K Matsumoto

Y Toya

S Shimasaki
email: shimasaki@t.kagawa-nct.ac.jp

S Taniguchi

Electrification of Droplet by Applying High-Voltage for Spherical Silicon Solar Cell

Keywords: droplet, capillary jet, spherical silicon solar cell, electrification

Spherical silicon solar cell is solar cell that uses silicon spheres of 1 mm in diameter. Purpose of my research is establishment of a manufacturing technique of the spherical silicon. The spherical silicon is produced by the dropping solidification method: a capillary jet of liquid metal is split into droplets and the droplets are solidified during free fall. Minagawa et al. found that mono-dispersed droplets could be generated by applying an intermittent electromagnetic pinch force to a liquid metal jet. However, the droplets frequently coalesced during free fall before completing its solidification, therefor mono-dispersed spherical silicon could not be obtained. In order to solve this problem, an electrification method was investigated in this study. The droplets are electrified by applying a high voltage. It is expected that a repulsive force will act upon the charged droplets and prevent them from coalescence. Experiments on droplets electrification were carried out with capillary jet of water and liquid gallium. Droplet charges were measured by an electrometer. It was found that the droplet charge depends on some parameters such as a jet diameter, shape of an electrode and a value of applied voltage. The experimental results were compared with a theoretical analysis, which has proposed by Brandenberger et al., and the agreement between the two was quite well. It was concluded that a droplet charge can be estimated by the theoretical model.

Introduction

After the Great East Japan Earthquake, solar power generation in Japan has been promoted shifts from nuclear power generation to solar power generation. Among solar power generation, the spherical silicon solar cell attracts much attention. The spherical silicon solar cell is a solar cell that uses silicon spheres of 1 mm in diameter (see, Fig. 1) , and has several advantages as follows: (1) Productivity is high because the spheres are directory produced from liquid silicon. (2) Material yield of silicon is high because there is no cutting process of silicon ingot. However, the spherical silicon solar cell has still a problem of production process. A mass-production method of silicon spheres at low cost has not been established yet. In the dropping method [2,3], silicon droplets are formed by disruption of a capillary jet of molten silicon. Then the molten droplets are allowed to solidify in free fall through a cooling tower and the solidified particles are collected. The problem of the process is that the size of the particles is not uniform, because the jet disruption is not uniform and the droplets coalesce each other in free fall. As a result, the productivity of the spheres is decreased.

Principle of droplet electrification

Coalescence of droplets can be prevented by electrification of droplets [4]. The principle of droplet electrification is shown in Fig. 2. A capillary jet is surrounded by a grounded cylindrical electrode. The jet has a high-electrostatic potential which is maintained by a high-voltage power supply of several thousands volts. The capillary jet have an equipotential surface charge, and the charge will be trapped by droplets which are detached from the tip of the jet. The charged droplets have a repulsive force which prevents them from coalescence.

Calculation method of electric charge

An electric charge of droplet is estimated by the theory derived by Brandenberger et al. [4]. The strength of the electric field can be expressed in Gauss's law: The electric charge per 1 of droplet q [C] can be estimated by assuming that the length of the electrode Lelec is equal to the optimal wavelength λop, which has the fastest growing rate of disturbance for the capillary jet. Integration of Eq. ( 1) and substitution of λop for Lelec in Eq. (1) will give the following equation:

Q rL r E  

Experimental method

Experiment of droplet electrification with high-voltage application was carried out. The liquid, which was eclectically connected to the high-voltage power supply, was issued from the nozzle and the capillary jet was formed. The cylindrical electrode was arranged in the vicinity of the break-up point of the liquid jet, and the electrode was connected to ground.

Cold model experiments were carried out. Water and gallium were used as liquid. Experimental conditions are shown in Table 1. Effects of the experimental conditions of liquid medium, nozzle diameter, inner diameter of cylindrical electrode and applied voltage on electrification of droplets were examined.

Experimental results

Figure 4 shows the temporal change of the electrified amount per unit time, which was measured by the electrometer. The applied voltage of the jet was changed with time from 0.5 kV to 3.0 kV as indicated in the figure. As seen in the figure, the curve of the electrified amount per unit time (namely, current) was a step-like function depending on the applied voltage. A droplet charge q can be obtained by divining a current value of each step by a frequency of droplet generation. Figure 5 shows the results, a droplet charge was proportional to an applied voltage. The experimental values agreed well with the calculated values of Eq. ( 2). experimental charge, Q / nC calculated charge, q / nC water (djet = 0.92 mm) • water (djet = 0.56 mm) ▲ gallium (djet = 0.48 mm) ■ Fig. 6: Relationship between calculated charge and experimental charge.

Conclusions

Following conclusions were obtained from the experimental results:  It was confirmed that droplets can be electrified by applying a high-voltage.



The electric charge of droplet could be measured, and it was found that the charge was varied depending on several conditions such as liquid medium of the jet, nozzle diameter, inner diameter of the cylindrical electrode, and applied voltage.



It was possible to estimate the electric charge of droplet by Eq. ( 2).

