10TH INTERNATIONAL SYMPOSIUM ON PARTICLE IMAGE VELOCIMETRY - PIV13 Delft, The Netherlands, July 1-3, 2013
Use of synchronised PIV to measure a pulsed flow velocity field in a discs and
doughnuts column
Paisant Jean-Franc¸ois1, Amokrane Abdenour1, Lamadie Fabrice1, Charton Sophie1, Randriamanantena Tojonirina1and Duhamet Jean2
1CEA, DEN, DTEC, SGCS, LGCI, F-30207, Bagnols/C `eze, FRANCE, jean-francois.paisant@cea.fr 2CEA, DEN, DTEC, F-30207, Bagnols/C `eze, FRANCE
ABSTRACT
The study presented here deals with the pulsed flow in a discs and doughnuts column. We aim for the understanding of the hydrodynamic properties of the one-phase flow. We experimentaly investigate the pulsation intensity effects on flow structure. Velocity fields from the PIV process reveal that the use of pulsation intensity (Amplitude Frequency) can only be limited to qualitative analysis. No obvious tendency of the flow structure and intensity could be linked to it. Moreover, an unexpected eddy stemming from the lack of centenring of the packings appeared in the measured velocity fields, highlighting the asymmetry of the flow.
1. Introduction
The separation of actinides contained in the fission products is one of the main issue of the recycling process for nuclear industry. This separation is made by an hydrometallurgical process. Liquid-liquid extraction is the step on which we focus on. Extracting actinides (uranium, plutonium, . . . ) contained in nitrical acid (HNO3) by the extractive phase (T PH 30%TBP) takes place at the interface of the two phases. The efficiency of the chemical process is then naturally driven by the amount of interface. Pneumatic pulsed column with discs and doughnuts are one of contactors chosen by nuclear industry. These devices are gravitationnal, the heavy phase flows from top to the bottom of the column crosscurrently to the light one. There are two operating modes. In the first one, the aqueous phase is the continous one, the organic phase is dispersed in it. The second operating mode is the opposite. Pulsation gives the energy required to establish and sustain of the emulsion.
Phenomena such as deformation, breakage or coalescence monitor the evolution of the exchange area. These phenomena depend of the flow in the column. Hydrodynamic is then essential to design and optimize these operating systems.
The present study is focused on the behavior of the pulsed flow in a laboratory scale column. Data coming from the PIV acquisitions are used to validate a numerical model.
Such a flow is complex, pulsation and geometry induce temporal and spatial periodicity. The axisymmetry of the flow has been highlighted by the phd work of C.Daniel [3]. However, to our knowledge, PIV study on frequency f and amplitude A influence on flow structure does not exist in litterature. The interest of the present work is to explore the influence of the pulsation on the flow. The existence of specific Amplitude, frequency or intensity A f related flow structures is investigated. Similarly, the axisymmetry hypothesis is checked in different operating conditions.
2. Experimental facility 2.1 Used liquids
For this study, we used two liquids : hydrogenate tetrapropylene1(TPH) and an aqueous solution of potassium thyocianate (KSCN). Extractive molecule is contained in the organic solvent (TPH) which is used in nuclear industry. KSCN is used to match the refractive indices of water and glass. As our device is made in glass; index matching prevent measurements from being affected by optical distorsion. Physical properties of the liquids can be seen in the table 1.
2.2 Experimental
1hydrocarbure fraction based on dodecane
Density Viscosity refractive index
kg m3 103Pa s
T PH 760 1.26 1.425
H2O KSCN 64% 1, 400 2.58 1.476
(a) Column scheme (b) Experimental devices
Figure 1: Experimental facility.
```` ```` ``` Amplitude Frequency 0.5 Hz 1 Hz 2 Hz 1 cm ∅ 1.6 ms 0.85 ms 2 cm 3.5 ms 0.8 ms 0.6 ms 4 cm 1.1 ms 0.64 ms ∅
Table 2: Choice of the ∆t.
