Radioactive Particle Tracking (RPT) and Benchmarking for High Velocity Conditions
2.2 Radioactive particle tracking technique
2.2.1 Methodology
this technique is very expensive and bulky. Radioactive particle tracking (RPT) offers all the requirements of this work and also well-proven technique. Therefore, RPT is used for this investigation. In the subsequent sections, methodology, hardwares and softwares for implementation of RPT are discussed. Further, experiments to quantify the accuracy of RPT technique are also discussed.
distance - count map is generated. In experiments, tracer particle is free to move, similar to the phase of interest and it radiates gamma-rays (photons) during its path which is absorbed by the detectors placed around the vessel. The best possible position of tracer is obtained by comparing the counts recorded by the detectors during experiments and from the distance-count map generated during the calibration. Since the frequency of data acquisition is known, time differencing of the two successive instantaneous particle position gives instantaneous velocity (Lagrangian velocity). Ensemble averaged velocity is obtained by dividing the whole column into the small cells, much like the way it is done in computational fluid dynamics (CFD). The difference between instantaneous velocity and ensemble averaged velocity gives fluctuation velocity. RPT measurement not only provide, instantaneous, mean and fluctuation velocities, it also provides PDF of instantaneous velocities at all the locations. Flow chart explaining the methodology of obtaining various quantities is given in Figure 2.6. Various turbulence parameters calculated from the velocities are given in Table 2.1. In addition to turbulence parameters, dispersion coefficient, autocorrelation coefficient, hurst exponent, time of flight, circulation time distribution, RTD can also be calculated (Larachi et al., 1997; Degaleesan, 1997; Roy et al., 2000; Bhusarapu, 2005).
Table 2.1 Quantities calculated from RPT experimental data (Roy et al., 2005;
Upadhyay, 2010) Instantaneous Velocity
Where-
Ensemble average velocity
Fluctuating velocity component Azimuthally average velocity component
RMS velocity Stress
Fluctuating kinetic energy per unit volume
2 2 1
2 [ cos( ) ]
vr r r
t
2 2 1
2 [ sin( )]
v r
t
z
v z t
2 2
1 2 1 2 1 2
1 2 cos( )
r2 r r r r
( , , ) , 1
( , , ) 1 ( , , )
( , , )
N i j k
q q n
n
v i j k v i j k
N i j k
'( , , ) ( , , ) ( , , )
q q q
v i j k v i j k v i j k
dim
dim
( )
1
( )
1
( , , ) ( , , ) ( , )
( , , )
T i
q az j
q T i
j
v i j k N i j k v i k
N i j k
'2 RMS
q q
v v
' '
( , , ) ( , , )
qs p v i j k v i j kq s
'2 '2 '2
1 [ ]
2 p r z
KE v v v
Figure 2.6 Flow chart for RPT data acquisition and processing 2.2.2 Implementation of RPT
In this section, various hardware and electronics, mechanism involved in the photon detection, approximations involved in the implementation are explained.
Radioactive tracer particle
For tracking the phase of interest suitable radioactive particle (gamma-ray source) is chosen based on the criteria given by Roy et al. (2002) and Upadhyay (2010). In literature, different ways are followed to make a tracer particle for RPT experiments. For liquid tracking, small amount of radiotracer, mostly Scandium-46, is enfolded by hollow polypropylene or polystyrene balls. The density of this polymeric ball is matched by keeping some air gap inside the ball before sealing (Devanathan, 1991; Degaleensan, 1997; Upadhyay, 2010).
For solid tracking, size and density difference between the solids and tracer is adjusted by RPT Experiments
(Counts from detector)
Calibration (Distance count map)
Instantaneous position of the particle
Instantaneous Velocity
Ensemble Average Velocity
Fluctuating Velocity
Kinetic Energy Stress
Root Mean Square Velocity Reconstruction
Algorithm
Azimuthally Average Velocity
r, θ
r, z i, j, k i, j, k+1 i, j, k+2 Eulerian grid
coating with polypropylene or encapsulating the tracer particle in suitable material like polypropylene or aluminum beads. Roy (2000) and Bhusarapu (2005) have implemented RPT technique in liquid-solids and gas-solids circulating fluidized bed respectively to track the motion of the solid. Roy (2000) has packed Scandium-46 isotope granules in an aluminum bead of same size as of the solids used in the experiments. Density of the prepared tracer particle is matched with solids used in experiment by maintaining the air gap before sealing the aluminum bead. Bhusarapu (2005) has used Scandium-46 metal particle as a tracer but in order to match the density of Scandium-46 particle with glass beads used in experiment, a thin layer of neoprene was quoted on the tracer particle.
However, in case of solid tracking ideally, material of construction of tracer particle and solids present in the system should also be matched for better prediction of solid-solid interactions and solid fluctuation. Hence, in current work small amount of scandium powder is doped in the glass beads (explained in detail later). The prepared particle is sent to the nuclear reactor (situated in BARC, Mumbai) for irradiation to make it radioactive.
The tracer particle prepared in this way not only ensures similar size and density as of the other solids (glass beads) used in the experiment but also maintain the material of construction same as of the other solids. Hence, a better prediction of solid-solid interactions and solid fluctuations can be expected.
Photon detection and counting
A basic pulse detection system and schematic are shown in the Figure 2.7. It consists of scintillation detector, high voltage supplier, amplifier, single channel analyzer/
discriminator and data acquisition system. Tracer particle may emit single or multiple energy gamma rays e.g. Caesium-137 emits single gamma-ray with energy peak at 669 keV, scandium-46 emits two gamma rays with 100% probability with energy peak at 889
light flash is converted to the electric pulse by photocathode. Electric pulse is amplified and sent to the discriminator. According to the voltage received, the signals are discriminated and counted. The number of photopeak counts recorded in a sampling time is given by Larachi et al. (1994), which is shown in equation 2.1
1 absabs
C TvA
vA
(2.1)
Where, T = Sampling time (s)
v = Number of gamma ray photons emitted per disintegration A= Source strength (Ci)
= Photo peak fraction
= Detector dead time (s)Figure 2.7 Schematic diagram of data acquisition system
Scintillation detectors
Scintillation means “light flash”. Scintillation detector consists of scintillating crystal intimately attached with photomultiplier tube as shown in the Figure 2.8. When gamma ray falls on the crystal material, it transfers its energy to the metal present in the crystal, which excites the crystal material. While returning to normal state it releases light, this light is detected by photocathode. This photocathode emits electrons when it detects light. PMT consists of series of dynodes maintained in higher potential than the previous dynode to attract the electrons. When the electrons hits dynode, secondary electrons are emitted, thereby gets multiplied. Finally, once electrons reach anode of PMT an electric signal is produced. NaI (Tl) is the commonly used detector for gamma ray detection.
Figure 2.8 Schematic diagram of NaI(Tl) detector Multichannel analyzer
Multichannel analyzer (MCA) separates the incident photons according to their energy level. It converts analog signal to digital and bins according to their energy level. Each channel corresponds to the range of energy level to be binned. Increasing the number of
channels increase the resolution of spectra. Figure 2.9 shows the energy spectra of scandium-46 with peak from MCA with 1024 channels.
Figure 2.9 Energy spectra of Scandium - 46 Single Channel Analyzer
From the spectrum it can inferred that, it contains all the energy levels including Compton backscattered photons and electronic noise. These unwanted photons can be removed through single channel analyzer (SCA) by using a threshold. Single channel analyzer (SCA) discriminates the incident photons between the LLD (Low level discriminator) threshold and ULD (Upper level discriminator) threshold. SCA can be used for high frequency acquisition, as there are only threshold limits, it process the data faster than MCA.