Numerical and experimental studies of Electra's scalloped transmission foil cooling with small impinging jets

Numerical and experimental studies of Electra's scalloped transmission foil cooling with small impinging jets

Fusion Engineering and Design 88 (2013) 3152–3156 Contents lists available at ScienceDirect Fusion Engineering and Design journal homepage: www.else...

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Fusion Engineering and Design 88 (2013) 3152–3156

Contents lists available at ScienceDirect

Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes

Numerical and experimental studies of Electra’s scalloped transmission foil cooling with small impinging jets Bo Lu a,∗ , S.I. Abdel-Khalik a , Dennis L. Sadowski a , Kevin G. Schoonover b a b

G.W. Woodruff School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, GA 30332, United States SCG LLC, Atlanta, GA, United States

h i g h l i g h t s • • • •

Obliquely impinging jets were investigated for cooling the scalloped hibachi foil of Electra gas laser. CFD simulations were performed to optimizing the jet impingement configurations. Experimental Nusselt numbers were obtained at various jet velocities, jet diameters and jet-to-surface spacing and an empirical correlation was derived. The studies show that impinging jets can effectively improve the average heat transfer coefficients of the Electra hibachi foil without severe deterioration of the laser quality.

a r t i c l e

i n f o

Article history: Received 4 April 2013 Received in revised form 20 September 2013 Accepted 23 September 2013 Available online 17 October 2013 Keywords: Inertial fusion energy Gas laser Scalloped hibachi foil Heat transfer enhancement Impinging jets

a b s t r a c t The 5 Hz rep-rate operation of the Electra KrF laser necessitates the cooling and protection of the transmission foil that is subject to the pulsating electron beam bombardment. The pulsed volumetric heating from the e-beam attenuation heats up the foil (∼2.54 × 10−5 m thick) rapidly and often causes the foil to fail, increasing the operation cost and down time for the laser. Various methods have been investigated forheat transfer enhancement. While elevated heat transfer was achieved, the previous methods assume a flat foil shape. The actual foil shape is scalloped due to the pressure difference across the foil during the laser operation. Also a new “scalloped” foil design was proposed for thermal stress reduction. This paper investigates the applicability of small locally impinging jets to cooling the scalloped-shaped foil. The jets were formed through a line of small circular openings on two stainless-steel jet tubes aligned with the foil edges having the two columns of jets impinging on the foil obliquely in a staggered pattern for improved coverage. CFD simulations were used to optimize jet configurations. Experiments were performed that utilize a scalloped foil strip which matched the foil shape between two neighboring supporting ribs in the Electra hibachi. Jet diameters and jet velocities were varied at a surface heat flux greater than 20.0 kW/m2 . Substantial heat transfer enhancement with impinging jets was observed. Average Nusselt numbers were correlated with jet Reynolds number and the normalized jet-to-foil distance. The study indicates that the impinging jets can effectively enhance heat transfer for the scalloped foil and can be a promising method for actual foil coolingof KrF lasers, including Electra. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Electra is an electron-beam pumped krypton fluoride (KrF) gas laser developed for inertial fusion energy research at the Naval Research Laboratory [1,2]. Operating at a nominal rep-rate of 5 Hz with laser energy output from 400 to 700 J, the laser has the capability of being scalable to 15–25 kJ energy output for a future inertial

∗ Corresponding author. Present address: Institute of Plasma Physics, Chinese Academy of Sciences, Hefei, Anhui 230031, China. Tel.: +86 55165591309; fax: +86 55165591310. E-mail address: [email protected] (B. Lu). 0920-3796/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.fusengdes.2013.09.004

fusion power plant. As one of the main components, the laser cell (30 × 30 × 100 cm) was designed to provide a 100 ns-long laser pulse at 5 Hz when an e-beam (500 kV and 100 kA) excites KrF gas and its subsequent deexciatation amplifies the input low-energy laser (Fig. 1). A thin stainless steel foil (25 ␮m) is used to separate the laser gas from the vacuum environments on the diode side and act as the e-beam transmission foil. In order to withstand the pressure differential, the large laser window was designed as a hibachi-like structure for support and seal, resulting in 24 uniformly spaced foil spans. Although stainless steel is good material for transmission foil for its excellent mechanical strength and durability, chemical compatibility with fluorine, and good e-beam transmission efficiency,

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Fig. 2. Jet impinging configuration for the scalloped foil (1/8W means the jet impinging location from the foil edge, where W is the foil width. The jet-to-foil distance L and the impingement angle  were also defined).