Experimental facility is made of a pulsed discs and doughnuts column. It consists of three elements: two settlers of diameter 89.4 mm and a central section with a diameter of 50 mm and a height of 716 mm. Settlers are respectively at top and bottom of the main column. The stack of discs and doughnuts is inside the main body. This stack is build with 13 series of discs and doughnuts. Optical deformation occurs when light beams pass through the column which is in glass (borosilicate). This deformation is due to the difference between the refractive index of air, glass and TPH. In order to reduce this diopter effect, a double rectangular casing is added to the barrel and filled up in with the continuous phase.
The studied fluid initially injected in the column by a pomp is pulsed by a pneumatic blower via a pulse leg partially filled with liquid. Indeed, mecanic pulsation is avoided in nuclear industry to reduce the number of nuclear waste by reducing the number of parts in direct contact with radioactivity. Our pneumatic blower allows temporal impulse and discharge control with various pulsation shapes: symmetrical and asymmetrical. Amplitude of pulsation is controlled by air pressure. This amplitude is measured in the pulse leg with a ruler.
A photography and scheme of our facilty can be seen on the figure 1, more details can be found T.Randriamanantena [4]. 2.3 Acquisition
Camera, software DAVISTM and synchronizing unit PTU (Programming Time Unit) are provided by LavisionTM. Camera (captor
sCMOS) has a resolution of 2560 2120 pixels, with a pixel size of 6.5 6.5 µm2. The lighting is assumed by a two cavity Laser Nd:Yag LitronTM. His wavelength is 532 nm. The seeding is realized with silver coated hollow sphere of diameter 10 µm provided
by LavisionTM. The PTU allows to take synchronized images with the double flash laser. Before data acquisition, a calibration step is
performed. After calibration, a pixel represents a real area of 38.54 38.54 µm2. Time step ∆t between two images has been selected depending on the amplitude and frequency. Choices of ∆t are summarized in the table 2. These choices come from primary acquisitions and ensure a movement of 10 px for the maximum speed observed. Image acquisitions are synchronized with the flow in order to allow velocity fields averaging on the periods also called phase averaging method. The principle is in a first time to record N cycles of T periods. Each cycle consisting of M PIV double images. In a second time, we calculate PIV instantaneous field for each instants of each periods. M sequences of N images are established. Every series include instaneous velocity field corresponding to the same instant of a period. Finally, a statistical average is applied on these series. The mean velocity field u ¡ is defined at the moment t0P v0;Tw by:
xupx,t0qy 1 N N ¸ i1 upx,t0q (1)
In order to record our images at the same instant t0from one cycle to another, the recording is triggered by the crossing of a differential pressure threshold. This pressure difference is measured in the ”pulse leg” by a differential presure sensor linked to two tee fittings from the ”pulse leg”. The tee fittings are located on either side of a convergent divergent located itself between the pulsed leg and the column. This pressure difference is then converted into an electrical signal by the sensor. This signal is very noisy and its amplitude very low. That’s why an electronic analogic card filters and amplifies the signal before sending it to the trigger device. It transforms the signal into a good sine wave. Then a logical TTL impulse is sent by a TTL pulse generator (StanfordTMDG535 unit) from the sine
wave to the PTU. This impulse is sent when the sine wave exceeds a threshold that is set depending the AF flow parameter. Finally, an acquisition sequence of one period is triggered by the PTU. Evolution of these differents signals is summarized by the image 2.
-4 -2 0 2 4 6 0 100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500 1600 1700 1800 1900 2000 2100 Voltage Time (ms) INVERSION INVERSION INVERSION INTAKE BACK FLOW
Signal from the pressure sensor Filtered and amplified signal TTL impulse
Threshold
Figure 2: Evolution of the signal over the time.
Cycles recorded by this way are only comparable for a given amplitude and a given frequency. Indeed, for electronic reasons the trigger threshold changes according to the amplitude and frequency. Amplitude of the electronic signal is actually changed by the pulsation intensity through the pressure difference. As the threshold sets the 0 instant of the cycle, this one moves from an acquisition sequence to another.
