A new laser-based technique for simultaneous time-resolved point measurements of flow temperature and velocity using thermographic phosphor tracer particles

Abstract

Turbulent flows involving heat transfer or chemical reactions are important processes in the operation of numerous devices such as engines, and heating and cooling systems. Laser-based measurements of flow temperature and velocity have aided our understanding of the underlying flow physics in such processes. Recently, micron-size thermographic phosphor tracer particles, which are solid materials with temperature-dependent luminescence properties, have been exploited for simultaneous temperature and velocity imaging. However, the measurement strategy, which requires pulsed lasers to illuminate and excite the particles, is typically implemented at low temporal (~10 Hz) and spatial resolutions (>400 µm). Also, it is difficult to implement the technique for near-wall measurements or where limited optical access is required. In this dissertation, an alternative and complementary single-point measurement technique is presented, also based on thermographic phosphor particles. Here, particles seeded in a flow are probed individually when crossing a probe volume formed using continuous wave (CW) lasers. Using photomultiplier tubes to detect the scattering and luminescence signals from the same particle, velocimetry and thermometry are performed simultaneously, at sampling rates up to kHz’s and spatial resolution of 150 µm using a combined laser Doppler velocimetry and phosphor thermometry technique. The development of this measurement technique, based on the two-colour ratio strategy in phosphor thermometry is first described. The technique is demonstrated, using the phosphor BaAl10Mg17:Eu2+, in a heated jet from 293 - 670 K with temperature precision of 4-8%, and accuracy better than 2%. The utility of the technique is further demonstrated for near-wall measurement with accurate measurements performed as close as 200 µm from a heated surface. Another temperature evaluation strategy, which exploits the temperature dependence of the luminescence lifetime, by probing the phase-shifted luminescence from the same phosphor particles when using a modulated excitation source, is also described. The concept is demonstrated in a heated jet above 600 K, with a measurement precision as high as 1% obtained at 840 K. A discussion on applications and future developments of the concept of the ‘thermographic laser Doppler velocimetry’ is also provided.