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EVEDA lithium test loop
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Measurement of lithium target surface velocity in the IFMIF/EVEDA lithium test loop Takuji Kanemura a,∗ , Hiroo Kondo a , Tomohiro Furukawa a , Yasushi Hirakawa a , Eiji Hoashi b , Sachiko Yoshihashi c , Hiroshi Horiike c , Eiichi Wakai a a
Japan Atomic Energy Agency, 4002 Narita, O-arai, Higashi-Ibaraki-gun, Ibaraki 311-1393, Japan Osaka University, 2-1 Yamada-oka, Suita, Osaka 565-0871, Japan c Fukui University of Technology, Gakuen 3-6-1, Fukui-shi, Fukui 910-8505, Japan b
h i g h l i g h t s • • • • •
The objective is to measure the free-surface velocity field of the IFMIF Li target. The Li target has an important role to remove 10 MW heat input from a deuteron beam. The free-surface of the Li target is under the most severe heat load condition. Measured surface velocities are almost equal to cross-sectional average velocities. It was confirmed that the IFMIF Li target has adequate heat removal performance.
a r t i c l e
i n f o
Article history: Received 19 August 2015 Received in revised form 26 October 2015 Accepted 26 October 2015 Available online xxx Keywords: IFMIF EVEDA Lithium Jet Surface velocity Measurement
a b s t r a c t In the framework of the Engineering Validation and Engineering Design Activities (EVEDA) project of the International Fusion Materials Irradiation Facility (IFMIF), we measured surface velocity fields of a lithium (Li) target at the EVEDA Li test loop under specifically-designated IFMIF conditions (target speeds of 10, 15, and 20 m/s, vacuum pressure of 10−3 Pa, and Li temperature of 250 ◦ C). In the current design of the IFMIF, the free surface of the Li target is under a most severe heat load condition with respect to Li boiling. The objective of this study is to measure the actual free-surface velocity under these IFMIF conditions to evaluate the heat removal performance of the Li target. The measured results (using the surface-wave tracking method that our team developed) showed two-dimensional time-averaged velocity distributions around the IFMIF beam footprint being virtually uniform, and close to the cross-sectional average velocity. The uniformity of the velocity distributions was less than 1 m/s. The comparison between the measured and analyzed surface velocity at the beam center showed that the analysis accurately predicts the measurement results within a margin of 3%. Finally, it was confirmed that the Li target delivers adequate heat removal performance in the IFMIF as designed. © 2015 Elsevier B.V. All rights reserved.
1. Introduction The International Fusion Materials Irradiation Facility (IFMIF) is an accelerator-based neutron source utilizing Li(d, xn) reactions, which is undergoing development in the framework of the Engineering Validation and Engineering Design Activities (EVEDA) project [1]. A liquid Li wall jet is planned to serve as a 10-MW deuteron beam target, which flows at a nominal velocity of 15 m/s (operational range: 10–16 m/s) under a vacuum pressure of 10−3 Pa at the nozzle-exit, at a Li temperature of 250 ◦ C.
∗ Corresponding author. E-mail address: [email protected] (T. Kanemura).
In addition to the role of neutron production, secure removal of 10 MW heat-equivalence from the deuteron beam under a vacuum condition of 10−3 Pa is another vital role of the Li target. The boiling point (Tb ) of Li at a pressure of 10−3 Pa, is 344 ◦ C. Accordingly, the degree of subcooling T = Tb − TLi , where TLi is the Li temperature at a point of interest) is just 94 ◦ C, even under no heat-input conditions. To prevent Li boiling, the Li target is designed to flow along a concave back wall [2]. The highest temperature of Li, resulting from energy deposition from the deuteron beam, is predicted to be located at the Bragg peak of a deuteron energy deposition profile in Li. The static pressure inside Li around the Bragg peak must be increased to prevent Li-boiling. The system’s static pressure is designed to be increased by a centrifugal force acting radially
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Please cite this article in press as: T. Kanemura, et al., Measurement of lithium target surface velocity in the IFMIF/EVEDA lithium test loop, Fusion Eng. Des. (2016), http://dx.doi.org/10.1016/j.fusengdes.2015.10.030
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on the Li flowing along a concave wall, resulting in a T at the Bragg peak exceeding a few-hundred ◦ C. In contrast, the free surface of the Li target is continuously exposed to a vacuum environment of 10−3 Pa. Considering the continuity of pressure across the liquid–gas interface (neglecting the Laplace pressure caused by surface tension), the static pressure of the Li surface is always 10−3 Pa. In the current design, the free surface of the Li target is under the most severe heat load condition with respect to Li boiling; [2,3] showed that the T is minimized at the free surface at any velocity, and decreases with a decrease in target velocity (T = 22 ◦ C at 10 m/s, 41 ◦ C at 15 m/s, and 58 ◦ C at 20 m/s). The relationship between T and free-surface velocity (us ) can be assumed to be linear; therefore, we can estimate that the allowable minimum us is approximately 4 m/s. This value, being adequately low compared with the operational velocity, gives us the impression that there is considerable margin before attaining Li-boiling. However, considering the error of numerical calculations and actual velocity fluctuations, we have to measure actual freesurface velocity under the IFMIF condition from the viewpoint of validation of the Li target. Hence, the overall objective of this study is to measure the freesurface velocity field of the Li target to evaluate the heat removal performance of the Li target. In the framework of the IFMIF/EVEDA project, we measured the surface velocity field around the area corresponding to the IFMIF beam footprint under the following IFMIF conditions: cross-sectional average velocities (Um ) of 10–20 m/s, a vacuum condition of 10−3 Pa, and a Li temperature of 