In this study, 10 double images per cycle were taken due to technical limitations linked to images storage time. 250 periods were used for the phase averaging step. Following the evolution of the mean velocity at a point located above a disk and close to the wall, i.e the location of highest levels of velocity, it was observed that the standart deviation and the mean velocity do not change after 250 cycles. Which means adding new vector fields do not modify the mean velocity. In this study, we have taken 10 double images per cycle. This is a technical term due to the storage time of the images. We have chosen 250 periods to make our averaging. To do this choice, we followed the evolution of the mean velocity at a point. The point is located above a disk and close to the wall because it is a place where we have higher velocities. So the most important stress is located here. We can see on the graph 3 that the standart deviation and the mean velocity do not change after 250 cycles. That means adding new vector fields do not modify the mean velocity.
0.05 0.1 0.15 0.2 0 100 200 300 400 500 Velocity m.s -1 Number of images 250 Average velocity Standart deviation
Figure 3: Changes in the average velocity over the cycle number.
3. Results
3.1 Computation
Cross-correlation algorithme provided by DavisTMwas used to calculate PIV velocity fields. Each images was divided in interrogation
windows ranging from 64 64 pixels to 32 32 pixels. The velocity field is computed in multi-pass with an overlap. First pass has an overlap of 25% and the next have 50% which gives us a vector field with a 16 16 resolution. After the calculation, the averaging process described above is applied. For the post-treatment, we opted for a median filter which erases velocities as v{v R vvm ασ;vm ασw with vmmedian vector of neighborhood (here a eight-connexity), σ is the standart deviation of this neighborhood and α is a feedback coefficient, α 2. We apply a second criterion so as to avoid clearing good vectors: if v P vvm β;vm βw the vector v is kept.
3.2 R´esults
Regardless amplitude or frequency parameters or imposed flow, our velocity fields are composed of three easily readable states. These states are imposed by the pulsation and are:
an intake phase 4-(a)
a so called inversion phase 4-(b) a back flow phase 4-(c).
The three are shown on figure 4. Intake and back flow phases are easily understandable, they respectively correspond to the positive and negative acceleration imposed on the fluid by the pulse. The so called inversion phase corresponds to the instant when speed nullify. its brevety makes this instant very difficult to record. Instantaneous fields do not properly grab this state, most of the fields catch an intake or a back flow state. It is consequently well-founded to assert that the figure 4-(b) shows us a numerical artefact which comes from the averaging process.
The first remark that can be done on the instantaneous or average velocity field is the presence of an unwanted recirculation close to the wall. This one is visible on the figure 5. This unexpected flow structure appearing on all acquisitions, demonstrate the asymmetry of the flow in our experimental device. Other acquisitions on a perpendicular plan, on different heights helped us to find out the cause of this recirculation which is mainly produced by a bad centering of our discs and doughnuts. In fact, struts are only fixed on the top of the column which dooes not allow a good centering of the discs and dougnuts. The difference between real and ideal position is included between 0.04 mm and 0.1 millim. A more accurate value could not be obtained since the measurement is performed close to the wall which is the most optically distorted location in the column. In the end, the centering error represents 8% to 20% of the reference slack. The existence of this recirculation in industrial column is highly probable, since its slack tolerance can reach 30%. To discard the hypothesis of an optical amplification of the recirculation, PIV acquisitions with an aqueous solution of potassium thiocyanate with 64% of mass were performed. KSCN property is to enable the adjustment of the refractive index of water according to its concentration R.Budwig[2]. The refractive index of the resulting solution is n 1.476. Which is close the refractive index of the glass used to build the column. Acquisitions made with KSCN overcame the problems of optical distorsions close to the wall where the recirculation takes
(a) Intake (b) Inversion
(c) Back flow
Figure 5: Streamlines with unwanted flow.
place. Compared to the aquisitions with TPH, no changes were noticed neither in flow structure nor in the level of the recirculation velocity.
