Abstract
Abstract: This work experimentally investigated the flow characteristics in a facility with randomly packed spheres at a low-aspect ratio of 4.4. Velocity fields in the near-wall region and in the pores between spheres were obtained by employing the matched-index-of-refraction (MIR) and time-resolved particle image velocimetry (TR-PIV) techniques for Reynolds numbers of 340, 520, and 720. From the obtained TR-PIV velocity vector fields, flow characteristics including first- and second-order statistics, such as mean velocity, root-mean-square fluctuating velocity, and Reynolds stress profiles, were computed. The effects of the wall enclosure and Reynolds numbers on the flow patterns were investigated by comparing the computed flow statistics and evaluating two-point cross correlations of the velocities measured adjacent to and far from the wall. Comparisons of the mean velocities, root-mean-square fluctuating velocities, and Reynolds stress component showed an increase in flow mixing and turbulent intensity in the gaps between spheres in the packed bed. The re-circulation-region sizes, however, were found to be independent from an increase in Reynolds numbers. Finally, flow modal decompositions, such as the proper orthogonal decomposition (POD) and dynamic mode decomposition (DMD), were applied to the vorticity fields extracted from sub-regions located near and far from the wall to reveal the most dominant POD and DMD flow structures. Graphical abstract: [Figure not available: see fulltext.].
| Original language | English |
|---|---|
| Article number | 127 |
| Journal | Experiments in Fluids |
| Volume | 59 |
| Issue number | 8 |
| DOIs | |
| State | Published - Aug 1 2018 |
| Externally published | Yes |
Funding
This research is financially supported by the U.S. Department of Energy, NEAMS project and under a contract DE-NE0008550. Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. Acknowledgements This research is financially supported by the U.S. Department of Energy, NEAMS project and under a contract DE-NE0008550. complexities associated with non-invasively probing the micro-scale flow phenomena within the complex network of void spaces between pebbles have hindered the efforts to characterize the underlying transport phenomena. The geometrical complexity inside a PBR represents a challenge to the experimental and computational efforts to construct transport models, which have been previously built upon volume averages of micro-scale parameters. However, these models should accurately capture the flow behaviors. Multiple-point or full-field measurements of flow characteristics and heat transfers at a high level of spatial and temporal resolutions are needed to fully map the complex flow patterns and to provide data at high spatial density to permit accurate volume averaging in the pebble bed. Researchers at Texas A&M University have conducted isothermal measurements of pressure drops and velocity fields in a pebble bed experimental facility to support the research on advanced nuclear reactors sponsored by the Department of Energy (DOE). The general purpose of these tests is to perform high spatial and temporal resolution measurements of the pressure and velocity fields within the pebble bed reactor. The experimental activities provide an experimental database of pressure and velocity measurements suitable for validating system-level codes and developing the CFD models that are currently considered for pebble bed reactors (Calis et al 2001; Van Staden et al. 2002; Du Toit et al. 2006).
| Funders | Funder number |
|---|---|
| U.S. Department of Energy | |
| Nuclear Energy University Program | DE-NE0008550 |
| Texas A and M University |