 754 A1 also discloses indirect cooling of the rear wall of the compressor impeller or of the medium flowing through the separating gap. To this end, a feed and distributing device connected to the lubricating-oil system of the turbocharger is arranged on or in the casing part disposed at the rear wall and forming with the latter the separating gap. The oil used for the bearing lubrication serves as cooling medium, for which purpose the lubricating-oil circuit of the turbocharger is tapped. A disadvantage of this cooling is the relatively high oil demand and the heat quantity to be additionally dissipated by the oil cooler. This leads to an increased overall volume of the cooler. In addition, in the event of an accident with damage to the corresponding parts, there is an increased risk of deflagration. 
     U.S. Pat. No. 4,815,184 also discloses water cooling of the bearing housing of a turbocharger. However, this cooling serves to remove the carbonization risk of the lubricating oil remaining in the bearing housing of the turbocharger after shutdown of the latter. In contrast to the abovedescribed solutions of the prior art, the feeding of the cooling medium is not necessary during the continuous operation but rather when the turbocharger is shut off. This type of cooling of the bearing housing is therefore unable to provide any reference to indirect cooling of the flow in radial gaps formed between rotors and stators of turbomachines. In addition, this solution expressly does not deal with the cooling of the intermediate wall. 
     SUMMARY OF THE INVENTION 
     The invention attempts to avoid all these disadvantages. Its object is to provide a method of cooling the flow in radial gaps formed between rotors and stators of turbomachines, which method is improved with regard to its cooling effect. In addition, a simple, cost-effective and robust arrangement for realizing the method is to be specified. 
     According to the invention, this is achieved in that, in a method according to the preamble of claim  1 , water is used as cooling fluid for the stator part adjacent to the radial gap. 
     The water used as cooling medium has a somewhat higher density than the known lubricating oils as well as approximately twice as high a specific heat capacity. Since the heat flow to be dissipated via a cooling medium is in proportion to the product of density and specific heat capacity, a distinct advantage over oil cooling is obtained when using water. At the same mass flow and the same temperature of the water, a greater quantity of heat can thus be extracted from the medium flowing through the radial gap via the stator part to be cooled. The cooling effect on the regions of the rotor which are adjacent to the radial gap is therefore likewise greater. Conversely, to dissipate the same quantity of heat, a smaller mass flow of cooling water is required compared with the lubricating oil, as a result of which the feed and discharge device for the cooling medium may be of correspondingly smaller dimensions. 
     To this end, at least one recess is formed in the interior of the stator part adjacent to the radial gap or at least one cavity is arranged at the stator part. The recess or the cavity is connected to both a feed line and a discharge line for the cooling fluid. The cooling fluid is introduced or drawn off again via these lines. Depending on the rotor-side wall thickness, which is to be kept as small as possible, an improved cooling effect can be achieved by the guidance of the water directly adjacent to the radial gap in the interior of the stator part. If, however, instead of the recess in the stator part, the cavity described is formed at the stator part, simpler and more cost-effective manufacture can be realized with likewise good cooling effect. 
     In a system consisting of an internal combustion engine, a charge-air cooler and an exhaust-gas turbocharger, either fresh water from outside the system or advantageously water present in the system is used as cooling fluid. In the latter case, the cooling water located in a cooling-water circuit of the charge-air cooler is used for this purpose, and this cooling water is branched off upstream of the charge-air cooler. In this case, the fixed stator part is a casing part of a radial compressor, and this casing part defines the radial gap relative to the rotor, i.e. relative to the rotating compressor impeller of an exhaust-gas turbocharger. 
     Formed as a recess of the stator part is a tube integrally cast in the stator part, as a result of which a simple and robust cooling arrangement is obtained. As an alternative to this, at least one groove is arranged in the stator part, at least one tube which serves as recess being inserted and cast in each groove. Of course, a stator part having at least one corresponding integrally cast core, which is removed in order to form the recess, is far simpler in production. 
