Characterisation of the luminescence properties of BAM:Eu2+ particles as a tracer for thermographic particle image velocimetry

Abstract

Thermographic phosphor particles are seeded into the flow as tracers for simultaneous temperature and velocity measurements in fluids. Several studies using different phosphors as gas-phase tracers have been published in recent years. However, little is known about their emission characteristics when they are dispersed as individual particles in the fluid. In this paper, the luminescence properties of BAM:Eu2+ particles, a phosphor with favourable characteristics (short luminescence lifetime, blue emission spectrum, high quantum efficiency), are thoroughly investigated in the gas phase. Using a recently developed particle-counting tool, the emission intensity per particle is measured over a wide range of conditions, including for various temperatures from 300 to 920 K, in air and in pure nitrogen. The luminescence emission per particle is shown to drop with temperature, but to be insensitive to the seeding density and to the oxygen content over the tested range. Together with a spectroscopic study, and a statistical error analysis, these results are used to predict the temperature precision of the technique under various conditions for different filter combinations and to assess the current upper temperature limit of this phosphor for practical applications. Potential additional sources of uncertainty are also investigated, including cross-dependencies of the measured intensity ratio on the seeding density, excitation fluence and oxygen partial pressure in the gas phase. Only a weak dependence on the laser fluence is observed, while the measured intensity ratio is shown to be insensitive to both seeding density and the oxygen volume fraction. Finally, the saturation behaviour of the phosphorescence emission is examined, through theoretical considerations and measurements performed with different excitation schemes in an attempt to increase signal levels. In conclusion, this paper confirms that BAM:Eu2+ is a very suitable tracer for measurements in turbulent flows up to 900 K.