Heat transfer in packed beds of crushed rock
The use of packed beds of crushed rocks with air as the heat transfer medium has been proposed as a cost-effective, widely available method to store thermal energy at high temperatures. To design such a system, it is important to understand the properties of the crushed rocks, as well as the thermal performance and pressure drop characteristics of the packed bed. Various parameters influence the thermal performance of a packed bed, such as flow velocity, bed porosity, particle shape, particle size, particle roughness and particle aspect ratio. In this study, the thermal performance of the packed bed of crushed rocks is evaluated while specifically investigating the influence of the flow direction/ particle orientation through the packed bed. The results are intended to provide experimental data for validating computational fluid dynamics (CFD) simulations of packed beds of crushed rocks, with the ultimate goal of improving their design. In order to keep most particle parameters constant during all tests and due to the challenges associated with generating a suitable mesh for CFD simulations of irregular crushed rocks in a packed bed, a representative particle shape was required. This representative shape needed to preserve key geometric parameters such as volume, surface area, true sphericity, inscribed radius ratio, and aspect ratio. To achieve this, a set of 100 crushed rock particles was 3D scanned, and the average geometric parameters calculated, for which an ellipsoidal particle with similar geometric parameters was selected. An experimental setup was developed that enables non-invasive testing of the heat transfer coefficient and allows the packed bed to be rotated to vary both the azimuth and elevation angles, while maintaining a constant bed length. The experimental results showed that the particle orientation/ flow direction does not significantly influence the heat transfer coefficient in packed beds. The maximum difference observed in the heat transfer coefficient between all flow angles was only 12-13 %, occurring between the Reynolds numbers of 1600 and 3200. The experimental uncertainty in the heat transfer coefficient was ± 5.14 %. Based on the experimental data, a new Nusselt number correlation was developed. This correlation compared well with previously well-known Nusselt number correlations for spherical particles in packed beds, but also suggests that ellipsoidal particles yield slightly higher Nusselt numbers at high Reynolds numbers.
Networked Digital Library of Theses and Dissertations