Asymptotic Height-Dependent Heat Transfer in Vertical Granular Flows Due to the Janssen Effect

Concentrated solar power (CSP) incorporates direct thermal energy storage, providing scalable and cost-effective renewable generation with enhanced grid dispatchability. To achieve higher system efficiency and lower levelized cost, the next-generation CSP needs to operate above 700 ^oC, posing grand challenges on heat transfer media selection, component design, and characterization techniques. Granular material has emerged as a promising stable and economical high-temperature heat transfer medium in CSP applications, which carries the concentrated solar radiation to particle bed heat exchangers. Two key thermophysical properties dictate particle-to-wall heat transfer in granular flows: bulk effective thermal conductivity (k_eff) and near-wall thermal resistance (denoted as an effective air-gap thickness D_air). When particles enter the heat exchanger, there forms a long and narrow granular column filling the particle channel. Due to the existence of particle-wall frictional interactions, the normal stress will saturate exponentially rather than increase linearly along the column, known as the Janssen effect in granular physics. Since particle packing structure is sensitive to subtle pressure changes, the Janssen effect will lead to variations of k_eff and D_air along the flow, which has never been studied previously but its knowledge is important for practical particle-bed heat exchangers with lengths on the scale of meter.
In the presented work, we utilized the modulated photothermal radiometry (MPR), a non-invasive frequency-domain technique, to measure the k_eff and D_air of flowing ceramic particles in a 1-meter-long vertical channel at 300 ^oC. We found asymptotically increasing k_eff and decreasing D_air along the flow direction owing to the Janssen effect. The Janssen effect is confirmed by similar asymptotic trends from granular column apparent mass measurements as well as separate MPR heat transfer measurements on particle beds under various compressive pressures. Furthermore, local heat transfer coefficient along the 1-meter-long channel was estimated based on spatially resolved k_eff and D_air by the MPR. This work reveals for the first time the impact of the over century-old Janssen effect on heat transfer in granular media, and can lead to a better understanding of heat transfer in granular flows.

References:
[1] Zhang, Xintong, et al. "Micromechanical origin of heat transfer to granular flow." Physical Review E 109.4 (2024): L042902.
[2] Zhang, Xintong, et al. "Impact of Janssen Effect on Thermal Transport in Granular Flow." Under review. 2024.