( 3 )Fig. 1 :

 31 Fig. 1: Schematic diagram of spherical silicon solar cell [1].

Fig. 2 :

 2 Fig. 2: Prevention of droplets coalescence by electrification.

  where, εe dielectric constant [F/m], E(r) the strength of the electric field [V/m], r the distance from central axis [m], Lelec the length of electrode [m], Q charge to be given to the jet [C].

  applied voltage [V], relec the internal radius of electrode [m], rjet the radius of jet [m]. The optimal wavelength λop can be expressed in following equation derived by Weber [where, d0 the diameter of a nozzle [m], η viscosity [Pa•s], ρ solution density [kg/m 3 ], γ surface tension [N/m].

Fig. 4 :

 4 Fig. 4: Temporal change of electrified amount per unit time (nozzle diameter: 0.92 mm, inner diameter of electrode: 17.45 mm).

Fig. 5 :

 5 Fig. 5: Relation between applied voltage and droplet charge (nozzle diameter: 0.92 mm, inner diameter of electrode: 17.45 mm).Comparison between experimental and calculated values of droplet charge for various conditions is shown in Fig.6. The horizontal axis is observed value and the vertical axis represents calculated value. The calculated values agreed with the observed values within an error of about 10 %. It can be concluded that the droplet charge can be predicted by Eq. (2).

Table 1 :

 1 Experimental conditions.

	liquid	water	gallium
	nozzle diameter,
	jet r [mm]	0.56, 0.92	0.48
	inner diameter of cylindrical electrode, elec r [mm]	8.35, 11.55, 17.45	6.6
	voltage, Φ [kV]	0.5, 1.0, 1.5, 2.0, 2.5, 3.0

Acknowledgment

This work was supported by JSPS KAKENHI, Grant-in-Aid for Young Scientists (A), Grant Number 25709071.


N Yoshikawa

T Sumi

S Mikoshiba

Y Itagaki

S Taniguchi

Exchange and removal of Cs ion in clay minerals by microwave application

Keywords: cesium, layered silicate, vermiculite, microwave, ion exchange, chloride additives

Many kinds of clay minerals consist of layered silicates, and cations exist in the inter-layer clearance. Some of them are tightly fixed in them, because they are bound due to the electrostatic force. Especially, ion radius of Cs fits the size of some sites on the layer crystal. Therefore, Cs ions, once contained in clay minerals, they are not dissolved into water. One of the possible methods for the Cs removal is to exchange it with some other cations. In this study, vermiculite was selected as an example of clay minerals. The vermiculite particles were suspended in water and doped with Cs. After separating the sediments (doped vermiculite) from the water and dried, microwave irradiation of the doped vermiculite was performed with addition of some chloride compounds (salts) in a powder state (solid state). Degree of the ion exchange was determined by measuring the amount of Cs re-dissolved into water. It was observed that microwave heating of vermiculite with KCl and/or CaCl 2 addition enhanced the ion exchange reaction, comparing with the electric heating cases. On the other hand, addition of the chlorides improved the microwave heating ability, although KCl and/or CaCl 2 itself is not heated well by microwave. In this study, influence of KCl addition was mainly investigated. Electric conductivity of the mixture up to 5MHz was measured and shown to be higher than that of vermiculite. Dielectric loss factor (ε r ") of the mixture increased as the temperature increase more in the mixture than vermiculite alone. Increase in both parameters indicates the improved microwave heating characteristics.

Introduction

Clay minerals consist of layered silicates [1], which are bound together by Van der Waals force, and there are interlayer clearances. In the clearance, interlayer water exists and also there are some cations which are bound to the layered crystals by electrostatic force. Among them, ion radius of cesium (Cs) is known to fit size of some sites on the layer crystal and hence tightly bound to the silicate layers. Therefore, once Cs ions are incorporated into clay minerals, they are not easily removed out. Radioactive Cs (especially Cs 137 ) causes a serious problem.

Ion exchange reaction is one of the methods to remove Cs from clay minerals via substituting the intercalated Cs ions with some cation. Microwave application is attempted to enhance the reaction, as it is expected microwave has some interaction with the inter-layer water, and promote the ion exchange.

This study is performed to examine the possibility of removing of Cs from the Cs-doped clay soil by iron exchange. Microwave processing is attempted to apply to this process, because the objects can be treated in a short time due to the rapid heating characteristics. It is also attempted to investigate the possibility to take advantage of microwave processing to promote exchanging reaction of the Cs ions with other cations, as indicated by previous reports [2,3]. The clay particles and the chlorides (and some compounds) are mixed in powder (solid) states and they are treated by microwave irradiation.

In this study, vermiculite is selected for the model material of the clay minerals, and exchange reaction of ions by microwave irradiation is investigated. Non-radioactive Cs was used for doping with the vermiculite. In order to interpret the microwave heating behavior, the permittivity (high temperature) and the conductivity measurement (room temperature) are performed.

