atw 2017-06

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atw Vol. 62 (2017) | Issue 6 ı June RESEARCH AND INNOVATION 418 Acknowledgement This study was sponsored by the Ministry of Trade, Industry and Energy (MOTIE) and was supported by Nuclear Convergence and Original Technology Development Program Grant funded by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) (Grant code: 20111520100030) Nomenclature P pressure (Pa) h heat transfer coefficient (W/m2°C) W weight fraction (w/o) T temperature (°C) k thermal conductivity (W/m°C) N number of pipe ( - ) L length (m) D diameter (m) ρ density(kg/m 3 ) c specific heat (J/kg-°C) q heat flux (W/m 2 ) g gravity (m/s 2 ) h latent heat (J/kg) μ viscosity (kg/m-s) Q heat transfer rate (W) Subscripts nc non-condensable gas ( - ) b boiling region of pipe ( - ) c condensation region of pipe ( - ) hot outside boiling region of pipe ( - ) cold outside condensation region of pipe( - ) P/D Pitch-to-Diameter ratio ( - ) References [1] G.H. Nam, J.S. Park, S.N. Kim. Conceptual Design of Passive Containment Cooling System for APR-1400 using Multi-Pod Heat Pipe, Nuclear Technology. 189 (2015) 278–293. [2] H. Imura. Heat Transfer in the Two- Phase Closed Thermosiphon, Trans. JSME, Vol. 45, pp.712-722, 1979. [3] H. Imura. Critical Heat Flux in a Closed Two-Phase Thermosyphon, Int. J. Heat Mass Transfer, Vol26, No.8, pp. 1181-1188, 1983. [4] I. Khazaee, R. Hosseini, S.H. Noie. Experimental investigation of effective parameters and correlation of geyser boiling in a two-phase closed thermosyphon, Applied Thermal Engineering, Vol. 30, pp. 406-412, 2010. [5] S. Khandekar, et. al. Thermal performance of closed two-phase thermosyphon using nanofluids, Int. J. Thermal Science, Vol. 47, 659-667, 2008. [6] Y.G. Lee, et. al. An experimental study of air-steam condensation on the exterior surface of a vertical tube under natural convection conditions, Int. J. Heat and Mass Transfer, Vol. 104, pp. 1034-1047, 2017. [7] A. Dehbi. A generalized correlation for steam condensation rates in the presence of air under turbulent free convection, Int. J. Heat and Mass Transfer, Vol. 86, pp.1-15, 2015. [8] J.C. de la Rosa, A, Escriva. Review on condensation on the containment structures, Nuclear Energy, Vol. 51, pp. 33-36, 2009. Authors Kyung Ho Nam Korea Atomic Energy Research Institute 111, Daedeok-daero 989beon-gil Yuseong-gu, Daejeon, Korea Sang Nyung Kim Kyunghee University 1732, Deogyeong-daero Giheung-gu, Yongin-si, Gyeonggi-do, Korea Displacement of Cryomodule in CADS Injector II Yuan Jiandong, Zhang Bin, Wang Fengfeng, Wan Yuqin, Sun Guozhen, Yao Junjie, Zhang Juihui and He Yuan 1 Introduction As Cryomodule can easily reduce higher power consumption and length of an accelerator, make the accelerator can be run continuously, it is becoming increasingly important in the superconducting linac [1]. Due to the invisibility and coupled with ultra-low temperature characteristics (4 k), Cryomodule is the key points and difficulties for a superconducting linear accelerator. The Chinese academy of sciences institute of modern physics is developing an accelerator driven subcritical system (CADS) Injector II [2].CADS will accelerate protons with a beam current of 10mA to about 1.5 GeV to produce neutrons for the transmutation of nuclear waste [3]. To avoid generating beam orbit distortion, the magnet magnetic center must be on the beam axis, so the displacement of cold components has extremely requirements [4]. From the theoretical point, there are generally three approaches to deal with the displacement on the working condition [5]. One is to maintain the alignment upon the cooldown. In this approach, the structure is designed so that the cooldown is absolutely symmetric. The other is to allow realignment once cold. In this approach, components must be realigned after they reached their final cryogenic temperature. As we all know that both the above two situations cannot easily be reached. The last approach is to allow the components to change in a predict able and repeated way. There are four different methods to realize this objective currently. The European organization for nuclear research developed a double-sided Brandeis CCD Angle Monitor (BCAM) [6]. The Japanese high-energy accelerator research organization adopted white light interferometer (WLI) [7]. German electron synchrotron [8], the institute of high energy physics Chinese academy of sciences [9] and Fermi national accelerator laboratory [10] employed a Wire Position Monitor (WPM) to monitor the contraction. The France large national heavy-ion accelerator adopted a micro-alignment telescope to align Cryomodule intuitively [11]. However, these above methods only investigated the cryo-displacement, did not concern the effect of the negative pressure of the vacuum. Ref [12] (D. Passarelli) have estimated the pressure distribution inside the cavity string used a mathematical model. Ref [13] analyzed the displacement induced by temperature differences, but did not correlate the cryo-vacuum displacement. In this paper, we present a detailed description of the principle of the vacuum cryo-environments firstly; and then we take out the simulation of vacuum and cryo-displacement Research and Innovation Displacement of Cryomodule in CADS Injector II ı Yuan Jiandong, Zhang Bin, Wang Fengfeng, Wan Yuqin, Sun Guozhen, Yao Junjie, Zhang Juihui and He Yuan