250 ◦ C. 2. Surface-wave tracking (SWAT) method 2.1. Outline The measurement technique employed for this endeavor was the surface-wave tracking (SWAT) method, developed by the authors [4]. The SWAT method is based on the particle image velocimetry (PIV) technique that is typically applied to water (transparent fluid) for measuring a two- or three-dimensional velocity field using cross-correlation values in two interrogation regions of double-images seeding tracer particles. PIV is a powerful method to investigate a flow field, but has a fundamentally unavoidable problem for application to Li flow. Tracer particles cannot be seeded into a Li circulation loop. Therefore, we have developed an original SWAT method to measure surface-wave velocities. The SWAT method enables us to measure a two-dimensional surface-wave velocity distribution using the cross-correlation analysis of intensity patterns generated by free-surface waves, without the seeding of any tracer particles. The measurement procedure of the SWAT method is described and illustrated in [4]. By analyzing a pair of images taken sequentially with a time interval t, the movement vector (d) at a given measurement point can be obtained. Thus, the velocity vector (u) can be calculated as follows: u=˛
d t
(1)
where ˛ is the image scale (m/pixel). Furthermore, a subpixel analysis was employed to improve the measurement resolution, which is defined by 0.1˛/t [4]. 2.2. SWAT method notes The following two points must be emphasized when using the SWAT method. First, the relationship between surface-wave velocity (uwave ) and surface velocity of the jet (us ). In principle, we measure uwave , not us , with the SWAT method. When a free-surface flow is in a
laminar condition, uwave is separable into the jet surface velocity and the phase speed of waves (cwave ), as uwave = cwave + us . Our previous study [5] conducted in the Osaka University Li loop has experimentally proven that the above relationship (uwave = cwave + us ) holds true when Um ≤ 8 m/s. In this velocity range, the free-surface of a Li flow produced by an IFMIF-type nozzle is in a laminar condition. In contrast, at higher velocities (e.g., Um ≥ 10 m/s), the boundary layer at the nozzle exit becomes turbulent and the produced free-surface is in a turbulent condition [6]. When the latter is at hand, free-surface fluctuations are no longer waves, but are instead caused by turbulent fluid motions beneath the free surface. In this case, irregular patterns appearing on the free surface travel at the surface velocity with no or negligible phase velocity (i.e., uwave ≈ us ). In another previous study [4], it was experimentally proven that this relation (uwave ≈ us ) holds true when Um ≥ 10 m/s. This is the actual scenario applicable to the present study; thus, the measurement results in this study are characterized by free-surface velocity, not surface-wave velocity. Second, is the effect of surface wakes on measurement results. It was found that if a surface wake exists inside a measurement area, then the measured velocity vectors are directed toward the edge lines of the surface wakes and the their magnitudes are reduced to some extent (5–10%) compared with Um [4]. This may be caused by the secondary flow induced by the surface wake. When using the SWAT method, we assume the measured velocity (u) has only the X and Y components, and not the Z component, as u = ux ix + uy iy . If a relatively large locally-stationary free-surface deformation induced by the surface wake exists, the velocity near that deformation always has three components, expressed as follows: u = ux ix + uy iy + uz iz . This results in measured velocities being lower than real surface velocities. In such cases, the actual relationship between the measured velocity and the surface velocity is presently unknown. Thus, we must eliminate or essentially overlook the subject result measured in the region where the surface wake exists. 3. Experiment 3.1. EVEDA Li test loop Experiments were conducted in the EVEDA Li test loop (ELTL) constructed in an earlier stage of the present project [7]. The main Li loop of the ELTL supplies liquid-Li to the target assembly (TA), where the Li target is generated. The maximum flow rate of the main pump is 3000 L/min, which is equivalent to a target speed of 20 m/s. The space above the Li free-surface in the TA can be evacuated using a turbo-molecular pump on the order of 10−3 Pa. The TA of ELTL can generate a concave Li jet 25 mm thick (full scale of the IFMIF) and 100 mm wide (1/2.6 scale of the IFMIF) at the nozzle exit. Because of the reduced flow channel width, the area relevant to the beam footprint is 50 mm in height and 40 mm in width around the beam center [8]. The TA is shown in the right side of Fig. 1. It mainly comprises a nozzle and a concave flow channel with a gradually changing curvature-radius. The nozzle has two contraction parts, whose contraction ratios are 4 in the first upstream part, 2.5 in the second part, and 10 in total. A flow straightener consisting of a honeycomb and three perforated plates is used in the upstream of the nozzle. The TA is equipped with a large diameter port for measurement of free-surface flow. 3.2. Experimental setup and conditions Measurement instruments are a high-speed video (HSV) camera (Fastcam SA5, Photron Ltd.), a camera lens (Nikon AF Micro-Nikkor 200 mm F/4D IF-ED with Kenko TELEPLUS PRO 300 AF 2.0× DGX),
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size of interrogation area was determined to be approximately 12 mm, which is 3–4 times larger than the dominant wavelength of free-surface waves, which is the optimal value revealed in our preliminary experiments (If the size becomes smaller, the information which is contained in the interrogation area is not enough to identify a specific wave pattern. If the size becomes larger, the amount of information is enough, but the spatial resolution of velocity distribution measurement is reduced accordingly). 4. Results and discussion 4.1. Free-surface images
Fig. 1. Experimental setup (unit: mm). Table 1 Experimental conditions.
