Airtight external shroud for a turbomachine turbine wheel

1. An external shroud for a turbomachine turbine wheel, the shroud being substantially cylindrical, the shroud comprising:

ring sectors positioned circumferentially end to end, each ring sector including

a circumferentially directed plate,

a circumferentially directed sheet brazed to an internal surface of the plate, and

a block of abradable material fixed to the sheet,

wherein a circumferential dimension of the plate of each ring sector is substantially identical to a circumferential dimension of the block of abradable material, and

wherein, over an entire axial dimension of the ring sector, the sheet and the block of each ring sector are offset with respect to the plate of the ring sector in a circumferential direction so that a circumferential end part of the plate of each ring sector overlaps a corresponding end part of the sheet and the block of an adjacent ring sector.

2. The external shroud as claimed in claim 1 , wherein the plate and the block of each ring sector are circumferentially offset from each other by an angle α smaller than about 2°.

3. The external shroud as claimed in claim 1 , wherein the circumferential end part of the plate of each ring sector overlaps the corresponding end part of the block of an adjacent ring sector over a circumferential distance smaller than about 10 mm.

4. The external shroud as claimed in claim 1 , wherein the circumferential end part of the plate of each ring sector is separated by a small radial clearance from the corresponding end part of the block of an adjacent ring sector.

5. The external shroud as claimed in claim 1 , wherein the sheet has a thickness or radial dimension smaller than about 1 mm.

6. A turbomachine turbine comprising:

rotor wheels,

wherein at least one of these rotor wheels is surrounded by an external shroud as claimed in claim 1 .

7. A turbomachine comprising: a turbine as claimed in claim 6 .