Abstract:
A diffuser arrangement through which a fluid may flow is provided. The diffuser includes an outer diffuser comprising an inner surface, and a flow-guiding device, which is configured such that at least part of the boundary layer flow forming on the inner surface of the outer diffuser can be accelerated in the main flow direction, so that a flow separation is prevented on the inner surface of the outer diffuser. Also provided are an exhaust steam plenum of a steam turbine and an exhaust gas plenum of a gas turbine, both including a diffuser arrangement.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is the US National Stage of International Application No. PCT/EP2008/052222, filed Feb. 25, 2008 and claims the benefit thereof. The International Application claims the benefits of European Patent Office application No. 07005175.0 EP filed Mar. 13, 2007, both of the applications are incorporated by reference herein in their entirety. 
     
    
     FIELD OF INVENTION 
       [0002]    The invention refers to a diffuser arrangement and especially to an exhaust steam plenum of a steam turbine or an exhaust gas plenum of a gas turbine with the diffuser arrangement. 
       BACKGROUND OF INVENTION 
       [0003]    A diffuser is a passage which is exposable to throughflow by fluid and which in the case of separation-free throughflow decelerates the fluid by means of cross-sectional widening and, in accordance with Bernoulli&#39;s theorem, reduces the kinetic pressure of the fluid to the benefit of the static pressure. 
         [0004]    The quality of the diffuser is described by means of the pressure recovery coefficient, which is defined by 
         [0000]        C   p =( p   out   −p   in )/( p   total,in   −p   in ) 
         [0005]    and the overall or total pressure p total,in , the static pressure p in  at the diffuser inlet and the static pressure p out  at the diffuser outlet. 
         [0006]    Diffusers are used for example in pipelines for pressure recovery or for constant bridging of cross-sectional widenings (transition diffuser). In the case of pipelines with circular cross section the diffusers are axially symmetrically formed. In  FIG. 4 , a longitudinal section of an axially symmetrical diffuser  101  is shown, and schematically shows the flow which typically occurs within it. The diffuser  101  has an inlet cross section  102  and an outlet cross section  103 , the area ratio of which is greater than one. Upstream of the diffuser  101 , a cylindrical inflow pipe is arranged, through which flows an inflow  108 , and downstream of the diffuser  101  a cylindrical outflow pipe is arranged, through which flows an outflow  109 . 
         [0007]    On account of the adherence of the fluid on the diffuser wall, a boundary layer develops in the flow close to the wall. In the lower half of the diffuser  101  which is shown in  FIG. 4 , five characteristic velocity profiles  110  to  114  are shown along the main flow direction, wherein the first of the two velocity profiles  110 ,  111  show the flow close to the wall in the inflow pipe and the three velocity profiles  112  to  114  which follow upstream show the flow close to the wall in the diffuser  101 . 
         [0008]    Since the flow in the diffuser  101  is decelerated, the flow velocity of the main flow decreases in the flow direction, as a result of which by fulfilling the first main theorem of thermodynamics the static pressure of the flow correspondingly increases in the flow direction. According to Prandtl&#39;s boundary layer theory, the static pressure in the boundary layer is constant transversely to the flow direction. 
         [0009]    On account of the deceleration effect of the diffuser  101  and the adherence condition on the diffuser wall, the flow velocity of the flow close to the wall decreases. After negotiating a specific flow path the gradient of the flow velocity transversely to, and on, the diffuser wall is zero. This position is a separation point  105  of the flow, which is shown in the boundary layer profile  113 . 
         [0010]    At the separation point  105 , the flow moves away from the diffuser wall towards the middle of the diffuser  101 , wherein downstream of the separation point  105  in the wall proximity a backflow develops which forms a separation bubble  106 . The separation bubble  106  brings about a narrowing of the cross section of the diffuser  101  which is effectively exposed to throughflow so that the main flow in the region of the separation bubble  106  is accelerated. As a result, in the main flow the kinetic energy is increased and the flow is reapplied in the outlet pipe at a reapplication point  107 . 
         [0011]    The degree of opening of the diffuser  101  which is shown in  FIG. 4  substantially determines the shape and the size of the separation bubble  106  and the position of the separation point  105  and of the reapplication point  107  which possibly occurs. The higher the degree of opening of the diffuser  101 , the further upstream the separation point  105  lies. 
