Application of nuclear techniques in two-phase liquid-solid particles hydrotransport investigations

The paper presents gamma radiation application to two-phase flow investigation in a vertical pipeline, where the flow of solid particles transported by water was examined by use of both: radiotracers and gamma-absorption method. The simultaneous use of two methods allows analyzing of important parameters of solid particles hydrotransport. In the described experiments as solid phase the ceramic models representing natural polymetallic ocean nodules were used. Radiotracers allow to track the movements of selected models, representing specified grain size and the designation of its velocity. However gamma-absorption method enables measurement of average solid-phase velocity. For analysis of electrical signals obtained from scintillation detectors the cross-correlation method has been applied.


Introduction
Two-phase liquid-solid particles flow occurs often in the mining industry.An example would be vertical pipeline hydrotransport of minerals, e.g.polymetallic nodules.Nodules are porous organic and mineral compositions which contain a various metals (mostly Mn, Si, Fe, Al, Na, Mg, Ni, K, Cu) [1][2][3][4].They occur on the bottom of seas and oceans, but the richest deposits can be found at the Pacific Ocean bed in depth about 5000 m in the so called Clarion-Clipperton zone.The ocean nodules usually have the form of irregular grains with the diameter of several mm up to 0.5 m and density of about 2 g / cm 3 in wet state.The mining of nodules using the hydraulic method requires nodules vertical transport by water to sea level in extremely hard and varying environment.Determination of the velocity of nodules of various sizes in the vibrating pipeline at a significant depth is a very difficult task and requires the use noninvasive measurement techniques [5,6].
One of these techniques, which are employed for many years in measurements of two-phase flows in pipelines and open channels, is a method using radioactive isotopes.In nuclear techniques radiotracer is injected under certain conditions into the flow and/or sealed radioactive sources are used [7][8][9][10][11][12][13][14][15][16][17][18][19].In both cases as detectors of radiation the scintillation probes are mounted outside of the analyzed stream.
Typically in such measurements the mutually delayed stochastic signals are provided by detectors located on the outer walls of the pipeline.Measured time delay of signals is applied to determine the velocity of the marked phase (radiotracers method) or averaged velocity of the solid phase (absorption method).In the carried out experiments solid phase has been modeled using ceramic particles representing three grain sizes of natural polymetallic ocean nodules.The processing of stochastic signals from scintillation probes requires the use of statistical methods in the time and frequency domain [20][21][22][23][24][25][26].The cross-correlation function (CCF) is the most known methods of time delay estimation applied for stationary random signals.
Radiotracer idea is based on marked particle tracking using a scintillation probes placed along the pipeline [7,8,18].
The principle of both methods and typical gammaradiation measuring equipment for liquid-solid particles flow evaluation in a vertical pipeline is presented in figure 1.Two sealed radioactive sources in collimator (3) emit gamma radiation beams shaped by the collimator.Photons pass through the pipeline (7) with the multiphase mixture.The changes in the intensity of radiation are recorded by the scintillation probes of the absorption set (1) with collimators (2) and next converted into output electrical impulses [7].Count rates I x and I y at the outputs of probes depend on the flowing medium content in the test cross section.In the present investigations a linear 241 Am J-ray sources with an activity of 100 mCi and probes with 2" NaI(Tl) scintillation crystal were used.Two such sets were mounted on the pipe with the distance of L 1 = 90 mm between them.Based on the time delay of signals I x and I y and the distance L 1 one can calculate the average velocity of the solid phase.
Probes for radiotracer measurements (5) are based on the longer distance of L 2 = 1882 mm.These probes record the radiation intensity I v and I z from the marked particle (6).Velocity of the marked particle can be determined as before by determining time delay between signals.
The current study has been performed for a mixture of water and artificial nodules at volumetric concentration 0.05.The density of the particles representing the nodules was 2000 kg / m 3 .
Table 1 presents the characteristics of solid particles and figure 2 shows image of particles before experiments.

Experimental set-up
The measuring system presented in figure 1   The key part of the installation is a vertical pipeline of length 7.75 m and with an internal diameter of 150 mm made of acrylic glass.The laboratory set-up has been described in details in papers [5,13,16].The view of some parts of the experimental installation together with measuring section and gamma-absorption set as well as probe for radiotracer experiments is shown in figure 4. The data acquisition equipment was comprised of the dedicated 8-channel EC Electronics counter of HSC 8000 connected to PC using a USB port.

