Abstract
Recent studies have used structured laser illumination planar imaging (SLIPI) combined with phosphor thermography to remove multiple scatter effects and near-wall reflections, which lead to biases in temperature measurements and reduced spatial resolution. We show that for the typical non-linear pump-signal range under which thermographic phosphors are used, errors may arise in the reconstruction of the temperature field using SLIPI. In this study, synthetic laser induced phosphorescence (LIP) images are generated numerically by adapting the synthetic PIV image generator for the purpose. The simulations are combined with phosphorescent signal yield functions obtained from experimental data to investigate the application of SLIPI to gas-phase phosphor thermography. We conclude that whilst SLIPI is effective in removing scattering noise for phosphors for which the two-colour signal ratio is insensitive to the laser fluence, it creates a bias in the temperature measurement otherwise. We also show that the extent of multiple scatter in LIP images is always overestimated by SLIPI, owning to the non-linear emission behaviour and particle image diffraction.
Highlights
An alternative solution to multiple scattering in laser-induced phosphorescence is to combine it with structured laser illumination planar imaging (SLIPI), as demonstrated by Zentgraf and co-workers [4]
Based on previous literature and the authors’ experience in conducting LIP, the application of SLIPI may be necessary when: (a) reducing seeding density is not an option, for example where temperature gradients are high; (b) the experimental test conditions lead to a large scale seeded co-flow region which may introduce severe multiple scattering; and (c) direct incidence of laser light on surfaces cannot be avoided by adjusting the incidence angle or by other strategies such as using UVabsorptive paint
The purpose of this study is to investigate the effects of SLIPI signal retrieval on the true signal when realistic curves for phosphorescence yields are used, rather than the effectiveness of SLIPI in removing IDC
Summary
The excitation and emission spectra of some phosphorescent tracers may partially overlap, and the photons in the overlapping spectral range emitted from the laser plane may be re-absorbed by particles outside the excitation plane and lead to a secondary emission, which may carry different temperature information, yet contribute to the total signal. An early report on the multiple scatter effect for phosphor particles in dispersed form [2] showed how the phosphor signal could be captured from outside the illuminated area by a trimmed laser sheet, suggesting that multiple scattering leads to reduced spatial resolution. It was reported that the seeding density within the cross-flow should be 10 times lower than that in the jet so as to create an unbiased measurement Both signal to noise ratio and spatial resolution severely deteriorate at such low seeding rates. It was suggested by [6] that an in-situ calibration at exactly the same seeding density and flow condition would, in principle, allow a compensation for the effects of multiple scattering, this may not always be possible for internal flows
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