The fluidized particle-in-tube solar receiver concept is promoted as an attractive solution for heating particles at high temperature in the context of the next generation of solar power tower. Similar to most existing central solar receivers, the irradiated part of the system, the absorber, is composed of tubes in which circulate the fluidized particles. In this concept, the bottom tip of the tubes is immersed in a fluidized bed generated in a vessel named the dispenser. A secondary air injection, called aeration, is added at the bottom of the tube to stabilize the flow. Contrary to risers, the particle mass flow rate is controlled by a combination of the overpressure in the dispenser and the aeration air velocity in the tube. This is an originality of the system that justifies a specific study of the fluidization regimes in a wide range of operating parameters. Moreover, due to the high value of the aspect ratio, the particle flow structure varies along the tube. Experiments were conducted with Geldart Group A particles at ambient temperature with a 0.045 m internal diameter and 3 m long tube. Various temporal pressure signal processing methods, applied in the case of classical risers, are applied. Over a short acquisition time, a cross-reference of the results is necessary to identify and characterize the fluidization regimes. Bubbling, slugging, turbulent and fast fluidization regimes are encountered and the two operation modes, without and with particle circulation, are compared.

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The concept of solar receiver using fluidized particles as heat transfer fluid is attractive from the point of view of its performance but also of the material used. In this concept, the receiver is composed of tubes subjected to concentrated solar radiation in which the fluidized particles circulate vertically. Circulation in the tubes, immersed in a “nurse” fluidized bed, is ensured thanks to a controlled pressure difference imposed on the latter and secondary aeration. This ventilation located at the bottom of the absorber tubes makes it possible to control the fluidization regimes. The latter strongly influence the parietal heat transfers and therefore the performance of the receiver. In order to better understand the conditions of appearance of these regimes and to better identify them, a study at room temperature was carried out with a tube 45 mm in internal diameter and 3.63 m in height. The tube is instrumented with several pressure sensors distributed over its height. More than 170 experiments have been performed exploring wide ranges of particle and aeration flow rates, with and without particle circulation. Signal processing methods, classically used in the scientific literature of fluidized beds, are applied. Combined together, these methods have enabled the identification of bubbling, pistoning (of the wall and axisymmetric), turbulent fluidization and rapid fluidization regimes. The pooling of all this information allows the establishment of a diagram of the fluidization regimes and their transition, showing that the local slip velocity is the key parameter governing the structure of the flow.