Experimental part

Specimen used for this study is a commercial vermiculite for gardening. Chemical composition of vermiculite is expressed as follows [START_REF]Handbook of Clay[END_REF]: (Na,K,Ca 0.5 Mg 0.5 ) 0.6-0.9 (Mg,Fe 3+ ,Al) 2-3 (Si,Al) 4 O 10 (OH) 2 nH 2 O Well-defined initial state of the specimen was setup for the experiments, as follows: as-received vermiculite was gently ground with an agate mortar, then dispersed in purified water. Standard cesium solution (CsCl, 1000ppm, Wako, Tokyo, non-radioactive) for ICP analysis was diluted with purified water and doped. The Cs-doped suspension was filtrated and the sediments were dried at 60 o C for 24hrs. The amount of the doped Cs in vermiculite was estimated measuring the Cs concentration of the aqueous solution separated from the suspension by filtration. Concentration of the doped Cs was of the level of less1000ppb or less depending on the specimens. The reagent grade (assay 95%, Wako) of KCl and/or CaCl 2 salts were used by mixing with the Cs-doped vermiculite in powder state for the microwave heat treatment.

Chemical analysis

Cs concentration in aqueous solution was measured with ICP-MS (Agilent Technologies 7500 Series) for the purpose of detecting the very little amount of Cs. This analytical method was undertaken for the re-dissolution test of Cs into water (to examine the degree of ion exchange) from the heat treated powder specimens.

Microwave apparatus

Microwave heating was performed with a 5.8GHz single mode applicator (Maikuro-Denshi, MMG-607V) of TE103 cavity, at an input power less than 200W. The specimen powder (mixture) was contained in a fused silica cell having dimension of 10mm in diameter and 50mm in length (vermiculite weight, 0.1g), it was placed at the position of electric field maximum in the microwave cavity, and the resonance of the cavity was obtained tuning the movable plunger and adjusting the system impedance with a E-H tuner. The temperature of the specimen was measured with an optical method using sapphire rod above 350 o C (PhotriX / Luxtron Corp.), and with fluorescent sensor below 450 o C (Anritsu-Keiki, FS400-2W).

Dielectric properties of materials

In this study, permittivity measurement of the specimen was performed using co-axial wave guide method [START_REF] Kawahira | [END_REF] and cavity perturbation method at 2.45GHz. Temperature variation of the permittivity was recorded. The permittivity values are expected not different too much from the range around 3GHz up to 6GHz. On the other hand, impedance measurement was also made in a frequency range less than 5MHz down to 4Hz. The measurement was conducted at room temperature.

Results

Cs re-dissolution into water (Removal ratio) Fig. 1 illustrates comparison of the Cs removal ratio (vermiculite with addition of CaCl 2 and CaCO 3 ) by microwave and by electric furnace heating at different heating temperature. After heating, they were suspended in water. Re-dissolved amount of Cs in water was measured and the removal ratio of Cs was evaluated comparing with the doped amount of Cs. It can be seen that by microwave heating below 670 o C, the removal ratio larger than the electric furnace cases. At 1000 o C, the ratio becomes less. Probably, thermal effect becomes overwhelming at this condition and/or by the enhanced sintering kinetics to be shown later. Vermiculite and its mixture with 67wt%KCl were heated at different temperatures by microwave in powder states. The results are plotted in Fig. 2. The dissolution (removal) ratio increased by mixing with KCl, which is evident even without microwave heating (room temperature plots). On the other hand, highest ratio was obtained when heating to 670 o C (Cs melting point), and it decreased in the specimens heated to 1000 o C. The reason in this case was considered to be related with the fact that sintering of the vermiculite was promoted as the photographs shown in Fig. 3. Microwave heating behavior Microwave heating behavior of vermiculite and its mixture with KCl was investigated. The attained maximum temperature by application of 140W microwave input was compared among various specimens. The results are listed in table 1. The maximum temperature rather depends on the skill of manipulation of movable plunger and E,H tuner, however, this results provide a good measure of microwave absorbability of each specimen. Although higher temperature can be attained if larger microwave power is applied, it is evident that addition of KCl improves the microwave absorbability very much. On the other hand, however it was found that KCl itself is not a good absorber, as mentioned before.

Table 1: Temperature attained by microwave heating of various specimens at the input power of 140W (measured with a sapphire rod).

Permittivity and high frequency impedance of powder mixture Microwave energy (power) is absorbed in substances according to Eq. 1, where E and H are electric and magnetic field, respectively, µ is a magnetic permeability. As the present case, the materials are non-magnetic, therefore, the third term can be neglected. So, the heating mechanisms are ascribed to the first (dielectric) and the second (induction current) heating mechanisms.

P = 1 2 ωε " E 2 + 1 2 σ E 2 + 1 2 ωµ " H 2 (1)
For the purpose of interpreting the heating behavior of the above listed specimens, permittivity and impedance measurement were performed, and comparison was made among vermiculite, vermiculite mixing with 30mass%KCl (mixture) and KCl. The heating ability increased by addition of cations as listed in Table 1, which might be partially related with the increase in the electric conductivity (σ) and partially with the dielectric loss factor (imaginary part of permittivity (ε")). Dependence of the conductivity on frequency below 5MHz is shown in Fig. 4. The conductivity becomes higher by addition of KCl to vermiculite and much higher than that for KCl. Next, high temperature measurement of permittivity is performed and ε" values at different temperature are shown in Fig. 5. The dielectric loss factor increased as an increase of temperature [START_REF]Microwave Processing of Materials[END_REF], indicating the promoted (accelerating) microwave heating, as an increase of temperature. Though it is not possible to measure the conductivity at GHz, the electric conductivity can be estimated using a value of imaginary part of permittivity (ε r ") to be ~3[S/m] at 800 o C using a relation (σ =ωε 0 ε r ", ω =2π⋅2.45⋅10 9 , ε 0 is a permittivity of vacuum). The dielectric conductivity is much lower comparing with the metals. The mixed powder was shown to increase as the heating temperature (shown in Fig. 5). Therefore, it is inferred that contribution of the conduction loss due to long ion transport is not large. However, the dielectric interaction resulted in stimulating the exchange reaction and might be related to the dielectric loss. The detailed mechanisms are required to be investigated in the future study. vermiculite and with 33wt% KCl addition.