Flow condition
Cross-sectional average Li speed, Um [m/s] Gas pressure, P [Pa] Li temperature, T [◦ C] Measurement area [mm]
SWAT measurement condition
The size of interrogation area [mm × mm] Frame rate [Hz] Time interval, t [ms] Exposure time [s] Recording time [s] Image scale, ˛ [mm/pixel] Velocity resolution [m/s] Measurement uncertainty
10, 15, 20 10−3 250 Square area around the beam center: approx. −40 ≤ X ≤ 40 and −40 ≤ Y ≤ 40 11.52 × 11.52 (64 pixels × 64 pixels) 5000 0.2 (= 1/5000 s) 11 (10 m/s), 6.9 (15 m/s), and 4.5 (20 m/s) 5 (= 25,000 images) Approx. 0.18 0.09 (= 0.1˛/t) 1% RD (reading value)
four 210-W metal halide lamps (LS-M210, Sumita Optical Glass, Inc.), and two 350-W metal halide lamps (LS-M350, Sumita Optical Glass, Inc.). A sketch of the camera position is given in Fig. 1 (the lamps are not illustrated). The camera was installed approximately 2 m away from the Li target. Here we define the X and Y coordinates as the streamwise and spanwise directions, respectively, and the origin of the X and Y coordinates is located at the beam center. The area corresponding to the IFMIF beam footprint is the rectangle of −25 mm ≤ X ≤ 25 mm in the X direction and −20 mm ≤Y ≤ 20 mm in the Y direction (Y = ±50 mm denotes the locations of the side walls) for the present ELTL case. The key parameters associated with the experiment are provided in Table 1. Target speed (Um ) is calculated by dividing the Li flow rate (F) [measured with the electromagnetic flowmeter (EMF)], by the cross-sectional area (A) of the nozzle outlet of the TA (Um = F/A). The measurement area in the SWAT was approximated as a “square”-area surrounding the beam center. The exposure time was set to be sufficiently short to prevent motion-blur (the utilized exposure time was selected such that the traveling distance of the free surface during that time does not exceed one pixel). The time interval (t) of 0.2 ms (= frame rate of 5 kHz) was selected to reduce the measurement resolution to a sufficiently small value. Furthermore, wave decay due to viscous dissipation during the time interval must be considered. The modulus of decay () can be estimated to be more than 13 ms in the same manner as described in [4]. This value is sufficiently longer than the time interval of 0.2 ms. Thus, deformation of the surface shape due to viscosity can be ignored in the present setting of the frame rate. The recording time was ultimately set to 5 s, whereby 24,999 successive velocity fields were accordingly obtained per single measurement. The
Fig. 2 shows typical samples of the free-surface images taken for the SWAT measurement. The insides of the yellow dot-anddash rectangles correspond to the area(s) relevant to the beam footprint in the IFMIF. The red dots denote the beam center(s). The measurement areas are enclosed by the white dashed lines. The scale of free-surface fluctuations decreased with higher velocities. At Um = 20 m/s, the free-surface structures appeared to be characterized by very tiny turbulent fluctuations and did not display the appearance of waves. It was observed that at all velocities, the free-surface structures were initially small just after the nozzle exit, became larger (wavelengths became longer) to some extent along the streamwise direction, and finally became small again around the beam footprint. The other wave pattern, an oblique wave pattern, can be observed near both side walls. In particular, at Um = 10 m/s, the oblique wave patterns are clearly seen in the regions enclosed by the dotted green lines in Fig. 2 (a). These are the patterns of surface wakes initially originated at the corners between the nozzle edge and the side walls as shown in Fig. 4 of [8]. The wake profile spread more into the flow center region at a lower Um . As shown in Fig. 2 (c), at Um = 20 m/s, wake patterns are no longer observed. 4.2. Two-dimensional velocity distribution Fig. 3 shows two-dimensional velocity distributions with velocity vectors. The contour indicates the norm of the velocity vectors: U = u = u2x + u2y . The effect of the surface wakes generated at the corners between the nozzle edge and the side walls is clearly seen near both side walls. The wake effect seems to range from both side walls to the edges of the beam footprint at Um = 10 m/s. As Um increases, the uniformity of the velocity field similarly increases because the spreading angle of the surface wake becomes narrower. The nonuniformity of the velocity distribution inside the area relevant to the IFMIF beam footprint is less than 1 m/s even if Um = 10 m/s, and is thus not problematic for the IFMIF. These measurement results are consistent with the observation results in Fig. 2. 4.3. Time-averaged velocity along the streamwise direction Fig. 4 shows the X and Y components of the time-averaged velocity along the X direction at Y = 0 (flow center line). Fig. 4(a) and (b) shows the X component (ux ) and the Y component (ux ), respectively. The bars indicate root-mean-square (RMS) values for each velocity and location. One can see from Fig. 4(a) that free-surface velocities nearly recover to Um at all the velocities (the free-surface velocity is initially zero at the nozzle exit and gradually increases along the flow direction), and that the distribution of ux is almost uniform along the flow center line. Fig. 4(b) conveys that uy is nearly a zero value on average. Table 2 shows the list of velocities measured at the beam center under the IFMIF condition as a summary of the measurement
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Fig. 2. Free-surface images taken for SWAT. (For interpretation of the references to color in the text, the reader is referred to the web version of this article.)
