In-cylinder thermographic PIV combined with phosphor thermometry using ZnO:Zn

Two-dimensional thermographic particle image velocimetry (T-PIV) is presented for the in situ measurement in optically accessible internal combustion (IC) engines. Temperature and velocity measurements are combined using thermographic phosphor particles as tracers for PIV. For three commercially available phosphors (BAM:Eu2+, ZnO, and ZnO:Zn), temperature sensitivity, luminescence intensity at high temperatures and laser-fluence dependence were evaluated for phosphor-coated surfaces in a high-temperature cell. ZnO:Zn was identified as the best-suited candidate for engine in-cylinder measurements and further analyzed in the aerosolized state at temperatures up to 775 K to generate calibration data required for signal quantification in engine experiments. T-PIV was successfully applied in the IC engine to simultaneously obtain instantaneous two-dimensional velocity and temperature fields using the intensity-ratio method. Despite a measurement uncertainty (±1σ basis) of only 3.7 K at 317 K (1.2%) to 24.4 K (4.2%) at 575 K, this technique suffers from low signal intensities due to thermal quenching at increasing temperatures, which leads to reduced accuracy as the piston approaches top dead center. Thermographic measurements were successful to visualize local temperature changes due to evaporative cooling after fuel injection. The measured mean gas temperatures agreed well with zero-dimensional simulations that use additional wall-temperature measurements from thermographic phosphor measurements based on the lifetime method as input for heat transfer calculations.

Three-dimensional temperature and velocity measurements in fluids using thermographic phosphor tracer particles

Many flows of technical and scientific interest are intrinsically three-dimensional. Extracting slices using planar measurement techniques allows only a limited view into the flow physics and can introduce ambiguities while investigating the extent of 3D regions. Nowadays, thanks to tremendous progress in the field of volumetric velocimetry, full 3D-3C velocity information can be gathered using tomographic PIV or PTV hence eliminating many of these ambiguities (Discetti and Coletti, 2018; Westerweel et al., 2013). However, for scalar quantities like temperature, 3D measurements remain challenging. Previous approaches for coupled 3D thermometry and velocimetry combined astigmatism PTV with encapsulated europium chelates particles (Massing et al., 2018) or tomographic PIV with thermochromic liquid crystals particles (Schiepel et al., 2021). Here we present a new technique based on solid thermographic phosphor tracer particles, which have been extensively used for planar fluid temperature and velocity measurements (Abram et al., 2018) and are applicable in a wide range of temperatures. The particles are seeded into a gas flow where their 3D positions are retrieved by triangulation from multiple views and their temperatures are derived from two-colour luminescence ratio imaging. In the following, the experimental setup and key processing steps are described before a demonstration of the concept in a turbulent heated jet is shown.

Thermal and optical characterization of up-converting thermographic phosphor polymer composite films

Up-converting thermographic phosphors are of significant interest due to specific advantages for temperature measurement applications over traditional contact-based methods. Typically, infrared excitation stimulates visible fluorescence only from the target phosphor and not the surrounding medium. This is in contrast to ultraviolet excitation which may also produce interfering luminescence from cells and other biological tissue in the vicinity, for instance. When traversing a material, usually infrared losses due to scattering and absorption are less than for ultraviolet wavelengths. An example is human skin. This investigation follows logically from earlier efforts incorporating thermographic phosphors into elastomers and aerogels and their function as a reusable temperature sensor has been previously demonstrated by the authors. Layered phosphor/PDMS/aerogel composites are also currently under investigation by the authors for heat flux sensing. For maximum utility and understanding; physical, optical and thermal properties are characterized over a wide range of temperatures. Y2O2S:Yb,Er and La2O2S:Yb,Er up-converting phosphor composites with a fixed doping concentration were synthesized for this study and fully characterized as a function of temperature. The excitation/ emission characteristics of the powder alone and the prepared composites were investigated between -50 °C and +200 °C in an environmental chamber and the decay behavior of each sample type was measured. Here, the authors report on decay behavior and emission intensity of the PDMS composites as a function of temperature. Results were compared with powder –only parameters and are reported here.