Investigation of Thermographic Phosphors for Gas-Phase Temperature Measurements in Combustion Applications

Witkowski, D. Investigation of Thermographic Phosphors for Gas-Phase Temperature Measurements in Combustion Applications. University of Wisconsin-Madison, 2017.

The feasibility of a planar gas temperature diagnostic, termed aerosol phosphor thermometry (APT), was investigated for combustion applications. APT has several advantages over other thermometry methods, such as the potential to measure both the reactants and products of a combusting flow, and the capability of providing simultaneous spatially-resolved planar temperature and velocity measurements. Unfortunately, thermal quenching of the phosphor signal due to nonradiative relaxation at elevated temperatures has limited the state-of-the-art for accurate single-shot APT to measurements below approximately 800 K. Therefore, the primary focus of this work was to establish a methodology that utilizes configurational coordinate diagrams in combination with host-referred binding energy diagrams to systematically select new phosphors for high-temperature thermometry applications. Oxide hosts doped with trivalent ions were investigated, and based on the analysis Ce3+ doped ortho-phosphates were selected for testing. All selected phosphors had high measured quenching temperatures (T50>800 K), partially validating the methodology. One particular phosphor, Ce:GdPO4, had a quenching temperature of T50=1000 K and demonstrated usable signal levels out to 1300 K, representing a substantial improvement on the current state-of-the-art from a temperature quenching perspective.

Following this, an experimental setup designed to characterize the properties of thermographic phosphors in an environment representative of APT applications was presented. Luminescence imaging and spectrally-resolved measurements of aerosolized phosphor particles in a seeded jet were presented. A significant result of this work was the ability to quantitatively assess systematic errors due to radiative trapping in the measured spectra of the furnace by making a head-to-head comparison with data collected in the jet. Finally, the current viability of APT for high-temperature applications was assessed by using the data gathered throughout this work to make predictions of achievable temperature precision using different thermometry strategies. Utilizing the spectral temperature-sensitivity, it was found that even the best performing phosphors examined were not sufficient for high temperature (T > 1000 K) single-shot measurements. As a result, alternative approaches using the temperature sensitivity of the luminescence signal level were investigated and shown to potentially offer significantly enhanced temperature precision, although these new proposed methods will need to be tested and validated in the future.