Discussion on ion exchange kinetics

Removal of Cs was caused by ion exchange with the additive cations (Ca 2+ in Fig. 2 case). The lower temperature enhancement of the exchange reaction by microwave is noticeable and its mechanism has to be discussed. The same tendency was obtained in the case of KCl addition.

According to the dissolution ratio (removal ratio) of Cs into water (Fig. 1, 2), it is supposed that ion exchange occurred between the intercalated Cs and K. In discussing the possibility of the exchange reaction, the followings are taken into consideration:

It is known well that the interlayer clearance of vermiculite contains water and some cations. The capacity of cation incorporation is characterized in terms of Cation Exchange Capacity (CEC). Vermiculite is known to have large CEC among various minerals, having a value above 100 cmol c kg -1 , which is favorable to hold fertilizing ions such as NH 4+ etc. Among various cations, K ion is known to have the highest CEC, indicating the possibility of very large amount of K intercalation in vermiculite.

Although it is reported [START_REF] Maes | [END_REF] that the exchange selectivity of K + is not higher than Cs + , K + has the largest uptake among the foreign cations competed with Cs in the report [8], on the other hand. As it has to be considered that the degree of the exchange reaction depends on the minerals tested (as the naturally occurring matters), it was demonstrated that the ion exchange occurred in the vermiculite by KCl addition effectively. And the exchange ratio increased by heating to the high temperature. (There are much less attempts reported at the elevated temperature above 300 o C.) The removal ratio (up to 30% in Fig. 3) is still low for the actual operation, however, it is believed that the ratio could be improved by further exploring the experimental conditions. In addition, the enhanced kinetics will be discussed in terms of microwave effect by comparing with that of the conventional heating in future studies.

Conclusion

This study was conducted to remove intercalated Cs ions from vermiculite by ion exchange with Ca and K. CaCl 2 and KCl was mixed with Cs incorporated vermiculite and heated by microwave. Cs removal ratio increased by the ion exchange, the reaction of which was enhanced by microwave heating. On the other hand, it was shown that microwave absorbability was enhanced with mixing of KCl. Imaginary part of permittivity (loss factor) was altered by mixing with KCl in GHz range, the conductivity and permittivity increased at the lower (MHz) range at room temperature. Temperature dependence of permittivity is suggested to be important. Dissociation experiments of Cs indicated that treatment of heating to higher temperature of vermiculite mixture enhanced the removal of Cs.

Fig. 1 :

 1 Fig. 1: Comparison of Cs removal rate between Fig. 2: Cs removal rate with and the microwave and conventional heating without addition of KCl (with addition of CaCl 2 and CaCO 3 )

Fig. 3 :

 3 Fig. 3: Photographs of vermiculite + 67wt% KCl mixture (a) before and (b) after heated by microwave at 1000 o C.

Fig. 4 :Fig. 5 :

 45 Fig. 4: Conductivity dependence of vermiculite Fig. 5: Temperature dependence of ε r " and with KCl additions on frequency.vermiculite and with 33wt% KCl addition.

Acknowledgement

This study was supported by grant-in-Aid for Scientific Research (B) 24360285 by JSPS.


T Wondrak
email: t.wondrak@hzdr.de

M Ratajczak

K Timmel

F Stefani

S Eckert

Recent progress on contactless inductive

Keywords: flow measurement, contactless inductive flow tomography, continuous casting