Fig. 1. Schematic of the Electra krypton fluoride gas laser cell [1].

the repetitive and pulsed operation causes substantial volumetric heating to the thin foil and often causes the foil to fail. Various heat transfer enhancement schemes have been studied to check their feasibility in cooling the hibachi foil. An actuated louver was shown to be able to reduce the foil temperature, but produced a large inhomogeneity in the gas field and low laser quality [3–5]. Two-phase mist cooling was very efficient, but the two-foil configuration necessary to contain the cooling medium would decrease laser efficiency and might bring about serious outcomes if the foil broke and liquid entered the laser cell [6,7]. A method using a nearwall planar jet seemed applicable, since the jet could use the same type as the laser gas [8,9]. Good heat transfer enhancement was observed but the method was not efficient enough because of the jet deceleration and large flow rate. Impinging jets, which were widely used in industry for cooling heated surfaces, were also investigated and seemed promising for the flat foil [10]. Average heat transfer enhancement of 60–700% depending on jet velocities was achieved. Since the jets were injected locally at a low flow rate, the laser quality was not affected substantially [9]. Recently, to further improve the performance of Electra, an advanced hibachi foil design was proposed, where the foil is curved and pre-stressed into a scalloped shape before being pressurized [11,12]. This concept was shown to have much smaller thermal stresses than the flat design, wherein the stress (von Mises) of scalloped foil was not only reduced in magnitude (a factor of ∼330 less) and but also more uniform than the flat foil for nominal operation [12]. While the scalloped design resulted in stress reduction, pulsed heating due to e-beam attenuation would still be a potential catastrophic failure mechanism, and active foil cooling would still be needed. To provide a useful reference for future Electra operation, impinging jet cooling was investigated here for the scalloped foil design. Both numerical and experimental studies were conducted at various jet velocities, jet size and jet impingement configuration. Section 2 describes the experimental setup. CFD simulations for jet impingement configuration optimization are presented in Section 3. Experiment results are summarized and discussed in Section 4. Finally the conclusions are given in Section 5. 2. Experimental setup The experiments in the current study use the same setup used in the previous study for the flat foil, except that the flat foil fixture was replaced with a scalloped one [10]. The scalloped foil strip usestype304 stainless steel at 0.0254 m thick. The electrically heated foil was approximately 4.45 cm wide with a radius of curvature of 4.14 cm, which is close to the proposed Electra scalloped hibachi design. The foil is thermally insulated on the back. As in the flat foil experiments, the foil was also fixed in a vertical rectangular channel, through

which a background air flow of approximately 4.0 m/s is imposed to simulate the laser gas flow. Experiments were performed at a heat flux of ∼20 kW/m2 , which is expected to be a characteristic value for the average foil heat loading during normal operations [13]. Foil temperature was imaged using an infrared thermography camera (FLIR PM280) with a spatial resolution of ∼1 mm. The camera was focused to measure about one third of the foil in the center. Since the jet supply should not block the e-beam pathway, the jets were produced through two rows of circular openings on two jet tubes that were vertically parallel to the foil’s long edges. When used on Electra, each jet tube would be located just in front of a hibachi rib. The stainless-steel jet tube had an OD of ∼9.6 mm and wall thickness of ∼0.5 mm. Circular openings were machined along a straight line at an 1.25-cm interval, so a line of jets was formed on each tube [10]. For each foil span, two jet tubes were used to cool the foil with a total number of 51 jets. There are 25 jets on one side and 26 on the other and they were interlaced to have an overall jet-to-jet spacing of ∼0.63 cm. Three sets of jet tubes were prepared with jet diameters (d) of 0.8, 1.2 and 1.6 mm, respectively. Fig. 2 shows a top view of the jet setup, in which jets impinged on the 1/3 foil width and the definition of jet-to-foil distance (L) and jet impingement angle () were also illustrated. Note left and right jets were actually offset vertically by ∼0.63 cm. Jet diameter and temperature were used as the characteristic length and reference temperature when evaluating local heat transfer coefficients and Nusselt numbers as follows, h=