results. The X component (ux ) is almost equal to the cross-sectional mean velocity (Um ), and the Y component (ux ) is almost zero. 4.4. Comparisons between measurement results and analytical results We can analytically predict the velocity profile in the Li target as derived in [9]. In [9], it was assumed that the free-surface velocity at the nozzle exit is not zero, but rather, Um . Moreover, it was also asserted that no side wall effects exist and that the subject flow is completely two-dimensional. In reality, the free-surface velocity is zero at the nozzle exit, and then accelerated toward a value near Um due to viscosity influences. This acceleration occurs and finishes
Fig. 3. Two-dimensional surface velocity field.
just after the nozzle exit; thus, it was ultimately ignored to simplify the analysis. Therefore, overall, the aforementioned assumptions are proven to function well because the predictability of target thicknesses is excellent, as shown in [9]. Table 3 shows the results of the comparison between the measured and analyzed surface velocities at the beam center
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(X, Y) = (0, 0). The analysis accurately predicts the measurement results within 3%, even at a Um = 20 m/s. 5. Summary In this study, we described the experimental results of surface velocity for a Li target measured at the EVEDA Li test loop (ELTL) using the surface-wave tracking (SWAT) method under the IFMIF conditions of P = 10−3 Pa and T = 250 ◦ C. Two-dimensional, time-averaged velocity distributions around the IFMIF beam footprint showed that the effect of surface wakes generated at the corners between the nozzle edge and the side walls was clearly observed near both side walls. The wake-effects appear to range from both side walls to the edges of the beam footprint at Um = 10 m/s. The spread-angle of the wakes becomes narrower at higher velocities. In the IFMIF beam footprint, the time-averaged velocity distribution is almost uniform and close to the cross-sectional average velocity Um . The nonuniformity of the velocity distribution is less than 1 m/s, even if Um = 10 m/s, and is therefore non-problematic. At the beam center, the streamwise component of the velocity vector is 10.3 m/s, 14.7 m/s, and 19.5 m/s at Um = 10, 15, and 20 m/s, respectively. The comparison between the measured and analyzed surface velocities at the beam center [(X, Y) = (0, 0)] demonstrated that the analysis efficiently predicted the measurement results within a 3% margin. Thus, it was successfully confirmed that the Li target would deliver adequate heat removal performance in the IFMIF, as designed. Acknowledgements The authors would like to express their gratitude to Dr. J. Knaster, the leader of the IFMIF/EVEDA Project, and Drs. F. Groeschel and F.S. Nitti, the former Li Target Group of the IFMIF/EVEDA Project, for their support of this present research endeavor. Fig. 4. Velocity profile as a function of X; shadow areas denote non-irradiated areas in the IFMIF.
Table 2 Measured velocities at the beam center under the IFMIF condition. Um [m/s]
Velocity component
Average [m/s]
RMS [m/s]
10
ux uy
10.3 0.2
0.8 0.8
15
ux uy
14.7 0.0
0.4 1.0
20
ux uy
19.5 0.0
0.6 0.9
Table 3 Comparisons between measured and analyzed surface velocities at the beam center. Um [m/s]
Measurement [m/s]
Analysis [m/s]
10 15 20
10.3 14.7 19.5
10.2 15.1 20.1
References [1] J. Knaster, et al., IFMIF: overview of the validation activities, Nucl. Fusion 53 (2013) 116001, 18 pp. [2] M. Ida, H. Nakamura, H. Nakamura, H. Takeuchi, Designs of contraction nozzle and concave back-wall for IFMIF target, Fusion Eng. Des. 70 (2004) 95–106. [3] J. Knaster, et al., Assessment of the beam–target interaction of IFMIF: a state of the art, Fusion Eng. Des. 89 (2014) 1709–1716. [4] H. Sugiura, T. Kanemura, et al., J. Nucl. Sci. Technol. 48 (2011) 1230–1237. [5] H. Sugiura, et al., Development of velocity measurement on a liquid lithium flow for IFMIF, Fusion Eng. Des. 84 (2009) 1803–1807. [6] K. Itoh, et al., Initial free surface instabilities on a high-speed water jet simulating a liquid-metal target, Fusion Technol. 36 (1999) 69–84. [7] H. Kondo, T. Furukawa, Y. Hirakawa, K. Nakamura, M. Ida, K. Watanabe, T. Kanemura, et al., IFMIF/EVEDA lithium test loop: design and fabrication technology of target assembly as a key component, Nucl. Fusion 51 (2011) 123008, 12 pp. [8] H. Kondo, et al., Validation of IFMIF liquid Li target for IFMIF/EVEDA project, Fusion Eng. Des. (2015), http://dx.doi.org/10.1016/j.fusengdes.2015.01.036. [9] T. Kanemura, et al., Measurement of Li target thickness in the EVEDA Li test loop, Fusion Eng. Des. (2015), http://dx.doi.org/10.1016/j.fusengdes.2015.06. 060.
