Simultaneous temperature and velocity imaging in turbulent flows using thermographic phosphor tracer particles
Creators: name: Fond, Benoit
Publisher: name: Imperial College London
Year: 2014

description: Combined measurements of velocity and temperature are essential to improve our understanding of turbulent flows involving heat transfer or chemical reactions. However, performing such measurements is a very difficult task. The presence of particles, which are seeded into the flow as tracers for the flow velocity, strongly interferes with classic optical thermometry techniques such as Rayleigh scattering. A review of the current approaches shows that a technique that can measure both quantities simultaneously, in two dimensions and over a wide range of flow conditions is yet to be found. An alternative approach to this problem, presented in this dissertation, uses tracer particles made of temperature-sensitive luminescent material, which are capable of also indicating the gas temperature. Thermographic phosphors are shown to be clear candidates for this concept. Made of ceramic material, they are chemically inert and survive low and high temperature environments. The temperature has a strong influence on the luminescence process allowing various ways to perform thermometry Currently, phosphors are used for surface temperature measurements, but a phosphor suitable for two-dimensional measurements in turbulent flows must meet stringent requirements in terms of luminescence properties. In this respect, the temperature dependence of the emission spectrum, a high quantum efficiency and a short lifetime are essential. Micrometre-size refractory particles are widely used for PIV and are able to follow the fluid motion without slip for a wide range of fluid velocities and turbulence intensities However, for the concept to be valid, the ability of phosphor particles to follow fluctuations in the gas temperature must be demonstrated. Using theoretical heat transfer models, it is shown that the temperature response of a particle is faster than its velocity response irrespective of the gas temperature. These response times have a quadratic dependence on the particle diameter so only small particles can be used. Various aspects of the practical implementation of the flow measurement concept, such as the excitation, particle seeding, detection, image processing and calibration, are considered, tested and developed, with the objective of providing high signal levels and to permit precise, accurate, and highly resolved measurements. In order to determine whether a sufficient signal level can be obtained for a reasonable particle seeding density, i.e. that does not have any effect on the gas properties, a particle counting tool is implemented. This system is used to characterise the phosphorescence intensity of 2 μm diameter particles made of BAM:Eu2+, a phosphor with very advantageous properties for flow measurements. It is shown that a seeding density comparable to that of conventional PIV and relatively small laser fluence provide sufficient signal levels for precise single shot measurements. The technique is demonstrated in a turbulent heated jet from 300 K to 700 K. Single shot measurements of temperature and velocity are presented with a single-shot, single-pixel temperature precision of 2-5 %, a temperature accuracy of 2%, and a spatial resolution of 400 μm. An additional concept is explored. By seeding two streams with different materials, the phosphorescence signal can be used to visualise the turbulent mixing between the streams. This concept is demonstrated in the same turbulent heated jet. Future developments and applications of the thermographic phosphor tracer particle concept are discussed. Owing to the very wide variety of thermographic phosphors, the results presented in this dissertation constitute a solid foundation for the expansion of this promising technique.