Abstract:
A gas turbine having a turbine stage is presented. The gas turbine has a guide vane that can be operated cooled by a cooling medium, and a cooling medium feed device for feeding the cooling medium into the interior of the guide van, which, in the region of its rear edge, has on the pressure side at least one cooling medium outlet opening, through which the cooling medium can flow out of the interior of the guide vane into the main flow. The cooling medium feed device has a mass flow control device for controlling the mass flow through the at least one cooling medium outlet opening, with which the mass flow through the at least one cooling medium outlet opening can be increased during operation of the gas turbine under part-load compared to operation of the gas turbine under full load.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is the US National Stage of International Application No. PCT/EP2012/053971 filed Mar. 8, 2012 and claims benefit thereof, the entire content of which is hereby incorporated herein by reference. The International Application claims priority to the European application No. 11161499.6 EP filed Apr. 7, 2011, the entire contents of which is hereby incorporated herein by reference. 
     
    
     FIELD OF INVENTION 
       [0002]    The invention relates to a gas turbine having a turbine stage which has guide blades operable with cooling by a cooling medium and has a cooling medium supply device for supplying the cooling medium into the interior of the guide blades, and to a method for cooling the turbine stage. 
       BACKGROUND OF INVENTION 
       [0003]    A gas turbine is coupled, for example in a power plant for the generation of electrical energy, to a generator and is thus operated both at part load and also at full load at the same rotational speed. The gas turbine has a compressor, a combustion chamber and a turbine, wherein ambient air is drawn in and compressed by the compressor, said ambient air being heated in the combustion chamber by way of the combustion of a fuel. The heated and compressed air is expanded in the turbine with work being performed, wherein the generator is driven by means of the excess work thereby gained. The turbine is conventionally of axial type of construction, wherein the guide blade rows and rotor blade rows are arranged in an alternating fashion and one behind the other in a main flow direction. To attain the highest possible thermodynamic efficiency of the gas turbine, it is desirable for the gas turbine to be operated with as high a turbine inlet temperature as possible. The maximum admissible turbine inlet temperature is determined by the thermal load capacity of the turbine, in particular of the guide blade rows and of the rotor blade rows of the turbine. 
         [0004]    An increase in the maximum admissible turbine inlet temperature is possible if for example the first guide blade row directly downstream of the combustion chamber outlet is cooled. For the cooling of the guide blade row, it is known for example from GB 1 338 354 and GB 938 247 for the guide blades of the guide blade row to be of hollow form and to be traversed by a flow of cooling air which is extracted for example from the compressor. 
         [0005]    If the gas turbine is operated at part load, the turbine inlet temperature and the overall pressure ratio of the gas turbine decrease in relation to full-load operation of the gas turbine, whereby the thermodynamic efficiency of the gas turbine is disadvantageously decreased in part-load operation. Furthermore, in part-load operation of the gas turbine, the overall mass flow of the main flow through the gas turbine is reduced, whereby it is disadvantageously the case that the relative approach flow angles to the turbine rotor blades, in particular to the turbine rotor blades of the rotor blade row downstream of the first guide blade row, differ from the approach flow angles in the design state. This leads to an incorrect impingement of flow on the turbine rotor blades in part-load operation of the gas turbine, whereby the exertion of work on the turbine rotor blades is disadvantageously reduced. Furthermore, in part-load operation of the gas turbine, the fuel supply into the combustion chamber is reduced, whereby the flame temperature in the gas turbine decreases. This can result in disadvantageous combustion instabilities in the combustion chamber, which adversely affect the operation and the availability of the gas turbine in part-load operation. 
         [0006]    If the gas turbine is switched entirely from full-load operation to part-load operation, the individual stage pressure ratios and thus the overall pressure ratio of the gas turbine decrease, and the exhaust-gas temperature increases in the case of a turbine inlet temperature remaining substantially the same. This can give rise to an operating state in which the exhaust-gas temperature exceeds a maximum admissible maximum value. An adequate reaction to this would, in said part-load operation, be to reduce the turbine inlet temperature in order that the exhaust-gas temperature is returned to a value less than or equal to its maximum admissible maximum value, this however disadvantageously being associated with a decrease in the thermodynamic efficiency of the gas turbine. 