Introduction

In continuous casting any kind of flow monitoring of the liquid steel in the mold would be highly desirable for increasing the efficiency of the process and the quality of the final product. The flow field in the upper part of the mold is most critical for the quality of the produced steel. Here, some unfavorable flow conditions can lead to slag entrainment and surface defects [1]. In order to control the flow structure in the mold electromagnetic brakes (EMBr) or electromagnetic stirrers are frequently used [2]. Yet, the specific effects of those magnetic actuators are poorly understood due to the lack of adequate flow measurement systems. One possible measurement technique is the contactless inductive flow tomography (CIFT) [3,4,5] which provides at least a rough picture of the flow by applying primary magnetic fields to the melt and by measuring the flow induced magnetic field perturbation outside the fluid volume. These measurements are utilized to infer the flow field by solving a linear inverse problem. Appropriate regularization methods, like Tikhonov regularization in combination with the Lcurve technique, are necessary in order to mitigate the non-uniqueness problem. In order to demonstrate the applicability of CIFT for continuous casting, a measurement system for a 1:10 scale model (Mini-LIMMCAST) of a continuous slab caster has been set-up at Helmholtz-Zentrum Dresden -Rossendorf (HZDR) [6,7]. In a variety of experiments the CIFT-reconstructed GaInSn flow in the mold was shown to agree reasonably with corresponding UDV measurements [8,9]. In these first experiments we used a static excitation magnetic field and recorded the magnetic field by Fluxgate probes with a dynamic range of 6 orders of magnitude. However, these measurements with DC excitation were extremely sensitive to external interferences. An AC excitation field with low frequency is recommended to make the CIFT measurements more robust to fluctuations of the environmental magnetic field. This allows for separating the flow induced magnetic field from the environmental noise. The typical excitation frequency is in the range of a few Hz, so that the skin effect is negligible and the magnetic field can still penetrate all the liquid. In the AC case, induction coils can be used which are well applicable in high temperature environments. A further advantage of induction coils is that they do not saturate in strong static magnetic fields, which are widely used in the continuous casting process for the purpose of electromagnetic braking [10]. In this paper we will present the latest developments for the CIFT measurements in the presence of a strong magnetic field generated by an electromagnetic brake.

Experimental setup and CIFT

At HZDR, several liquid metal facilities have been setup-up for the systematic investigation of the flow structures as they are typical for the continuous casting process, and of various means for controlling them [6]. Figure 1(a) shows a schematic sketch of the mold of the smaller of these facilities, called Mini-LIMMCAST (scale 1:10), which is operated with the eutectic alloy GaInSn that is liquid at room temperature. The tundish is represented by a stainless steel cylinder which contains about 3.5 l of the metallic alloy. The melt is discharged through an acrylic glass tube (SEN), with the inner diameter of 10 mm, into the mold with a rectangular cross section of 140 x 35 mm 2 (also made from acrylic glass). From this mold the liquid metal flows over a dam into a storage vessel. The vertical position of the dam controls the free surface level in the mold. The experiments presented here were performed in a discontinuous mode. The liquid level in the tundish and the mold were monitored using a laser and an ultrasonic distance sensor, respectively. The liquid flow rate can be derived from the descent of the surface level in the tundish. Additionally, a magnetic ruler type brake was installed along the wide faces of the mold in order to validate the functioning of CIFT for such a configuration (Fig. 1(b)). The brake comprises pole shoes which increase and form the magnetic field of the brake and are made of material with very high magnetic permeability. The effects of this type of magnetic brake were already examined using Ultrasound Doppler Velocimetry (UDV) and compared to numerical calculations [11,12,13]. CIFT relies on inverting an integral equation system which can be derived from Ohm's law for moving conductors by applying Biot-Savart's law and solving the arising Poisson equation for the potential for insulating boundaries [3,4]. The intrinsic non-uniqueness problem of the emerging linear inverse problem, which mainly concerns the detailed depth-dependence of the velocity, is circumvented by utilizing the so-called Tikhonov regularization by minimizing, in parallel to the mean magnetic field deviations, the kinetic energy of the flow [4,14]. For the restricted problem of inferring the flow in the mold of continuous slab casting it is sufficient to reconstruct only the two-dimensional flow field parallel to the wide faces of the mold, since this represents the dominant and most interesting flow structure [7]. In the presented case the applied magnetic field is generated by two rectangular coils which are mounted above and below the pole shoes of the electromagnetic brake, as seen in Fig. 1(b), in order to be closest to the region of the jet. The magnetic field sensors are placed along the centreline of the narrow faces of the mold, since the strongest induced magnetic field is located there. The measured induced field component is directed normal to the wall. For the application of an AC excitation magnetic field we are using 7 pickup coils with a diameter of 28 mm and 340 000 turns along each narrow face of the mold [10]. The induced voltages of the pickup coils are recorded by a 24-bit analog-digital converter system from Labortechnik Tasler. In the post-processing step the amplitude and the phase of the signal in comparison to the current through the excitation coil is calculated using the quadrature demodulation. The typical excitation frequency is in the range of a few Hz, so that the skin effect is negligible and the magnetic field can still penetrate all the liquid. Another important advantage of AC excitation is the reduction of the sensitivity of the measurement system to fluctuations in the environmental magnetic field.

CIFT in the presence of an electromagnetic brake

Due to the high magnetic permeability and the high electrical conductivity of the pole shoes strong eddy currents are induced by the excitation magnetic field, even at low frequencies in the order of 1 Hz. These eddy currents, in turn, generate an additional magnetic field which can be detected by the pickup coil. Fortunately, due to the fact that the magnetic field of the eddy currents has a phase angle of 90 degree relative to the current in the excitation coil, this contribution to the measured induced magnetic field can be filtered out. In this way, we can safely calculate the magnetic field inside the mold as resulting exclusively from the excitation coil. In principle, the magnetic pole shoes could also deform the induced magnetic field. We checked this effect with a simulation using Opera3D for our setup, and it turned out to be negligible [10].

