Laser doppler measurements of flow in freeboard of a fluidized bed

Abstract

One of the factors which reduces combustion efficiency in a fluidized-bed combustor is the elutriation of fine coal particles from the freeboard above the bed surface. To our knowledge, data about the real local flow behavior of the two-phase flow in the freeboard are lacking in the literature. This paper concerns the measurement of the gas and the elutriated particle velocities in the freeboard of a cold fluidized bed using Laser Doppler Anemometry. A special electronic system has been devised to record simultaneously the gas and particle velocities and to indicate the size of the particles. In this paper the apparatus and some preliminary measurements are reported. The most significant observation is that the gas velocity profiles exhibit minima near the bed center and maxima near the walls.

Initial particle velocity spatial distribution from 2-D erupting bubbles in fluidized beds

“…Early measurements in fluidized beds were restricted to the freeboard region using point measurements techniques, such as X-ray photography [6], cine films [11], laser Doppler anemometry (LDA) [12] and hot wire anemometry [9]. These yields a time history in a point of the bed, normally far from the bed surface, from which mean and RMS-fluctuation velocities can be calculated.…”

Two‐phase flow behavior in the freeboard of a gas‐fluidized bed

“…Although Levy and Lockwood (1983) reported a "crater" type distribution across the freeboard, they did not make measurements at low freeboard levels. From Figure 7a it is seen that a t z = 7.4 cm, the maximum is on the centerline, indicating the migration of the bubbles toward the center as they move up in the dense bed.…”

“…The RMS fluctuation velocity follows a similar decay pattern and reaches a constant value at about the freeboard height where the local mean gas velocity reaches the superficial gas velocity. Levy and Lockwood (1983) reported that the axial mean gas velocity was constant. This is probably because they did not seed the air flow in the LDV measurements; the particles were too coarse to follow the gas fluctuations.…”

“…They did not measure the gas velocity or turbulent fluctuations. Levy and Lockwood (1983) were the first to use a nonintrusive technique to measure the gas velocity and the particle velocity fields in the freeboard simultaneously. They used a 5 mW helium-neon based laser Doppler velocimeter with a pedestal amplitude detection facility to discriminate gas and particle velocities.…”

Measurements and numerical predictions of gas vortices formed by single bubble eruptions in the freeboard of a fluidised bed

“…Further research by Levy and Lockwood (1983) explored the gas and particle velocities in the freeboard region using laser Doppler anemometry (LDA). Two distinct experiments were performed in a bubbling fluidized bed: single bubble release and free bubbling.…”