q Tf − Tj

Nud =

hd k

(1)

(2)

where h is the heat transfer coefficient, Nud is Nusselt number; q is surface heat flux, Tf and Tj are temperatures of the foil and jet, respectively; d is jet diameter, and k is thermal conductivity of the fluid. 3. CFD optimization of jet impingement cooling The effectiveness of jet impingement cooling is mainly governed by several parameters including jet Reynolds number (Red ), jet-tofoil spacing (L), jet impingement angle and jet-to-jet pitch [14]. To reduce the degrees of freedom in the study, jet-to-jet spacing on each jet tube was fixed at 1.25 cm. Jets were designed to impinge on the foil obliquely. To achieve the maximum heat transfer enhancement, the jet tubes were mounted as close as possible to the foil to reduce jet-to-foil spacing. In the current study, the minimum perpendicular distance between the tube surface and the foil was ∼0.5 cm. With these limitations, the study was focused on the effect of the jet Reynolds number and jet tube orientation on the cooling effectiveness. Changing the jet tube orientation simultaneously changed the jet-to-foil spacing and jet impingement angle. To find the optimal jet impinging configuration, a twodimensional CFD model was developed that neglected the

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Fig. 3. CFD simulation model for jet impinging configuration.

Fig. 5. Velocity magnitude contour for two different jet impinging locations.

Fig. 4. Comparison of foil temperature and heat transfer coefficient for different jet impinging locations (simulation).

background flow (Fig. 3), since its effect was only adding extra cooling capability. Only one jet from the left jet tube was simulated, since the jets from the right jet tube were not on the same altitude. Two cases wereexamined, in which the jet stagnation point on the foil is located at 1/8 and 1/3 off oil width from the edge, respectively. For jets impinging further toward the center, the effect of increase in jet-to-foil distance would exponentially decrease the heat transfer coefficients. The jet would first impinge on the foil and form a wall jet to cool the entire foil. A room-temperature jet of 1.2 mm diameter and 30.0 m/s velocity was simulated to cool the foil which was heated volumetrically at 7.8 × 108 kW/m3 (or 20.0 kW/m2 ). Fig. 4 compared the foil temperature and surface heat transfer coefficients for the above two cases. It showed that when jets impinged near the edge, the foil edge was better cooled and the foil temperature increases downstream from the jet. When jets were directed toward the inner portion of the foil, the edge temperature was

high since the jet spread did not cover that area. However, the entire foil was cooled more uniformly than the previous case. It was interesting to note that the average temperature and heat transfer coefficient were nearly the same for both cases. Fig. 5 also shows the contour of velocity magnitude for two different impinging locations. Upon impingement, the jet further spread toward the right and subsequently formed a curved wall jet that cooled the curved foil. It could be seen that when the jet impinged near the edge of the foil, the wall jet developed fully and finally covered the entire surface. For the other case, since the jet was directed toward the foil center, the foil portion behind the jet impingement location was left unprotected by the wall jet. This could be detrimental if the heat flux was high and hot spots formed. Although the current simulation indicated that cooling effects were similar in both cases, jet impingement at the edge would be more desirable to provide a better wall jet coverage for the foil. Experiments conducted in the current investigation mainly used the 1/8 width configuration and varied the jet diameter and jet velocities.

