Abstract:
A hydraulic turbine including: a passageway permitting liquid to pass through the turbine; a draft tube defining a portion of the passageway through which liquid normally flows in a vortex flow path during optimal turbine operating conditions; a rotatable runner mounted upstream of the draft tube and rotating about a central axis passing through the runner and extending into the draft tube; at least one nozzle head device positioned relative to the central axis of the runner and adjacent to an upper portion of the draft tube, the at least one nozzle head device has at least one nozzle from which a corresponding control jet of high velocity liquid is injected axially downstream of the runner and into liquid flowing into the upper portion of the draft tube during part load turbine operation, so as to mitigate breakdown of the vortex flow path.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a hydraulic turbine having one or more control jets of liquid injected with high velocity axially downstream of the turbine runner and into an upper portion of the turbine draft tube, during part load operation of the turbine, to control the swirling flow and mitigate both helical vortex breakdown and its associated pressure fluctuations. 
       BACKGROUND OF THE INVENTION 
       [0002]    Until recently, hydraulic turbines have been operated close to peak efficiency. In the neighborhood of this optimum operating point, dynamic forces on the turbine components are generally low, with the exception of transient conditions such as load rejection and surge. 
         [0003]    The variable demand on the energy market, as well as the limited energy storage capabilities, requires a great flexibility in operating hydraulic turbines. As a result, hydraulic turbines tend to be operated over an extended range far from the best efficiency point. In particular, Francis turbines, which have a fixed-pitch runner, have a high level of residual swirl at the draft tube inlet as a result of the mismatch between the swirl generated by the wicket gates (guide vanes) and the angular momentum extracted by the turbine runner when operating at part load conditions. In the turbine draft tube the flow exiting the runner is decelerated, thereby converting the excess of the kinetic energy into static pressure. The decelerated swirling flow often results in breakdown of the normal vortex associated with flow of liquid in the draft tube which gives rise to the development of a central quasi-stagnation region in the draft tube. The vortex breakdown is now recognized as a primary cause of severe pressure fluctuations or pressure pulsations experienced in the draft tube of a hydraulic turbine operating at part load. The pressure pulsations are believed to be caused by the transformation of an axis-symmetrically swirling vortex flow into one or more precessing helical vortices as the operating condition shifts towards part load. The precessing motion of the helical vortex results in a fluctuating pressure on any stationary point of the draft tube. In addition, a limited quantity of air or water vapor in the liquid flow provides a degree of elasticity, termed cavitation compliance, and this elasticity can lead to a form of resonance in the draft tube excited by the precessing inhomogeneous pressure field associated with the core of the spiral vortex flow. 
         [0004]    Many different solutions have been proposed with respect to the problem of draft tube instability including altering blade design, the introduction of vanes in the draft tube, and the injection of air into a recirculation region surrounded by the vortex rope. The air injection produces an essentially axis-symmetrical stable flow, or a hollow air core surrounded by the swirling water flow. The air injection changes the breakdown of the vortex form from a spiral to a bubble. The injection of relatively small amounts of air have small effects on the efficiency of the turbine operation while considerably reducing the part load pressure swings. However, the vortex rope and the excitation of the rope continues to exist. 
         [0005]    Accordingly, there is a need to develop hydraulic turbines for present day hydroelectric facilities that operate efficiently not only under normal load conditions but also at low or partial load conditions, without being subjected to the severe pressure fluctuations originating in the draft tube as a result of helical vortex breakdown in the decelerated swirling flow downstream of the runner. 
       SUMMARY OF THE INVENTION 
       [0006]    The present invention relates to controlling swirling flow downstream of a hydraulic turbine runner by the axial injection of high velocity jet or jets of liquid at the runner outlet or draft tube inlet. The control jet or jets of liquid act on the flow of liquid in the draft tube by mitigating breakdown of the vortex flow path of this liquid and thereby diminishing or eliminating draft tube pressure pulsations experienced during part load operation of the turbine. 
