Patent Application
20090255268
This invention relates generally to turbo machinery and specifically, to the cooling of combustor liners in gas turbine combustors.
Conventional gas turbine combustion systems employ multiple combustor assemblies to achieve reliable and efficient turbine operation. Each combustor assembly includes a cylindrical liner, a fuel injection system, and a transition piece that guides the flow of the hot gases from the combustor to the inlet of the turbine. Generally, a portion of the compressor discharge air is used to cool the combustor liner and is then introduced into the combustor reaction zone to be mixed with the fuel and burned.
In systems incorporating impingement cooled transition pieces, a hollow sleeve (also referred to herein as “transition sleeve”) surrounds the transition piece, and the sleeve wall is perforated so that compressor discharge air will flow through the cooling apertures in the sleeve wall and impinge upon (and thus cool) the transition piece. This cooling air then flows along an annulus between the sleeve and the transition piece. This so-called “cross flow” eventually flows into another annulus between the combustor liner and a surrounding flow sleeve (also referred to herein as a “liner sleeve”). The liner sleeve is also formed with several rows of cooling holes about its circumference, the first row located adjacent a mounting flange where the liner sleeve joins to the transition sleeve.
Even though there is a strong crossflow resulting from the transition piece cooling flow, the negative impact of the crossflow on the impingement cooling flow may be minimized by the use of collars or cooling conduits, also referred to as “thimbles”, that are inserted into the cooling holes in the combustor liner sleeve, through which the cooling jets pass. These thimbles provide a physical blockage to the cross flow which forces the crossflow into the desired flow path while simultaneously ensuring that the cooling jets effectively impinge on the combustor liner surface to be cooled.
The thimbles or collars are preferably mounted in each hole of at least the first row of holes at the aft end of the liner sleeve, adjacent a mounting flange where the combustor liner and transition piece are joined. This arrangement decreases the gap between the jet orifices and impingement surface; blocks the cross flow that deflects the jets and forces the cross flow into the desired flowpath for the subsequent jet rows; allows the diameter of the jets to be smaller and thereby reduce cooling air; and provides consistent and accurate control over the location of jet impingement. It also stabilizes unwanted axial oscillation of the first row of jets and prevents the formation of a thick boundary layer (and resulting reduced heat transfer) upstream of the first row of jets. The use of thimbles as described above is disclosed in commonly-owned U.S. Pat. No. 6,484,505.
There remains a need, however, for even more effective impingement cooling of combustor liners by cooling jets directed at the liner surface, but without creating high thermal gradients along the liner.
In one exemplary but non-limiting embodiment, the thimble geometry is altered so that the walls of the thimble diverge in a direction from the thimble inlet to the thimble outlet. In other words, the thimbles have a truncated-cone shape, such that the cooling jets spread radially outwardly as they flow towards the combustor liner, thus providing more uniform cooling of the aft section of the liner.
Accordingly, in one aspect, the invention relates to a cooling arrangement for a turbine combustor liner comprising: a combustor liner; a flow sleeve surrounding at least a portion of the combustor liner with a flow annulus therebetween, the flow sleeve having a plurality of rows of cooling holes formed about a circumference thereof for directing cooling air into the flow annulus and toward the combustor liner; wherein at least one thimble is fitted within a respective one or more of the cooling holes, the at least one thimble extending in a radial direction toward the combustor liner, and having a peripheral wall diverging in a direction of flow of the cooling air.
In another aspect, the invention relates to A method of cooling a combustor liner surrounded by a flow sleeve comprising: forming plural cooling holes in the flow sleeve; and fitting thimbles in at least some of the cooling holes, each of the thimbles having a diverging peripheral wall in the direction of flow of cooling fluid through the thimbles toward the combustor liner.
The invention will now be described in connection with the figures identified below.
With reference to
Flow from the gas turbine compressor exits an axial diffuser 16 and enters into a compressor discharge case 18. About 50% of the compressor discharge air passes through apertures 20 formed along and about a transition piece impingement sleeve 22 for flow in an annular region or annulus 24 between the transition piece 10 and the radially outer transition piece impingement sleeve 22. The remaining approximately 50% of the compressor discharge flow passes into flow sleeve holes 34 and mixes with the air from the transition piece from annulus 30 and eventually mixes with the gas turbine fuel in the combustor.
It can be seen from the flow arrow 32 in
The impingement cooling flow in the first row of holes 34 in the liner sleeve (the row of holes closest to the mounting flange 26) is particularly subject to disruption by the crossflow from the annulus 24. The cross flow impacts on the first row cooling jets exiting the holes 34, bending them over and degrading their ability to impinge upon the liner 12. Depending on the relative strengths of the cross flow and jets, the jet flow may not even reach the surface of the combustor liner 12. Because the impingement jets are high velocity, there is a characteristic zone of low static pressure behind the jets and near the liner sleeve entrance holes. The cross flow accelerates toward the low pressure zone, leading to a velocity gradient across the liner sleeve/liner annulus 30. The resulting low velocity and thickened boundary layer near the liner surface has very poor heat transfer effectiveness.
To neutralize the negative impact of the crossflow on the cooling jets, cooling thimbles 36 as shown in
In accordance with an exemplary but non-limiting implementation of this invention, and with reference to
The cross section of the tubular peripheral wall 48 is shown to be circular but other cross-sectional shapes could be used, e.g., square, triangular, airfoil-shaped, semi-circular and the like, so long as there is divergence in the flow direction. The divergent or expanding thimble design cools the aft section of the liner effectively by directing and spreading the jet towards the liner in a manner that does not produce meaningful thermal gradients along the liner, and thus produces more uniform cooling of the aft section of the liner.
While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.
