Session: 06-04: Heat Transfer in CSP Applications 2

Paper Number: 131247

131247 - Probing Thermal Transport in Fluidized Bed Using Modulated Photothermal Radiometry 

Abstract: 

Fluidized beds have been widely used in various thermal processes due to their excellent heat and mass transfer characteristics. In the concentrated solar power (CSP) applications, fluidized bed receivers and heat exchangers can be operated at high temperatures to obtain high particle-wall heat transfer coefficients (HTCs) with certain particle materials and sizes. However, the complexity of the fluidization process has made it difficult to gain a clear understanding of the heat transfer mechanisms. For example, while it has been recognized that the primary thermal resistance from a wall to a fluidized bed lies in the near-wall regime, this resistance has not been separately quantified from the total thermal resistance, as the state-of-the-art thermal metrology lacks spatial resolution. This lack of understanding has hampered the improvement of the overall HTC. This paper presents an experimental study on separately characterizing heat transfer in the near-wall regime and the bulk regime of a fluidized bed using modulated photothermal radiometry (MPR). The MPR is a non-contact frequency domain technique using an intensity-modulated laser as the heat source and the surface infrared emission as thermometry, which has been developed by us to measure the thermal transport properties of flowing liquids and moving particle beds in our previous studies. The thermal penetration depth of the laser heating is varied by controlling the modulation frequency of the laser, and thus the measurement can simultaneously resolve the thermal resistances of the near-wall regime and the fluidized bed. With the MPR technique, we measured fluidized silica sands with a mean diameter of 164 μm in a vertical channel at 300 ^oC and various gas velocities. Our results show that the near-wall thermal resistance is substantially increased with gas velocity and eventually becomes a constant after the particles are fully fluidized, which partially offsets the benefit of higher HTC brought by higher gas velocity. We also used the MPR to quantify the improvement in particle-wall heat transfer in an inclined channel. We found that an 8^o inclination towards the heat exchanging side led to a lower near-wall thermal resistance and a higher HTC at high gas velocities. This work demonstrates that the MPR technique is a useful tool to separate the important near-wall thermal resistance from a bulk particle bed, which not only advances our understanding of heat transfer in fluidized beds, but may also contribute to a better design of fluidized bed heat exchangers with higher HTC.

Presenting Author: Xintong Zhang University of California, San Diego

Presenting Author Biography: Xintong Zhang is a Ph.D. student at the Department of Mechanical and Aerospace Engineering, University of California, San Diego. He obtained his B.S. degree at the Department of Automotive Engineering, Tsinghua University in 2019. He is currently pursuing his Ph.D. degree in Prof. Renkun Chen's group. His major research topic is the experimental characterization and modeling of heat transfer in granular media.

Authors:

Xintong Zhang University of California, San Diego
Sarath Adapa University of California, San Diego
Zhiwen Ma National Renewable Energy Laboratory
Renkun Chen University of California, San Diego