Hybrid Radial-Axial Flow for Enhanced Thermal Performance in Packed Bed Energy Storage

Abstract

In this work, a hybrid radial-axial (HRA) system is used to store thermal energy in a packed bed. The heat transfer fluid (HTF) is delivered via a perforated radial pipe placed at the center of the packed bed along the axial length. Hot fluid flows from the center toward the wall through the holes (like other radial systems), but then leaves via the traditional axial flow exit, creating the HRA flow configuration. A computational fluid dynamics (CFD) model is used to analyze the thermal performance of the packed bed during the charging process utilizing the new HRA system. Alumina beads of 6 mm were filler materials and air was HTF with inlet temperature of 75°C for proof of concept. The present paper focuses on two aims: (1) utilizing CFD models to analyze flow and temperature profiles in the packed bed; (2) comparing the model results to experimental results published in a previous HRA flow study and to traditional axial flow. Two HRA configurations were considered based on previous experimental designs, one with uniform holes in the central pipe (R₁) and one with gradients in the hole sizes to promote even flow from the central pipe into the bed (R₂). The numerical results agree with the experimental results in both cases. The HRA system performance depends on the flow profile created by the hole designs, and it can perform better than the axial flow depending on the design of the radial pipe. Design R₂, which promotes even flow from the central pipe into the bed, has higher charging efficiency than standard axial flow methods. For HRA design R₂ at 0.0048 m³/s (7 SCFM, standard cubic feet per minute), numerical results for charging efficiency were 75.5% versus 73.8% for traditional axial flow. For HRA design R₂ at 0.0061 m³/s (9 SCFM), numerical charging efficiency was 80.5% versus 78.1% for traditional axial flow. These results are consistent with experimental data.