Accurate surface temperature measurements present vast difficulties in numerous technical applications, especially when imaging fast temperature changes. One example is spray-induced surface cooling, where temperature variations occur on the sub-millisecond timescale. Phosphor thermometry relies on changes in the photoluminescence properties (typically the lifetime or the emission spectrum) of phosphor materials for temperature determination. For temperature imaging in situations where short measurement durations are critical such as on fast moving objects or short and intense heat transfer events, the spectral method is preferred. However, in many situations, fluorescence signals over a broad spectral range originate from fuel, flame radicals, optical windows or even the chemical binder used to coat the phosphor material and may interfere with the measurements. In this work, we investigate a delay strategy for the spectral method that prevents interference from fluorescence sources by using a phosphor with a microsecond-range-lifetime. By applying a short delay (<1 µs), between excitation and detection, short-lived interfering fluorescence signals can be efficiently avoided. The temporal and spectral content of the fluorescence from the substrate, binder and fuel are investigated with a photomultiplier tube and a spectrometer respectively. The delayed gating strategy is then implemented for temperature imaging of the wetted side of a spray impinged surface with a tin-doped phosphor, (Sr,Mg)3(PO4)2:Sn2+; selected for its suitable lifetime (26 µs @ 300 K) and high temperature sensitivity. Calibration measurements for coated surfaces show that by avoiding fluorescence from the binder, the temperature sensitivity is improved from 0.3% K−1 to 0.8% K−1 at 293 K. Furthermore the calibration curve repeatability between two similar coatings is enhanced. Finally, temperature measurements of the cooling induced by a fluorescing impinging gasoline spray was successfully performed with this strategy owing the same level of measurement precision (0.5 K) as with a non-fluorescing fuel (n-hexane UV grade).

The wall cooling induced by spray impingement is investigated using phosphor thermometry. Thin coatings of zinc oxide (ZnO) phosphor were applied with a transparent chemical binder onto a steel surface. Instantaneous spatially resolved temperatures were determined using the spectral intensity ratio method directly after the injection of UV-grade hexane onto the surface using a commercial gasoline injector. The investigations showed that 2D temperature measurements with high spatial and shot-to-shot precision of, respectively, 0.5 and 0.6 K can be achieved, allowing the accurate resolution of the cooling induced by the spray. The presence of a liquid film over the phosphor coating during measurements showed no noticeable influence on the measured temperatures. However, in some cases a change in the intensity ratio at the spray impingement area, in the form of a permanent "stain", could be observed after multiple injections. The formation of this stain was less likely with increasing annealing time of the coating as well as lower plate operating temperatures during the injection experiments. Finally, the experimental results indicate a noticeable influence of the thickness of the phosphor coating on the measured spray-induced wall cooling history. Hence, for quantitative analysis, a compromise between coating thickness and measurement accuracy needs to be considered for similar applications where the heat transfer rates are very high.

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