Abstract:
An axial flow turbine is described having a casing defining a flow path for a working fluid therein, a rotor co-axial to the casing, a plurality of stages, each including a stationary row of vanes circumferentially mounted on the casing a rotating row of blades, circumferentially mounted on the rotor, with an inner face of the casing exposed to the working fluid having one or more essentially circumferential grooves of increasing depth each ending in an extraction port with a bore.

Description:
TECHNICAL FIELD 
       [0001]    This invention relates generally to a system for cooling axial flow turbines, particularly low-pressure steam turbines. More specifically it relates to a system for cooling last stage blades in low-pressure steam turbines, in particular where such last stage blades are made from composite materials. 
       BACKGROUND 
       [0002]    The rotating blades of low-pressure steam turbines induce tremendous centrifugal forces into the rotor. This can be a limiting factor in designing the turbine for maximum efficiency. A solution is to use lower density blade materials as such blades exert less force into the rotor. This solution can, however, only be applied if the low-density material has adequate mechanical properties. While using titanium is presently regarded as the method of choice, future alternatives may have even better strength to weight ratios. Among the possible alternatives are blades of composite materials, examples of which is disclosed the published United States patent application US 2008/0152506 A1 and the published international patent applications WO 2011/039075 A1 and WO 2010/066648 and the Swiss Patent Number CH 547943. 
         [0003]    Composite materials are typically less temperature resistant than metals. This can be a problem, in particular during low volume flow operation and full speed conditions. Under such conditions not enough heat is carried by the volume flow through the turbine and particularly the last stage blades become susceptible to windage heating of the blade tip area. Normal blade temperatures typically do not exceed 65° C. However, last stage blade tip temperatures can exceed 250° C. under windage conditions without corrective means. At such temperatures, the mechanical properties of composite material are significantly impacted and they may suffer permanent degradation. 
         [0004]    A solution to windage heating is provided by Patent application No. US2007/292265 A1. The solution comprises injecting a cooling medium in the vicinity of the last stage tip region. The medium, which includes either steam or water, may be injected from the casing either fore or aft of the blade tip. As an alternative, or in addition, a small extraction groove for extracting flow through the outer sidewall may be provided near the blade tip just forward of the blade. 
         [0005]    In view of the prior art it is seen as an object of the present invention to provide more efficient means and methods of cooling the tips of turbine blades, in particular last stage blades of composite materials. 
       SUMMARY 
       [0006]    According to an aspect of the present invention, there is provided an axial flow turbine having a casing defining a flow path for a working fluid therein, a rotor co -axial to the casing, a plurality of stages, each including a stationary row of vanes circumferentially mounted on the casing a rotating row of blades, circumferentially mounted on the rotor, with an inner face of the casing exposed to the working fluid having one or more essentially circumferential grooves of increasing depth each ending in an extraction port with a bore. 
         [0007]    The grooves follow typically a circumferential line around the inner face of the casing. However the may also deviate by preferably only up to 10 degrees from the circumferential line. If deviating, the grooves deviate preferably in general flow direction through the turbine. 
         [0008]    The inner face of the casing in this invention can be the inner face of any part mounted onto the actual inner face of the casing such as diaphragms, vane carriers, heat shield etc. The grooves are machined into the face of the part which is exposed to the flow of the working fluid. 
         [0009]    Preferably, the depth of the groove start at zero depth. The depth best increases smoothly to avoid the formation of vortices or other obstacles to a smooth extraction of working fluid. 
         [0010]    The bore of the extraction port is preferably oriented tangentially to the groove to take advantage of the flow direction of the steam at low volume flow conditions in the turbine. 
         [0011]    In a preferred variant of the invention there are two grooves in diametrically opposing positions along essentially the same circumference. For ease of manufacture, it is best to design the one or more grooves such that they end at a joint line of the casing and bores for the extraction ports at the opposite side of the joint line. In this manner the bore can be implemented by drilling through the face of the joint. 