         [0012]    On account of the narrowing of the effective cross section of the diffuser  101 , the separation bubble  106  reduces the pressure recovery effect of the diffuser  101  compared with a diffuser in which the flow is fully applied. 
         [0013]    In order to create a laminar boundary layer flow in a diffuser, a bladed wheel which is seated on a hub, the blades of which, being shrouded by a diffuser plate, are connected on the blade tip side by means of a ring, is known from laid-open specification DE 1 628 337. A ring of stator blades is arranged on the ring in such a way that this widens the jet flow which flows off the bladed wheel while maintaining the boundary flow which is guided by the diffuser plate. In addition to the stator blades, this is especially achieved by the ring having a corresponding cross-sectional shape which, moreover, benefits the course of the entrained flow filaments and blows these out at higher velocity. 
         [0014]    Furthermore, for avoiding flow separations in a diffuser, a pipe which is arranged parallel to the diffuser wall and which extends along the flow direction, is known from JP 08 260905. On account of the diverging cross section of the diffuser and of the correspondingly diverging pipe which is parallel to it, the flow cross section of the annular passage which is formed between diffuser wall and the pipe is increased so that medium flowing in the annular passage is decelerated. 
         [0015]    A steam turbine or a gas turbine is run at partial load, base load and overload. In the construction and design of the steam turbine or gas turbine, their individual components can be geometrically designed in an optimized manner only at a single operating point for example with regard to efficiency or aerodynamic or thermodynamic effectiveness. This has the result that at other operating points, which are not identical to the design operating point, the components cannot operate in an optimum manner. 
         [0016]    This state also applies to an exhaust steam plenum of the steam turbine or to an exhaust gas plenum of the gas turbine. The exhaust steam plenum or the exhaust gas plenum is conventionally constructed as an axial diffuser. 
         [0017]    As a rule, the axial diffuser is geometrically designed in an optimized manner with regard to the base load so that at partial load and overload the axial diffuser cannot be operated in an optimized manner. 
         [0018]    At the inlet of the axial diffuser, in the case of optimum design, a lower static pressure exists than at the outlet. As a result of lowering the pressure at the diffuser inlet, which at the same time represents the exit of the blading, the last rotor blade ring is brought to a higher power output. 
         [0019]    The mass flow of the flow which flows through the axial diffuser is lower in the partial load range than in the base load range, as a result of which the average flow velocity in the axial diffuser in the base load range is higher than in the partial load range. As a result, the flow in the axial diffuser in the partial load range is more prone to separation than the flow which occurs in the axial diffuser at base load. 
         [0020]    Therefore, the pressure recovery in the axial diffuser at partial load is lower compared with the pressure recovery at base load. This has the result that at partial load the turbine output is lowered compared with the turbine output at base load. The influence of an improvement in the pressure recovery of a gas turbine diffuser of c p =0.1 was estimated by Farohki at 0.8% of the delivered turbine output. This connection is similarly applicable to axially exhausting steam turbines. 
         [0021]    A reduction of the degree of opening of the axial diffuser could provide a remedy in this case since the flow is decelerated less sharply as a result and is therefore prone to separation to a lesser degree. However, the overall length of the axial diffuser is consequently extended, as a result of which the total overall length of the steam turbine or gas turbine is disadvantageously increased. 
       SUMMARY OF INVENTION 
       [0022]    It is the object of the invention to create a diffuser arrangement, the pressure recovery of which is high and its overall length short. 
         [0023]    The diffuser arrangement according to the invention is exposable to throughflow by fluid and has an outer diffuser which has an inner surface, and a flow-accelerating device which is installed in such a way that at least some of the boundary layer flow which develops on the inner surface of the outer diffuser can be accelerated in the main flow direction so that a flow separation on the inner surface of the outer diffuser is prevented. 
         [0024]    If the fluid flows through the outer diffuser, then it is decelerated in the main flow direction, as a result of which the boundary layer flow which develops on the inner surface of the outer diffuser is principally prone to separation. The separation would emanate from a point at which the kinetic energy of the flow is zero. 
         [0025]    By means of the flow-accelerating device according to the invention at least some of the flow close to the wall is accelerated so that the kinetic energy of the flow close to the wall is increased. As a result, the effect of the kinetic energy of the flow close to the wall not being zero at any point is prevented, as a result of which a flow separation on the inner surface of the outer diffuser is prevented. Therefore, the diffuser arrangement has a high pressure recovery. 
