The present invention relates generally to gas turbines, for example, for electrical power generation, and more particularly to cooling the stage one nozzles of such turbines.
The traditional approach for cooling turbine blades and nozzles was to extract high pressure cooling air from a source, for example, from the intermediate and final stages of the turbine compressor. In such a system, a series of internal flow passages are typically used to achieve the desired mass flow objectives for cooling the turbine blades. In contrast, external piping is used to supply air to the nozzles, with air film cooling typically being used and the air exiting into the hot gas stream of the turbine. In advanced gas turbine designs, it has been recognized that the temperature of the hot gas flowing past the turbine components could be higher than the melting temperature of the metal. It was therefore necessary to establish a cooling scheme to protect the hot gas path components during operation. Steam has been demonstrated to be a preferred cooling media for cooling gas turbine nozzles (stator vanes), particularly for combined-cycle plants. See, for example, U.S. Pat. No. 5,253,976, the disclosure of which is incorporated herein by this reference. For a complete description of the steam-cooled buckets, reference is made to U.S. Pat. No. 5,536,143, the disclosure of which is incorporated herein by reference. For a complete description of the steam (or air) cooling circuit for supplying cooling medium to the first and second stage buckets through the rotor, reference is made to U.S. Pat. No. 5,593,274, the disclosure of which is incorporated herein by reference.
Because steam has a higher heat capacity than the combustion gas, however, it is considered inefficient to allow the coolant steam to mix with the hot gas stream. Consequently, in conventional steam cooled buckets it has been considered desirable to maintain cooling steam inside the hot gas path components in a closed circuit. Nevertheless, certain areas of the components in the hot gas path cannot practically be cooled with steam in a closed circuit. For example, the relatively thin structure of the trailing edges of the nozzle vanes effectively precludes steam cooling of those edges. Accordingly, air cooling is used to cool those portions of the nozzle vanes. For a complete description of the steam cooled nozzles with air cooling along the trailing edge, reference is made to U.S. Pat. No. 5,634,766, the disclosure of which is incorporated herein by reference.
In a typical closed loop steam or air cooled nozzle design such as that briefly described above and disclosed in the above-mentioned patents, the steam or air is used to cool the nozzle wall via impingement, or convection in the case of the trailing edge cavity. In some cases, with this kind of cooling scheme, the thermal gradient in the nozzle wall can reach very high levels, which can cause low LCF (Low Cycle Fatigue) life for local regions of the nozzle wall.
The present invention modifies the typical closed loop steam or air cooled nozzle design by introducing cooling media, e.g. air, film cooling to greatly reduce local thermal gradient, which, in turn, will increase the local LCF life. More specifically, the invention is embodied in the addition of at least one air pocket to a closed loop steam or air cooled nozzle for providing a cooling air source for film cooling of the airfoil surface in regions where low LCF life would otherwise exist due to high thermal gradient. The air pocket is located inside one or more cavities of a closed loop steam or air cooled gas turbine nozzle. Air is piped into the air pocket and then flows out into the hot gas path through air film holes that are defined in the airfoil wall to communicate the air pocket to the vane exterior.
Thus, in an embodiment of the present invention, there is provided a cooling system for cooling the hot gas components of a nozzle stage of a gas turbine, in which closed circuit steam or air cooling and/or open circuit air cooling systems may be employed. In the closed circuit system, a plurality of nozzle vane segments are provided, each of which comprises one or more nozzle vanes extending between radially inner and outer walls. The vanes have a plurality of cavities in communication with compartments in the outer and inner walls for flowing cooling media in a closed circuit for cooling the outer and inner walls and the vanes per se. This closed circuit cooling system is substantially structurally similar to the steam cooling system described and illustrated in the prior referenced U.S. Pat. No. 5,634,766, with certain exceptions as noted below. Thus, cooling media may be provided to a plenum in the outer wall of the segment for distribution to chambers therein and passage through impingement openings in a plate for impingement cooling of the outer wall surface of the segment. The spent impingement cooling media flows into leading edge and aft cavities extending radially through the vane. At least one cooling fluid return/intermediate cooling cavity extends radially and lies between the leading edge and aft cavities. A separate trailing edge cavity may also provided. The flow of cooling air in a trailing edge cavity per se is the subject of a U.S. Pat. No. 5,611,662, the disclosure of which is incorporated herein by reference. The cooling air from that trailing edge cavity flows to the inner wall, for flow through a passage for supplying purge air to the wheelspace, or into the hot gas path. To cool the airfoil surface in regions where low LCF life will otherwise exist due to high thermal gradient, at least one air pocket is located inside one or more of the aforementioned cavities of the closed loop steam or air cooled gas turbine nozzle. Air is piped into the air pocket and then flows out into the hot gas path through air film holes defined in the airfoil wall, to communicate the air pocket to the vane exterior and thereby create a cooling air film to cool the airfoil surface.
