Thermographic particle image velocimetry: from phosphorescence to incandescence
Creators: name: FAN, LUMING
Publisher: name: Apollo - University of Cambridge Repository
Year: 2019
Subjects: PIV
phosphorescence
LII
Laser diagnostics
combustion
heat transfer
two phase flow

description: Thermographic PIV is a promising technique that enables simultaneous temperature and velocity imaging in flows with intentional seeded phosphor particles. This is highly attractive to researchers in fluid mechanics and combustion, as it directly visualizes the heat and mass transfer process in turbulent flows/flames. However, three problems for this technique are: (a) it requires two lasers and three cameras running simultaneously, making it a high-cost technique; (b) several recent studies reported the multiple scattering effects for gas-phase phosphor thermometry, which may severely bias the temperature measurement for certain flow configurations; and (c) the phosphorescent emission disappears at high temperature due to thermal quenching, which limits the temperature measurements to mostly non-reacting cases below 1100 K. This dissertation is aimed at providing solutions to the issues described above. To reduce the cost of the current thermographic PIV setup, a simplified version is proposed which uses a double-pulsed laser with UV capacity and two CCD cameras operating in the double-frame mode. This experiment proves that, apart from Mie scattering, phosphorescence image pairs can also be used to perform cross-correlation and calculate the vector field. Therefore both velocity and temperature field can be extracted from phosphorescence emissions excited by a single laser (UV-PIV). Thermographic PIV with this simplified setup is demonstrated on an electrically heated air jet, and 3 K accuracy is achieved in the core region of the jet, by comparing with a thermocouple scan. A novel calibration process is also proposed to eliminate the influence of non-uniform laser profile on the temperature measurements. The same technique is also applied to visualize heat transfer in an impinging jet. By correlating the instantaneous gaseous temperature fields with the averaged \textit{Nu} profiles derived from the wall temperature, the role of vortical structures in heat transfer is investigated and discussed. During the application of thermographic PIV, the problem of multiple scattering emerged and has been reported by several studies, especially for cases where an excessive seeding is used. Multiple scattering was found to reduce the spatial resolution and bias the temperature measurements. A recent study demonstrated that the Structured Laser Illumination Planar Imaging (SLIPI) technique could effectively remove multiple scattering and near-wall effects from the LIP image. However, it is well known that the emission spectrum of some most commonly used thermographic phosphors is sensitive to the changes in laser fluence, whilst SLIPI intentionally modulates the laser profiles and thus may bring in uncertainty into the temperature retrieval. This has yet not been discussed in the literature. In this dissertation, a numerical analysis is conducted, by generating artificial laser induced phosphorescence images, to investigate the effects that SLIPI may have on the temperature measurements. To implement simultaneous temperature and velocity measurements in flames, an entirely new approach of thermographic PIV is proposed in this dissertation. This new version is based on laser-induced incandescence (LII), rather than phosphorescence. Submicron black particles are seeded into a flame, and further heated by a high-energy top-hat laser sheet to several thousands kelvin. The particle temperature $T_\mathrm{p}$ can be measured by two-color pyrometry, where the temperature increase $\Delta T$ due to laser absorption can be determined by conducting an {\em in-situ} calibration. Thus the local temperature $T_0$ can be indirectly determined by subtracting $\Delta T$ from $T_\mathrm{p}$. The same particle can also be used as PIV tracers. The concept and fundamentals of this new thermographic PIV approach are described in this thesis. The combination of LII and PIV is also applied as a tool to measure the gas-phase velocity in a two-phase flow, which is a canonical problem for multiphase flow studies.
description: Part of the research presented in this thesis was funded by University Technology Malaysia (UTM) under grant number RG84263