4. Experimental studies Based on the numerical simulations, experiments were conducted at a fixed jet tube-to-foil surface distance and angle between the jet direction and the foil while varying the jet diameters and velocities. A sample thermal image of the foil temperature was shown to illustrate the cooling pattern of the foil with a jet velocity of only 10.0 m/s (Fig. 6). Upon impingement, the jet spread toward the center of the foil and the upward background cross flow tended to spread the jet in the flow direction as indicated by the deformed shape. Fig. 6 shows that the foil was not cooled as uniformly as desired due to non-uniform jet velocity distribution, although a header was added to the jet supply system to distribute the gas evenly between the two jet tubes. However, asymmetric cooling effect still existed between the left and right portion of the foil, leading to the uncertainty in the average heat transfer enhancement of the foil.

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Fig. 8. Heat transfer coefficient variation along three analysis lines.

velocity of only ∼10.0 m/s, heat transfer at the foil was significantly enhanced. For the centerline, the enhancement was ∼15% and for the two lines at the edges, the average heat transfer coefficients were 60–100% higher. In assessing the cooling effectiveness, average heat transfer enhancement was of bigger interest for the engineering design considerations. Fig. 9(a) plotted the average Nusselt number vs. jet Reynolds number at different jet diameters. The results show that over the relatively narrow range of Reynolds numbers investigated

Fig. 6. Sample thermal image of the foil.

A detailed foil temperature profile along the three vertical lines shown in Fig. 6 was plotted in Fig. 7. Similar to the flat foil cooling experiment [10], periodic temperature distribution along the foil was seen, although the peak temperature differed due to variation in the jet velocities. The foil was cooled in a very similar fashion on both edges, while the foil temperature on the centerline is much higher because jet cooling effectiveness decreased with the distance from the stagnation point. Fig. 8 compared the heat transfer coefficient for the three analysis lines. To illustrate the jet cooling effectiveness, the heat transfer coefficients when the jets were off were also plotted. It could be seen that even with a low jet

Fig. 7. Foil temperature profiles along three analysis lines.

Fig. 9. Average Nusselt number as a function of Reynolds number (experiments and fitting).

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left portion was not impacted substantially and the average heat transfer coefficient is not enhanced. 5. Conclusions

Fig. 10. Comparison of average Nusselt number at different jet impinging locations.

here, high Reynolds number led to a linear increase in heat transfer enhancement and at the same jet Reynolds number, larger jets were more efficient. Average Nusselt number was also well correlated with jet Reynolds number (Red )and the normalized jet-to-foil spacing (L/d) as often used in normally impinging jets, L/d

Nud,ave = 0.0027Red0.91 Pr 0.33 e−0.0081

(3)

where Pr is the Prandtl number (∼0.67). Fig. 9(b) shows a plot of the correlation against the experimental Nusselt numbers normalized using the exponential dependence on L/d and showed good agreement between the correlation and the experiments. Compared to the correlations obtained in experiments where the surface was uniformly covered by an array of normally impinging jets, a slightly larger exponent (0.91) in the Reynolds number was obtained (an exponent in the range of 0.7–0.8 was reported [14–16]). The difference was probably caused by the distinct experimental conditions used in the current investigation, where jets were impinging along the edges of the foil obliquely and an external cross flow was applied. Both conditions assisted in inducing a larger spread in the jet impingement region, which in turn contributed to the elevated heat transfer rate over the entire surface and was reflected in the more sensitive dependence on Reynolds number. A comparison of average Nusselt number at two different jet impingement locations also is shown in Fig. 10, where jets impinged near the edge and on the centerline, respectively. It was found that for the same Reynolds number, higher Nusselt numbers were obtained when jets were directed toward the edges, in which the wall jets could cover the foil entirely. For jets impinging on the centerline, the left portion of the foil is not cooled by the jets and the jet-to-foil length was also larger. The difference would be more pronounced at higher Reynolds number, because the entire foil was better cooled due the jet spread after the jets impinge on the edge. On the other hand, even at high Reynolds number, cooling of the

