Surface temperature measurement on engine components by means of irreversible thermal coatings

Thermal paints are used by most aeronautical manufacturers in dedicated tests to record temperature profiles over the surface of engine components. The analysis of these coatings, which irreversibly change colour with temperature, is at present performed by a skilled operator who draws isothermal lines directly on the components surface. The technique suffers from a lack of accuracy and automation, as only some temperature levels can be determined and there is no convenient means to insert them in a 3D numerical description of the object. This paper first presents the characteristics and implementation of thermal paints. Two major advances are introduced regarding the image processing techniques developed to obtain temperature from colour and the use of a 3D scanner to digitize the outer shape of the components. The analysis is made quicker, more accurate and consistent than the former visual interpretation: temperature information is directly mapped on the 3D mesh and is ready for input in thermal models. The key points of the technique are discussed and illustrated by results on test pieces and turbine blades.