Characterization of the Solids Flow in Circulating Fluidized Beds By High-Frequency-Radar Measurements

This work presents results from the latest developments in the use of the radar technology for the diagnosis of solid particles flow in fluidized beds. We apply a radar system at submillimeter wave-frequencies (0.34 THz) operated in the frequency-modulated continuous-wave pulse-Doppler mode, which was previously shown to be able to measure concentration and velocity of small solid particles. The radar system used provides a time resolved (up to 30 Hz) measurement along the beam (4–6 cm wide and 0.5 cm longitudinal steps, yielding a spatial resolution of 6.3–14 cm³). This work further explores the capabilities of the radar technology with the aims of both expanding the validity range of the technique and examining the solids flow in specific regions of a circulating fluidized bed. With this purpose, measurements along the vertical and horizontal directions in riser (3.1 m high and 0.45 m² in cross section, using 106 µm glass beads as bed material) were performed, for which fluidization velocity and total solids inventory were varied to achieve a variety of flow conditions.

We report the following achievements:

• A more accurate evaluation than the previous work with respect to the agreement between the solids concentration values measured by means of radar and pressure, especially for concentration levels >1·10^-1 m³/m³ for which the penetration of the radar is shortened. This was attained by performing radar measurements along the horizontal direction in the riser.
• Confirming the validity of the radar technique for measurement of local concentrations by combining measurements performed in the vertical and horizontal directions.
• Calculation of solids mass fluxes in both horizontal and vertical directions at specific locations in the bed by combining the measurements of solids concentration and distribution of solids velocity.
• Evaluating the solids concentration, velocity and flux profiles near the vertical walls and close to the riser exit.