Diagnostics of particles and droplets under high-flux radiation

Abstract

Radiation heating is important in many engineering processes including gasification, fuel production, materials processing, and concentrated radiation collection. Its high efficiency enables it to consistently drive thermal reactions operated under extreme conditions. High-flux radiation delivers energy directly to the target bodies and improves heat transfer efficiency. Traditional radiation simulators, which have been normally used to radiatively heat the targets, usually have some major difficulties with the power control and projection. Therefore, this thesis utilizes a multi-diode laser system to provide a uniform and well-controlled radiation flux up to 28.87 MW/m2. It serves to demonstrate the radiation heating of particles, aggregates, tablets, and droplets. Laser, optical, and conventional techniques are applied to reliably record the temperature changes in the target bodies, determine the thermal characteristics such as heating rates, and investigate the heat transfer process and optical properties of the selected targets. According to the researched targets, this thesis is divided into four parts: In the first part, the well-controlled radiation was employed on two types of phosphors, BaMg2Al10O17:Eu (BAM) and ZnO:Zn, to investigate temperature imaging of mobile aggregates under high fluxes and to evaluate the radiation trapping at room temperature. For temperature imaging, the real-time temperatures of fluidized BAM aggregates were recorded using the planar laser-induced phosphorescence technique. The maximum temperature of an aggregate was 1063 K, and the largest average temperature was 723 K. For non-thermal radiation trapping, the optical properties of BAM and ZnO:Zn were derived from the developed collision-based Monte Carlo ray-tracing model. The extinction coefficient of ZnO:Zn was 50% smaller than that of BAM. Radiation trapping was predicted in the direction of the radiation path. For ZnO:Zn, 70% of flux was scattered when the distance increases to 0.5, while for BAM, the distance was 0.15. In the second part, the high-flux radiation was applied on biomass tablets to investigate the thermal processes and improve the efficiency of biomass utilization. To avoid any physical contact, each biomass tablet was suspended using a home-built acoustic levitator. Three different thermal processes, i.e., the initial fast-heating process within 1 s, the secondary slow-heating process from 1 to 3 s, and the final ignition starting at 3 s, were identified from the time-resolved temperature profiles. The ignition temperature was determined around 430 to 450 K. In the third part, the high-flux radiation was tested on acoustically levitated liquid droplets. A newly developed hydrochar slurry fuel was proved to be pseudo-plastic and desirable for potential liquid fuel applications. The maximum heating rate of a single 50 wt.% slurry droplet within the first 0.15 s was measured above 400 K/s. The maximum surface ignition time of suspended and irradiated hydrochar slurry droplets was 0.37 ± 0.01 s, and the surface ignition temperature was 375 ± 15 K, independent of heat flux. In the fourth part, the high-flux radiation was employed in the measurement of emissivity and absorption function of two solid particles, Al2O3 and SiC, at elevated temperatures. The values of emissivity were determined to be 0.75 ± 0.015 for Al2O3 and 0.92 ± 0.012 for SiC from 300 to 1200 K. The absorption function of Al2O3 at 910 nm was nonlinearly increased with the temperature, while that of SiC dropped slightly. The modeling results of micro-sized particles suggest that the temperature rise-time of the two materials have significantly different dependencies on the radiation flux. The outcomes of this thesis will benefit studies done on heat transfer, optical properties, and thermal processing in the energy field, especially those high-temperature reactions associated with high-flux radiation heating.