1. Field of the Invention
The present invention relates to the cooling of components subject to the impingement of hot gases, for example in turbomachines, the coolant exiting the components through small holes to film-cool the heated surfaces of the components. In particular, it relates to measures capable of reducing the likelihood of blockage of such holes by environmental debris entrained in the flow of coolant or hot gas.
2. Background Information
Typical examples of such components are air-cooled nozzle guide vanes and high pressure turbine rotor blades, which are situated directly downstream of a gas turbine engine's combustion chambers. The film cooling holes are arranged in spanwise rows along the flanks of the aerofoil portions of the blades or vanes so that the streams of cooling air emerging from the holes onto the external surface can collectively protect it from direct contact with the hot gases and carry heat away by merging together to form a more-or-less continuous film of cooling air flowing next to the surface. The process of merging of the individual streams can be aided by flaring the apertures in the external surface so as to encourage the streams of cooling air to fan out towards each other.
One problem with operation of engines containing such blades and vanes is that the film cooling holes have been subject to blockage by dust entrained in the cooling air systems, particularly in middle eastern countries. Because of the high temperatures at which these components operate, small dust particles which strike the edges of the holes, due to vorticity of the air flow through or over the entrances and exits of the holes, become slightly plastic and stick to the edges; this accretion process can continue over many hours' service until blockage occurs. Blockage can occur either internally of the aerofoil at the film hole inlets, or on the outside of the aerofoil at their outlets, through it is most serious at their inlets because of the difficulty of cleaning the insides of blades and vanes. Blockage can also occur to a lesser extent part-way through the hole.