A comparison of laser Doppler and single-fibre reflection probes for the measurement of the velocity of solids in a gas-solid circulating fluidized bed

Abstract

In this study, an attempt has been made to compare an intrusive solids velocity measurement technique with the laser Doppler anemometer (LDA) which is a typical non-intrusive solids velocity measurement technique. Single-fibre reflection (SFR) probe and LDA measurements were directly compared under identical operating conditions in the upper dilute zone of a circulating fluidized bed (CFB). This experimental unit was a cold model CFB (inner diameter of the riser, 0.4 m; height, 15.6 m). The measurements were taken at a height of 6 m above the distributor. Quartz sand with a mean diameter of 166 µm was fluidized at a superficial gas velocity of 4 m s −1. The cross-sectional average solids volume concentration was 1.2 vol.%. The highest instantaneous local solids concentration was 5 vol.% close to the riser wall. LDA measurements were taken with a miniaturized measuring head (diameter, 14 mm) operating in the backscatter mode and inserted in a probe casing (diameter, 22 mm). The focal point of the laser beams was located at a distance of 19 mm from the probe casing window. This allows the LDA measuring head to be moved through the fluidized bed and valid LDA measurements to be acquired inside relatively dense suspensions. The fibre-optical probe measurements were taken with a two-channel SFR probe (fibre: diameter, 600 µm; probe: diameter, 8 mm; centre-to-centre distance between the two sensing surfaces, 2.3 mm). The measurements at a superficial gas velocity of 4 m s −1 showed mean solids velocities between 10 m s −1 upwards and 3ms −1 downwards. The LDA system faces problems in the regions of higher solids concentration located close to the riser wall where data acquisition rates decrease considerably. The good agreement between the local average solids velocities acquired with the two systems proves that intrusive probe measurements cause negligible distortion of the solids velocity Pattern in a CFB environment.