         [0012]    The one or more grooves in conjunction with the extraction port are best adapted to remove working fluid from a volume in the vicinity of the tip of the blades for the purpose of cooling the tips of rotating blades, particularly blades of composite material, for which heating is a more severe problem than for metal blades. Hence the preferred position of the grooves is located between vanes and blades of the last stage of the turbine. 
         [0013]    The above and further aspects of the invention will be apparent from the following detailed description and drawings as listed below. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    Exemplary embodiments of the invention will now be described, with reference to the accompanying drawings, in which: 
           [0015]      FIG. 1  is a schematic axial cross-section of a turbine; 
           [0016]      FIG. 2A  shows an enlarged view of the last stage of the turbine of  FIG. 1 ; 
           [0017]      FIG. 2B  is a circumferential cross-section along line A-A′ of  FIG. 2A ; 
           [0018]      FIGS. 2C and 2D  are axial cross-sections along line B-B′ and C-C′ of  FIG. 2B , respectively; and 
           [0019]      FIGS. 3A and 3B  illustrate the flow through the turbine at full volume flow and at low volume flow, respectively. 
       
    
    
     DETAILED DESCRIPTION 
       [0020]    Aspects and details of examples of the present invention are described in further details in the following description. Exemplary embodiments of the present invention are described with references to the drawings, wherein like reference numerals are used to refer to like elements throughout. In the following description, for purposes of explanation, numerous specific details are set forth to provide a thorough understanding of the invention. However, the present invention may be practiced without these specific details, and is not limited to the exemplary embodiments disclosed herein 
         [0021]      FIG. 1  shows an exemplary multiple stage axial flow turbine  10 . The turbine  10  comprises a casing  11  enclosing stationary vanes  12  that are circumferentially mounted thereon and rotating blades  13  that are circumferentially mounted on a rotor  14  with the rotor resting in bearings (not shown). The casing  11 , vanes  12  and blades  13  define a flow path for a working fluid such as steam therein. Each blade  12  has an airfoil extending into the flow path from the rotor  14  to a tip region  131  wherein the tip region  131  is defined as the top one third of the airfoil part of the blade  13 . The blade  13  can be made of metal, including metal alloys, composites including layered composites that comprise layered carbon fibre bonded by resins or a mixture of both metal and composites. The multiple stages of the turbine  10  are defined as a pair of stationary vane and a moving blade rows wherein the last stage of the turbine  10  is located towards the downstream end of the turbine  10  as defined by the normal flow direction (as indicated by arrows) through the turbine  10 . The turbine  10  can be a steam turbine and in particularly a low pressure (LP) steam turbine. As LP turbine, it is followed typically by a condenser unit (not shown), in which the steam condensates. 
         [0022]    The last stage of the turbine  10  with the last row of vanes  12  and blades  13  is shown enlarged in the following figures. The  FIG. 2B  shows a cross-section of part of the turbine along the line A-A′ of  FIG. 2A . Before the last blades  14  a pair of shallow grooves  111  are machined into the inner face of the casing  11  (or of a vane carrier, if the vanes are not mounted directly onto the casing). The depth of each groove  111  increases gradually in direction of the rotation of the blades  13  from zero to a final depth d after approximately one half turn. At the final depth d the groove enters into an extraction hole or channel  112 . 
         [0023]    The extraction hole  112  is tangentially to the groove  111  such that the opening of the channel is essentially perpendicular to groove. The extraction hole releases the steam into a water cooled mixing chamber or directly into a condenser. 
         [0024]    The extraction hole or channel  112  can be shut using a valve  113  or other suitable means. In normal operations the extraction channels is closed and opened only when the extraction is required, i.e under low flow volumes or when the temperature of the blades is rising beyond their operational limits. 