Please cite this article in press as: T. Kanemura, et al., Measurement of lithium target surface velocity in the IFMIF/EVEDA lithium test loop, Fusion Eng. Des. (2016), http://dx.doi.org/10.1016/j.fusengdes.2015.10.030
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Measurement of lithium target surface velocity in the IFMIF/EVEDA lithium test loop Takuji Kanemura a,∗ , Hiroo Kondo a , Tomohiro Furukawa a , Yasushi Hirakawa a , Eiji Hoashi b , Sachiko Yoshihashi c , Hiroshi Horiike c , Eiichi Wakai a a
Japan Atomic Energy Agency, 4002 Narita, O-arai, Higashi-Ibaraki-gun, Ibaraki 311-1393, Japan Osaka University, 2-1 Yamada-oka, Suita, Osaka 565-0871, Japan c Fukui University of Technology, Gakuen 3-6-1, Fukui-shi, Fukui 910-8505, Japan b
h i g h l i g h t s • • • • •
The objective is to measure the free-surface velocity field of the IFMIF Li target. The Li target has an important role to remove 10 MW heat input from a deuteron beam. The free-surface of the Li target is under the most severe heat load condition. Measured surface velocities are almost equal to cross-sectional average velocities. It was confirmed that the IFMIF Li target has adequate heat removal performance.
a r t i c l e
i n f o
Article history: Received 19 August 2015 Received in revised form 26 October 2015 Accepted 26 October 2015 Available online xxx Keywords: IFMIF EVEDA Lithium Jet Surface velocity Measurement
a b s t r a c t In the framework of the Engineering Validation and Engineering Design Activities (EVEDA) project of the International Fusion Materials Irradiation Facility (IFMIF), we measured surface velocity fields of a lithium (Li) target at the EVEDA Li test loop under specifically-designated IFMIF conditions (target speeds of 10, 15, and 20 m/s, vacuum pressure of 10−3 Pa, and Li temperature of 250 ◦ C). In the current design of the IFMIF, the free surface of the Li target is under a most severe heat load condition with respect to Li boiling. The objective of this study is to measure the actual free-surface velocity under these IFMIF conditions to evaluate the heat removal performance of the Li target. The measured results (using the surface-wave tracking method that our team developed) showed two-dimensional time-averaged velocity distributions around the IFMIF beam footprint being virtually uniform, and close to the cross-sectional average velocity. The uniformity of the velocity distributions was less than 1 m/s. The comparison between the measured and analyzed surface velocity at the beam center showed that the analysis accurately predicts the measurement results within a margin of 3%. Finally, it was confirmed that the Li target delivers adequate heat removal performance in the IFMIF as designed. © 2015 Elsevier B.V. All rights reserved.
1. Introduction The International Fusion Materials Irradiation Facility (IFMIF) is an accelerator-based neutron source utilizing Li(d, xn) reactions, which is undergoing development in the framework of the Engineering Validation and Engineering Design Activities (EVEDA) project [1]. A liquid Li wall jet is planned to serve as a 10-MW deuteron beam target, which flows at a nominal velocity of 15 m/s (operational range: 10–16 m/s) under a vacuum pressure of 10−3 Pa at the nozzle-exit, at a Li temperature of 250 ◦ C.
∗ Corresponding author. E-mail address: [email protected] (T. Kanemura).
In addition to the role of neutron production, secure removal of 10 MW heat-equivalence from the deuteron beam under a vacuum condition of 10−3 Pa is another vital role of the Li target. The boiling point (Tb ) of Li at a pressure of 10−3 Pa, is 344 ◦ C. Accordingly, the degree of subcooling T = Tb − TLi , where TLi is the Li temperature at a point of interest) is just 94 ◦ C, even under no heat-input conditions. To prevent Li boiling, the Li target is designed to flow along a concave back wall [2]. The highest temperature of Li, resulting from energy deposition from the deuteron beam, is predicted to be located at the Bragg peak of a deuteron energy deposition profile in Li. The static pressure inside Li around the Bragg peak must be increased to prevent Li-boiling. The system’s static pressure is designed to be increased by a centrifugal force acting radially
http://dx.doi.org/10.1016/j.fusengdes.2015.10.030 0920-3796/© 2015 Elsevier B.V. All rights reserved.