     An additional advantage is achieved by virtue of the fact that the cooling fluid, before the water cooling of the stator part adjacent to the radial gap, is used for the indirect cooling of the diffuser, receiving the main flow of the working medium downstream of the point at which the leakage flow is branched off, and of the diffuser plate delimiting the diffuser relative to the bearing housing. Effective cooling of the material of the turbomachine can thus also be achieved in this downstream region. In addition, the heat flow from the diffuser to the stator part adjacent to the radial gap is thus reduced. 
     In an especially advantageous manner, a second cooling fluid is used in addition to the water cooling and is directed into the radial gap, in which case air is preferably used. On account of the double cooling of the radial gap, the temperature of the rotor, which is subjected to high thermal loading, can be further reduced. To this end, at least one feed passage as well as a discharge device for the second cooling fluid are arranged at the radial gap. 
     By the feed of the second cooling fluid being partly or even completely shut off, the cooling effect can be adapted in a simple manner to the conditions to be expected during operation of the turbomachine or also to the actual temperature conditions. 
    
    
     BRIEF DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Several exemplary embodiments of the invention are shown in the drawing with reference to an exhaustgas turbocharger connected to an internal combustion engine. In the drawing: 
     FIG. 1 shows a schematic representation of the exhaustgas turbocharger connected to the internal combustion engine; 
     FIG. 2 shows a partial longitudinal section through the radial compressor of the exhaust-gas turbocharger; 
     FIG. 3 shows a representation according to FIG. 2 but in a second exemplary embodiment; 
     FIG. 4 shows a representation according to FIG. 2 but in a third exemplary embodiment; 
     FIG. 5 shows a representation according to FIG. 2 but in a fourth exemplary embodiment; 
     FIG. 6 shows a representation according to FIG. 2 but in a further exemplary embodiment; 
     FIG. 7 shows a representation according to FIG. 2 but in a next exemplary embodiment; 
    
    
     Only the elements essential for the understanding of the invention are shown. The direction of flow of the working media is designated by arrows. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1, in a schematic representation, shows an exhaust-gas turbocharger  2  interacting with an internal combustion engine  1  designed as a diesel engine. The exhaust-gas turbocharger consists of a radial compressor  3  and an exhaust-gas turbine  4 , which have a common shaft  5 . The radial compressor  3  is connected to the internal combustion engine  1  via a charge-air line  6 , and the exhaust-gas turbine  4  is connected to the internal combustion engine  1  via an exhaust-gas line  7 . A charge-air cooler  8  is arranged in the charge-air line  6 , i.e. between the radial compressor  3  and the internal combustion engine  1 . The charge-air cooler  8  has a cooling-water circuit  9  with a feed and discharge (not shown). 
     The radial compressor  3  is equipped with a compressor casing  10 , in which a rotor  11  designed as compressor impeller and connected to the shaft  5  is arranged. The compressor impeller  11  has a hub  13  fitted with a multiplicity of moving blades  12 . A flow passage  14  is formed between the hub  13  and the compressor casing  10 . Downstream of the moving blades  12 , a radially arranged, bladed diffuser  15  adjoins the flow passage  14 , the diffuser  15  in turn opening out into a spiral  16  of the radial compressor  3 . The compressor casing  10  mainly comprises an air-inlet casing  17 , an air-outlet casing  18 , a diffuser plate  19  and a stator part  20  designed as an intermediate wall for a bearing housing  21  of the exhaust-gas turbocharger  2  (FIG.  2 ). 
     On the turbine side, the hub  13  has a rear wall  22  as well as a fastening sleeve  23  for the shaft  5 . The fastening sleeve  23  is accommodated by the intermediate wall  20  of the compressor casing  10 . Another suitable compressor-impeller/shaft connection may of course also be selected. Likewise, the use of an unbladed diffuser is also possible. 
