Non-intrusive temperature measurement of particles in a fluidized bed heated by well-characterized radiation

Attempts to determine in-situ and instantaneous temperatures of micron-sized particles and aggregates within a tempestuous environment has proven to be mind-boggling. Presently, most of the available techniques employ a system that provides the direct temperature measurement of particles suspended in a turbulent flow. However, these techniques have limited spatial and temporal resolution. Furthermore, recently published studies have directed that the effect of particle mass loading on the solid-phase heat transfer should not be neglected. Recent technological advances have led to the development of laser diagnostic measurement techniques. These technique provides the in-situ and high-speed measurement of an increasing number of parameters with high temporal resolution. Specifically, the laser-induced phosphorescence (LIP), which utilizes the temperature-dependent properties of thermophosphors. Unfortunately, this technique is yet to employed to resolve particle temperature in an environment in which the particles are at a considerably different temperature from that of the adjacent flow, notably, in a system where the particles are concurrently heated with a source of high flux radiation.

To this note, a team of researchers led by Professor Zeyad Alwahabi at the University of Adelaide in Australia reported a direct, non-intrusive, temporally resolved, in-situ particle temperature measurement simultaneously with radiation attenuation. To be specific, they aimed at demonstrating a temporally-resolved, volume-averaged application of LIP with particles heated with well controlled high flux radiation. Moreover, they purposed to experimentally verify the relationship of gas and particle temperature, where a two-phase thermometry is performed simultaneously. Their work is currently published in the research journal, International Journal of Multiphase Flow.

The research technique employed entailed the application of LIP to provide non-intrusive, temporally resolved and in-situ measurement of suspended particles as a function of heat flux and radiation attenuation. Next, the researchers performed excitation at 355 nm with a repetition rate of 1.67 Hz. The particles were then transported with dry air within an optically-accessible fluidized bed and heated with a well-defined source of high-flux radiation from a 3 kW solid-state solar thermal simulator radiation. Lastly, the researchers simultaneously measured the temperatures of the particles and gas with the former being determined from thermo-phosphorescent emissions following the excitation at 355 nm.

The authors observed that the mass loading, in addition to the irradiation flux, had a major impact on the particle’s temperatures. In addition, the researchers were able to establish the relationship between particle and gas temperature. Moreover, they also noted that the maximum particle temperature was 465.8 °C recorded at a specific radiation flux and 40% radiation attenuation. Eventually, it was found that, on average, the measured gas temperature was 44 °C lower than the measured particle temperature.

To sum up, Professor Zeyad Alwahabi and his colleagues study successfully demonstrated temperature measurements of irradiated particles in a fluidized bed using laser-induced phosphorescence. The main observation was that the irradiation flux and mass loading were seen to play vital roles in particle and gas temperature rise. Altogether, the technique portrayed here is applicable to planar imaging and therefore has potential to be employed together with other diagnostics methods, such as a particle image velocimetry.