Patent Application
20150198063
This application claims priority to European application 14151063.6 filed Jan. 14, 2014, the contents of which are hereby incorporated in its entirety.
The invention relates to a cooled stator heat shield for a gas turbine and a gas turbine equipped with such a cooled stator heat shield.
In the installed state, stator heat shields are situated on a stator and/or on a housing of a gas turbine. They are usually mounted on a guide vane carrier and form a radial border for a hot gas path of the gas turbine in the area of the rotor blades of a rotor of the gas turbine. As a rule, a plurality of such stator heat shields are arranged adjacent to one another in the circumferential direction with regard to an axis of rotation of the rotor, thereby forming a closed ring of individual stator heat shields. The individual stator heat shields here form ring segments. The stator heat shields protect the housing and/or the guide vane carriers from exposure to the hot gas of the gas turbine. The outside of the stator heat shields is exposed to the hot gas, while the inside of the respective stator heat shield facing away from the hot gas path is exposed to a suitable cooling air to cool the respective stator heat shield. Due to this cooling, the lifetime of the stator heat shields can be increased. Fundamentally, however, there is a need for increasing the lifetime of such stator heat shields further.
Cooling of stator heat shields, particularly of first stage is a very challenge task. Cooling effectiveness is limited to convective cooling scheme, since film cooling of hot gas exposed surface is not applicable at area where the rotating blade passes the stator heat shields. This is for two reasons. Firstly, the complex flow field in the gap between stator heat shields and blade tip does not allow for cooling film development and resulted film effectiveness is very low and extremely hard to predict and measure. Secondly, in case of rubbing events cooling openings are often closed by this event, thus preventing required cooling air outflow that would have detrimental effect on the whole cooling system and significantly reduce lifetime.
The common practice for stator heat shield cooling is to use extensive impingement cooling with cooling air discharged from side faces. For example, a component of a gas turbine engine, i.e. the heat shield, is disclosed in US20120251295 A1. The component includes an external wall which, in use, is exposed on one surface thereof to working gas flowing through the engine. The component further includes effusion cooling holes formed in the external wall. In use, cooling air blows through the cooling holes to form a cooling film on the surface of the external wall exposed to the working gas. The component further includes an air inlet arrangement which receives the cooling air for distribution to the cooling holes. The component further includes a plurality of metering feeds and a plurality of supply plena. The metering feeds meter the cooling air from the air inlet arrangement to respective of the supply plena, which in turn supply the metered cooling air to respective portions of the cooling holes. The cooling scheme of US20120251295 A1 is well robust but due to limitation of the impingement system for cooling of large areas, low coolant consumption is not achievable.
US6354795 B1 proposes impingement cooled stator heat shields with cooling air ejection at hot gas exposed surface in one possible arrangement. However, this disclosure does not propose high heat transfer utilization rates, since cooling air is discharged to the flow path right after the impingement without passing through any additional channels to cover higher cooled area. Thus this scheme has high coolant mass flow rate per square unit and does not support significant saving of cooling air.
With further development of gas turbines, it is focused on the raise of cyclic parameters (pressure ratio and hot gas temperature), that would lead to increase of hot gas thermal exposure of all cooled parts with highest impact on heat shields, since they are only convectively cooled. To turn the lifetime of stator heat shield back to the acceptable level, that would require to increase cooling flow rates by opening discharge areas, or increase air to hot gas pressure ratio by using air from higher compressor stages. Both these actions would lead to detrimental impact on turbine and engine efficiency. Under such situation, it is required to provide improved stator heat shields which could enhance the cooling effect to a higher level while achieve substantial coolant savings.
A turbine shroud cooling assembly for gas turbines is described in document EP 2 657451 A2, where a typical impingement cooling features is combined with cooling micro channels. Because the impingement is not exposed to the hot gas washed surface there is only a low cooling efficiency. Furthermore, the arranged micro channels do weaken the hot gas washed wall, which is not robust due to high possibility of internal leakages. U.S. Pat. No. 8,449,246 B1 describes an impingement cooling exposed to “cold” walls and therefore providing very minor effect on overall metal temperature reduction. US2006/210390 A1 suggests a pure serpentine channel cooling system with very smooth channels which cannot result in an achievement of high cooling effectiveness with low coolant consumption rates. The cooling scheme using serpentine channels with heat transfer enhancing elements which is disclosed in US2010/183428 A1 implies lots of pressure loss generators, especially bends. Thus this cooling scheme requires excessive coolant to hot gas pressure ration and cannot be implemented for the leading edge regions of heat shields.