         [0007]    The secondary air consumption is determined, as per GB 1 338 354, by the cooling air consumption which is required at rated load. As a result of the temperature decrease at part load, it is then possible for a part of the cooling air to be saved. The saved fraction is then discharged as “jet flap air” at the trailing edge of the guide blades for the purpose of diversion manipulation. A change in the secondary air consumption as a function of the operating state is not provided in GB 1 338 354. According to GB 938 247, however, a deactivation of the cooling of the turbine guide blades may be provided. 
       SUMMARY OF INVENTION 
       [0008]    It is an object of the invention to provide a gas turbine and a method for cooling a turbine stage of the gas turbine, wherein the gas turbine can be operated at part load in a stable manner and with high thermodynamic efficiency. 
         [0009]    The gas turbine according to the invention having a turbine stage has a guide blade operable with cooling by a cooling medium and has a cooling medium supply device for supplying the cooling medium into the interior of the guide blade, which guide blade has, in the region of its trailing edge and on its pressure side, at least one cooling medium outlet opening through which the cooling medium can be caused to flow out of the interior of the guide blade into the main flow, wherein the cooling medium supply device has a mass flow control device for controlling the mass flow through the at least one cooling medium outlet opening, by means of which mass flow control device the mass flow through the at least one cooling medium outlet opening can be increased in part-load operation of the gas turbine in relation to full-load operation of the gas turbine. The method according to the invention for cooling the turbine stage of the gas turbine has the steps: operating the gas turbine at part load; controlling the mass flow control device such that the mass flow through the at least one cooling medium outlet opening is increased in relation to the mass flow through the at least one cooling medium outlet opening in full-load operation of the gas turbine. 
         [0010]    The guide blade that is operable with cooling by the cooling medium is designed such that, in full-load operation, with a supply of a correspondingly adequately large mass flow of the cooling medium, it can be operated at the design point with an adequately long service life. In the design of the guide blade, it is taken into consideration that the mass flow of the cooling medium is large enough that thermal overloading of the guide blade is prevented owing to the cooling action by the cooling medium on the guide blade. The profiling of the guide blade is selected such that, taking into consideration the cooling action by the cooling medium and the aerodynamic influence thereof, the guide blade meets defined design requirements at the design point. 
         [0011]    As a result of the pressure-side outflow of the cooling medium through the at least one cooling medium outlet opening in the region of the trailing edge of the guide blade, the diversion action of the guide blade is intensified in relation to the diversion action of the guide blade with little or no outflow of the cooling medium through the at least one cooling medium outlet opening in the region of the trailing edge of the guide blade. Thus, if the mass flow of the cooling medium flowing out of the at least one cooling medium outlet opening at the pressure side is increased by means of the mass flow control device during operation of the gas turbine, the diversion action of the guide blade is consequently intensified. 
         [0012]    The increase of the mass flow of the cooling medium would not be required with regard to the required cooling action by the cooling medium. The increase of the mass flow of the cooling medium yields an intensification of the diversion action of the guide blade. Thus, during operation of the guide blade with increased mass flow of the cooling medium that flows off the guide blade on the pressure side in the region of its trailing edge, the diversion angle of the guide blade can be adjusted through corresponding selection of the mass flow of the cooling medium, even though the guide blade is operated with sufficient cooling by the cooling medium. 
         [0013]    This is advantageous in particular in part-load operation of the gas turbine, in which the diversion action of the guide blade is disadvantageously reduced. Said reduction can advantageously be counteracted by virtue of the mass flow control device for controlling the mass flow through the at least one cooling medium outlet opening being used to increase the mass flow through the at least one cooling medium outlet opening in part-load operation of the gas turbine in relation to full-load operation of the gas turbine. In this way, in part-load operation of the gas turbine, the diversion action of the guide blade can be advantageously increased, wherein the guide blade is nevertheless cooled to an adequate extent. Furthermore, through corresponding actuation of the mass flow control device, the mass flow of the cooling medium through the at least one cooling medium outlet opening can be adjusted such that the stage pressure ratio across the guide blade and the rotor blade is increased. The increase in the stage pressure ratio permits an increase in the turbine inlet temperature while maintaining the same exhaust-gas temperature, resulting in an increase in the thermodynamic efficiency and the specific power of the gas turbine. 
