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

• Applicant: Professor Dr.-Ing. Frank Beyrau
• Subject Area: Technical Thermodynamics ; Energy Process Engineering
• Term: from 2019 to 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 427979038

Turbulent flows are inherently 3-dimensional. Over the past decade, the development of Tomographic Particle Image Velocimetry (PIV) has enabled three-dimensional velocity measurements, thereby facilitating tremendous progress in the understanding of turbulent flow structures. In many turbulent heat transfer processes, whether naturally occurring (e.g., natural convection in the ocean, atmosphere or mantle) or induced to improve the efficiency or reliability of devices (e.g., gas turbines and electronic circuits), knowledge of the velocity field alone is insufficient to unambiguously describe the flow, and simultaneous temperature measurements are highly desirable. This project proposes a novel concept for simultaneous three-dimensional temperature and velocity measurements based on combining thermographic phosphor particles with 3-dimensional particle-based velocimetry techniques. Unlike the three-dimensional scalar measurement concept based on tomographic reconstruction of volumetric signals from multiple views, here the temperature of individual micron-size thermographic phosphor particles is probed. Particle locations can be accurately determined from eithertriangulation or Tomographic-PIV reconstruction so that a 3-dimensional temperature field will be obtained. This concept allows high spatial resolution and only requires the addition of two imaging sensors and a UV laser to excite the particles and form spectrally filtered images of their luminescence for ratio-based thermometry. Furthermore, this additional view has a short depth of field that can be used to enhance the quality of the velocity measurements by decreasing the amount of ghost particles. In this project we will set-up a 6-camera system in combination with thick light sheets (~7-10 mm) using laser and camera equipment already available at the applicants lab. Initial measurements will be performed in a turbulent heated jet. Since this standard test case has well-defined isothermal regions, it can be used to assess the measurement performance in terms of temperature precision and detect potential positioning errors. First, imaging tools for low particle image densities (0.005 particles per pixels) will be developed using triangulation for particle positioning, and simple pinhole projections for luminescence signal assignment. Methods for measurements at higher particle image densities based on tomographic reconstruction algorithm, and more accurate imaging models will then be developed. As a demonstration, this 3D temperature and velocity diagnostic will then be applied to measure the wake behind a heated cylinder, providing the simultaneous visualisation of isothermal and iso-vorticity surfaces, and demonstrating the importance of such measurements for the understanding of complex 3-dimensional heat transfer phenomena. Such investigations are crucial, e.g. for the fundamental understanding of natural convection, or to the improvement of industrial cooling devices.

• DFG Programme: Research Grants
• Co-Investigator: Professor Dr. Benoît Fond