Centripetal turbines generally comprise a turbine wheel mounted within a turbine housing, the inner wall of which defines an annular inlet passageway arranged around the turbine wheel and a generally cylindrical axial outlet passageway extending from the turbine wheel. The arrangement is such that pressurised gas admitted to the inlet passageway flows to the outlet passageway via the turbine wheel, thereby driving the turbine wheel.
Where the outlet passageway meets the inlet passageway the inner wall of the turbine housing curves radially outwards forming a curved annular shoulder. The radially outer edges of the turbine wheel blades are profiled to substantially follow the profile of the housing, having a first portion in the region of the inlet passageway which is typically straight, a second curved portion which follows the contour of the curved annular shoulder, and a third substantially straight portion which extends into the outlet passageway.
The turbine blades are designed to follow closely the profile of the housing in order to minimise the gap between the two which is necessary to maximise efficiency. However, minimising the gap between the tips of the turbine blades and the inner wall of the housing is problematical because of the differential thermal expansion of the various turbine components as the turbine temperature rises to its operating temperature.
Conventionally turbines have been constructed with a clearance gap between the blade tips and the housing to allow for the differential expansion. However, given that turbines are generally designed for operating over a range of temperatures a compromise must be reached; either a gap large enough to allow for differential expansion at all extreme operating temperatures must be provided, which will result in an undesirably large gap at certain operating temperatures, or only a relatively small clearance gap may be provided and it be accepted that at least in some, albeit transient, operating conditions the turbine blades will rub against the housing (this could obviously result in rapid wear and in some cases damage to the turbine components).
Various approaches have been adopted to tackle this problem, one such approach being to coat the inner wall of the turbine housing with an annular layer of an abradable material adjacent the turbine blade tips, i.e. covering the curved internal shoulder and that part of the outlet passageway which surrounds the turbine wheel. This allows the turbine to be constructed with essentially zero clearance between the turbine wheel and the housing, with the turbine wheel effectively machining its own clearance as it rotates. Various different materials have been proposed as suitable abradable coatings, see for example U.S. Pat. No. 5,185,217.
Whilst the above solution is effective, it is also relatively expensive both in terms of the abradable materials used and the associated processes of coating the turbine housing with a given abradable layer.