To make matters even more complicated, flow oscillations generate a strongly varying induced magnetic field under the influence of the strong static magnetic field of the electromagnetic brake, too. If the frequency of the typical flow oscillations is in the same order as that of the CIFT excitation current, both signals superimpose and cannot be separated. This problem can be circumvented by using a higher excitation frequency which is in our case 20 Hz. We measured the magnetic field for two different cases when the electromagnetic brake is switched off and when generating a static magnetic field of 310 mT. It is known from UDV measurements [13] that for an active magnetic brake with insulating walls the jet leaves the SEN horizontally and becomes very unstable, whereas with the brake switched off the flow remains rather stable. Figure 2 shows the fields of the sensors 1-7 for these two cases. For the brake being switched off, Fig. 2(a) shows the time dependent signals. One can clearly see that each sensor has a constant mean value with superimposed slight oscillations and a very precise reference at the beginning. If the magnetic brake is switched on (with insulating walls), we get an unstable double-roll flow structure in the mold, which is clearly mirrored in the measured induced magnetic field (Fig. 2(b)). Intense long-term fluctuations with periods in the order of several seconds are also observed, in particular in sensors 6 and 7. These measurements demonstrate that it is indeed possible to measure safely the induced magnetic fields which are 6-7 orders of magnitude smaller than the magnetic field of the brake. The next step will be the reconstruction of the velocity.

LIMMCAST

Motivated by those promising experiments at the Mini-LIMMCAST facility (scale 1:10), we have started to implement CIFT at the larger LIMMCAST facility (scale 1:2). One goal is the demonstration of the scalability in order to make sure that CIFT is able to reconstruct the flow in a real casting machine. The LIMMCAST facility is operated with SnBi at 250 °C. An electromagnetic pump is used to convey continuously the liquid metal into the tundish from which the melt pours through a pipe with an inner diameter of 21 mm into the mold with a cross section of 400 x 100 mm 2 . The flowrate is controlled by a stopper rod [6]. Fig. 3(a) shows a schematic sketch of the excitation coil and the magnetic field sensors which are arranged in a very similar manner as for Mini-LIMMCAST, except that for the first tests we mounted the sensors only along one side of the mold, as depicted in Fig. 3(a). Special thermal insulation had to be installed to prevent the sensors from heating [10].

The excitation current for CIFT had a frequency of 3 Hz and an amplitude of 17 A. Interestingly, in spite of the lower electrical conductivity of SnBi (about 0.4 of the conductivity of GaInSn) and the larger distance of the sensors of about 10 cm from the fluid, the induced magnetic field is in the same order of magnitude as in case of GaInSn.

Conclusions

In this paper, we have demonstrated the suitability of CIFT for flow measurements in two model experiments related to the continuous casting of steel. Also in the case of an active electromagnetic brake, the induced magnetic field is measurable. The ultimate goal is to employ the method to the hot melts in real steel casters, but this is still a long way to go.

Fig. 1 :

 1 Fig. 1: Schematic sketch of the sensor arrangement of CIFT for the Mini-LIMCAST facility (a) and photograph of the facility (b), (excitation coil -X, brake coils -B, EMBr -E, sensing coils -S).

Fig. 2 :

 2 Fig. 2: Measurement of the induced magnetic field in the case of the EMBr switched off (a) and the EMBr switched on with a static magnetic field of 300 mT with insulating walls (b) using an excitation frequency of 20 Hz.

Fig. 3 (Fig. 3 :

 33 Fig. 3: A typical experiment at the LIMMCAST facility with CIFT [10].

Acknowledgment

Financial support of this research by the German Helmholtz Association in the frame of the Helmholtz-Alliance LIMTECH is gratefully acknowledged.


S Franke

T Gundrum

G Gerbeth

J-C Willemetz

Applications of Ultrasonic Doppler Velocimetry to flow measurements in hot liquid metals

Keywords: Flow measurements, ultrasound Doppler method, liquid metal, channel flow, high temperature melts 0

The successful application of the ultrasound Doppler method at hot channel flows by means of commercial high temperature probes is presented. To obtain sufficient Doppler signals, different problems have to be solved: the transmission of the ultrasonic beam through the channel wall made of stainless steel, the acoustic coupling between the transducer and the channel wall, and the wetting of the inner surface of the wall by the liquid metal, respectively. An integrated sensor concept and method are figured out to meet these requirements. The feasibility of this sensor concept is demonstrated at experiments in metallic melts at temperatures up to 230°C. Measurements are performed at a circular channel flow at the LIMMCAST facility at HZDR applying an eutectic bismuth-tin alloy. In addition, a lead-bismuth flow in a rectangular channel profile measured at the METAL:LIC loop at the Institute of Physics Riga (IPUL) is presented in this report.

