Experimental and Numerical Investigation on Particle-gas Flows in Open Vortex-based Solar Cavity Receivers

Abstract

The thesis presents a systematic study of particle-gas flow behaviour within an open vortex-based solar cavity receiver, termed a Solar Expanding-Vortex particle Receiver-Reactor (SEVR) that is under development at the Centre for Energy Technology (CET) at the University of Adelaide. This class of solar receivers is configured to be used in a central tower system in the field of a concentrated solar power (CSP) system. Concentrated solar radiation is introduced through the open aperture of the receiver, heating the particles in a suspension to provide good heat transfer to the gas phase. The complexity of the gas and particle flow behaviour within it, such as particle egress through the aperture and convective heat losses under solar radiation, means many gaps in understanding remain. Potential safety risks, environmental pollution, and thermal efficiency issues all result from particle egress through the open aperture. Therefore, it is both essential and critical to mitigate, or even eliminate, particle egress from an open receiver. This thesis presents a detailed characterisation of the flow behaviour of the particle-laden flow within a lab-scale open vortex-based solar cavity receiver using both experimental and numerical approaches, development and understanding of a further novel control strategy for particle egress mitigation, and scale-up analysis for various types of heat transfer media, including, for example, steam. A laser-based Mie scattering method was used in the experiment, for which a laser sheet was aligned to be parallel with, and 3 mm downstream from, the aperture plane. The Mie scattered signals were used to obtain the trajectory component of each individual particle, and particle numbers which, in turn, allow the total number of egressed particles to be measured. Numerical simulations were performed with the computational fluid dynamics (CFD) package ANSYS/CFX. The CFD model was developed and validated against gas phase velocity profiles from previous measurements, together with particle planar velocities and particle egress rates, which were both measured in the current project. Therefore, a level of confidence in the present CFD model was obtained and the validated CFD model has been applied for further numerical analysis. Both the experimental and numerical results quantify the influence of the flow conditions and the receiver geometry on the particle egress through the aperture. Among all the parameters evaluated, the decreasing aperture-to-cavity diameter ratio from 0.5 to 0.25 was found to have a controlling effect on mitigating particle egress. However, the aperture diameter of a solar receiver is compromised by the incident radiation profile, which will cause heat spillage losses for a small aperture. The second most significant method in mitigating particle egress is to increase the outlet-to-inlet mass flow rate ratio of the gas phase, by applying an induced flow through the outlet. With the increase in this ratio, more gas flows carrying particles were drawn through the outlet port, resulting in less particle egress. However, the ratios of 1.1 and 1.3 led to a net inflow through the aperture. That is, external air is drawn through the aperture, entering the receiver, resulting in a potential reduction in the outlet temperature, and thereby a reduction in the thermal efficiency of the device. The novel concept of an aerodynamic barrier, termed a ‘buffer chamber’, was proposed, developed, and demonstrated to balance the trade-off between the particle egress through the aperture (hence the convective heat losses) and the optical/thermal efficiency of the receiver. It was found that the use of the buffer chamber mitigates particle egress by 86% and increases the outlet temperature of the gas phase up to 643 K by reducing heat losses through the aperture. Particle egress was found to be eliminated under certain conditions for some of the cases evaluated. On the other hand, incident solar radiation was inhibited, lowering both the thermal and the optical efficiencies of the system. A preliminary numerical analysis was performed for three configurations with an air-particle flow, with air only, or with steam as the heat transfer media, to investigate the technical feasibility of upscaling this receiver with the aerodynamic barrier to 50 MW. A more realistic solar radiation profile was adopted to mimic that from a heliostat field on a clear day. For the receiver configuration of an air-particle heater, the energetic thermal efficiency was compromised with the flow residence time and the mass flow rate, featuring a highest value of 83%. For a steam heater configuration, an increase in outlet temperatures and thermal efficiencies were identified with a maximum thermal efficiency of 88%. These results further indicate that an air-particle heating configuration and a steam heating configuration that both absorb solar radiation directly are suitable for the concept of such a vortex-based solar cavity receiver.