Phosphor thermometry for high temperature gas turbine applications

In the paper developments and some preliminary experiments concerning the application of thermographic phosphors for surface temperature and heat flux measurements in gas turbine combustion chambers are presented. When illuminated with UV light (typically from a pulsed laser) these material exhibits phosphorescence, which is temperature dependent by virtue of variations in the relative intensity of distinct emission lines or of variations in the time constant of the exponential emission decay which occurs once excitation has ceased. Both modes of sensitivity have been investigated with a range of phosphors including are YAG:Tb, YAG:Dy and Y₂O₃:Eu. Of these YAG:Dy has been shown to be best suited to measurements in combustion chambers. It is sensitive to temperatures in the range 300 to 1500 K, emits light in the blue green region of the spectrum where blackbody radiation is relatively weak and exhibits both modes of sensitivity. In addition a number of techniques for laying down phosphor coatings on metal substrates are reviewed and an experimental investigation of two techniques is presented Firstly, the use of chemical binders similar to those used in thermal paints and, secondly, a new cost effective coating technique referred to as Electrostatic assisted Combustion Chemical Vapour Deposition (EACVD) which has been developed in the Materials Department at Imperial College. The latter allows thin layers of phosphors to be laid down without the need for a chemical binder and with close control of coating composition and thickness. To demonstrate the technique a 2 μm thick layer of Y₂O₃:Eu has been laid down on a nimonic substrate and a performance comparison made with a similar coating laid down using the alternative chemical binder technique. The EACVD coating has been shown to be more robust than those laid down with chemical binders whilst the coating shows similar temperature sensitivity