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A single, fully wettable, neutrally buoyant, small radioactive particle and 16 scintillation detectors are utilized to monitor particle motion in gas-liquid bubble columns of different heights and diameters and at different gas superficial velocities. Instantaneous velocities, time-averaged velocities, Reynolds stresses and eddy diffusivities are computed from the particle position versus time data. Anisotropy, indicated by the data, was confirmed by Hurst's rescaled R/S analysis of the fluctuating velocities. For the radial direction, Hurst's exponent is about 0.5 in agreement with a random walk (diffusion)-type mechanism, while in the axial direction, Hurst's exponent of about 0.7 indicates the persistence of long-range effects. NotationD H Nexp RL R(t,s) rms S ( t, s) s t TL V' u~ u X(t) X*(t) position vector eddy dispersion coefficient, (cma/s); also column diameter, (m) Hurst's exponent number of independent experiments autocorrelation coefficient accumulated range of fluctuation velocities, (m/s) root mean square sample normalized standard deviation number of recorded (sampled) fluctuating velocities time, (s); also number of sampled fluctuating velocities integral time scale of turbulence, (s) Lagrangian fluctuating velocity, (m/s) superficial gas velocity, (m/s) index of summation random variable, i.e. fluctuating velocity component, (m/s) record of random variables (i.e. fluctuating velocities) defined by Eq. (5)

The development of a noninvasive PC-based computer automated radioactive particle tracking (CARPT) facility for investigation of phase recirculation and turbulence in multiphase systems such as fluidized beds and bubble columns is presented. In this facility, the motion of a single radioactive particle, which is dynamically similar to the recirculating phase, is monitored by an array of scintillation detectors which surround the test section. An on-line computer is used to map the flow field of the recirculating phase. The data acquisition is achieved by commercially available nuclear instrumentation via a modular, high-speed GPIB-CAMAC system through assembly language software. Using CARPT, solids’ motion in gas-fluidized beds and liquid motion in bubble columns have been investigated. The capabilities and versatility of the CARPT facility is described by illustrating some typical results for mean recirculation in gas-fluidized beds with and without internals and in a gas-liquid bubble column. The results include the mean circulation profiles and turbulence parameters such as the Reynolds normal and shear stresses and the turbulent eddy dispersion coefficients. Potential applications of CARPT technique to other recirculating systems are also discussed.

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