Optical fiber probe particle velocity measurement

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

Horio et al. (1992a) O3 Optical fiber probe applied to measure particle and gas velocities simultaneously ozone injection upstream of two optieal probes, tracer gas velocity measured using UV light, optical fiber and photomultiplier... 

Figure 3.31 Single fiber optic probe used for particle velocity measurement. (From Sekoguci et al., 1985. Copyright 1985 by American Society of Mechanical Engineers, New York. Reprinted with permission.)...

Another method of particle velocity measurement is the double tip optical fiber, which consists of two groups of optical fibers with a certain distance between each other, as shown in Fig. 6. When a solid particle passes the probe tip, two signals with a certain time interval in between would be detected. This time lag can be computed by means of the cross-correlation function (Qin and Liu, 1982) ... 

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). 

Both the transmission-type probe and the reflection-type probe, need be calibrated for their measuring range in local solids concentration. The calibration of optic fiber probes is known to be a difficult problem. Calibration methods fall into two categories the first is to calibrate a probe against agitated or fluidized liquid—solid systems the second is to use particle free-fall in gas—solid systems or the traditional pressure drop method for fluidized solids the third is in a flow system with particle density deduced from mass flux of particles and measurement where phase velocities were nearly equal. 

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. 

FIGURE 4-24 Different types of fiber optic probes for particle velocity measurement. 

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. 

Patrose B, Caram HS. Optical fiber probe transit anemometer for particle velocity measurements in fluidized beds. AIChE J 28 604-609, 1982. 

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. 

Rathbone et al. (1989) used a similar probe to determine particle velocity parallel to the surface. The difference is that the central fiber is the sensor of the Fiber Optic Doppler Anemometry (FODA), and it transmits light from a 5 mW He-Ne laser. Particles illuminated by the central fiber scatter light to the surrounding fibers as they passed the probe. When the particles passed over two fibers in succession, the transit time at cross correlation maximum can be obtained. In principle, by correlating the strongest signals from the fibers, it should be possible to determine the direction or different components of particle velocity. However, the measurement was carried out in a two-dimensional fluidized bed, and relatively few data were obtained. 

FIGURE 4-37 Typical particle velocity distribution as measured by a five-fiber optic probe (Sand, dp of 150 im, by free fall system). 

The mean particle velocity is estimated as Mp = AL/tinax) where r ax is the delay time at which the 0i2(t) reaches a maximum. To determine the velocity, one also needs the effective separation distance between the two probes. This generally differs from the actual separation distance and needs to be obtained via calibration. Different probes have been used to measure local time mean particle velocities in CFB risers. Hartge et al. (1988) used a pair of 1 mm diameter fibers separated by 4.4 mm. Horio et al. (1988, 1992) employed a probe with two detection fibers and a single emission fiber in between. To capture individual particle motion, Ohki and Shirai (1976) recommended that the cross-correlation is improved if the diameter of the optical fiber is the same as the particle diameter, with the separation distance between the receiving fibers of similar magnitude. 

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