Three main points can be underlined concerning this recirculation. First, it exists regardless of the amplitude or frequency for the intake and back flow steps. Second, it moves with the different cycle time step and rotates versus the main flow. In the intake step, it is above the doughnuts. In the back flow step, it is below the dougnuts. Third, recirculation is confined in an angular sector around 90degrees. Several outcomes can be formulated. The flow theoritically axisymmetric is a 3D flow. Therefore, if an accurate modeling of the flow is expected, a 3D calculation has to be done. Besides, an higher viscous breakage rate could be expected in this area. Indeed, the recirculation rotates against the main flow, creating an area with greater shear. A futur modelling work will be performed to check this assumption.
Second remark, we notify an irrelevance of the pulsation intensity as criterion to compare different flow regimes. The flow structure remains unchanged for any pulsation intensity. The only perceptible change is on the velocity levels. The figure 6 shows this evolution of velocity scale. For the same pulsation intensity, higher velocities are observed for high frequencies. For the studied range of frequency and amplitude a modification of pulsation intensity only impacts the scale of velocity. As can be seen on 6, for an intensity of A f 4 cm·s1, the scale are quite the same. But, for intensities 2 cm · s1and 1 cm · s1, it is completely different, we have great variations for a same intensity and do not have a clear difference between the rate A f 2 cm·s1with f 0.5Hz A 2 cm and the rate A f 1 cm·s1. Furthermore, A.Amokrane et al. [1] also notified this on a study of residence time of droplet, he observed that intensity doesn’t allow to conclude on the residence time. Two different intensities lead to two differents residence time, but for a same intensity amplitude is needed.
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0 1 2 3 4 5 6 7 8 9 Velocity m.s -1 Cycle instant 2Hz 2cm, Af=4m.s-1 2Hz 1cm, Af=2m.s-1 1Hz 4cm, Af=4m.s-1 1Hz 2cm, Af=2m.s-1 1Hz 1cm, Af=1m.s-1 0.5Hz 4cm, Af=2m.s-1 0.5Hz 2cm, Af=1m.s-1
(a) Maximal velocities
0.05 0.1 0.15 0.2 0.25 0 1 2 3 4 5 6 7 8 9 Velocity m.s -1 Cycle instant 2Hz 2cm, Af=4m.s-1 2Hz 1cm, Af=2m.s-1 1Hz 4cm, Af=4m.s-1 1Hz 2cm, Af=2m.s-1 1Hz 1cm, Af=1m.s-1 0.5Hz 4cm, Af=2m.s-1 0.5Hz 2cm, Af=1m.s-1 (b) Average velocities
4. Conclusion
We presented here a synchronisation process which allows to record average velocity fields for a pulsed flow by the mean of PIV. This process is built in several steps which are recording an electronic signal that characterizes the flow, processing this signal, and triggering the acquisition. With this process, we have seen two phenomena inside the column. First, an asymmetry of the flow due to a shortcoming centering of discs and doughnuts. Second, we find an irrelevance of pulsation intensity to characterize the flow. It is necessary to add at least the frequency to the pulsation intensity so as to properly describe a flow regime.
REFERENCES
[1] A. Amokrane, S. Charton, F. Lamadie, J. Becker, J.P. Klein, and F. Puel. Study of the dispersed phase behaviour in a pulsed column for oxalate precipitation in an emulsion, 2012.
[2] R. Budwig. Refractive index matching methods for liquid flow investigations. Experiments in Fluids, 17:350–355, 1994. [3] C Daniel. Dynamique des ´ecoulements liquide-liquide oscillants en g´eom´etrie chican´ee. PhD thesis, Institut Nationnal
Polytechnique de Toulouse, 2002.
[4] T. Randriamanantena. Caract´erisation et mod´elisation du comportement de la phase dispers´ee dans les colonnes puls´ees. PhD thesis, Universit´e Pierre et Marie Curie, Paris IV, 2011.