         [0007]    By “high velocity” it is meant that the axially directed jet of liquid has a velocity that is greater than the mean axial velocity of liquid flowing at the runner outlet so as to provide the benefit of the present invention. This high velocity of the liquid control jet or jets may be as much as about 2 to 4 times or more greater than the mean axial velocity of the liquid flowing at the runner outlet. It should be understood that the high velocity of the liquid control jet or jets will vary depending on the location of injection of the liquid control jet or jets and the number of control jets utilized. It should be further understood that by making reference to a control jet or jets of high velocity liquid being injected axially of the turbine it is meant the control jet or jets, may be directed along the turbine axis, parallel to the turbine axis, or converging on a focal point adjacent the upper portion of the draft tube lying on the turbine axis or a parallel axis adjacent to the turbine axis. Further the jet or jets may be located offset from the turbine axis by, for example but not limited thereto, 10 percent of the diameter of the turbine runner and still be considered to be located relative to a central axis of the turbine runner. 
         [0008]    The control jet or jets are preferably operated when the turbine is operating at part load conditions. The control jet or jets are injected from at least one nozzle head device positioned relative to a central axis for the turbine runner and adjacent to an upper portion of the draft tube whereby the jet or jets of liquid are injected downstream of the runner. The at least one nozzle head device has at least one nozzle from which a corresponding high velocity liquid control jet is emitted into the draft tube. In one embodiment, the turbine has a rotatable runner mounted above the draft tube and the runner has a crown portion that houses the at least one nozzle head device. In an alternative embodiment, the at least one nozzle head device may be supported in an upper portion of the draft tube below and spaced from the crown of the turbine runner. 
         [0009]    The at least one nozzle head device may comprise a single nozzle or a plurality of nozzles arranged in one or more circular arrays, or a single annular nozzle. 
         [0010]    In accordance with the present invention there is provided a hydraulic turbine comprising a passageway permitting liquid to pass through the turbine and a draft tube defining a portion of the passageway through which liquid normally flows in a vortex flow path during optimal turbine operating conditions. A rotatable runner is mounted upstream of the draft tube and rotates about a central axis passing through the runner and extending into the draft tube. At least one nozzle head device is positioned relative to the central axis of the runner and adjacent to an upper portion of the draft tube. The at least one nozzle head device has at least one nozzle from which a corresponding control jet of high velocity liquid is injected axially downstream of the runner and into liquid flowing into the upper portion of the draft tube during part load turbine operation so as to mitigate breakdown of the vortex flow path. 
         [0011]    In accordance with the present invention there is provided a method of controlling part load operation of a hydraulic turbine during part load conditions having a runner, a draft tube located downstream of the runner and a liquid passageway extending through the runner and the draft tube. The method comprises the step of injecting one or more control jets of high velocity liquid axially of the turbine, downstream of the turbine runner and into at least an upper portion of the draft tube. 
         [0012]    It is envisaged that the method may further include the step of locating the one or more control jets centrally of the runner prior to the step of injecting. Further, the jet or jets may be located offset from the turbine axis. 
         [0013]    Further during the step of injecting the one or more control jets, the one or more control jets may be injected in one direction selected from the group consisting of along an axis of the turbine, parallel to the turbine axis, converging on a focal point adjacent the upper portion of the draft tube lying on the turbine axis and a parallel axis adjacent to the turbine axis. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    For a better understanding of the nature and objects of the present invention reference may be had to the accompanying diagrammatic drawings in which: 
           [0015]      FIG. 1A  is an elevation view, partially in cross-section, of a Francis turbine showing a nozzle head device for emitting a control jet positioned in the crown above the draft tube; 
           [0016]      FIG. 1B  illustrates an alternative embodiment for the nozzle head device for emitting a control jet where the nozzle head device is in the runner crown exclusively; 
           [0017]      FIG. 2  illustrates an alternative embodiment for the nozzle head device for emitting a control jet where the nozzle head device is positioned in the draft tube spaced below the crown; 
           [0018]      FIGS. 3A and 3B  show comparative velocity contours for water flowing through the draft tube; 
           [0019]      FIGS. 4A and 4B  show comparative pressure contours for water flowing through the draft tube; 
           [0020]      FIGS. 5A through 5D  show alternative embodiments for the arrangement of nozzles in the nozzle head device. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0021]    The present invention relates to a hydraulic turbine having one or more liquid control jets axially directed downstream of a turbine runner and into an upper portion of a draft tube. The present invention is intended for use in fixed-pitch hydraulic turbines and preferably finds application in propeller and Francis type turbines. 
