THERMO-HYDRAULIC ANALYSIS OF FORCED FLOW HELIUM COOLED CRYOPANELS OF CRYOPUMP USING VENECIA CODE

SAMIRAN SHANTI MUKHERJEE et al. THERMO-HYDRAULIC ANALYSIS OF FORCED FLOW HELIUM COOLED CRYOPANELS OF CRYOPUMP USING VENECIA CODE SAMIRAN SHANTI MUKHERJEE, VISHAL GUPTA, RANJANA GANGRADEY, JYOTI SHANKAR MISHRA, PRATIK NAYAK, PARESH PANCHAL, SUBHADIP DAS, JYOTI AGARWAL Institute for Plasma Research Gandhinagar, India Email: samiran@ipr.res.in Abstract Cryoadsorption cryopump with large pumping speeds application has been developed at the Institute for Plasma Research (IPR). These pumps are cooled with liquid helium for cryopumping panels below temperature 5K to adsorb hydrogen and helium gases and with gaseous helium for thermal shields at around 80K during fusion reactor relevant future applications. These panels are coated with activated carbon as sorbent. Sorbent with micro-pores adsorbs gases and the pores get saturated after certain duration of pumping operation. During regeneration by increasing the panel temperature adsorbed gases get removed. A cycle of operation is thus followed comprising, cool down from 80K to ~K and warm up from ~K to 80K during the normal operation cycle of the cryopump. Cryopanels and shielding panels are mostly hydoformed quilted stainless steel panels with sheet thickness of.5mm. Hydroformed panel of size 000mm (l) x 200mm (w) with the 3mm thickness connected by inlet and outlet tubes are used as K cryopanel. Thermohydraulic analyses are carried out in Venecia software developed by Alphysica for the 2 Panel cryopump for different operational schemes. To investigate the necessary mass flow rates and cool down time, optimized selection of the cryopanel arrangements, flow paths and manifolds is required. Results of cool-down time, mass flow requirement and temperature profile is presented for different cooling and regeneration scheme.. INTRODUCTION For large pulse duration plasma machines and future coming up experimental fusion reactor like ITER, pumping of enormous scale gas loads is a demanding requirement []. Among all the other auxiliary subsystems, large pumping capacity vacuum subsystem will be one of the key requirements for successful steady operation of the fusion machines. The pumping requirements in ITER like machines is huge (greater than 50 cubic meters per second) and are required for torus, cryostat and neutral beam injectors [2]. The gases to be pumped are mainly helium and various isotopes of hydrogen. After a rigorous study at KIT Germany, for more than 0 years, cryopump design for ITER scenario was completed [3] and the first delivery of cryopump to ITER site, Cadarache FE was completed during August 207[]. Institute for Plasma Research (IPR) is working for the development of the technologies in national level relevant to nuclear fusion that will lead us to build fusion power reactors in near future [5][6]. Aiming that, prototype development of various auxiliary technologies is on-going at IPR. In recent years, a prototype Cryoadsorption cryopump with large pumping speeds application has been successfully tested at the IPR [7]. These pumps are designed to adsorb hydrogen isotopes and exhaust helium gases in order to maintain high vacuum conditions for subsequent plasma confinement. The pump mainly consists of ) thermal shield and baffles, cooled down to 80K temperature. 2) Activated carbon coated hydro formed panels connected in series and/or parallel cooled down at around.5k temperature. The porous activated carbon materials provide enormous pumping speed when cooled to sufficiently low temperatures. On regeneration of such cryopanels, the clogged gases are removed and pores become fresh to hold yet another cycle of gas load. In the process the panel undergoes thermal cycling comprising cool down to.5k by liquid helium and warm up by gaseous helium up to 80K. Preestimating the optimum cool down and warm up time scales are important to study regeneration stages and to estimate the pumping time scales. Design and analysis for 2-panel cryopump with expecting pumping speed greater than 50,000 litres per second is on-going at IPR. This cryopump will be tested stand-alone integrated to 80K gaseous helium supply plant and liquid helium plant to demonstrate the technical feasibility and indigenous capability. 2. GEOMETRICAL ARRANGEMENTS AND THE COOLING SCHEMES As an overview, Fig. shown the stand alone test scheme of the cryopump integrated with K liquid helium plant supply system and 80K gaseous helium supply system. Super critical helium at.k and bar pressure will be used to cool down the cryopanel assembly from 80K to less than 5K temperatures, whereas the 80K gaseous helium supply system will be used to warm up the panel till 80K for fast regeneration and cool down

FIP/P3-50 the panel assembly from 300K to 80K temperatures. On the other hand, all the hydroformed thermal shields will be kept at constant temperature of 80-85K to provide radiation shielding to the K components. The internal distribution of the 80K system is not shown in the Fig.. At actual it will have a distribution line, through which 80K gaseous helium will be supplied to the K system as well as thermal shields. Present study will help to understand the mass flow requirement for the K panel assembly and the required cool down and 80K warm up time. The 80K plant available at IPR is capable of delivering 50-20 gm/s variable mass flow rate at 80K and delivers at 5 bar pressure. FIG.. Lay-out diagram of the cryopump assembly with the K liquid helium and 80K gaseous helium supply system. A B C FIG. 2. A. Flow diagram of the cooling scheme for cryopump panel assembly showing different inlet supply arrangements B. Cryopanel CAD model C.3D CAD model of the cryopanel assembly of 2 panels with flow connectors and headers For estimating the cool down and warm up time for the 2 panel cryopanel assembly thermohydraulic analysis is being carried out in Venecia Code developed by Alphysica. 2 panels will be arranged in four parallel channel manifolds and each channel will have 6 cryopanels in series as shown in Fig.2A. Each individual panel will