         [0014]    It is preferable for the mass flow control device to have a throttle valve for mass flow control and for the guide blade to be designed, in full-load operation, for a mass flow of the cooling medium which is throttled by the throttle valve. It is furthermore preferable for the guide blade to be designed, in part-load operation, for a mass flow of the cooling medium which is not throttled by the throttle valve. The cooling medium is preferably compressor exit air, and the mass flow control device is preferably fed with the compressor exit air from the compressor exit. 
         [0015]    It is preferably provided that, in part-load operation of the gas turbine, the compressor exit air is conducted so as to bypass the combustion chamber. The compressor exit air is conducted so as to bypass the gas turbine of the combustion chamber in part-load operation for example in order that, during the decrease in turbine inlet temperature, an excessively intense drop in flame temperature is prevented and combustion instabilities are avoided. The compressor exit air conducted so as to bypass the combustion chamber in part-load operation is surplus and is thus preferably available to the cooling medium supply device for the cooling of the guide blade. 
         [0016]    In part-load operation, the mass flow control device is preferably operated without throttling, wherein the compressor exit air conducted so as to bypass the combustion chamber is used for the cooling of the guide blade. By contrast, if the gas turbine is operated at full load, the throttle valve of the mass flow control device is placed into a throttling position, whereby the mass flow of the cooling medium is reduced. Said reduction in the mass flow of the cooling medium should preferably be selected such that both adequate cooling and also an adequate diversion action of the guide blade are provided at all times during the operation of the gas turbine. 
         [0017]    The guide blade preferably has a cooling medium inlet, through which the cooling medium can be caused to flow into the interior of the guide blade, and a cooling medium reservoir, by means of which the cooling medium is provided to the cooling medium inlet by the mass flow control device. The cooling medium reservoir is advantageously provided as a buffer vessel for the cooling medium, such that it is possible in particular for a large transient demand for cooling medium by the guide blade to be compensated by the cooling medium reservoir. Furthermore, the flow speeds in the cooling medium reservoir are low, such that flow losses in the cooling medium reservoir are low. 
         [0018]    The mass flow control device is preferably fed from a compressor plenum of the gas turbine. It is in particular the case that that fraction of the compressor exit air which is conducted so as to bypass the combustion chamber in part-load operation of the gas turbine is conducted into the compressor plenum. Furthermore, the compressor exit pressure prevails in the compressor plenum, such that, in the compressor plenum, the cooling medium is provided with an adequately high pressure for corresponding throttling preferably by means of the throttle valve. 
         [0019]    The cooled guide blade is preferably the first guide blade, arranged immediately downstream of the combustion chamber of the gas turbine, of the first turbine stage of the gas turbine. Furthermore, in the method according to the invention, it is preferable for the mass flow to be set such that the part-load operation of the gas turbine is optimized with regard to the thermodynamic efficiency and/or with regard to the flow impingement angle on the rotor blade arranged downstream of the guide blade and/or with regard to the exhaust-gas temperature of the turbine of the gas turbine and/or with regard to the flame stability in the combustion chamber of the gas turbine. 
         [0020]    In a known way, with decreasing gas turbine load, the absolute value of the compressor mass flow and the absolute values of the secondary mass flows also decrease. To then nevertheless effect a higher consumption of cooling air in part-load operation, provision is made such that, with a decrease in the gas turbine load, the percentage value of the cooling medium mass flow—in relation to the compressor intake mass flow—is increased. This increases the relative consumption of cooling medium in part-load operation in relation to full-load operation. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0021]    The invention will be explained in more detail below on the basis of the appended schematic drawing. The FIGURE shows a longitudinal section through an embodiment of the gas turbine according to the invention in the region of the combustion chamber and of the turbine. 