         [0022]    Referring to  FIG. 1A  there is shown an exemplary hydraulic turbine installation  10  suitable for use in the generation of hydro-electricity. The turbine installation  10  comprises a Francis turbine  12  having a crown  14 , runner blades  16 , and a band  18 . The Francis turbine runner  12  is adapted to rotate within a stationary casing  42 . Below the Francis turbine runner  12  is located a draft tube  22 . It should be understood that while a Francis turbine runner is shown the runner could also be a propeller type runner. Only a portion of the draft tube  22  is shown. The upper portion  24  of the draft tube  22  is shown to have a vertically extending central axis  26 . The axis  26  is also the central axis for the runner  12  and the axis of the hydraulic turbine  10 . 
         [0023]    The upper portion of the crown  14  is connected to shaft  28 . The shaft  28  has a coupling flange  30  which is connected by bolts  32  to a coupling flange  34  of a generator shaft  36 . Rotation of the Francis runner  12  causes the shaft  28  to rotate and hence, the generator shaft  36  to rotate. The generator shaft  36  is connected to a generator (not shown) which generates electricity in response to the rotating action of the Francis turbine  12 . 
         [0024]    The Francis turbine  12  rotates as a result of water moving along water passageway  40 , from the spiral casing  42  past stay vanes  44 , past wicked gate  46 , the runner blades  16  and into the draft tube  22 . 
         [0025]    In accordance with the embodiment shown in  FIG. 1A , a control jet  50  is injected into the water flowing in the top portion  24  of the draft tube  22 . A nozzle head device  52  comprise an end portion of conduit  58  that passes through the centre of the crown  14 . The head device  52  has a nozzle  54  from which the control jet  50  is injected into the water or liquid flowing through the draft tube  22 . The crown  14  has a crown tip portion  56  that houses the nozzle head device  52 . 
         [0026]    Water under pressure is supplied to the crown tip portion  56  and the nozzle head device  52  by the conduit  58  passing through shaft  28 . The conduit  58  is connected to radially inwardly extending conduits  60 . The shaft  28  has an outside wall  62  with one or more liquid ports  64  contained therein. The radially inwardly directed conduits  60  are connected with the nozzle  54 , via conduit  58  and nozzle head device  52  and transport high pressure water from the liquid ports  64  to the nozzle  54 . In the illustrated embodiment the inlet ports  64  are located between the coupling flanges  30  and  34  which is also referred to herein as being located on the outside wall of the shaft  28  as the coupling  30  forms part of the shaft  28 . While the shafts  28  and  36  are illustrated as being solid, in practice, they are typically hollow. 
         [0027]    A stationary liquid collection chamber  66  is mounted in surrounding sealing relation with the hollow shaft  28 , or in the illustrated embodiment the coupling flanges  30 ,  34 . Stationary liquid collection chamber  66  directs pressurized water into the ports  64 . The stationary liquid or water collection chamber  66  is connected to a liquid or water bypass supply conduit  68  at an end portion  70  thereof. The liquid bypass supply conduit  68  has an opposite end portion  72  coupled in sealing relation with the spiral casing  42  and communicates liquid from the spiral casing  42  to the liquid collection chamber  66 . A regulating valve  74  is located in the pressurized liquid supply conduit  68 . The valve  74  controls the flow rate of the liquid supplied to the liquid chamber and is adapted to switch the control jet from an off condition to an on condition and to vary the flow rate of the control jet depending on the part load operating conditions of the turbine installation  10 . In the embodiment shown, the regulating valve  74  is located upstream of the nozzle  54  so as to control the flow rate of the water and hence the resultant velocity of the “high velocity” control jet  50  emitted from nozzle  54 . It should be understood that the regulating valve  74  is able to switch the supply of water to liquid collection chamber  66  off when the turbine is operating at optimal load conditions. As a result no water is emitted from nozzle  54 . In the embodiment of  FIG. 1A , the control jet  50  is emitted from the nozzle  54  of nozzle head device  52  which is positioned centrally of, and adjacent to, upper portion  24  of draft tube  22  within the crown  14 . Producing the water jet at the crown tip takes advantage of the hollow turbine shaft, and benefits from a high-pressure water supply from upstream in the turbine spiral casing. 