Introduction

Liquid metal technologies gain in importance for the metal industry, for example, the production of cast parts, the continuous casting of steel and the crystal growth at the semiconductor industry. As well the role of liquid metal technologies grows steadily at the branch of energy industry. Present and future examples may be found at the nuclear power industry, more precisely at fast breeder power plants and fusion reactors. However, also renewable energies may benefit from liquid metal technologies, for example with respect to the application of liquid metal coolants at concentrated solar power plants. Applications in the energy sector typically utilize liquid metals as coolant, however, future prospects consider liquid metals also for energy storage systems. For the last two decades the ultrasound Doppler method has developed to a very powerful tool to investigate the velocity structure of liquid metal flows as reported, for example, by Takeda [1] for mercury and by Brito [2] for liquid gallium. However, in case of hot metallic melts the user is confronted with a number of specific problems: First of all, the application of the ultrasonic transducers is usually restricted by temperature. Furthermore, the transmission of a sufficient amount of ultrasonic energy from the transducer to the fluid has to be guaranteed. Here, the acoustic coupling and the wetting conditions have to be considered as important issues. Moreover, a sufficiently high concentration of scattering particles has to be provided to obtain reliable Doppler signals from the fluid. First successful velocity profile measurements at higher temperatures were published by Eckert [3]. The flow profile of liquid sodium in a square channel cross section was measured at temperatures up to 150°C by means of commercial ultrasonic probes. A specific sensor development for high temperature applications focuses on the use of acoustic wave guides [4]. In this concept the acoustic energy propagates inside a construction comprising a coiled, thin foil of stainless steel. The temperature gradient along the wave guide impedes that the temperature of piezo ceramic of the probe exceeds its Curie point. The reliability of the waveguide probe has been demonstrated by successful measurements in lead-bismuth at 300°C and copper-tin at 650°C. Despite the advantages of the waveguides, there are still shortcomings of this technology. Apart from high costs of the waveguide probes an inconvenient limitation is the measuring depth since the transition of the ultrasonic pulse from the waveguide into the fluid generates a strong reflection signal in the ultrasonic echo. This interface reflection may be mirrored into the range where the velocity profile is measured disturbing the reconstruction of the velocity profile. Due to technical limitations it causes difficulties to filter these interface reflections. The best approach for compensation is to delay the interface reflection by extending the length of the waveguide. However, the sound velocity of steel is very high, much higher than the sound velocities of typically applied liquid melts implying that the waveguide must be much longer than the measuring depth. Hence, in this report our sensor concept given in [3] will be extended to measure instantaneous flow velocities at liquid metals up to temperatures of 230°C without the application of waveguide sensors. The feasibility of the concept is demonstrated by measurements of channel flows at the LIMMCAST and the METAL:LIC facility, which are operated at temperatures around 200°C with Sn-Bi alloys and Pb-Bi alloys, respectively.

Method of measurement

The measurements are carried out using the Doppler device DOP3010 (Signal Processing SA) equipped with specific high-temperature transducer probes (TR0405LTH) specified up to a maximum temperature of 230°C. The selection of an ultrasonic frequency of 4 MHz with a piezo diameter of 5 mm considers various aspects with respect to the spatial and temporal resolution, the velocity limitation, the multiple reflection characteristic as well as constructional issues. Owing to the high temperature, the abrasive character of the metal melt, wetting issues and safety reasons the ultrasonic transducer probe cannot be brought in direct contact to the fluid. Therefore, a special probe socket (fig. 1) is designed to protect the sensor and to provide reliable measuring conditions. The socket consists of an open stainless steel cylinder with a thin plate at the cylinder front. The transducer face is directly attached to this acoustically transmitting front plate through which the measurement is conducted. Because of cleaning and constructional issues the inner diameter of the socket cylinder is distinctly larger than die probe diameter. The proper alignment of the probe is guaranteed by a duct attached to the probe. The duct is also intended as counterpart for a spring mechanism pressing the probe at the polished inner wall of the front plate. This mechanism ensures a stable acoustic coupling despite thermal expansion. A spout at the channel wall incorporates the probe socket and aligns it in a specific angle Θ with respect to the channel axis (fig. 1). The axial flow component in a rectilinear channel can be derived by taking into account the correction factor cos Θ. The acoustic transmission among transducer face and front plate of the socket is supplied by an acoustic couplant known from non-destructive testing technique which can withstand temperatures of about 230°C. Further aspects of acoustic transmission have to be considered to guarantee a sufficient amount of ultrasonic energy along the acoustic path in the fluid: The thickness of the front plate has to meet a multiple of the half wave length of the ultrasonic wave in the wall material in order to maximize the acoustic transmittance [3]. According to this, for the front plate (also made of stainless steel) a wall thickness of 2.9 mm is selected to match for ultrasonic frequencies of 1, 2 and 4 MHz. Tolerances of the resonant wall thickness arising from temperature changes or an imperfect fabrication can be compensated by a fine tuning of the emitter frequency. In addition, a sufficient wetting of the outer wall of the front plate with liquid metal has to be guaranteed using a specific preparation: After mechanical polishing the surface is electroplated with nickel. Afterwards this nickel layer is coated with bismuth-tin eutectic to act as a protection layer against oxidation. The nickel layer promotes the wetting of the stainless steel socket with the liquid metal. An appropriate density of scattering particles is essential for reliable velocity profiles. However, it is very difficult to add artificial scattering particles to the metal melt. Instead, natural scattering particles are available for that purpose supposed to be oxides or local segregations of the eutectic alloy. These particles apparently deposit at the channel wall after a while. Here, it is necessary to occasionally stir up the particles into the fluid volume by high flow rates. Using the mean flow profiles (see figure 4) the volume flow rate Q versus the speed of rotation (in rpm) of the EM pump (see figure 5) is calculated assuming an almost axisymmetric flow profile. For this purpose the distorted velocity data at the channel walls are interpolated by using a cubic spline with the velocity directly at the walls set to zero. The interpolated curves are denoted as dashed line in figure 4. The maximum negative velocity gradients measured at the wall regions were selected as a criterion for truncation.