Advanced scalloped hibachi foil design of the Electra gas laser was proposed to reduce the stresses applied to the foil. To further ensure the steady operation of the scalloped foil, bench-top heat transfer experiments using impinging jets were conducted to investigate heat transfer enhancement from obliquely impinging jets. CFD simulations were used to optimize the experimental configuration, where jets were arranged as close to the foil as possible and directed at the edges of the foil. Experimental results indicate that heat transfer was significantly enhanced with the injection of two rows of interlaced impinging circular jets, with the extent of enhancement depending on the jet Reynolds number. Although impinging jets may not provide as uniform a cooling pattern as desired, it is shown to be very effective in enhancing the average heat transfer enhancement without deteriorating the laser quality. By reducing the jet spacing, it may be possible to achieve a more uniform foil cooling. This, however, may increase the pumping power requirements for the cooling system, and hence, reduce the overall laser efficiency. Acknowledgement Financial support by the Naval Research Laboratory through the hibachi foil cooling project was acknowledged. References [1] J.D. Sethian, M. Friedman, J.L. Giuliani Jr., R.H. Lehmberg, S.P. Obenschain, P. Kepple, et al., Phys. Plasmas 10 (2003) 2142–2146. [2] P.M. Burns, M. Myers, J.D. Sethian, M.F. Wolford, J.L. Giuliani, S.P. Obenschain, et al., Fusion Sci. Technol. 52 (2007) 445–453. [3] F. Hegeler, J.L. Giuliani, J.D. Sethian, M.C. Myers, M.F. Wolford, M. Friedman, Proceedings of 15th IEEE International Pulsed Power Source, 2005, pp. 1294–1297. [4] F. Hegeler, J.L. Giuliani, J.D. Sethian, M.C. Myers, M.F. Wolford, P.M. Burns, et al., IEEE Trans. Plasma Sci. 36 (2008) 778–793. [5] P.M. Burns, M. Myers, J.D. Sethian, M.F. Wolford, J.L. Giuliani, R.H. Lehmberg, et al., Fusion Sci. Technol. 56 (2009) 346–351. [6] V. Novak, D. Sadowski, S. Shin, K. Schoonover, S.I. Abdel-Khalik, Fusion Sci. Technol. 47 (2005) 610–615. [7] V. Novak, S.I. Abdel-Khalik, D.L. Sadowski, K.G. Schoonover, Fusion Sci. Technol. 52 (2007) 483–488. [8] B. Lu, S.I. Abdel-Khalik, D.L. Sadowski, K.G. Schoonover, F. Hegeler, P.M. Burns, et al., Fusion Sci. Technol. 56 (2009) 441–445. [9] B. Lu, S.I. Abdel-Khalik, D.L. Sadowski, K.G. Schoonover, J. Fusion Energy 30 (2011) 453–458. [10] B. Lu, S.I. Abdel-Khalik, D.L. Sadowski, K.G. Schoonover, Fusion Eng. Des. 87 (2012) 352–358. [11] R. Jaynes, T. Albert, F. Hegeler, J.D. Sethian, Proceedings of 16th IEEE International Pulsed Power Source, 2007, pp. 826–830. [12] A. Aoyama, J. Blanchard, J.D. Sethian, N. Ghoniem, S. Sharafat, Fusion Sci. Technol. 56 (2009) 435–440. [13] J.L. Giuliani, F. Hegeler, J.D. Sethian, M.F. Wolford, M.C. Myers, S. Abdel-Khalik, et al., J. Phys. IV France 133 (2006) 637–639. [14] H. Martin, in: T. Irvine Jr., J. Harnett (Eds.), Advances in Heat Transfer, vol. 13, Academic Press, New York, 1977, pp. 1–60. [15] K. Jambunathan, E. Lai, M.A. Moss, B.L. Button, Int. J. Heat Fluid Flow 13 (1992) 106–115. [16] R. Viskanta, Exp. Thermal Fluid Sci. 6 (1993) 111–134.