More specifically, in a preferred embodiment of the present invention, there is provided a closed circuit stator vane segment comprising radially inner and outer walls spaced from one another, a vane extending between the inner and outer walls and having leading and trailing edges, the vane including discrete leading edge, trailing edge and intermediate cavities between the leading and trailing edges and extending radially of the vane, an insert in the leading edge cavity for receiving cooling media and having impingement openings for directing the cooling media against interior wall surfaces of the leading edge cavity for impingement cooling of the vane about the leading edge cavity, an insert in the intermediate cavity for receiving cooling media and having impingement openings for directing the cooling media against interior wall surfaces of the intermediate cavity for impingement cooling of the vane about the intermediate cavity, the trailing edge cavity lying in communication with a cooling air inlet for receiving cooling air therefrom and having an outlet one of at a trailing edge thereof and at a radially inner end thereof, for directing spent cooling air one of into the hot gas path exterior to the vane and into a wheelspace between adjacent turbine stages, and wherein at least one air pocket is defined in a wall of at least one of the cavities. Each air pocket is substantially closed with respect to the respective cavity, is in flow communication with a source of cooling air, and has at least one outlet aperture for flow communication between an interior of the pocket and the exterior of the vane, to cool the airfoil surface.
The present invention may further be embodied in a closed circuit cooling system for cooling the hot gas components of nozzle stages of a gas turbine, particularly the first nozzle stage, in combination with an open circuit air cooling system for certain of those components. More particularly, nozzle vane segments are provided having the necessary structural integrity under high thermal fluxes and pressures affording a capacity of being cooled by a cooling medium, preferably steam, flowing in a pressurized closed circuit and in combination with open air circuit cooling. Thus, the present invention provides, in at least the first stage of a turbine, a plurality of nozzle vane segments each of which comprise one or more nozzle vanes extending between radially outer and inner walls. The vanes have a plurality of cavities in communication with compartments in the outer and inner walls for flowing a cooling media, preferably steam, in a closed-circuit path for cooling the outer and inner walls and the vanes, per se. Impingement cooling is provided in the leading cavity of the vane, as well as in the intermediate, return cavity(ies) of the first stage nozzle vane. Inserts in the leading and aft cavities comprise sleeves that extend through the cavities, spaced from the walls thereof. The inserts have impingement holes in opposition to the walls of the cavity it whereby steam flowing into the inserts flows outwardly through the impingement holes for impingement cooling of the vane walls. Return channels are provided along the inserts for channeling the spent impingement cooling steam. Similarly, inserts in the return, intermediate cavity(ies) have impingement openings for flowing impingement cooling medium against the side walls of the vane. Those inserts also have return cavities for collecting the spent impingement cooling steam and transmitting it to the steam outlet.
The open circuit air cooling system is provided for air film cooling of the airfoil surface in regions where low LCF life will otherwise exist due to high thermal gradient. More particularly, at least one air pocket is defined in or along at least a portion of the wall of at least one cavity of the segment and a plurality of rearwardly directed openings are defined through the wall for communicating the air pocket with the segment exterior. Cooling air exiting these openings film cools the exterior surface of the vane along the leading edge and/or intermediate cavity(ies). A conventional open air cooling system may also be provided for cooling the trailing edge cavity.