       
    
    
     DETAILED DESCRIPTION OF INVENTION 
       [0022]    As can be seen from the FIGURE, a gas turbine  1  has a housing  2  in which there are arranged a compressor (not shown), a combustion chamber  3  and a turbine which has multiple turbine stages  8 ,  9 . Downstream of the compressor exit there is arranged a diverting diffuser (not shown) which issues into a compressor plenum  5  which is formed as a cavity in the housing  3  and in which the combustion chamber  3  is arranged. During the operation of the gas turbine  1 , ambient air is drawn in, and compressed to a compressor exit pressure, by the compressor. From the compressor exit, the compressor exit air enters the combustion chamber interior  4  of the combustion chamber  3  and is mixed with a liquid or gaseous fuel. Thus, in the combustion chamber interior  4 , a combustible mixture is generated which is ignited and burned in the combustion chamber  3 . The combustion in the combustion chamber interior  4  takes place substantially isobarically. At the combustion chamber outlet  6  of the combustion chamber  3 , the mixture is at a high temperature correspondingly effected by the combustion, and said mixture is conducted via a transfer duct  7  to the turbine of the gas turbine  1 . 
         [0023]    In the FIGURE, a first turbine stage  8  and a second turbine stage  9  of the turbine are illustrated. Each turbine stage  8 ,  9  has a guide blade row  10  and a rotor blade row  12 . The guide blade row  10  of the first turbine stage  8  is formed by a multiplicity of identical guide blades  11  arranged equidistantly over the circumference. The rotor blade row  12  of the first turbine stage  8  is formed by identical rotor blades  13  arranged equidistantly over the circumference. The guide blades  11  and the rotor blades  13  are of axial type of construction. The guide blades  11  are held, at their radially outer ends, by a guide blade carrier  14 . The rotor blades  13  are each equipped, at their radially inner ends, with a blade root which engages in a positively locking manner with a rotor  15  of the gas turbine  1 . Each guide blade  11  has a blade aerofoil with a trailing edge  16 , a leading edge and a suction side (not shown) and a pressure side  18 . In the region of the leading edges of the guide blades  11 , the hot gas mixture conducted in the transfer duct  7  enters into the first turbine stage  8 , wherein the hot gas mixture is diverted by the guide blade row  10 , and said hot gas mixture is expanded in the rotor blade row  12  with work being performed. 
         [0024]    The higher the turbine inlet temperature of the hot gas mixture, the higher the thermodynamic efficiency of the gas turbine  1 . Thermal loading limits of the material of the guide blades  11  yield a maximum admissible turbine inlet temperature. To increase the maximum admissible turbine inlet temperature, the guide blades  11  are cooled during operation of the gas turbine  1  in order to lower the thermal load of the guide blades  11 . For this purpose, the guide blades  11  are of hollow design, wherein the guide blades  11  are traversed by a flow of a cooling medium for cooling purposes. 
         [0025]    The gas turbine has, as a cooling medium supply device, a bleed line  19 , a throttle valve  20 , a supply line  21 , a cooling air reservoir  22 , a cooling air inlet  23 , and a chamber  24 , wherein the throttle valve  20  is designed as a mass flow control device. The bleed line  19  is connected in fluid-conducting fashion to the compressor plenum  5  and merges via the throttle valve  20  into the supply line  21 . The supply line  21  issues into the cooling air reservoir  22 , which is arranged radially outside the guide blade ring  10  and which is in the form of an annular chamber. Arranged radially directly to the outside of the guide blades  11  and concentrically with respect to the cooling air reservoir  24  is the chamber  24 , which issues into the interior spaces of the guide blades  11 . The radial distance between the chamber  24  and the cooling air reservoir  22  is bridged, for each guide blade  11 , by the cooling air inlet  23 . 
