The objective of the present work was to research the storage behavior of a fluidized bed filled with a granular phase change material (PCM) with a small particle diameter (d p = 0.54 mm). The performance of the fluidized bed was compared to that of well-known storage methods such as fluidized beds with sand and packed beds based of sand and PCM. For this purpose, heating experiments were conducted in a cylindrical bed with air as the working fluid.The influence of the bed height and flow rate on the storage and recovery efficiencies of the fluidized bed of PCM was analyzed. Additionally, the stability of the PCM during various charging-discharging cycles was studied.The results indicate that this PCM is an alternative material that can be used in fluidized bed systems to increase the efficiency of storing thermal * Corresponding author. NOTICE: this is the author's version of a work that was accepted for publication in Chemical Engineering Journal. Changes resulting from the publishing process, such as peer review, editing, corrections, structural formatting, and other quality control mechanisms may not be reflected in this document. Changes may have been made to this work since it was submitted for publication. The cycling test shows that the PCM is stable under bubbling conditions up to 15 cycles, which corresponds to approximately 75 hours of continuous operation. A definitive version was subsequently published in Chemical Engineering

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This work presents an experimental study to determine the capacity of a phase change material (PCM) in granular form to be used in fixed and bubbling fluidized beds for thermal energy storage. The experimental measurements are focused on determination of the heat transfer coefficient between a heated surface immersed in the bed and the granular PCM. The flow rate is varied to quantify its influence on the heat transfer coefficient. The PCM used is Rubitherm GR50 with a phase change temperature of approximately 50• C. The PCM is available in two different particle sizes, 0.54 mm and 1.64 mm, of which the finer is used in the fluidized bed and the coarser is used in the fixed bed. In addition, the results obtained for the PCM are compared with the heat transfer coefficients measured for sand, a material commonly used for thermal storage. * Corresponding author. NOTICE: this is the author's version of a work that was accepted for publication in Applied Thermal Engineering. Changes resulting from the publishing process, such as peer review, editing, corrections, structural formatting, and other quality control mechanisms may not be reflected in this document. Changes may have been made to this work since it was submitted for publication. A definitive version was subsequently published in Applied Thermal Engineering 78 (2015) pp. 373-379. doi:10.1016/j.applthermaleng.2014.12.044 In comparing the heat transfer coefficients for fixed and fluidized beds, the heat transfer coefficients in the fluidized bed with PCM are nearly three times higher than those for the fixed bed at the same gas flow rate. This increase in the heat transfer is a result of two main factors: first, the continuous renewal of PCM particles from the heated surface when they are fluidized, and second, the large quantities of energy in latent form absorbed by the PCM. In the fixed bed there is no renovation of particles, consequently only a small percentage of particles are able to change its phase. Hence, there is no increase in the heat transfer coefficient due to this fact.

Diameter and velocity of bubbles from a three-dimensional two-fluid model simulation of a cylindrical fluidized bed are presented. Two methods for obtaining the bubble size and velocity are compared: i) estimation from the chord lengths and velocities of the detected bubbles using information from two virtual voidage probes (pierced bubble method) and ii) calculation from the bubble volume and velocity directly obtained from the instantaneous 3D voidage field (tomography method). The Maximum Entropy method (MaxEnt) is employed to convert probability density functions of chord lengths into the corresponding diameter distributions. The algorithm for the direct evaluation of the bubble volume and velocity, based on the tomography reconstruction of the 3D field, is explicitly explained and used to evaluate the results obtained from the virtual void probe signals. Results show a good agreement between the bubble sizes obtained using the MaxEnt treatment of the chord lengths and the directly obtained bubble sizes, which confirms the robustness of the MaxEnt method to infer bubble behavior in 3D bubbling beds. In particular, the mean bubble diameter obtained with the MaxEnt method applied to chord lengths was less than 4.5% different to the result from the tomography reconstruction. It was found that the bubble velocities obtained from virtual voidage probes are higher than the bubble velocities calculated with the tomography method, but the differences were not greater than 17% in the worst case. The probability density functions of bubble size and velocity obtained with the two methods were similar in terms of the location of the most probable values and the variation of the distribution with the distance to the distributor.

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