Experimental and Numerical Study of Free-falling Particle-laden Flows Relevant to Solar Particle Receivers
Date
2023
Authors
Han, Shipu
Editors
Advisors
Tian, Zhao Feng
Sun, Zhiwei
Nathan, Graham
Sun, Zhiwei
Nathan, Graham
Journal Title
Journal ISSN
Volume Title
Type:
Thesis
Citation
Statement of Responsibility
Conference Name
Abstract
This thesis presents the experimental and numerical studies of particle-laden flow characteristics and particle impact processes relevant to multistage particle solar receivers. Specifically, the particle velocity, particle size distribution, and inter-particle and particle wall collisions are investigated in the particle-laden flows within the four-way coupling regime (particle volume fraction greater than 10−3). The multistage particle receiver is a novel concept of solar particle receivers that comprises several regularly spaced flow retention devices, such as funnels and ledges, to periodically collect, retard, and release the dense stream of particles falling within a cavity under gravity. The configuration of the multistage particle receiver leads to complex flow characteristics, which triggers the increasing demand for advanced measurement methods to provide a reliable and well-resolved characterisation of velocity and size distributions of particles in conditions with high particle loadings. Furthermore, collisions of the particleladen flow with the retention devices introduce complicated interactions between particles and surfaces, resulting in an increased need to better understand the particle behaviours in the near-wall region. Therefore, this thesis serves to fill these gaps in advance of the measurement and understanding of the particleladen flows relevant to the multistage particle receiver. The thesis is divided into three parts. In the first part, the thesis presents the development and applications of a novel micro-focusing particle shadowgraphy (μ-PS) technique for measurements of particle velocities and sizes within four-way coupling particle-laden flows. The μ-PS technique was applied to a laboratory-scale free-falling particle curtain, whose volume fraction is up to 0.08 in the four-way coupling regime. Measurements of the lateral profile of particle velocity through the curtain revealed for the first time that the velocity varies by 40% from the centre to the edges of the curtain. The technique has also been applied to a high-temperature, industrial-scale reactor for in-situ measurements of the size and morphology of limestone particles. This is the first time that in-situ measurements have been performed at this scale. A spatial resolution of 1.7 μm/pixel was achieved in particle-laden flows with a particle volume fraction of up to 0.02, which allows a lower detection limit of 15 μm and an upper detection limit of 1 mm. Comparison of the in-situ result to particle size distribution obtained with an independent ex-situ laser diffraction measurement showed good agreement, with a discrepancy of only some tens of micrometres. Particle agglomerates were also detected by the μ-PS technique, resulting in a second peak at approximately 400 μm. This additional measurement data is not provided by other conventional ex-situ methods. In the second part, the thesis presents experimental investigations of particleparticle and particle-wall collisions in the near-wall region of the multistage particle receiver. Measurements of particle rebound velocity and rebound angle were carried out for the impact of particle-laden flows onto an inclined surface, with the volume fraction of the flow varied from 0.0024 to 0.024 in the four-way coupling regime. Statistical results revealed that an increase in particle loading leads to an increase in the probability that a layer of particles is formed near the point of impact. This layer absorbs energy, leading to reduced rebound speeds. Furthermore, the layer increases the range of rebound angle for impact angles close to the normal direction. Tilting the plate to be more vertical increases the establishment and the influence of the sliding layer, which transfers momentum to the rebounding particles, leading to an increase in rebound speeds but a decrease in the range of rebound angles. In the third part, the thesis presents a numerical model with temperaturedependent functions of material properties to study the effects of temperature on particle-wall collisions for individual particles. The coefficient of restitution (CoR, also known as e), of Carbo CP particles impacting on stainless-steel and ceramic refractory surfaces was predicted and compared to experimental measurements at 25◦C, 500◦C, and 850◦C. Comparisons revealed that the model is reliable at ambient temperature to predict the normal CoR for glancing impacts (impacts close to the vertical direction), while accurate descriptions of material properties, particularly for the tensile strength, are needed to improve the reliability of prediction. The model was found to underestimate the normal CoR for glancing impacts at both ambient and elevated temperatures, with a maximum discrepancy of 0.4, which is attributed to the inappropriate assumption that normal collisions are independent of tangential impacts. On the other hand, comparisons reveal that the model is reliable to predict the tangential CoR for impacts in the sliding regime (more oblique impacts) when the coefficient of dynamic friction is provided. However, a large discrepancy was found for impacts in the non-sliding regime due to the significant influence of particle angular velocity, which was neglected in the model. In summary, this thesis addresses the complexities of particle-laden flows in multistage particle solar receivers, providing valuable insights into particle behavior, collision dynamics, and the impact of temperature. The novel μ-PS technique enables high-resolution measurements, while experimental investigations reveal the effects of particle loading on collision outcomes. The numerical model enhances our understanding of particle-wall collisions but requires further refinement, particularly in predicting CoR under different conditions. Overall, this research significantly contributes to the understanding and characterization of particle-laden flows in the context of multistage particle receivers, offering a foundation for future advancements in this field.
School/Discipline
School of Electrical and Mechanical Engineering
Dissertation Note
Thesis (Ph.D.) -- University of Adelaide, School of Electrical and Mechanical Engineering, 2023
Provenance
This electronic version is made publicly available by the University of Adelaide in accordance with its open access policy for student theses. Copyright in this thesis remains with the author. This thesis may incorporate third party material which has been used by the author pursuant to Fair Dealing exceptions. If you are the owner of any included third party copyright material you wish to be removed from this electronic version, please complete the take down form located at: http://www.adelaide.edu.au/legals