     There is inevitably a separating gap between the rotating compressor impeller  11 , i.e. its rear wall  22 , and the fixed intermediate wall  20  of the compressor casing  10 , this separating gap being designed as a radial gap  24  in the case of a radial compressor  3 . The radial gap  24  accommodates a labyrinth seal  25 , which seals off the compressor casing  10  from the bearing housing  21 . An encircling recess  26  is formed in the intermediate wall  20  of the compressor casing  10  and is connected to both a feed line  27  and a discharge line  28  for a cooling fluid  29  (FIG. 2, FIG.  3 ). In order to achieve as high a cooling effect as possible at the adjacent compresser impeller  11 , the intermediate wall  20  is designed to be as thin as possible on the compresser-impeller side of the recess  26 . To this end, a thin-walled tube  30 , which is closed at both ends and the interior space of which forms the recess  26 , is integrally cast during the manufacture of the intermediate wall  20  (FIG.  2 ). 
     During operation of the exhaust-gas turbocharger  2 , the compressor impeller  11  draws in ambient air as working medium  31 , which passes as a main flow  32  via the flow passage  14  and the diffuser  15  into the spiral  16 , is compressed further there and finally, via the charge-air line  6 , is used for supercharging the internal combustion engine  1  connected to the exhaustgas turbocharger  2 . Beforehand, however, appropriate cooling of the working medium  31  heated up during the compression operation is effected in the charge-air cooler  8 . 
     On its way from the flow passage  14  to the diffuser  15 , the main flow  32 , heated in the radial compressor  3 , of the working medium  31  is also admitted as leakage flow  33  to the radial gap  24 , as a result of which the compressor impeller  11  is additionally heated. However, since the operating temperature is greatest in the outer region of the compressor impeller  11 , high material loading occurs there in particular. Cooling water branched off as cooling fluid  29  from the cooling-water circuit  9  of the charge-air cooler  8  is directed into the recess  26 , arranged directly adjacent to this critical region, of the intermediate wall  20 . Indirect cooling of the leakage flow  33  located in the radial gap  24  and thus also indirect cooling of the compressor impeller  11  therefore occur. In this case, the cooling fluid  29  is branched off upstream of the charge-air cooler  8 , so that effective cooling can be achieved with the relatively cold cooling water. After the cooling action, the cooling fluid  29 , which is now heated, is fed back into the cooling-water circuit  9  via the discharge line  28  downstream of the charge-air cooler  8  (FIG.  1 ). Of course, instead of the cooling water present in the system of internal combustion engine  1 , charge-air cooler  8  and exhaust-gas turbocharger  2 , fresh water may also be supplied as cooling fluid  29  from outside the system (not shown). 
     In a second exemplary embodiment, in which the radial gap  24  is not sealed off by means of a labyrinth seal  25  but with a sealing ring  34  arranged between the fastening sleeve  23  and the intermediate wall  20 , the recess  26  is formed by a core which is integrally cast into the intermediate wall  20  and then has to be removed again (FIG.  3 ). 
     In a third exemplary embodiment, a groove  35  is formed in the intermediate wall  20 . Two tubes  36  are inserted and cast into the groove  35 , the two tubes  36  having a connecting line  37 . The interior spaces of the tubes  36  in turn form the recess  26  (FIG.  4 ). Of course, a single tube  36  may also be arranged in the groove  35 . Likewise, two or more grooves  35 , which may also accommodate more than two tubes  36 , may be formed in the intermediate wall  20  (not shown). 
     As an alternative to the recess  26  in the intermediate wall  20 , a cavity  38 , in a fourth exemplary embodiment, is formed at the intermediate wall  20  and is closed off by a lid  39  on the turbine side (FIG.  5 ). Like the recess  26 , the cavity  38  is also connected to a feed line  27  and a discharge line  28  for the cooling fluid  29 . With this variant, the manufacturing outlay required to realize the cooling of the compressor impeller  11  can be advantageously reduced. The lid  39  and thus also the cavity  38  may of course also be arranged with the same function on the compressor side of the intermediate wall  20  (not shown). 