The analysis of signals
Signal pulses I x , I y , I v and I z from the four probes are counted within the selected sampling time Δt = 1 ms and create mutually delayed discrete stochastic signals x

(t), y(t), v(t) and z(t).
An examples of the time records v(t) and z(t) obtained in the WRS0057 experiment (particle size grade I) are presented in figure 5, where s1, s2, s3, s4 denote consecutive marked particle passes through a measuring section of the pipe.
For comparison, the signals x(t) and y(t) from the probes of absorption set recorded at the same experiment is shown in figure 6.
The signals recorded at two spatially separated location within the distance L 1 allow determination of the τ 0S time delay (transit time) necessary for transportation of the solid particles through the measuring section of the pipe.The particles average velocity υ S can be calculated from the following formula: The velocity of the marked particles υ T is calculated as follows:  The frequently used method of time delay estimation of the ergodic random signals x(t) and y(t) is based on the cross-correlation function (CCF) R xy (τ), defined by equation [20][21][22][23][24]: where T is the averaging time, τtime delay.The transportation time delay τ 0 is determined base on position of the CCF global maximum.

Results
The graphs of the cross-correlation functions obtained in the WRS0057 experiment are shown in figure 7. Figure 7a  The combined standard uncertainty u c (υ) of the solids velocity, with negligible small uncertainties of the acquisition set, depends on an inaccuracy of uncorrelated L and τ 0 determination [27]: where u(L) is the standard uncertainty of the distance between detectors.Table 2 shows the results of the marked solid particle velocity υ T obtained from radiotracer measurements and its uncertainty.The average particle velocity υ S calculated using absorption method is υ S = 1.323 m / s with uncertainty u c (X S ) = 0.031 m / s.From the analysis of figure 8 and Table 2 it can be seen that velocity of the marked particles are in the range of greater X S values of the CCF distribution determined for absorption measurement.Velocities for s1, s2, s3, s4 are arranged within ± 1V (where V is standard deviation) aforementioned CCF.
The tracer experiment gives the ability to track individual selected grains whose velocity depends on the position in the flow.In the case of large number of radiotracer experiments when the number of passes indicated in the installation of stones is over 50, the calculated velocity values should be close to the CCF distribution obtained from absorption measurements.

02145-p.4 6 Conclusions
Radiotracer experiments allow tracking the behavior of the selected grain size classes of the solid phase in the liquid-solids particle flow in vertical pipeline.This is important for the development of technology of hydrotransport of polymetallic nodules from the seabed.
The absorption measurements give an average velocity of the entire population of solid particles.The resulting distribution of the CCF can be considered as distribution of all possible time delays (velocities) of the moving grain population.
Comparison of the results of both methods indicates that the velocities of marked particles obtained for s1, s2, s3, s4 are arranged within ± 1V aforementioned CCF.
Measurements of radiotracers, due to the use of open radioisotopes must be performed under conditions of radiological protection.However, in many cases of flow measurements there is no alternative method that gives similar opportunities.

Figure 2 .
Figure 2. The image of the particles (right-hand side: the particle with a bore for the isotope Tc-99m).
has been installed in the experimental set-up located in the Water Laboratory of the Wroclaw University of Environmental and Life Sciences, Poland.The entire experimental installation presented in figure 3 has been used for investigation of the polymetallic nodules hydrotransport.

Figure 4 .
Figure 4. Test section view of the experimental set-up.

Figure 5 .
Figure 5.Time records of signal v(t) and z(t) obtained in the WRS0057 experiment.

Figure 6 .
Figure 6.Time records of signal x(t) and y(t) in obtained from absorption set in the WRS0057 experiment

Figure 7 .
Figure 7. CCF calculated in the WRS0057 experiment; (a) for signals from radiotracers (b) for signals from absorption set.For correct interpretation of absorption and radiotracers experiments simultaneously conducted for different distances L 1 and L 2 , the CCFs were converted to the function of inverse velocity.The converted CCFs, which include amplitude adjustment, are shown in figure 8.

Figure 8 .
Figure 8. CCF for signals obtained for marked particles (colored areas) and for signals from absorption set (solid line) in the WRS0057 experiments.

Table 1 .
Characteristics of the artificial nodules.

Table 2 .
Results of the marked solid particle velocity measurement in the WRS0057 experiment.