MEASUREMENT OF LOCAL PARTICLE VELOCITY

Particle velocity is one of the fundamental parameters in the study of fluid-particle flow systems. The initial application of optic fibers was devoted to the measurement of particle velocity in fluidization. Of interest to researchers are the instantaneous value, average value and profile of particle velocity, and at the same time, it is desirable to measure solids concentration as well as the distribution of particle diameters. The range of measurement should be as wide as possible, while the probe diameter should be as small as possible in order to minimize its influence on the flow field. Since the optic fiber probe can satisfy these requirements, it was widely applied and developed. 

Measurement of particle velocity with optic fibers is based on traversing a distance / by a particle between two known points over a transit time (or the time lag) t or the velocity V = l/t. However, the instantaneous velocity can be obtained only when the measuring distance between the two detector points is short enough to avoid interference. The diameters of the optic fiber and of the particle can both fit in the same range, and the response of probe against light signals is almost without time lag. Thus this kind of probes is very suitable for the measurement of instantaneous particle velocity. 

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Herbert, P.M., Gauthier, T.A., Briens, C.L., and Bergougnou, M.A., "Application of Fiber Optic Reflection Probes to the Measurement of Local Particle Velocity and Concentration in Gas—Solid Flow", Powder Tech., 80, 243 (1994). 

Herbert PM, Gauthier TA, Briens CL, Bergougnou MA. Application of fiber optic reflection probes to the measurement of local particle velocity and concentration in gas solid flow. Powder Technol 80 243-252, 1994. 

Radioactive tracers of various sizes and densities Measurement of local particle velocities in a cold CFB Weinell et al. (1995)... 

Guiraud P., Costes J., Bertrand J., Local measurements of fluid particle velocities in a stirred suspension, Chem. Eng. J. 68 (1997), p. 75-86... 

The flow patterns for single phase, Newtonian and non-Newtonian liquids in tanks agitated by various types of impeller have been repotted in the literature.1 3 27 38 39) The experimental techniques which have been employed include the introduction of tracer liquids, neutrally buoyant particles or hydrogen bubbles, and measurement of local velocities by means of Pitot tubes, laser-doppler anemometers, and so on. The salient features of the flow patterns encountered with propellers and disc turbines are shown in Figures 7.9 and 7.10. 

LDV was operated in the backward scatter mode to obtain simultaneous measurement of two-dimensional velocity components. The resultant signals are transferred to a data acquisition system and processed on-line by a computer. A frequency shifter is employed to measure vertical particle velocity in both directions. As shown in Fig. 9, the LDV measured local particle velocity agree well with those from the optical fiber probe (Yang et al., 1991). 

VELOCITY PATTERNS AND VELOCITY GRADIENTS. More details of the flow patterns, the local velocities, and the total flow produced by an impeller have been obtained by use of small velocity probes or by photographic measurements of tracer particles. Some of Cutter s results for a 4-in. flat-blade turbine in an... 

The application of optic fiber probes to the measurement of local concentration of solids and particle velocity, will be described below separately. 

Velocity and concentration profiles are two important parameters often needed by the operator of slurry handling equipment. Several experimental techniques and mathematical models have been developed to predict these profiles. The aim of this chapter is to give the reader an overall picture of various experimental techniques and models used to measure and predict particle velocity and concentration distributions in slurry pipelines. I begin with a brief discussion of flow behavior in horizontal slurry pipelines, followed by a revision of the important correlations used to predict the critical deposit velocity. In the second part, I discuss various methods for measuring solids concentration in slurry pipelines. In the third part, I summarize methods for measuring bulk and local particle velocity. Finally, I review models for predicting solids concentration profiles in horizontal slurry pipelines. 

To measure local particle velocity in slurry pipelines, Brown et al. (88) modified Beck et al. s (86, 87) conductivity method. They developed a new conductivity probe where four electrodes are mounted on an L-shaped probe. The probe has two field electrodes and two sets of sensor electrodes separated by a known distance (Figure 24). The probe is capable of measuring particle velocity in vertical and horizontal slurry... 

The temperature of the cylindrical sample surface is measured by means of thermocouples inserted in 0.5 mm diameter holes running parallel with the axis. The inlet and outlet temperatures of the coolant and its flow rate can be measured. A pitot tube located near the test surface measures the local gas velocity. Dew point measurements and particle size analysis, are carried out at the same time as the mass accumulation tests to provide additional relevant data. 

Laser Doppler velocimetry (LDV) enables measuring the local fluid velocities from the frequency of light reflection from fine tracer particles (called seed particles) in the fluid. LDV can also be used to detect the velocities of coarse particles or, with seeding, both gas and particle velocities. The wavelength of laser light reflected from a moving particle differs from that of... 

Mann and Crosby (1977) have employed a piezoelectric transducer for flow measurements in gas-solid flows. The principle of operation is based on the impact of individual particles with the transducer. This impact is recorded and the number of collisions per unit time can be transformed to a local particle velocity and solids flow rate (see Fig. 7-12). 

Old K, Walawender WP, EanLT (1977) The measurement of local velocity of solid particles. Powder Technol 18 171-178... 

The laser-Doppler anemometer measures local fluid velocity from the change in frequency of radiation, between a stationary source and a receiver, due to scattering by particles along the wave path. A laser is commonly used as the source of incident illumination. The measurements are essentially independent of local temperature and pressure. This technique can be used in many different flow systems with transparent fluids containing particles whose velocity is actually measured. For a brief review or the laser-Doppler technique see Goldstein, Appl. Mech. Rev., 27, 753-760 (1974). For additional details see Durst, MeUing, and Whitelaw, Principles and Practice of Laser-Doppler Anemometry, Academic, New York, 1976. 

The complete LDA system includes the appropriate transmission and detection optoelectronics, traverse mechanisms, computer-controlled signal processing, and a data acquisition and evaluation system. The LDA equipment is a powerful tool for the measurement of flow velocity and velocity fluctuation, as well as the local concentration of particles or droplets transported in the airflow. 

According to measurements made in the atmosphere, the Lagrangian time scale is of the order of 100 sec (Csanady, 1973). Using a characteristic particle velocity of 5 m sec", the above conditions are 100 sec and L > 500 m. Since one primary concern is to examine diffusion from point sources such as industrial stacks, which are generally characterized by small T and L, it is apparent that either one (but particularly the second one) or both of the above constraints cannot be satisfied, at least locally, in the vicinity of the point-like source. Therefore, in these situations, it is important to assess the error incurred by the use of the atmospheric diffusion equation. 

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