         [0028]    It should be understood that  FIG. 1A  illustrates an exemplary jet generation and control system comprising head device  52 , conduits  58 ,  60 , water collection chamber  66 , conduit  68 , and valve  74  and that alternative embodiments for supplying water under pressure to the head device  52  will be readily understood to a person skilled in the art. 
         [0029]    In  FIG. 1B  there is shown a cross-section through a turbine  10  comprising a spiral casing  111 , stay vanes  113 , guide vanes  112 , Francis runner  109 , and draft tube  114 . A passageway  115  extends through the turbine  110 . The runner crown  120  comprises a central cavity  122  connected with the high pressure side  124  of the runner  109  by open tubes  126 . A nozzle head device  128  has a needle  132  and a nozzle  130  from which the high velocity liquid control jet  50  is emitted into the upper portion  140  of the draft tube  114  from the runner crown  120 . An adjustment mechanism  134  is driven by an active control device  136  for adjusting the position of the nozzle head device  128  so as to control the speed of the high velocity liquid control jet  50 . 
         [0030]    One or more pressure sensors  142  are mounted in the draft tube  114  adjacent a sidewall thereof near the upper portion  140  of the draft tube  114 . The pressure sensors  142  measure the water pressure in the upper portion of the draft tube  114  and relay these measurements to the active control device  136  which in turn controls movement of the nozzle head device  128  in order to adjust the velocity of the high velocity liquid control jet  50 . When in an open position for the nozzle head device  128 , water from the high pressure side  124  of the runner  109  is supplied through tubes  126  the cavity or water chamber  122  to the nozzle  130 . As a result a liquid control jet  50  will be emitted from the crown  120  of the runner into upper portion  140  of the draft tube  114  also known as the draft tube inlet. The velocity or flow rate of the liquid control jet  50  is controlled by the position of the nozzle head device  128  depending on the pressure measured by the pressure sensors  142 . The nozzle head device  128  is closed when the turbine is not operating at partial load operating conditions. This system of jet generation has the advantages that the jet generation system concerns a single component of the turbine, namely the runner. The system of generation does not imply any water tightness problems, and can be implemented in situ for existing runners and all the system components, except for the runner cavities and tubes and can be considered to be a turnkey jet generation type of kit. 
         [0031]    Referring to  FIG. 2  there is shown an alternative embodiment wherein the nozzle head device  80  is spaced from the crown  14  of the turbine  10 . The nozzle head device  80  comprises a nozzle  82  from which the liquid jet  50  is injected vertically axially along the axis  26  of the draft tube  22  adjacent the upper portion  24  of the draft tube  22 . The nozzle head device  80  further comprises a plurality of supporting and liquid supplying conduits  84  interconnecting the head  82  with manifold  86  located on the outside wall of the draft tube  22 . The manifold  86  is connected via bypass supply piping  88  to the scroll  40 . A regulating valve  90  is located to control the pressure of the liquid or water supplied to nozzle  82  and hence the “high velocity” of the liquid control jet  50 . In this embodiment, water under pressure is supplied from the scroll  40  through the wall of the draft tube  22  and not through the crown  14  of the Francis turbine  12 . 
         [0032]    Referring to  FIGS. 3A ,  3 B,  4 A and  4 B there are shown computer simulations of velocity and pressure contours of water flowing in the draft tube  22  that occur at part load conditions.  FIGS. 3A and 3B  show velocity contours of water flowing within the draft tube at part load conditions. In  FIG. 3A , no control jet has been injected into the draft tube. In  FIG. 3B , a liquid control jet has been injected into the draft tube. In  FIG. 3A , where no jet is employed, there is shown a single helical draft tube vortex rope  94 . In  FIG. 3B  where the control jet is in operation, the central low pressure region indicated by the iso-surface has been greatly reduced and its shape has changed from a helical shape to a slightly off-centre extended cone  96 . In  FIG. 4A , the jet is not employed and the pressure contour shows strong circumferential variations in the vortex flow of the water in the draft tube at  98 , which associated with the precession motion result in severe pressure fluctuations. In  FIG. 4B , where the control jet is employed there appears to be no breakdown in the low pressure area associated with the vortex at area  100 . 
