Development of the thermographic laser doppler velocimetry technique

Ojo, Anthony O. and Fond, Benoit and Abram, Christopher and van Wachem, Berend G. M. and Heyes, Andrew L. and Beyrau, Frank; (2016) Development of the thermographic laser doppler velocimetry technique. In: Proceedings of the 18th International Symposium On Application of Laser And Imaging Techniques To Fluid Mechanics. Lisbon Simposia, Lisbon, Portugal. ISBN 9789899877788

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Abstract

Simultaneous measurements of flow temperature and velocity are crucial in characterising turbulent heat transport processes. The advancement of particle-based velocimetry methods has provided both qualitative and quantitative description of turbulent flows. In recent studies, the use of thermographic phosphors particles as flow tracers further supports these advancements due to the additional temperature information they provide. These particles have been employed to obtain planar measurements of flow temperature and velocity in an approach termed thermographic particle image velocimetry. Similarly, a point-based measurement approach has been demonstrated to achieve simultaneous measurements of these flow vector-scalar properties. This paper further describes and characterises the point-based joint measurement technique called thermographic laser Doppler velocimetry (thermographic LDV) technique for flow temperature and velocity measurements. The flow metrology uses both Mie-scattered light and the optical properties of the phosphorescence emission that results from successive interactions between continuous wave laser light and individual 2 µm BaMgAl10O17:Eu2+ thermographic phosphor particles, which are seeded into the flow as a tracer. Photomultipier tubes (PMTs) are used to detect the signals collected from the measurement volume. The flow velocity is determined from frequency of the Doppler bursts obtained when particles traverses the fringes of two crossed visible laser beams as in conventional LDV. Luminescence in the form of Gaussian bursts that occurs after excitation of the same particles by an overlapped UV laser beam is simultaneously detected. Flow temperatures are evaluated from these acquired luminescence signals using the two-colour ratio, where two PMTs, each fitted with interference filters, transmits different parts of the temperature dependent emission spectral profile. The ratio of the two detected intensities has a monotonic dependence on temperature and is used to infer the particle temperature using previously acquired calibration data. Potential cross dependencies that affect temperature measurements such as seeding density and laser fluence are investigated. The technique is then applied to acquire combined vector-scalar profile measurement at the exit of turbulent heated jet to evaluate the accuracy of the temperature measurements. A deviation better than 2% is achieved between mean temperature profile measurements obtained using a thermocouple and the point-based technique. Thermographic LDV is shown to serve as a valuable tool to turbulent heat transfer research.