Please cite this article in press as: T. Kanemura, et al., Measurement of lithium target surface velocity in the IFMIF/EVEDA lithium test loop, Fusion Eng. Des. (2016), http://dx.doi.org/10.1016/j.fusengdes.2015.10.030
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on the Li flowing along a concave wall, resulting in a T at the Bragg peak exceeding a few-hundred ◦ C. In contrast, the free surface of the Li target is continuously exposed to a vacuum environment of 10−3 Pa. Considering the continuity of pressure across the liquid–gas interface (neglecting the Laplace pressure caused by surface tension), the static pressure of the Li surface is always 10−3 Pa. In the current design, the free surface of the Li target is under the most severe heat load condition with respect to Li boiling; [2,3] showed that the T is minimized at the free surface at any velocity, and decreases with a decrease in target velocity (T = 22 ◦ C at 10 m/s, 41 ◦ C at 15 m/s, and 58 ◦ C at 20 m/s). The relationship between T and free-surface velocity (us ) can be assumed to be linear; therefore, we can estimate that the allowable minimum us is approximately 4 m/s. This value, being adequately low compared with the operational velocity, gives us the impression that there is considerable margin before attaining Li-boiling. However, considering the error of numerical calculations and actual velocity fluctuations, we have to measure actual freesurface velocity under the IFMIF condition from the viewpoint of validation of the Li target. Hence, the overall objective of this study is to measure the freesurface velocity field of the Li target to evaluate the heat removal performance of the Li target. In the framework of the IFMIF/EVEDA project, we measured the surface velocity field around the area corresponding to the IFMIF beam footprint under the following IFMIF conditions: cross-sectional average velocities (Um ) of 10–20 m/s, a vacuum condition of 10−3 Pa, and a Li temperature of 250 ◦ C. 2. Surface-wave tracking (SWAT) method 2.1. Outline The measurement technique employed for this endeavor was the surface-wave tracking (SWAT) method, developed by the authors [4]. The SWAT method is based on the particle image velocimetry (PIV) technique that is typically applied to water (transparent fluid) for measuring a two- or three-dimensional velocity field using cross-correlation values in two interrogation regions of double-images seeding tracer particles. PIV is a powerful method to investigate a flow field, but has a fundamentally unavoidable problem for application to Li flow. Tracer particles cannot be seeded into a Li circulation loop. Therefore, we have developed an original SWAT method to measure surface-wave velocities. The SWAT method enables us to measure a two-dimensional surface-wave velocity distribution using the cross-correlation analysis of intensity patterns generated by free-surface waves, without the seeding of any tracer particles. The measurement procedure of the SWAT method is described and illustrated in [4]. By analyzing a pair of images taken sequentially with a time interval t, the movement vector (d) at a given measurement point can be obtained. Thus, the velocity vector (u) can be calculated as follows: u=˛
d t
(1)
where ˛ is the image scale (m/pixel). Furthermore, a subpixel analysis was employed to improve the measurement resolution, which is defined by 0.1˛/t [4]. 2.2. SWAT method notes The following two points must be emphasized when using the SWAT method. First, the relationship between surface-wave velocity (uwave ) and surface velocity of the jet (us ). In principle, we measure uwave , not us , with the SWAT method. When a free-surface flow is in a
laminar condition, uwave is separable into the jet surface velocity and the phase speed of waves (cwave ), as uwave = cwave + us . Our previous study [5] conducted in the Osaka University Li loop has experimentally proven that the above relationship (uwave = cwave + us ) holds true when Um ≤ 8 m/s. In this velocity range, the free-surface of a Li flow produced by an IFMIF-type nozzle is in a laminar condition. In contrast, at higher velocities (e.g., Um ≥ 10 m/s), the boundary layer at the nozzle exit becomes turbulent and the produced free-surface is in a turbulent condition [6]. When the latter is at hand, free-surface fluctuations are no longer waves, but are instead caused by turbulent fluid motions beneath the free surface. In this case, irregular patterns appearing on the free surface travel at the surface velocity with no or negligible phase velocity (i.e., uwave ≈ us ). In another previous study [4], it was experimentally proven that this relation (uwave ≈ us ) holds true when Um ≥ 10 m/s. This is the actual scenario applicable to the present study; thus, the measurement results in this study are characterized by free-surface velocity, not surface-wave velocity. Second, is the effect of surface wakes on measurement results. It was found that if a surface wake exists inside a measurement area, then the measured velocity vectors are directed toward the edge lines of the surface wakes and the their magnitudes are reduced to some extent (5–10%) compared with Um [4]. This may be caused by the secondary flow induced by the surface wake. When using the SWAT method, we assume the measured velocity (u) has only the X and Y components, and not the Z component, as u = ux ix + uy iy . If a relatively large locally-stationary free-surface deformation induced by the surface wake exists, the velocity near that deformation always has three components, expressed as follows: u = ux ix + uy iy + uz iz . This results in measured velocities being lower than real surface velocities. In such cases, the actual relationship between the measured velocity and the surface velocity is presently unknown. Thus, we must eliminate or essentially overlook the subject result measured in the region where the surface wake exists. 3. Experiment 3.1. EVEDA Li test loop Experiments were conducted in the EVEDA Li test loop (ELTL) constructed in an earlier stage of the present project [7]. The main Li loop of the ELTL supplies liquid-Li to the target assembly (TA), where the Li target is generated. The maximum flow rate of the main pump is 3000 L/min, which is equivalent to a target speed of 20 m/s. The space above the Li free-surface in the TA can be evacuated using a turbo-molecular pump on the order of 10−3 Pa. The TA of ELTL can generate a concave Li jet 25 mm thick (full scale of the IFMIF) and 100 mm wide (1/2.6 scale of the IFMIF) at the nozzle exit. Because of the reduced flow channel width, the area relevant to the beam footprint is 50 mm in height and 40 mm in width around the beam center [8]. The TA is shown in the right side of Fig. 1. It mainly comprises a nozzle and a concave flow channel with a gradually changing curvature-radius. The nozzle has two contraction parts, whose contraction ratios are 4 in the first upstream part, 2.5 in the second part, and 10 in total. A flow straightener consisting of a honeycomb and three perforated plates is used in the upstream of the nozzle. The TA is equipped with a large diameter port for measurement of free-surface flow. 3.2. Experimental setup and conditions Measurement instruments are a high-speed video (HSV) camera (Fastcam SA5, Photron Ltd.), a camera lens (Nikon AF Micro-Nikkor 200 mm F/4D IF-ED with Kenko TELEPLUS PRO 300 AF 2.0× DGX),
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size of interrogation area was determined to be approximately 12 mm, which is 3–4 times larger than the dominant wavelength of free-surface waves, which is the optimal value revealed in our preliminary experiments (If the size becomes smaller, the information which is contained in the interrogation area is not enough to identify a specific wave pattern. If the size becomes larger, the amount of information is enough, but the spatial resolution of velocity distribution measurement is reduced accordingly). 4. Results and discussion 4.1. Free-surface images
Fig. 1. Experimental setup (unit: mm). Table 1 Experimental conditions.