         [0033]    By injecting a control jet of high velocity liquid axially into the draft tube, the precession frequency is altered in the draft tube and, eventually, by eliminating the quasi-stagnant central region, the control jet prevents or reduces development of vortex ropes in the draft tube liquid flow. As a result, the control jet addresses directly the vortex rope occurrence development thus mitigating the main source of pressure fluctuations, or at least it alters the precession frequency and reduces significantly the pressure fluctuations amplitude. Injecting a control jet of liquid is different from the air admission at the tip of the crown since the control jet of liquid is aimed at controlling or eliminating the vortex breakdown. Further when the control jet is not needed during turbine operation, the control jet can be switched off. 
         [0034]    By avoiding the helical vortex breakdown the overall performance of the draft tube at part load is significantly improved by reducing the hydraulic losses due to severe flow non-uniformities and unsteadiness. 
         [0035]    The control jet provides an active control of the swirling flow downstream of the runner. The control jet uses a fraction of the overall turbine discharge. The jet discharge bypasses the turbine bladed region and produces no power at the turbine shaft. However, the reduction in efficiency as a result of the jet discharge bypassing the turbine bladed region is lower than expected. This is because of reduction of hydraulic losses due to the precessing vortex rope compensating for hydraulic energy spent on the jet. In addition, the control jet has the benefit of diminishing the severe pressure pulsations and the draft tube instability at partial discharge. 
         [0036]    Referring to  FIGS. 5A through 5D  there is shown embodiments of the header portion  82  for a single nozzle head device  80  from  FIG. 2 . It should be understood that multiple nozzle head devices could be employed or multiple head portions  82  for each nozzle head device could be employed for alternative embodiments. However, in  FIG. 5A , a single nozzle  102  for emitting the control jet  50  is located to direct the jet  50  along axis  26  of the draft tube. Alternatively, this jet  50  could be directed along an axis parallel to and offset from the axis  26 . In  FIG. 5B , the plurality of nozzles  102  are arranged in a circular array about the central axis  26 . This will result in a plurality of jets being emitted from the nozzles  102 . Alternatively the jets could be arranged in a circular array an axis parallel to and offset from the axis  26 . The nozzles  102  can be arranged to emit the jets either parallel to the axis  26  or the jets may converge towards each other with a focal point lying on the axis  26 , or the jets may be directed to focus on an axis parallel to the central axis  26 . In  FIG. 5C , two circular arrays of nozzles  102  are arranged concentrically about the central axis  26 . In  FIG. 5D , a single nozzle in the form of annular ring  102  is arranged around the axis  26 . In alternative embodiments to  FIGS. 5A to 5D , the location of the nozzles can be chosen to direct the control jet or jets to be offset from the control axis  26  by as much as 10% of the diameter of the draft tube  22 . 
         [0037]    The utilization of the control liquid jet or jets of the present invention: a) successfully addresses directly the main cause of the flow instability, rather than the effects; b) does not require geometrical modifications of the runner outer shape; c) is continuously adjustable according to the operating conditions, and can be switched-off when it is not needed; and, d) although a fraction of the discharge may bypass the bladed region, the overall turbine efficiency suffers marginally, and may be improved, due to improvement in both runner and draft tube efficiencies when the control jet is on at part load operating conditions. 
         [0038]    While the invention has been described in connection with what is presently considered to be the most practical embodiments of the hydrodynamic approach of controlling the swirling flow and mitigating the helical vortex breakdown together with the associated severe pressure fluctuations by using axial high velocity liquid control jet or jets, it is to be understood that the invention is not to be limited thereto, but on the contrary, is intended to cover various modifications and equivalent arrangements as would be understood by a person skilled in the art of hydraulic turbines.