         [0025]    In  FIG. 2C , which shows a cross-section along line B-B′ of  FIG. 2B , the groove  111  has approached close to half its final depth d. In  FIG. 2D , which shows a cross-section along line C-C′ of  FIG. 2B , the groove  111  is shown at the point of entering the extraction hole or channel  112 . 
         [0026]    The groove  111  and the extraction hole  112  are oriented such that hot steam having a circumferential velocity component due to the rotation of the turbine is diverted from a volume close to the tip of the last stage blades  13  and guide by the grooves into the tangential extraction hole. 
         [0027]    The groove  111  and the extraction hole  112  are preferably located between the axial positions of the row of vanes  12  and blades  13  as volumes of hot steam are found to circulate in that volume. The width of the groove and the and the extraction hole  112  are design parameter and can in an extreme case take up most of the inner surface of the casing between the blades and vanes but are likely to be much smaller for typical turbines as in actual use today. 
         [0028]    As shown by the comparison of  FIGS. 3A and 3B  the flow through the turbine can changes significantly as the mass flow volume drops from its operational level to a lower level such as less than 50 percent of the normal mass flow, or even less than 30 percent of the normal mass flow. It is found that under such low volume operations the flow through the turbine, which is usually optimized for the operation mass flow levels, changes to leave pockets where the flow has only a small axial component. 
         [0029]    As shown in  FIG. 3A  the turbine has a smooth flow field as indicated by the stream lines under normal flow volumes. The flow has a predominant axial velocity component in direction to the exit of the turbine. When the flow volume through the turbine is reduced as is the case for example during start-up, run-out, load change or emergency situations the flow pattern changes to a more complex picture as illustrated in  FIG. 3B . 
         [0030]    Under reduced flow conditions, there are steam volumes with a small axial components. The volumes tend to have a much larger circumferential component as for example the volume  31  in  FIG. 3B , which circulates predominantly into and out of the paper plane while have only a small circulation in axial direction. Thus wet film scraping bores which are used in turbines are rendered inefficient under low loads, as these devices typically depend on a axial flow velocity to catch the film. 
         [0031]    By making use of the circumferential velocity hot steam can be extracted even with an adverse back pressure from the condenser unit of the turbine. 
         [0032]    Estimates show that by extracting about 1% of the mass flow using a groove of 300 mm width and a maximum depth d of 20 mm the temperature of a last stage blade can be reduced from 178 degrees C. to 166 degrees C. This value can be further increased by extracting more albeit at the expense of reducing the overall efficiency of the turbine. 
         [0033]    It is advantageous from a manufacturing point of view to have the bores for holes  112  start at the split between the upper and lower half of the turbine casing  11 . However the bores can be placed in principle at any point along the circumference of the casing or vane carrier. It is also possible to increase the number of grooves from 2 to 3, 4 or more along the same circumferential line. In such a variant of the invention, the gradient of the grooves is steeper to achieve the same target depth d after less than a half turn. 
         [0034]    It can be further advantageous to place the extraction grooves and channels at locations other than between the last stage vanes and blades or to have extraction grooves and channels at more than just one location. It is further possible to place the two grooves and extraction channels as described above not along a single circumferential line but slightly staggered along the axial length of the turbine. 
         [0035]    The present invention has been described above purely by way of example, and modifications can be made within the scope of the invention, particularly as relating to the shape, number and design of the extraction grooves and channels. The invention also consists in any individual features described or implicit herein or shown or implicit in the drawings or any combination of any such features or any generalization of any such features or combination, which extends to equivalents thereof. Thus, the breadth and scope of the present invention should not be limited by any of the above-described exemplary embodiments. 
         [0036]    Each feature disclosed in the specification, including the drawings, may be replaced by alternative features serving the same, equivalent or similar purposes, unless expressly stated otherwise. 
         [0037]    Unless explicitly stated herein, any discussion of the prior art throughout the specification is not an admission that such prior art is widely known or forms part of the common general knowledge in the field.