The photoluminescence properties of the phosphor (Sr,Mg) 3 (PO 4) 2 :Sn 2+ were investigated for luminescence thermometry in fluids. The luminescence emission intensity at 300 K is on the order of 10 6 photons per particle and the lifetime is 26 µs. With increasing temperature, the wide emission band exhibits a pronounced blue shift which can be exploited for temperature imaging using a two-colour ratiometric approach. The measured temperature sensitivity in an imaging configuration is 0.6%/K at 300 K. The T 50 quenching temperature is 650 K, similar to the phosphor BaMgAl 10 O 17 :Eu 2+ , but the estimated temperature sensitivity at 700 K is a factor 7 higher (0.36%/K). Moreover, the excitation laser fluence has a negligible effect on the measured temperature (0.6 K for a 10% change in the fluence), a five-to-tenfold improvement over phosphors previously investigated for fluid thermometry. The phosphor (Sr,Mg) 3 (PO 4) 2 :Sn 2+ therefore offers significant improvements for thermometry applications in turbulent flows in the 300-900 K range.

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compared to 355 nm. A single-shot, single-pixel temperature precision of ±2-3 • C (1σ ) can be achieved over a temperature range spanning 50 • C. The technique was applied to a convection experiment to measure the temperature fields in a buoyant thermal plume, demonstrating the suitability of these imaging diagnostics for the investigation of thermal convection and heat transfer.

Temperature field measurements in liquids are demonstrated using zinc oxide (ZnO) thermographic phosphor particles. The particles are added to the liquid as a tracer. Following laser excitation, the temperature-dependent luminescence emission of the particles is imaged and the temperature is determined using a two-colour intensity ratio method. In this work, the particle size requirements for accurate temperature tracing in turbulent liquid flows are calculated using a numerical heat transfer model. Spherical 5 µm-diameter particles are shown to have a 95% response time <35 µs in water between 0-100 °C, suitable for accurate tracing in turbulent flows. A method for preparing particle-liquid mixtures using ultrasonic dispersion was developed. The dispersions were characterised using scanning electron microscope imaging and laser diffraction particle sizing, indicating that the particle size is 1-2 µm. The particle luminescence properties were investigated using spectroscopic and particle luminescence imaging techniques. Using 355 nm laser excitation, the luminescence signal is shown to be the same in water and in air. However, 266 nm excitation is used to avoid spectral overlap between Raman scattering from water and the detected ZnO luminescence emission. It is shown that 266 nm excitation can be used for temperature measurements in water using mass loads as low as 1-5 mg/L, corresponding to measured particle number densities 0.5-2.5x10 12 particles/m 3. In this range, the measured intensity ratio is independent of the mass load. The dependence of the intensity ratio on the laser fluence is also less pronounced using excitation at 266 nm compared to 355 nm. A singleshot, single-pixel temperature precision of 2-3 °C can be achieved over a temperature range spanning 50 °C. The technique was applied to a convection experiment to measure the temperature fields in a buoyant thermal plume, demonstrating the suitability of these imaging diagnostics for the investigation of thermal convection and heat transfer.

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