         [0026]    Furthermore, the outer diffuser of the diffuser arrangement can have a large degree of opening without a flow separation occurring in it. Consequently, the outer diffuser and therefore the diffuser arrangement has a shorter overall length. 
         [0027]    The flow-accelerating device has a flow guiding device which extends inside the outer diffuser, and by its outer surface, which faces the inner surface of the outer diffuser, and a section of the inner surface of the outer diffuser, forms a nozzle passage through which the part of the boundary layer flow can flow. 
         [0028]    Therefore, the flow-accelerating device is formed by the nozzle passage which is defined by the flow guiding device interacting with the inner wall of the outer diffuser. As a result, the effect is achieved of the flow close to the wall, i.e. just the flow portion with otherwise low kinetic energy, being accelerated directly on the inner surface of the outer diffuser. Consequently, a separation in the diffuser arrangement is effectively prevented. 
         [0029]    Furthermore, the extent of the flow-guiding device in the main flow direction lies in the region of 5% to 40% of the extent in the main flow direction of the outer diffuser. As a result, the flow-guiding device is arranged entirely inside the outer diffuser and can be accurately placed on any section on the inner wall of the outer diffuser on which a separation of the fluid flow is to be expected. Therefore, the flow-guiding device can be purposefully arranged on a section where separation is a risk, as a result of which an effective prevention of flow separation is achieved and therefore the disturbance of the main flow by the flow-guiding device is low. 
         [0030]    It is preferred that the flow-guiding device by its inner surface which faces away from the outer surface forms an inner diffuser through which the fluid flow can flow and in so doing can be decelerated in the main flow direction. 
         [0031]    Therefore, in addition to the nozzle effect in the outer region the flow-guiding device also has a diffuser effect in the inner region so that the flow through the diffuser arrangement is sharply decelerated. As a result, the effect is achieved of the pressure recovery of the diffuser arrangement according to the invention being high. 
         [0032]    It is preferred that the outer diffuser and the flow-guiding device are axially symmetrically formed and are concentrically arranged around a common symmetry axis. 
         [0033]    Furthermore, it is preferred that the nozzle passage is formed as an annular passage. 
         [0034]    From this, the diffuser arrangement is advantageously created as an arrangement of a plurality of diffusers and a nozzle. This arrangement is formed by a series-connecting of the three diffusers, specifically the region of the outer diffuser upstream of the flow-guiding device, the inner diffuser of the flow-guiding device, and the region of the outer diffuser downstream of the flow-guiding device, and a parallel-connecting of the nozzle passage to the inner diffuser of the flow-guiding device. As a result, a compact, simple and effectively operating division of the outer diffuser is achieved, wherein the diffuser arrangement has a compact type of construction. 
         [0035]    The flow-guiding device is preferably formed as a straight guide plate. 
         [0036]    As a result, the guide plate can advantageously be cost-effectively produced. 
         [0037]    Alternatively to this, it is preferred that the flow-guiding device is aerodynamically profiled. Consequently, the flow-guiding device has a low flow resistance. 
         [0038]    Furthermore, it is preferred that the flow-guiding device is arranged in the region of 80% to 100% of the passage height (radius) of the outer diffuser. 
         [0039]    Consequently, the flow-guiding device is advantageously effectively placed in the flow close to the wall and, as a result, is aerodynamically effectively placed. 
         [0040]    Furthermore, the flow-guiding device is preferably arranged in the region of the inlet cross section of the outer diffuser. 
         [0041]    As a result, it is advantageously made possible for the inlet flow into the outer diffuser from the flow-guiding device to already have an accelerated flow in the boundary layer region, which accelerated flow over the course along the inner surface of the outer diffuser is not therefore prone to separation. 
         [0042]    Furthermore, it is preferred that the flow-guiding device is pivotably mounted relative to the main flow. 
         [0043]    In this way, the effect is advantageously achieved of the flow-guiding device being able to be individually adjusted by pivoting with regard to the respective flow conditions inside the outer diffuser in such a way that the flow-guiding device is aerodynamically effective. 
         [0044]    An exhaust steam plenum of a steam turbine or an exhaust gas plenum of a gas turbine preferably features the diffuser arrangement according to the invention. 
