Abstract
The operating temperatures of gas turbines are dependent on the properties of the blade material. Since the advent of the jet engine, immense strides have been made in the improvement of metallic alloys and long-life gas turbines can now be designed to run at temperatures up to about 1,400° F, but this development is probably nearing its limit. In order to achieve a fuel economy comparable with that of a marine Diesel engine the operating temperature must be raised to something of the order of 2,200° F and this will clearly require cooling of the blades or alternatively the use of non-metallic materials.
The existence of high centrifugal stresses in rotor blades facilitates cooling by the thermosiphon principle and at the same time militates against the use of non-metallic materials which tend to be relatively weak in tension. The stator blades, however, offer a promising field for the application of non-metallic materials, and the present paper describes developments which have been carried out over the last eight years.
Numerous materials were tested in the laboratory under conditions designed to simulate the stresses and thermal shocks that will be encountered in service. Various methods of manufacturing the required blade shapes were also investigated, and the shapes themselves were modified to some extent so as to meet the special requirements of the materials. Cascades of blades were tested in a high velocity gas stream at temperatures up to 2,200° F. Several materials were found to have reasonably good thermal shock resistance, and creep strength at high temperature appeared to be the most serious limitation. At least one material has emerged which seems likely to fulfil the necessary requirements. A single-stage turbine embodying refractory stator blades in conjunction with a liquid-cooled rotor is now in course of development.
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