Temperature field measurements in liquids using ZnO thermographic phosphor tracer particles

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. The particle size requirements for accurate temperature tracing in turbulent flows are calculated using a numerical heat transfer model. Particle–water mixtures were prepared using ultrasonic dispersion and characterised using scanning electron microscope imaging and laser diffraction particle-sizing, indicating that the particle size is 1–2 m. The particle luminescence properties were characterised using spectroscopic and particle luminescence imaging techniques. Using 355 nm laser excitation, the luminescence signal is 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 ⁻¹ , corresponding to measured particle number densities 0.5–2.5 × \,10¹² 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 less pronounced using excitation at 266 nm 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.