Development and Application of Methods to Measure Temperatures of Flowing Particles in Suspension

Abstract

This thesis reports on the development and application of a new method for the measurement of particles transported within a moving fluid and subjected to high fluxes of radiant heating. The comprehensive understanding of heat transfer in particle-laden flows is important as it is a key factor in enabling optimisation of various industrial and scientific applications such as combustion, mineral processing plants, and pharmaceutical manufacturing, as well as aid in the development of new technologies based on the two-phase flow. However, one of the major factors that limits the furtherance of understanding of the field is the difficulty in measuring temperatures of moving, micron-sized particles. Previously, most publications on heat transfer in particle-laden flows focus on gas temperature measurements, where the temperatures of particles are inferred through fundamental heat transfer equations. However, this technique is not applicable in systems where a large disparity exist between the gas and particle temperatures, and does not take into account inter-particle relationships which could have a significant effect on the overall heat transfer where the interparticle spacing is sufficiently low. In order to distinguish between the particle and gas phase temperatures, a radiative heat source capable of delivering continuous heat fluxes of up to 36.6MW/m2 in the form of a Solid-State Solar Thermal Simulator (SSSTS) was used throughout this dissertation. This is because the SSSTS operates at a wavelength of 910nm, which is only absorbed by the particle phase and not the gas phase. Importantly, the operation of the SSSTS at this wavelength does not interfere with the excitation signal (355nm) used in the LIP technique. However, the performance of the SSSTS is not well understood due to the system being the first of its kind. Chapter 4 of this dissertation addresses this by characterising in detail the SSSSTS. The next part of this dissertation describes the development and application of single-shot, nonintrusive particle temperature measurement techniques based on laser-induced phosphorescence (LIP), a thermometry that makes use of the phosphorescent emission properties of thermophosphors (TPs) governed by the temperature-dependent Boltzmann distribution. Here, ZnO:Zn TPs were selected to be used as they have the highest temperature sensitivity below 625°C. The TPs were suspended in unsteady flow in an optically-accessible fluidised bed and subjected to high radiative heat fluxes of up to 21.1 MW/m2. Two types of thermometry are reported – with Chapter 5 describing the development of an in-situ, areaaveraged, temporally-resolved particle temperature measurement technique by analysing the change in phosphorescent emission spectra of the selected TP with respect to wavelength, collected using a fibre-optic cable connected to a spectrometer; and Chapter 6 detailing a single-shot, planar particle temperature measurement. For the planar thermometry, the phosphorescent emissions of the TPs were collected using a single ICCD camera fitted with an image splitter and two interference filters specifically selected. Each measurement derives from two 15mm × 10.8mm images collected simultaneously to avoid errors associated with timedelays and/or angular distortions. The resultant spatial resolution for each image was 51pixels/mm, with an average of 30 particles recorded within the imaging region. It was demonstrated that the particle temperatures measured with the LIP technique was found to be approximately 44°C higher on average than the gas temperatures measured with a thermocouple in the same system. A strong dependence of heat flux, as well as particle attenuation (mass loading) on particle temperature was also reported. Additionally, a maximum particle temperature rise of 350°C was recorded with a heat flux of 21.1 MW/m2, where the maximum particle residence time in the heating region is 0.05s. The next section of this thesis details the study of application of the developed thermometry technique in a laminar particle-laden jet flow issued from a 12.8mm pipe downwards into a wind tunnel and the particles radiatively heated by the SSSTS. The measured data were analysed by comparison with the results from a simple first-order analytical model that considers the radiative heating, convective cooling, radiative heat loss and heat gain of a single particle. It was found that heat flux, particle concentration and to a lesser extent, particle diameter all affect particle temperatures. At low heat fluxes, 𝑄̇𝑟𝑎𝑑 ≤ 6.1 MW/m2, particle concentrations and temperatures were found to be higher in jet edge, consistent with previous investigations. At heat fluxes above that, where 𝑄̇𝑟𝑎𝑑 > 6.1 MW/m2, thermophoresis was observed, as evidenced by the migration of the smaller particles to the jet edge where the local temperature is lower. The effect of buoyancy was also observed at 𝑄̇𝑟𝑎𝑑 ≥ 20.6 MW/m2, as evidenced by two distinct regions of high particle temperatures upstream from the heating region (one at the jet axis, and one at the jet edge). These results were presented in Chapter 7 of the present dissertation.