         [0045]    Furthermore, it is preferred that the flow-accelerating device is arranged on the inner surface of the outer diffuser in the region of its inlet. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0046]    Preferred exemplary embodiments of a diffuser arrangement according to the invention, with reference to the attached schematic drawings, are explained in the following text. In the drawing: 
           [0047]      FIG. 1  shows a longitudinal section through a first exemplary embodiment of the diffuser arrangement, 
           [0048]      FIG. 2  shows a longitudinal section through a second exemplary embodiment of the diffuser arrangement, 
           [0049]      FIG. 3  shows a longitudinal section through a third exemplary embodiment of the diffuser arrangement, and 
           [0050]      FIG. 4  shows a longitudinal section of a diffuser with schematic representation of the flow conditions. 
       
    
    
     DETAILED DESCRIPTION OF INVENTION 
       [0051]    As is apparent from  FIG. 1 , a diffuser arrangement  1  has an outer diffuser  2  which is axially symmetrically formed around its symmetry axis  3 . An inlet cross section  4  of the outer diffuser  2 , through which an inflow  5  flows into the outer diffuser  2 , lies in a plane which is perpendicular to the symmetry axis  3 , and its outlet cross section  6 , from which an outflow  7  discharges from the outer diffuser  2 , lies in another plane which is perpendicular to the symmetry axis  3  of the outer diffuser  2 . This outer diffuser has an inner surface  8  which delimits the inside space of the said outer diffuser  2 . 
         [0052]    The outer diffuser  2  is formed as a straight diffuser, i.e. the inner surface  8  of the outer diffuser  2  forms a truncated cone, wherein the cross-sectional area at the inlet cross section  4  is smaller than the cross-sectional area at the outlet cross section  6 . 
         [0053]    A flow-guiding device  9  is arranged inside the outer diffuser  2 . The flow-guiding device  9  is formed as a guide plate which is oblong in longitudinal section and which, axially-symmetrically arranged around the symmetry axis  3  of the outer diffuser  2  concentrically with the outer diffuser  2 , delimits a truncated cone-shaped annular passage which narrows in the flow direction. 
         [0054]    On its outer periphery the flow-guiding device  9  has an outer surface  10  which with regard to the inner surface  8  of the outer diffuser  2  is inclined in such a way that the annulus cross section decreases in the flow direction in a plane which is perpendicular to the symmetry axis  3  and formed between the flow-guiding device  9  and the outer diffuser  2 . 
         [0055]    That is to say, the outer surface  10  of the flow-guiding device  9  interacts with a section of the inner surface  8  of the outer diffuser  2  which lies opposite it in such a way that the annular passage, which lies between the flow-guiding device  9  and the outer diffuser  2 , forms a nozzle passage  11 . Therefore, the section of the inner surface  8  of the outer diffuser  2  which faces the outer surface  10  of the flow-guiding device  9  is an inner surface  12  of the nozzle passage  11 . 
         [0056]    Upstream, the flow-guiding device  9  is delimited by its leading edge  13  and downstream is delimited by its trailing edge  14 . An inlet cross section  15  of the nozzle passage  11  is located in the region from the leading edge  13  of the flow-guiding device  9  up to the inner surface  8  of the outer diffuser  2 , and the outlet cross section  16  of the nozzle passage  11  is located in the region of the trailing edge  14  of the flow-guiding device  9  up to the inner surface  8  of the outer diffuser  2 , wherein the cross-sectional area of the inlet cross section  15  is greater than the cross-sectional area of the outlet cross section  16 . 
         [0057]    Facing away from the outer surface  10  of the flow-guiding device  9 , this has an inner surface  17  which forms an inner diffuser  18 . The leading edge  13  of the flow-guiding device  9  is arranged in a plane which is perpendicular to the symmetry axis  3  and forms an inlet cross section  19  of the inner diffuser  18 , and the trailing edge  14  of the flow-guiding device  9  is arranged in a plane which is perpendicular to the symmetry axis  3  and forms an outlet cross section  20  of the inner diffuser  18 , wherein the inlet cross section  19  is smaller than the outlet cross section  20 . 
         [0058]    From  FIG. 2 , the aerodynamic effectiveness of the flow-guiding device  9  is evident. According to  FIG. 2 , the flow-guiding device  9  is formed as a profiled annular guide plate. 