Flow condition
Cross-sectional average Li speed, Um [m/s] Gas pressure, P [Pa] Li temperature, T [◦ C] Measurement area [mm]
SWAT measurement condition
The size of interrogation area [mm × mm] Frame rate [Hz] Time interval, t [ms] Exposure time [s] Recording time [s] Image scale, ˛ [mm/pixel] Velocity resolution [m/s] Measurement uncertainty
10, 15, 20 10−3 250 Square area around the beam center: approx. −40 ≤ X ≤ 40 and −40 ≤ Y ≤ 40 11.52 × 11.52 (64 pixels × 64 pixels) 5000 0.2 (= 1/5000 s) 11 (10 m/s), 6.9 (15 m/s), and 4.5 (20 m/s) 5 (= 25,000 images) Approx. 0.18 0.09 (= 0.1˛/t) 1% RD (reading value)
four 210-W metal halide lamps (LS-M210, Sumita Optical Glass, Inc.), and two 350-W metal halide lamps (LS-M350, Sumita Optical Glass, Inc.). A sketch of the camera position is given in Fig. 1 (the lamps are not illustrated). The camera was installed approximately 2 m away from the Li target. Here we define the X and Y coordinates as the streamwise and spanwise directions, respectively, and the origin of the X and Y coordinates is located at the beam center. The area corresponding to the IFMIF beam footprint is the rectangle of −25 mm ≤ X ≤ 25 mm in the X direction and −20 mm ≤Y ≤ 20 mm in the Y direction (Y = ±50 mm denotes the locations of the side walls) for the present ELTL case. The key parameters associated with the experiment are provided in Table 1. Target speed (Um ) is calculated by dividing the Li flow rate (F) [measured with the electromagnetic flowmeter (EMF)], by the cross-sectional area (A) of the nozzle outlet of the TA (Um = F/A). The measurement area in the SWAT was approximated as a “square”-area surrounding the beam center. The exposure time was set to be sufficiently short to prevent motion-blur (the utilized exposure time was selected such that the traveling distance of the free surface during that time does not exceed one pixel). The time interval (t) of 0.2 ms (= frame rate of 5 kHz) was selected to reduce the measurement resolution to a sufficiently small value. Furthermore, wave decay due to viscous dissipation during the time interval must be considered. The modulus of decay () can be estimated to be more than 13 ms in the same manner as described in [4]. This value is sufficiently longer than the time interval of 0.2 ms. Thus, deformation of the surface shape due to viscosity can be ignored in the present setting of the frame rate. The recording time was ultimately set to 5 s, whereby 24,999 successive velocity fields were accordingly obtained per single measurement. The
Fig. 2 shows typical samples of the free-surface images taken for the SWAT measurement. The insides of the yellow dot-anddash rectangles correspond to the area(s) relevant to the beam footprint in the IFMIF. The red dots denote the beam center(s). The measurement areas are enclosed by the white dashed lines. The scale of free-surface fluctuations decreased with higher velocities. At Um = 20 m/s, the free-surface structures appeared to be characterized by very tiny turbulent fluctuations and did not display the appearance of waves. It was observed that at all velocities, the free-surface structures were initially small just after the nozzle exit, became larger (wavelengths became longer) to some extent along the streamwise direction, and finally became small again around the beam footprint. The other wave pattern, an oblique wave pattern, can be observed near both side walls. In particular, at Um = 10 m/s, the oblique wave patterns are clearly seen in the regions enclosed by the dotted green lines in Fig. 2 (a). These are the patterns of surface wakes initially originated at the corners between the nozzle edge and the side walls as shown in Fig. 4 of [8]. The wake profile spread more into the flow center region at a lower Um . As shown in Fig. 2 (c), at Um = 20 m/s, wake patterns are no longer observed. 4.2. Two-dimensional velocity distribution Fig. 3 shows two-dimensional velocity distributions with velocity vectors. The contour indicates the norm of the velocity vectors: U = u = u2x + u2y . The effect of the surface wakes generated at the corners between the nozzle edge and the side walls is clearly seen near both side walls. The wake effect seems to range from both side walls to the edges of the beam footprint at Um = 10 m/s. As Um increases, the uniformity of the velocity field similarly increases because the spreading angle of the surface wake becomes narrower. The nonuniformity of the velocity distribution inside the area relevant to the IFMIF beam footprint is less than 1 m/s even if Um = 10 m/s, and is thus not problematic for the IFMIF. These measurement results are consistent with the observation results in Fig. 2. 4.3. Time-averaged velocity along the streamwise direction Fig. 4 shows the X and Y components of the time-averaged velocity along the X direction at Y = 0 (flow center line). Fig. 4(a) and (b) shows the X component (ux ) and the Y component (ux ), respectively. The bars indicate root-mean-square (RMS) values for each velocity and location. One can see from Fig. 4(a) that free-surface velocities nearly recover to Um at all the velocities (the free-surface velocity is initially zero at the nozzle exit and gradually increases along the flow direction), and that the distribution of ux is almost uniform along the flow center line. Fig. 4(b) conveys that uy is nearly a zero value on average. Table 2 shows the list of velocities measured at the beam center under the IFMIF condition as a summary of the measurement
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Fig. 2. Free-surface images taken for SWAT. (For interpretation of the references to color in the text, the reader is referred to the web version of this article.)