Results

A -LIMMCAST facility at HZDR

B -META:LIC test loop at IPUL

The METAL:LIC loop shown in figure 6 A technology is presented to perform flow velocity measurements at metal melts with temperatures up to 230°C using the ultrasound Doppler method. Commercial high temperature probes are deployed which do not suffer from the limitation of the measuring depth as in the case of acoustic waveguides. Our approach includes a specific design of a probe socket considering thermal and chemical aspects as well as issues of wetting and acoustic coupling. Our method is demonstrated at two eutectic alloys, the lead-bismuth and the tin-bismuth eutectic which may gain in interest for future technologies. The results of our investigations prove that not only the mean flow is gathered with an adequate performance but also instantaneous velocity profiles can be measured with high reliability assuming a sufficiently high concentration of scattering particles in the fluid.

Fig. 1 :

 1 Fig. 1: Probe socket design

  The LIMMCAST (Liquid Metal Model for Continuous Casting) facility at HZDR aims to model the essential features of the flow field in the continuous casting process. The low melting point alloy Sn60Bi40 (c s ≈ 2000 m/s) is applied as model liquid whose liquidus temperature of 170°C allows for an operation in the temperature range between 200 and 400°C. The facility contains about 250 l of SnBi. An electromagnetic pump is used to convey the liquid metal trough the loop.

Fig. 2 :

 2 Fig. 2: LIMMCAST facility: (a) general view, (b) test section Fig. 3: Exemplary instantaneous velocity profile vs. pipe diameter of SnBi flow at 205°C Moreover, the facility offers an additional test section with a horizontal, straight tube (diameter d 0 = 54.5 mm) installed for material tests or verification of various measuring techniques. A segment of this test section shown in figure 2 features two measurement spouts for the probe sockets aligned in an angle Θ = 45° with respect to the channel axis. The measurements presented here are obtained from these test points. Figure 3 shows an instantaneous flow velocity profile versus the pipe diameter d. The edges of the diagram conform to the boundaries of the channel. The profile reveals a high signal-to-noise ratio at a high temporal resolution of about 60 ms and a fluid temperature of 210°C. The wall regions of the profile are truncated since the flow velocity cannot be determined correctly there. Saturation effects of the transducer probe and flow perturbations created by the probe socket impede a velocity measurement at the front wall [3]. Multiple reflection paths owing to the divergence of the ultrasonic beam prevent the drop of the velocity profile to zero at the opposite channel wall [3].

Fig. 4 :Fig. 5 :

 45 Fig. 4: Mean flow velocity for various rotation speeds of the EM pump (velocity values are corrected with respect to the Doppler angle) Fig. 5: Volume flow rate Q vs. rotation speed f rot of the EM pump

  is a model experiment at the Institute of Physics of University of Latvia (IPUL) for studying a liquid metal target within the European Spallation Source project. The loop deploys a total mass of 1100 kg of lead-bismuth eutectic (LBE; Pb45Bi55; c s ≈ 1750 m/s) which is driven by an electromagnetic permanent magnets pump. The flow rate is determined by a difference pressure measurement at a Venturi tube. The target test section comprises a channel segment with a rectangular cross section of height × width = 80 × 100 mm². The measuring points are mounted directly after the channel slightly bends. The probe sockets are aligned in an angle of Θ = 60° with respect to the channel axis of this pipe segment. Mean velocity profiles are shown in figure7for four different flow rates Q measured by the Venturi sensor. Owing to the pipe bending, neither the velocity information nor the measuring depth p is corrected by the Doppler angle. The edges of the diagram conform to the channel boundaries. The measurements were performed at a melt temperature of about 210°C. The velocity profile for the flow rate Q = 6.67 l/s exposes distinct noise probably induced by a low tracer particle concentration.

Fig. 6 :

 6 Fig. 6: METAL:LIC loop with measurement locations Fig. 7: Mean velocity profiles measured at various flow rates ConclusionsA technology is presented to perform flow velocity measurements at metal melts with temperatures up to 230°C using the ultrasound Doppler method. Commercial high temperature probes are deployed which do not suffer from the limitation of the measuring depth as in the case of acoustic waveguides. Our approach includes a specific design of a probe socket considering thermal and chemical aspects as well as issues of wetting and acoustic coupling. Our method is demonstrated at two eutectic alloys, the lead-bismuth and the tin-bismuth eutectic which may gain in interest for future technologies. The results of our investigations prove that not only the mean flow is gathered with an adequate performance but also instantaneous velocity profiles can be measured with high reliability assuming a sufficiently high concentration of scattering particles in the fluid.

Acknowledgment

The authors from HZDR acknowledge additional financial support from the German Helmholtz Association in the framework of the Helmholtz Alliance LIMTECH.