         [0059]    For representing the flow conditions in the diffuser arrangement  1 , in  FIG. 2  flow lines  21  are drawn in the region of the flow-guiding device  9 , and a velocity profile  22  upstream of the flow-guiding device  9 , a velocity profile  23  at the trailing edge  14  of the flow-guiding device  9 , and also a velocity profile  24  downstream of the flow-guiding device  9  are shown. 
         [0060]    The flow lines  21  have a converging path in the main flow direction, as a result of which the flow acceleration which is induced by means of the flow-guiding device  9  is indicated. The velocity gradient, which is normal to the wall, on the inner surface  8  of the outer diffuser  2  is flatter in the case of the velocity profile  22  upstream of the flow-guiding device  9  than in the case of the velocity profile  23  at the trailing edge  14  of the flow-guiding device  9 , which is flatter than the velocity gradient, which is normal to the wall, of the velocity profile  24  downstream of the flow-guiding device  9 . 
         [0061]    Consequently, it is shown that the flow, which is guided by the flow-guiding device  9  through the nozzle passage  11 , is accelerated (energized). Therefore, the flow-guiding device  9  locally increases the velocity of the flow in the proximity of the inner surface  12  of the outer diffuser  2 . In the process, high-energy flow material from the core flow is deflected in the direction towards the inner surface  12  of the outer diffuser  2  and therefore is added to the boundary layer on the inner surface  12  of the outer diffuser  2 . As a result of this energizing, the boundary layer on the inner surface  12  of the outer diffuser  2  can overcome greater positive pressure gradients in the main flow direction without being separated from the inner surface  12  of the outer diffuser  2  in the process. 
         [0062]    As a result, the outer diffuser  2  reacts kindly to premature separation phenomena. Therefore, by provision of the flow-guiding device  9  in the outer diffuser  2  a higher pressure recovery of the outer diffuser  2  is achieved. 
         [0063]      FIG. 3  shows an exhaust gas plenum of a gas turbine, which is formed as the outer diffuser  2 . The outer diffuser  2  is arranged downstream of a turbine rotor  25  and guides away the outflow, which issues from the turbine rotor  25 , from the inlet cross section  4  of the outer diffuser  2  to the outlet cross section  6  of the outer diffuser  2 , recovering pressure. 
         [0064]    The turbine rotor  25  has a turbine rotor hub  26  which is continued by a cylindrical outer diffuser hub  27  with the turbine rotor hub  26 . 
         [0065]    The turbine rotor  25  has a multiplicity of turbine rotor blades  28  which on their radial outer ends have a blade tip  29 . The turbine rotor  25  is enclosed by a turbine casing  30 . During operation of the turbine rotor  25  this rotates around its rotational axis (not shown), while the turbine casing  30  remains stationary. Therefore, a gap  31  is provided between the turbine rotor blade tip  29  and the turbine casing  30  so that the turbine rotor blade tip  29  does not rub on the turbine casing  30  during operation of the turbine rotor  25 . 
         [0066]    In order to avoid rubbing of the rotor blades on the turbine casing  30  and to thereby avoid damage, a minimum distance as a gap  31 , the so-called clearance, is necessary between rotor blade  28  and casing  30 . Some of the mass flow can flow through this gap without power yield to the rotor blade  28  and leads to energizing of the boundary layer. Depending upon the configuration of this gap  31 , with or without sealing, mass flow can flow through to a greater or lesser extent. In order to avoid, or greatly delay, a subsequent separation of the flow in the diffuser, a further energizing of the boundary layer by means of the flow-guiding device  9  is desired. 
         [0067]    According to  FIG. 3 , a remedy is provided by arranging the flow-guiding device  9  close to the inner surface  8  of the outer diffuser  2  in the region of the inlet cross section of the outer diffuser  4 . The boundary layer which is disturbed by the leakage flow is accelerated in the main flow direction by the flow-guiding device  9  on the inner surface of the outer diffuser  2  so that the kinetic energy in this flow region is increased. As a result, the effect is achieved of the flow not separating in the outer diffuser  2  on the inner surface  8  of the outer diffuser  2 . Therefore, the flow losses in the outer diffuser  2  are low and the pressure recovery of the outer diffuser  2  is high.