(19c) Bubbling and Slugging of Geldart Group a Particles in Small Diameter Columns: Experiments and 3D Numerical Simulations

In order to increase Concentrated Solar Power (CSP) plants efficiency, current research aims at increasing operating temperatures of the heat transfer fluid, and consequently increasing the efficiency of the power cycle. In addition, heat transfer fluids classically used for CSP have several limitations: restricted range of operating temperature, safety issues, corrosion and maintenance costs. Since solid particles are not subject to such limitations, provide a good thermal capacity, and a low-cost heat transfer and storage; an innovative alternative is to use an air-fluidized dense particle suspension as the heat transfer fluid, also called Upflow Bubbling Fluidized Bed (UBFB) [1][2]. The Next-CSP (http://next-csp.eu) European project aims at improving the reliability and performance of CSP plants with this new technology, by demonstrating the concept feasibility with a 4MW_th working pilot, and upscale it to the industrial scale by designing a 150MW_elplant.

The key to the proposed process is the effective heat transfer from the solar heated surfaces to the heat transfer fluid, which is directly linked to the gas-solid hydrodynamics. Moreover, the use of a UBFB receiver limits the operable particles down to Geldart group A or near-B particles because of their low minimum fluidization velocity: since the fluidization air will exhaust the UBFB at the bed temperature, sensible heat loss increases significantly with increasing air velocity [3].

Heat Transfer Coefficient (HTC) was measured up to 400-1100 W/m²K during on-sun experiments with small 1m height receiver tubes [4] while industrial scale receiver tubes will reach up to 10m height. In addition, hydrodynamics in the UBFB are expected to be strongly dependent on the tube height. Indeed, it has been shown that common unrestrained bubbling can be transformed into slugging when the bubble size approaches the size of the tube [5], which is known to strongly reduce the HTC.

In order to better understand the process and to optimize the design of the solar receiver it is of paramount to know how particles behave inside the bundle of small tubes within it and consequently being able to limit the formation of slugs.

Experimental measurements of fluidization of Geldart group A powder in small internal diameter tubes were conducted [5]. Then three-dimensional numerical simulations were carried out using the unstructured parallelized multiphase flow code, NEPTUNE_CFD, based on a Eulerian n-fluid modelling approach, but first simulations had trouble to replicate the experimentally measured bed height due to an inaccurate inclusion of the Particle Size Distribution (PSD) span and particles non-sphericity into calculation model [6].

In this paper, a new methodology was developed to include the impact of the PSD span into the expression of the drag force. In addition, the impact of particles irregular non-spherical shape [7] was also taken into account into the drag force term calculation. Both these corrections were based onto experimental measurement of morphology and laser diffraction particles size analysis. Also, the impact of non-sphericity onto particles-particles collisions and wall-particles collisions was evaluated.

Those three-dimensional numerical simulations were compared with experimental measurements of bed expansion, slugs size and frequency, and with high speed imaging analysis of the particles local motion. Fluidization velocities ranged from 7xU_mf to 56xU_mf, corresponding to the velocity range reached in the industrial scale receiver. Therefore numerical models were found in good agreement toward experiments concerning their ability to represent wall effects and slugging.

Acknowledgements

This project has received funding from the European Union’s Horizon 2020 research and innovation program under grant agreement No 727762, Next-CSP project. This work was granted access to the HPC resources of CALMIP under the allocation P1132 and CINES under the allocation gct6938 made by GENCI.

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