Patent Application
20130011248
The present disclosure is directed to a geometry to minimize feature related thermal stresses in a monolithic ceramic or Ceramic Matrix Composite (CMC) shroud used in a gas turbine engine.
The incorporation of a ceramic or CMC shroud in the hot section of a gas turbine engine as a replacement of a similar metallic component is beneficial. A metal shroud requires substantial cooling in order to withstand the high temperature in the turbine section. Often a metal shroud is composed of multiple arc sections held by a complex assembly of support hardware to provide close positional tolerance and circularity to provide gap control with the rotor blade tips. A one piece ceramic shroud, by virtue of its inherent thermal material properties and high temperature capability, provides the ability to reduce the running blade tip clearance and reduced cooling air, providing improvements in efficiency and increased power.
From a thermal stress point of view, a simple ring structure provides the most stable structure with the least distortion and lowest thermal stress. Various techniques exist for shroud support which require designing and fabricating the shroud ring structure with features onto which to support and locate the shroud within the metal assembly. Features may range from tabs or protrusions on the outer diameter surface or on one or both end faces, to slots or grooves on the outer diameter surface of the shroud. Although consideration needs to be made for attachment and assembly of the ceramic shroud, structural considerations also need to be made to insure ceramic integrity and life.
In accordance with the present disclosure, there is provided a shroud comprising a ring with a plurality of protrusions and a plurality of slots and each of said protrusions having an arc length and parallel sides.
Further in accordance with the present invention, there is provided an engine broadly comprising a rotor and ceramic shroud surrounding said rotor and said shroud comprising a ring with a plurality of protrusions and a plurality of slots and each of said protrusions having an arc length and parallel sides.
Other details of the monolithic ceramic or CMC shroud having reduced thermal stresses are set forth in the following detailed description and the accompanying drawings in which like reference numerals depict like elements.
There is shown in
From a thermal stress point of view, the ideal structure would be a simple ring structure. This is because such a structure provides the most stable structure with the least distortion and lowest thermal stress. For purposes of holding and positioning the shroud in the engine assembly, the ring structure of shroud 18 as shown in
A simple ring provides the hoop strength and the stable structure needed to withstand the thermal gradient that the shroud 18 will be exposed to. A ring structure with uniform thickness and no other features, such as tabs or slots, exposed to a uniform through thickness temperature gradient, can exhibit stresses considered acceptable for ceramics used for this purpose. The loading experienced in the ring structure from a through thickness temperature gradient is a bending load, with the hotter inner diameter surface in compression and the cooler outer diameter surface in tension. If the ring were unwrapped into a simply supported flat bar, the temperature gradient would cause a differential of thermal expansion from one side to the other and cause the bar to bend. To eliminate the deformation from the flat bar, a pure moment, equal and opposite, would need to be applied at each end of the bar. But if the bar is shaped into a hoop and the ends joined to form a ring with uniform cross section, the bending loading at each end of this joint is equal and opposite, and therefore prevents any deformation due to bending. The only deformation of the ring is thermal expansion typical of any heated volume of material.
A featureless ring, though, is not practical and some means are needed to provide support and location of the ring structure to the assembly. Various features pertaining to assembly have been proposed and may be valid for reasons specific to the design requirements. But once a feature is introduced into the structure of the ring and eliminates the constant cross section of the ring, the bending load in the ring becomes evident through higher strains and ring distortion. Depending on the feature, the distortion and localized high strain can cause increased stress and stress concentrations. These stress concentrations are most often associated with fillets at the attachment feature and are inherently tensile surface stresses or 1st principal stresses.
As can be seen from
As noted before, the sides 32 of the slots 34 are parallel. With a multitude of identical slots positioned around the circumference of the ring 38 forming the shroud 18, concentric and circumferential position of the ring 38 can be maintained with a mating ring 40 with mating features 42 that contact the parallel sides 32 of each slot as shown in
With slot arc lengths much less than protrusion arc lengths, or A1<<A2, than a majority of the ring 38 is dominated by the maximum ring thickness T2. This provides a stiffer cross section to withstand the bending load from the through thickness thermal gradient. This may cause a significant stress riser in the fillet region 33 of the slots 34 where the stress increases with decreasing slot arc length.
Conversely, as the arc length A2 of the protrusion 30 decreases and the arc length A1 of the slot 34 increases, then a majority of the ring 38 is dominated by the minimum ring thickness T1 and the stress concentrations decreases. The mounting feature becomes the parallel sides 32 of a protrusion feature 30 for when A1>>A2.
Curve 2 in
The maximum hoop stress of a plain ring thickness T1 is approximately 15% less than the lowest maximum stress condition, as shown by the dotted line 3 in
A featureless shroud ring is not a useful solution for assembly into the hot section of a gas turbine engine. Therefore, the geometry that provides the least thermally induced stress is a ring with a multitude of protrusions with an arc length as small as practical to provide adequate mounting support. From
While there has been discussed herein, a shroud 18 having 5 to 12 slots, determining the number of useful slots to use in a particular design application depends on issues including manufacturing cost. It follows that the more features the ring will have will in turn increase the cost of the shroud ring. Close tolerance control in ceramic components requires much of the machining of the contact surfaces and other critical features need to be done in the ceramic's hardened state. Therefore, to decrease manufacturing costs, as much machining in the green state should be performed prior to densification if the ceramic, leaving only a minimal amount of machining to obtain the required tolerance.
The shroud ring 18 may be formed from any suitable ceramic material known in the art. The shroud ring 18 may be a monolithic ceramic material.
There has been discussed herein a geometry to minimize feature related thermal stresses in a monolithic ceramic shroud. While the geometry has been described in the context of specific embodiments thereof, other unforeseen alternatives, modifications, and variations may become apparent to those skilled in the art having read the foregoing description. Accordingly, it is intended to embrace those alternatives, modifications, and variations, as fall within the broad scope of the appended claims.