         [0026]    The compressor exit air flows from the compressor exit via the diverting diffuser into the compressor plenum  5 , in which the compressor air is provided as the cooling medium (cooling air). From the compressor plenum  5 , the cooling air is bled off by means of the bleed line  19 . Depending on the throttling position of the throttle valve  20 , a corresponding cooling air mass flow is generated in the bleed line  19  and in the supply line  21 . From the supply line  21 , the cooling air flows into the cooling air reservoir  22  and is made available in the cooling air reservoir  22 . The pressure of the cooling air in the cooling air reservoir  22  is determined in particular by the cooling air mass flow flowing out of the cooling air reservoir  22  through the cooling air inlet  23 , the cooling air mass flow flowing into the cooling air reservoir  22  through the supply line  21 , and the throttling position of the throttle valve  20 . From the cooling air reservoir  22 , the cooling air flows via the cooling air inlet  23  into the chamber  24 , from where the cooling air flows into the interior spaces of the guide blades  11 . The guide blades  11  each have, in the region of their trailing edges  16  and on their pressure sides  18 , a substantially radially running row of cooling air outlet openings  25  through which the cooling air flows out of the interior of the guide blades  11  into the main flow. 
         [0027]    The cross sections of the bleed line  19 , of the supply line  21  and of the cooling air inlet  23  are dimensioned such that, in full-load operation of the gas turbine  1 , the guide blades  11  are supplied with a cooling air mass flow of such a magnitude that, when the throttling valve  20  is in a certain throttling position, adequate cooling of the guide blades  11  is provided. Here, the flow-off angle at the guide blades  11  is set such that the rotor blades  13  are impinged on, at their leading edges  17 , with an approach flow angle corresponding to the design approach flow angle. In the design of the guide blades  11 , it is taken into consideration that the mass flow of the cooling air is large enough that thermal overloading of the guide blades  11  is prevented owing to the cooling action of the cooling air on the guide blades  11 , and nevertheless the rotor blades  13  are impinged on by flow in an optimum manner. 
         [0028]    As a result of the pressure-side outflow of the cooling air through the cooling air outlet opening  25  in the region of the trailing edges  16  of the guide blades  11 , the diversion action of the guide blades  11  is intensified. Thus, in part-load operation of the gas turbine  11 , the throttle valve  20 , which in full-load operation is in a throttling position, is opened, whereby the mass flow of the cooling air through the cooling air outlet openings  25  is increased. As a result of this, the diversion action of the guide blades  11  is intensified. 
         [0029]    Such an increase of the mass flow of the cooling air would not be required with regard to the required cooling action by the cooling air. The increase of the mass flow of the cooling air additionally yields an intensification of the diversion action of the guide blades  11 , whereby the diversion angle of the guide blades  11  can be adjusted through corresponding selection of the mass flow of the cooling air by actuation of the throttle valve  20 . 
         [0030]    In part-load operation of the gas turbine, in which the diversion action of the guide blades  11  is disadvantageously reduced, the throttle valve  20  is opened, in the extreme case fully opened, whereby the mass flow through the cooling air outlet openings  25  is increased. As a result, in part-load operation of the gas turbine  1 , the diversion action of the guide blades  11  is advantageously intensified, whereby the leading edges  17  of the guide blades  13  are impinged on by flow in an optimum manner, and the guide blades  11  are adequately cooled. 
         [0031]    A greater amount of compressor air is available in the compressor plenum  5  in part-load operation of the gas turbine  1  than in full-load operation, because, in part-load operation, compressor exit air is conducted so as to bypass the combustion chamber  3  in order that, during the reduction in the turbine inlet temperature, an excessively intense drop in flame temperature is prevented and combustion instabilities are avoided. Said excess compressor exit air is advantageously available for being supplied to the guide blades  11 . In extreme part-load operation of the gas turbine  1 , the throttle valve  20  is set into a non-throttling position, such that the maximum cooling air mass flow is supplied to the guide blades  11 . By contrast, if the gas turbine  1  is operated at full load, the throttle valve  20  is placed into a throttling position, whereby the mass flow of the cooling air is reduced. Said reduction in the mass flow of the cooling air should be selected such that both adequate cooling and also an adequate diversion action of the guide blades  11  are realized at all times during the operation of the gas turbine  1 . 
         [0032]    The cooling air reservoir  22  acts as a buffer vessel for the cooling air, such that it is possible for a large transient demand for cooling air during the operation of the gas turbine  1  to be compensated by the cooling air reservoir  22 . Furthermore, the size of the volume of the cool.