atw Vol. 62 (2017) | Issue 6 ı June respectively; in the section IV, we compared the measured results with the simu lated ones; At last, we correlated the cryo-vacuum displacement. The deep investigation will benefit for the optimization [14] and upgrade [15-16] of Cryomodule design for the CIADS. 2 Heat transfer principle CADS injector II project includes 4 cells 6 cavities cryostats. Most cavities and magnets are working in the cryostat at liquid helium (LHE) temperature. The vacuum jacket of each cryomodule is rectangular box shaped of dimension 4.3 m × 1.7 m × 2.1 m. Their alignment will be carried out at room temperature first, and then after the compensation, the position error of the cavities and magnets shall be within ±0.1 mm. Finite element method was used here to analysis the thermal stress and displacement: we used Solid Works to construct model first; and then imported in ANSYS, meshing with four surfaces unit; finally simulated the thermal stress and cold displacement. Generally, heat transfer includes the sum of thermal radiation, convection, and sometimes conduction transfer. Usually, more than one of these processes occurs in a given situation. Since the Cryomodules are operated in a cryo-vacuum environment, there is no convective heat transfer in the static heat loads [17, 18]. There are only thermal radiation [19] from the “hotter” environment and direct thermal conduction through the cold mass supports, power coupler and the feedthrough [20]. Thermal conduction is the transfer of heat (internal energy) by microscopic collisions of particles and movement of electrons within a body [21]. According to Fourier's law, the heat flux resulting from thermal conduction is proportional to the magnitude of the temperature gradient, the thermal conductivity and the cross-sectional surface area. However, it is inversely proportional to the length of a conduction path and opposite to it in sign. In many cases, the analysis may be simplified by the use of thermal conductivity integrals. In this approach, the conduction heat transfer in one dimension is given by [22] (eq. 1): (1) Thermal radiation is an electromagnetic radiation generated by the thermal motion of charged particles in matter. All matter with a temperature greater than absolute zero emits thermal radiation. When the temperature of a body is greater than absolute zero, inter-atomic collisions cause the kinetic energy of the atoms or molecules to change. This results in charge-acceleration and/or dipole oscillation which produces electromagnetic radiation, and the wide spectrum of radiation reflects the wide spectrum of energies and accelerations that occur even at a single temperature. During the cool-down, the thermal shield receives radiative heat flux from both the external vacuum vessel and from the internal cold mass. In fact, the heat flux density due to radiation transport between two surfaces at different temperatures can be written as following [22] (eq. 2): (2) Where [22] the vector Q is the heat flux (in W/m 2 ) in the positive direction; λ is known as the conductivity constant or conduction coefficient (in w/m k); A is the total crosssectional area of conducting surface (in m 2 ); L is the thickness of conducting surface (in m). Where [22] σ = 5.67 10 is the Stefan-Boltzmann constant and ε is the effective emissivity of the king into account the view factor and surface emissivities. For a simple “feeling” of the order of magnitudes, it is useful to refer to the simple case of black body radiation (unitary emissivity) intercepted by a unitary surface at 2 K from a parallel plate at different temperatures. 2 Simulation A Vacuum displacement To guarantee an effective cool-down process for the Cryomodule, a high-vacuum level must be achieved [12]. The thorough stress and Strain analysis of the vacuum chamber under atmospheric pressure and selfgravity must be carried out. According to the Hooke's Law, the displacement ΔL (eq. 3) is proportional to the above composite forces F n , is inversely proportional to the cross section size A and the Young's Modulus E. (3) The transfer of energy to the gravitational field results in the deformation of vacuum. The Finite element model of the cold mass and its support are shown in Figure 1. The generalized Hooke's law can be used to predict the displacement (Formula 4) caused in a given material by an arbitrary combination of stresses. Where v is the Poisson ratio, F x , F y and F z are the combined forces of the x, y, z-direction respectively. (4) Given that 1.5 tons of gravity were exerted on the two insulating RESEARCH AND INNOVATION 419 | | Fig. 1. Model of the cryomudule. | | Fig. 2. Vacuum simulation. Research and Innovation Displacement of Cryomodule in CADS Injector II ı Yuan Jiandong, Zhang Bin, Wang Fengfeng, Wan Yuqin, Sun Guozhen, Yao Junjie, Zhang Juihui and He Yuan
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