     In the last-mentioned exemplary embodiments, the indirect cooling of the leakage flow  33  located in the radial gap  24  and thus also the indirect cooling of the compressor impeller  11  are essentially effected in a manner similar to the action described in the first exemplary embodiment. 
     In a further exemplary embodiment, the intermediate wall  20  is designed to be extended radially to the outside, so that it covers substantial regions of the diffuser  15 . To this end, the intermediate wall  20  has a corresponding outer ring  43 . An encircling cavity  44  is formed in the interior of the outer ring  43 . The feed line  27  for the cooling fluid  29  engages on the outer ring  43  and opens out into its cavity  44 , which is connected at the other end to the recess  26  of the intermediate wall  20  (FIG.  6 ). 
     In this solution, the cooling fluid  29 , starting from the feed line  27 , is first of all directed into the cavity  44  of the outer ring  43 , where it serves for the indirect cooling of the diffuser  15  or the diffuser plate  19 . Not until after that is the cooling fluid  29  directed into the recess  26  of the intermediate wall  20 . There, the indirect cooling, already described above, of the leakage flow  33  is effected. The recirculation of the cooling fluid  29  into the cooling-water circuit  9  is likewise realized via the discharge line  28 . 
     Of course, the intermediate wall  20 , as in U.S. Pat. No. 4,815,184, may also merge directly into the diffuser plate  19 , and the cavity  44  connected to the recess  26  of the intermediate wall  20  may be arranged in the diffuser plate  19  (not shown). 
     In a next exemplary embodiment, in addition to the indirect cooling already described, direct cooling of the leakage flow  33  is provided. To this end, a plurality of feed passages  40  opening tangentially to the rear wall  22  of the compressor impeller  11  into the radial gap  24  and intended for a second cooling fluid  41  are arranged so as to penetrate both the bearing housing  21  and the diffuser plate  19  (FIG.  7 ). The feed passages  40  are connected downstream of the charge-air cooler  8  to the charge-air line  6 , so that cooled charge air is used as second cooling fluid  41  (FIG.  1 ). 
     Pure film cooling of the entire rear wall  22  of the compressor impeller  11  is realized by the tangential introduction of the second cooling fluid  41 . The second cooling fluid  41  replaces the hot leakage flow  33 , so that the boundary layer forming on the rear wall  22  of the compressor impeller  11  is already formed from the start in particular by the cooled charge air. The subsequent drawing-off of the second cooling fluid  41  is effected via a discharge device  42  (not shown in any more detail) engaging in the intermediate wall  20  of the compressor casing  10 . This combination of indirect and direct cooling results in a special cooling effect, since the two cooling possibilities complement one another in their effect and thus provide for a very significant temperature reduction in the compressor impeller  11 . Of course, other cooling media may also be used as second cooling fluid  41 , an external supply of compressed air also being possible (not shown). 
     FIG. 1 additionally shows the arrangement of a control valve  45  in the feed passage  40  for the second cooling fluid  41 . The volumeric feed of the second cooling fluid  41  can be controlled by means of this control valve  45 , so that adaptation of the cooling effect to the conditions to be expected or to the actual temperature conditions during operation of the exhaust-gas turbocharger  2  is made possible. In this case, the control valve  45  may be actuated by hand as well as via a measuring and control unit (not shown). Possible measuring variables are the temperature of the charge air after the charge-air cooler  8  or even the temperature of the intermediate wall  20  itself. Of course, in this way, the feed of the second cooling fluid  41  may be prevented not only partly but also completely. In the latter case, only indirect cooling, i.e. water cooling, then takes place. 
     The abovedescribed cooling configurations may of course be combined with one another in any desired manner, irrespective of whether a labyrinth seal  25  is arranged in the radial gap  24 . During sole use of the intermediate-wall cooling, any increase in the compressor thrust and in the air leakages into the bearing housing  21  of the exhaust-gas turbocharger  2  is avoided from the outset.