Recent developments in phosphor thermometry

Reliable, fast and accurate temperature measurements are very important in numerous applications in science and technology. This is particularly so in energy converting prime movers, where extreme temperatures are required to realise high efficiency, thereby necessitating data concerning component and flow temperature to ensure reliable and safe operation. While many temperature measurement techniques exist, phosphor thermometry provides unique capabilities particularly suited to extreme environments. The development of the technique has enabled remote temperature measurements of surfaces [1, 2] in the presence of reactions or under erosive/corrosive conditions at high and low extremes of temperature, and of fluids in transparent gases or liquids [3]. Phosphor thermometry is an optical technique that can be used to record live, temporally resolved data but, where optical access is restricted or conditions too harsh, sensors can be adapted to 'remember' the conditions to which they have been exposed so that temperature data can be extracted later offline, using so-called thermal history sensors.

With phosphor thermometry applications growing in number and diversity in recent years, an international conference on phosphor thermometry was called to establish a focus for workers in the field to come together for dissemination of technical developments and novel implementations of these methods and to elucidate heat transfer, fluid-mechanical and other phenomena studied with them. The ICPT 2018 was held at Strathclyde University in Glasgow, Scotland during 25 July–27 July 2018. The conference served as an international forum for presentation, exchange of ideas and creation of knowledge in recent advances on various aspects of materials, theories, analyses, and applications of phosphor thermometry on surfaces and in fluid flows. The focus of the symposium was to report the latest progress in developing measurement techniques and to exchange new information among scientists and engineers working in universities, industries and other fields.

This special feature of Measurement Science and Technology contains eight papers that were selected from contributions to ICPT'18 that focus on new advancements in phosphor thermometry measurements and analysis techniques.

The paper by Allison [4] provides a fascinating history of the phosphor thermometry technique and shows that what may be thought of as an emerging measurement technique in fact has origins that stretch back at least as far as the 1930s. Moreover, interestingly, modern adaptations of the technique such as surface temperature mapping or flow temperature measurement are shown to have been envisaged by some of the earliest workers. Allison identifies three eras in the evolution of the technique. The first, 1930–1980, established the concept and included a limited range of esoteric applications. The second, 1980–2000, is characterised by advances in the instrumentation used (lasers, LEDs, fibre optics, CCD, etc) and the establishment of the core techniques (lifetime decay and intensity ratio measurement)—lead in no small part by Allison himself. The final era, 2000–present, has seen the evolution of new applications such as flow temperature measurements and new embodiments of the technique, such as in thermal history sensors. Indeed, as the paper shows, the technique is now so well established and diverse that a conference dedicated to its development and application was warranted.

Hertle et al [5] present a study of phosphor performance and design based on dysprosium-doped garnet and perovskite lattices. They highlight the importance of energy transfer to optically active dopants and the implications for excitation wavelengths, and further demonstrate how activator species, lutetium in this case, can be used to improve luminescent yield. By tuning the material composition and selecting the right excitation wavelength, the authors demonstrate a significant increase in the dynamic range of the phosphors considered. The paper serves to emphasise the fact that the study of phosphors in the context of temperature measurement is still at an early stage so that improvements in performance and, indeed, new candidate phosphor materials may be fruitfully pursued.

Sutton et al [6] describe an emerging approach for two-dimensional temperature mapping based on measuring the lifetime decay time constant rather than by a spectral approach based on the intensity ratio of two or more lines in the emission spectrum. The latter suffers from the need for precise alignment in the images from two separate cameras, which is difficult to achieve. Two-dimensional lifetime mapping uses a single camera, recording two or more images at different times during the decay and enabling a ratio to be formed that reflects the time constant. Sutton's embodiment of this technique enjoys the further advantage of using a simple monochrome camera that records images from sequential decays. They realised temperature measurements with an uncertainty of 0.5 °C at up to 450 °C and are seeking to extend the range to 1000 °C as part of an ongoing project.

Oketch et al [7] describe an application of the phosphor thermometry technique for surface temperature measurement on the cylindrical surface of a pipe under flame impingement with heat flux determined by simultaneous temperature measurement by thermocouple on the water-cooled inner surface. Surface temperature measurements at up to 600K were made using a ruby phosphor synthesised and deposited using the sol-gel technique and excited using a green LED. The work provides a clear demonstration of one of the key advantages of phosphor thermometry, which is that the emission/signal can easily be distinguished from background radiation such as that from a flame. The paper offers insights into the effect on stagnation point heat flux of different flame stabilisation mechanisms and points to how phosphor thermometry can be part of the standard toolkit of workers interested in the features and behaviour of reacting flows.

Mendieta et al [8] have developed a method to avoid the interference from short-lived fluorescence sources when performing fast, two-dimensional, phosphor-based surface temperature measurements. They showed that by applying a very short delay between detection and excitation (<1 µs) while using a short decay time phosphor, a good balance between signal collection, short measurement integration time and complete avoidance of short-lived interference sources can be obtained. This strategy was applied for the investigation of fuel-spray impingement cooling of a surface using a tin-based phosphor. Strong fluorescence from gasoline (using a 266 nm wavelength excitation) was efficiently avoided, obtaining the same spatial temperature precision (0.5 K) as when using non-fluorescent n-hexane for the same testing conditions. Additionally, suppression of fluorescence emissions stemming from the chemical binder, used to coat the phosphor material to the surface, resulted in improvements in both the coating's temperature sensitivity, 0.3%/K to 0.8%/K at 293 K, and temperature calibration repeatability.

Witkowski et al [9] describe a fluid thermometry method to exploit the overall decrease in luminescence intensity with increasing temperature caused by thermal quenching. To account for variation in the local particle seeding density in the flow, they propose referencing the luminescence signal against the scattering signal from the same particles. They show that uncertainty arising from the dependence of the luminescence/scattering ratio on the particle size does not influence the measurement precision using reasonable seeding densities of 200 particles mm⁻³. Demonstration measurements in a heated jet indicated an error of 15 K at 820 K, a significant improvement over two-colour measurements at these temperatures. With additional optimisation of the Ce-doped phosphors to delay thermal quenching to even higher temperatures, this method shows promise for precise measurements over 1000 K for the study of low-temperature reacting flows.

Stephan et al [10] employ the structured laser illumination planar imaging technique (SLIPI), in order to reduce the impact of multiple scattering and near-wall reflections on planar gas-phase phosphor thermometry. Conventional light-sheet excitation for flow thermometry can lead to errors due to the effect of multiple scattering of the signal from the seeded particles in the flow. The authors compare the effect of different reconstruction strategies using one-pulse and two-pulse SLIPI approaches. Furthermore, as the SLIPI approach comes with additional experimental complexity and reduced signal strength, a gradient-based threshold algorithm is developed in order to reduce the effect of multiple scattering when using conventional, non-structured light-sheet excitation. The effects of the different approaches are tested, evaluated and compared using a canonical flow configuration.

Fan and Hochgreb [11] investigate potential errors in the reconstruction of temperature fields using the abovementioned SLIPI technique. They show by means of a theoretical analysis that the multiple scattering contribution, as calculated by the SLIPI reconstruction, is always overestimated due to the non-linear emission behaviour of the phosphorescence and particle image diffraction.

The guest editors are very grateful to the authors for their effort and time spent in preparation of their manuscripts. We would like to thank the reviewers, who have voluntarily contributed their time for the evaluation of submitted work and which resulted in significant improvements to the manuscripts. Finally, the guest editors would like to thank the entire editorial board of Measurement Science and Technology for their patience and professional support.