It is therefore an object of the present invention to solve the aforementioned problems in the common practice for stator heat shield cooling.
The invention provides a cooled stator heat shield for a gas turbine, the gas turbine having a rotor defining an axis of rotation, the stator heat shield comprising a plurality of cooling units disposed in array substantially along the axis of rotation and covering the whole hot gas exposed surface of the heat shield , each of the cooling units comprises an outside part facing hot gas path of the gas turbine, and an inside plate positioned on the outside part and exposed to cooling air, wherein the inside plate comprises a plurality of inlet openings formed through the inside plate to introduce the cooling air into the outside part, thereby impingement cooling of the outside part,
the outside part comprises a cooling air channel formed therein with a first central portion to receive the cooling air, and a second spiral portion around the first central portion to convey the cooling air outward to a cooling air outlet of the second spiral portion.
The core of the invention is to combine impingement cooling with ribbed serpentine channels with the target to find a balance between maximum high heat utilization rates (thus minimum coolant consumption) together with high cooling effectiveness. Impingement provides best cooling effectiveness with small pressure drop, but is not suitable for large cooled areas, while the use of the disclosed serpentine channels allows to increase the cooled area with a certain amount of coolant, but provides excessive pressure drops. The cooling units are small-scale cellular (snail-like) cells, covering the whole hot gas exposed surface of the heat shield. Each individual cooling unit comprises impingement cooling features followed by a spiral (270 degrees) serpentine multipass. Both cooling features are directly faced hot gas washed wall.
According to one example embodiment of the present invention, in the cooling units positioned near the leading edge and trailing edge regions of the stator heat shield, the cooling air outlet is formed as a plurality of discharge holes through the undersurface of the outside part to feed the cooling air into the hot gas path.
According to one example embodiment of the present invention, in the cooling units positioned near the leading edge and trailing edge regions of the stator heat shield, the cooling air outlet is formed as a plurality of film cooling holes through the undersurface of the outside part.
According to one example embodiment of the present invention, a plurality of flow barrier elements are disposed inside the cooling air channel.
According to one example embodiment of the present invention, the flow barrier elements are selected from the group consisting of plain ribs, V shaped ribs, W shaped ribs, pins, vortex generators and dimples.
According to one example embodiment of the present invention, the stator heat shield is manufactured by casting or additive manufacturing method.
According to one example embodiment of the present invention, the additive manufacturing method includes selective laser melting.
The present invention also relates to a gas turbine comprising the stator heat shield of above.
The configuration of the stator heat shield in the present invention is able to optimize the thermal performance of every single cooling unit to collect required heat flux under the hot gas thermal conditions, resulting in maximum uniformity of metal temperatures and stresses in all locations, eliminating all critical zones and thus providing maximum lifetime of the stator heat shield while achieving the coolant savings. Further, the arrangement of the cooling units in the present invention allows to discharge the cooling air into the hot gas flow path at the leading edge and trailing edge regions of the stator heat shield. This would allow to keep the maximum operation pressure ratio and in consequence coolant flow velocities and heat transfer rates for highest cooling effectiveness. Initial investigation of the proposed cooling scheme in the present invention shows that the cooling air saving of 40% is expected compared to the common design.
The objects, advantages and other features of the present invention will become more apparent upon reading of the following non-restrictive description of preferred embodiments thereof, given for the purpose of exemplification only, with reference to the accompany drawing, through which similar reference numerals may be used to refer to similar elements, and in which:
a-4f show alternative structures for the flow barrier element in the invention;
According to
According to the present invention, the stator heat shield 10 comprises a plurality of cooling units 20 disposed in array substantially along the axis of rotation 9. As shown in
a-4f show alternative structures for the flow barrier element 28, where the flow barrier element 28 may be configured to be V shaped ribs, W shaped ribs, pins, vortex generators and dimples as shown by
It should be noted that the stator heat shield comprising a plurality of cooling units can be manufactured by casting or additive manufacturing method such as selective laser melting or any other appropriate means.
While the invention has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. Additionally, while various embodiments of the invention have been described, it is to be understood that aspects of the invention may include only some of the described embodiments. Accordingly, the invention is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 14151063.6 | Jan 2014 | EP | regional |
