An optical pyrometer to measure turbine blade surface temperature

The development of a cooled infrared pyrometric probe, with a high spatial and temporal resolving power, made it possible to establish by pyrometric methods surface temperature charts of the mobile blades of a turbine. The use of a sensor with a 1 sec time constant yields a resulting power on the blade surface of the order of 2 sq mm. This sensor is cooled at -40 C, which ensures a good detectivity and temperature measurements above 450 C. A water cooled metallic tube, ending by a right angle deflecting prism, makes up the light guide of the pyrometer and can be inserted between the various turbine stages. The probe has been designed to withstand the pressure and temperature conditions prevailing in the turbine blade test section, and was subjected without damage to 20 bar and 2100 K. The use of this pyrometer with the expermental turbine specially designed for qualifying new blade cooling methods has shown that pyrometry involves difficulties connected to: the non-zero blade reflection coefficient (emissivity lower than 1); and the gas radiation due to the gases. The above difficulties were analyzed and solutions were proposed; in particular a correcting program for the temperatures was perfected which takes into account the blade to blade mutual reflections.