Session: 05-08 Particles for Thermal Storage in CSP 1

Paper Number: 111527

111527 - Particle Dispersion and Heat Transfer in Fluidized Bed Heat Exchangers From Eulerian-Lagrangian Simulations 

High-temperature particle-based thermal energy storage (TES) at temperatures > 700°C is an effective method to provide around-the-clock heat from next generation concentrating solar power (CSP) plants [1]. However, challenges remain to designing cost-effective particle heat exchangers that extract the stored thermal energy into power cycle fluids for dispatchable electric power generation.  Narrow--channel counter-flow heat fluidized bed heat exchangers offer a promising approach to transferring heat out of hot particles, because the fluidized beds can achieve particle-wall heat transfer coefficients h_T,w as high as 1000 W m^-2 K^-1 with small particle diameters below 250 mm [1].  Particle-wall heat transfer is enhanced by the lateral dispersion across the depth of the bed channel, and experiments have shown that at intermediate excess fluidization velocities, h_T,w peaks at as much as 500% from packed bed values with bed depths of 12 mm [2]. However, particle dispersion over the longer length scales in the vertical direction negatively impacts particle-wall heat transfer by redistributing the thermal energy over the fluidized bed height and flattening the temperature.  This reduces the particle heat transfer by lowering the effective log mean temperature difference between the particles and the power cycle fluid.  Recent studies on finned walls have explored the effectiveness of extended surfaces to improve h_T,w and to suppress vertical dispersion to significantly improve particle-wall heat transfer.  A richer understanding of the particle-wall heat transfer mechanisms through detailed simulation can support better design of heat exchanger surfaces for increased particle-wall heat fluxes.

This presentation details a Multiphase Particle-In-Cell (MP-PIC) [3] method capable of simulating heat transfer in dense particle beds.  This simulation method is used to investigate the effects of particle dispersion on the particle-wall heat transfer in narrow fluidized bed heat exchangers with and without fins. The method employs a Lagrangian formulation to track as many as 1.5 million particles independently and an Eulerian formulation for the gas flow field and the particle stress used to model particle-particle interactions in order to enable simulation of the significant number of particles. The simulations explore heat transfer and dispersion with a fixed number of CARBOBEAD HSP 40/70 particles (mean diameter of 360 mm) and with a constant wall heat flux from one wall that is removed by an equal heat flux out the opposite (cooled) wall to keep the gas-particle system in thermal non-equilibrium.  Fluidizing air enters from the bottom of the bed at a fixed temperature and velocity, and fluidizes.  Bed simulations include those for flat walls and also for finned walls where the fins are modeled as internal boundaries that conform to the grid lines and have internal 1-D conduction.  Vertical particle dispersion primarily driven by the formation of bubbles is assessed through particle tracking and calculation of the dispersion tensor.  Comparisons of simulation with the smooth-walled and finned-wall channels show that the introduction of fins substantially reduces the size and life of bubbles, resulting in a substantial drop in the vertical particle dispersion coefficient.  Furthermore, the effect of the fins on the particle-wall heat transfer provides a basis for deriving fin effectiveness models for fluidized bed heat transfer and a basis for improving particle heat exchanger designs for next generation CSP plants. 

References:

[1] Ho, C.K., K.J. Albrecht, L. Yue, B. Mills, J. Sment, J. Christian, and M. Carlson, AIP Conference Proceedings, 2303(1): 030020, 2020.

[2] Fosheim, J.R.,  Hernandez, X., Abraham, J., Thompson, A., Jesteadt, B., and Jackson, G.S., AIP Conference Proceedings  2445(1): 160007, 2022

[3] Andrews, M.J. and P.J. O'Rourke, International Journal of Multiphase Flow, 22(2): 379, 1996.

Presenting Author: Federico Municchi Colorado School of Mines

Presenting Author Biography: Federico Municchi is a postdoctoral researcher at the Colorado School of Mines where he is studying high resolution computational simulations of multi-phase flows for particle energy storage and membrane distillation. Dr. Municchi received his Ph.D. in 2017 from the Graz University of Technology, after which he was a Postdoctoral Fellow at Purdue University and then the University of Nottingham before joining the school of Mines in Fall 2021.