results. The X component (ux ) is almost equal to the cross-sectional mean velocity (Um ), and the Y component (ux ) is almost zero. 4.4. Comparisons between measurement results and analytical results We can analytically predict the velocity profile in the Li target as derived in [9]. In [9], it was assumed that the free-surface velocity at the nozzle exit is not zero, but rather, Um . Moreover, it was also asserted that no side wall effects exist and that the subject flow is completely two-dimensional. In reality, the free-surface velocity is zero at the nozzle exit, and then accelerated toward a value near Um due to viscosity influences. This acceleration occurs and finishes
Fig. 3. Two-dimensional surface velocity field.
just after the nozzle exit; thus, it was ultimately ignored to simplify the analysis. Therefore, overall, the aforementioned assumptions are proven to function well because the predictability of target thicknesses is excellent, as shown in [9]. Table 3 shows the results of the comparison between the measured and analyzed surface velocities at the beam center
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(X, Y) = (0, 0). The analysis accurately predicts the measurement results within 3%, even at a Um = 20 m/s. 5. Summary In this study, we described the experimental results of surface velocity for a Li target measured at the EVEDA Li test loop (ELTL) using the surface-wave tracking (SWAT) method under the IFMIF conditions of P = 10−3 Pa and T = 250 ◦ C. Two-dimensional, time-averaged velocity distributions around the IFMIF beam footprint showed that the effect of surface wakes generated at the corners between the nozzle edge and the side walls was clearly observed near both side walls. The wake-effects appear to range from both side walls to the edges of the beam footprint at Um = 10 m/s. The spread-angle of the wakes becomes narrower at higher velocities. In the IFMIF beam footprint, the time-averaged velocity distribution is almost uniform and close to the cross-sectional average velocity Um . The nonuniformity of the velocity distribution is less than 1 m/s, even if Um = 10 m/s, and is therefore non-problematic. At the beam center, the streamwise component of the velocity vector is 10.3 m/s, 14.7 m/s, and 19.5 m/s at Um = 10, 15, and 20 m/s, respectively. The comparison between the measured and analyzed surface velocities at the beam center [(X, Y) = (0, 0)] demonstrated that the analysis efficiently predicted the measurement results within a 3% margin. Thus, it was successfully confirmed that the Li target would deliver adequate heat removal performance in the IFMIF, as designed. Acknowledgements The authors would like to express their gratitude to Dr. J. Knaster, the leader of the IFMIF/EVEDA Project, and Drs. F. Groeschel and F.S. Nitti, the former Li Target Group of the IFMIF/EVEDA Project, for their support of this present research endeavor. Fig. 4. Velocity profile as a function of X; shadow areas denote non-irradiated areas in the IFMIF.
Table 2 Measured velocities at the beam center under the IFMIF condition. Um [m/s]
Velocity component
Average [m/s]
RMS [m/s]
10
ux uy
10.3 0.2
0.8 0.8
15
ux uy
14.7 0.0
0.4 1.0
20
ux uy
19.5 0.0
0.6 0.9
Table 3 Comparisons between measured and analyzed surface velocities at the beam center. Um [m/s]
Measurement [m/s]
Analysis [m/s]
10 15 20
10.3 14.7 19.5
10.2 15.1 20.1
References [1] J. Knaster, et al., IFMIF: overview of the validation activities, Nucl. Fusion 53 (2013) 116001, 18 pp. [2] M. Ida, H. Nakamura, H. Nakamura, H. Takeuchi, Designs of contraction nozzle and concave back-wall for IFMIF target, Fusion Eng. Des. 70 (2004) 95–106. [3] J. Knaster, et al., Assessment of the beam–target interaction of IFMIF: a state of the art, Fusion Eng. Des. 89 (2014) 1709–1716. [4] H. Sugiura, T. Kanemura, et al., J. Nucl. Sci. Technol. 48 (2011) 1230–1237. [5] H. Sugiura, et al., Development of velocity measurement on a liquid lithium flow for IFMIF, Fusion Eng. Des. 84 (2009) 1803–1807. [6] K. Itoh, et al., Initial free surface instabilities on a high-speed water jet simulating a liquid-metal target, Fusion Technol. 36 (1999) 69–84. [7] H. Kondo, T. Furukawa, Y. Hirakawa, K. Nakamura, M. Ida, K. Watanabe, T. Kanemura, et al., IFMIF/EVEDA lithium test loop: design and fabrication technology of target assembly as a key component, Nucl. Fusion 51 (2011) 123008, 12 pp. [8] H. Kondo, et al., Validation of IFMIF liquid Li target for IFMIF/EVEDA project, Fusion Eng. Des. (2015), http://dx.doi.org/10.1016/j.fusengdes.2015.01.036. [9] T. Kanemura, et al., Measurement of Li target thickness in the EVEDA Li test loop, Fusion Eng. Des. (2015), http://dx.doi.org/10.1016/j.fusengdes.2015.06. 060.
Please cite this article in press as: T. Kanemura, et al., Measurement of lithium target surface velocity in the IFMIF/EVEDA lithium test loop, Fusion Eng. Des. (2016), http://dx.doi.org/10.1016/j.fusengdes.2015.10.030