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.
Highlights
Optical measurement techniques have a huge impact on our understanding of complex turbulent flow phenomena
Using a conventional particle image velocimetry (PIV) approach, visible laser light scattered by the particles is recorded to determine the velocity field
The deviations between the two cases are within the experimental uncertainty. This is a significant advantage of this tracer over most laser-induced fluorescence (LIF) tracers, which are strongly quenched by oxygen, see e.g. [17]
Summary
Optical measurement techniques have a huge impact on our understanding of complex turbulent flow phenomena. For all the phosphors, and other phosphors which could prove suitable for gas temperature measurements, there are no quantitative studies on the luminescence of dispersed particles in the gas phase This is in direct contrast with the well-understood physical processes of organic tracers used for flow measurements, see, e.g. the review article by Schulz and Sick [17], and so quantitative investigations are needed to compare different phosphors, predict signal levels for various practical applications and analyse any cross-dependencies of the measured temperature on other parameters such as the excitation fluence or seeding density. 4.1), including various temperatures up to 920 K, different bath gases (air and nitrogen) and excitation fluences Based on these results, and on emission spectra recorded at various temperatures, predictions of the achievable temperature precision are performed for several filter combinations in order to determine the upper temperature range of this phosphor
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