SAMIRAN SHANTI MUKHERJEE et al. have four parallel channels through which fluid will be passed through. Each cryopanel is 000 mm in length and 200 mm in width with hydroformed sheet thickness of.5 mm. The maximum bulge height is 9mm. These hydroformed cryopanels were developed indigenously [8] and technology has been transferred to the industry for its wide applications not only in nuclear fusion related technology but also for the space research and other application. These cryopanels and its prototypes were used as small scale miniature cryopump[9] to the larger scale applications. 3. MODELING AND THEORITICAL REFERENCES The present work has been carried out using the software package VENECIA which is the advanced version of the code Vincenta. With an extensive research and more than 0 years of experience in thermal hydraulic simulations for large magnet systems (ITER, KSTAR, JT-60) this code was developed and was validated [0]. For relatively complex geometries with efficient heat transfer requirements, liquid helium is not commonly used. In those cases a forced flow helium flow with temperature slightly higher than the normal boiling point.2k and pressure above the atmospheric but within the critical point is used. In this particular range, the Gruneisen parameter, isentropic compressibility, and velocity of sound are the three thermodynamic parameters, which are weakly non-catalytic near the critical point, and are used to describe fluid flow processes []. To define the simulation input, basic mathematical models is used for the cryogenic system like magnet system, cryopump system, cryo-distribution line etc. Defined mathematical models are described with set of algebraic, differential equations or in partial derivatives. The main function of the simulation is to solve those set of equations of continuity for D compressible fluid flow involving mass, momentum and energy conservation [2]. Transient parameters of compressible helium flow inside the channel simulated in this particular tool. Mass, momentum and energy conservation terms and material properties are linked under thermo-hydraulic conditions with different flows and solid materials. When the forced flow liquid helium is considered in the problem, the change in temperature, density and other fluid flow parameters are highly non-linear with respect to the time and space making the solution to fluid equations highly complex. Runga kutta th order numerical method is used for the thermal hydraulic analysis of two phase helium flow to know transient behaviour of the fluid. For carrying out the simulation a grid of the special distribution in D and time fraction is defined for the problem to solve in that particular space-time for zooming in the transients. Initial set of fluid parameters are then fed along with the different geometrical shape of the hydraulic structures. Upon solving the equations, simulation results are obtained graphically and numerical output of variations in thermal hydraulic parameters are obtained for every defined space-time step. Basic understanding as discussed above, numerical models set of equations used and their definitions are described by Nickolay A. Shatil [3] and how to define the numerical models for the simulations are elaborated in the Venecia user s guide version [2]. Simulations are carried out for different operation modes in order to get the full cryopump operation cycles 300K to 80K Cool Down: In this process 80K gaseous helium is supplied to the cryopanel assembly. During this mode, valve V2 (see Fig.2.A) is kept open and other fluid supply valve V and V3 is kept closed. 80K gaseous helium passes through the cryopump and cools down the system close to the 80K temperatures. 80K to.k Cool Down: In this process.k helium is supplied to the cryopanel assembly. During this mode, valve V3 (see Fig.2.A) is kept open and other fluid supply valve V and V2 is kept closed..k forced flow super critical helium passes through the cryopump and cools down the system below 5K temperatures. During this process the cryopump will be in operational mode and constant flow will be maintained during the operation time..k to 80K warm-up: In this process 80 gaseous helium is supplied to the cryopanel assembly. During this mode, valve V2 (see Fig.2.A) is kept open and other fluid supply valve V and V3 is kept closed. 80K forced flow gaseous helium passes through the cryopump and warms up the system close to 80K temperatures. During the sequential operation of cryopump 80K to.k cool-down and.k to 80K warm up is carried out for operation and 80K regeneration. 80K to 300K warm-up: In this process 300K gaseous helium is supplied to the cryopanel assembly. During this mode, valve V (see Fig.2.A) is kept open and other fluid supply valve V2 and V3 is kept closed. 300K gaseous helium passes through the cryopump and warms up the system close to 300K temperatures. This process is required whenever complete regeneration is necessary, in order to release the moisture, nitrogen and other residual gases. 3

FIP/P3-50 During the operation of the each mode, previous operation state is considered as initial boundary conditions. The weight of the cryopanel assembly is more than 0kg therefore required mass flow rate is reasonably high. Compared to the enthalpy of the overall system, the radiation heat load and the residual gas convection heat load component are negligible, therefore not considered for simulation. In the present simulation, required mass flow rate and time to cool down or warm up the system, are the major deliverable to attain the required operation conditions in different modes.. SOLUTION RESULTS During the modelling of the simulation, all the hydroformed cryopanels (as shown in Fig.2.B) are considered as straight channels. Each cryopanel have parallel channels of same hydraulic dimension. The main supply of fluid initially divides into parallel sections in the manifolds, thereafter supply fluid is fed to the panel section having 6 panels in series. A total of 2 panels were taken as the cryopump assembly as shown in Fig.2.C. As the present simulation aims the thermohydraulic analysis of the panel assembly, therefore transfer line section is not considered. It is assumed that, there will be bypass valve connected before that delivery of the fluid to the system. Hence, both the main fluid inlet and outlet pipe is assumed to be m in length. Details of the geometrical arrangements are put in table.. TABLE.VALUES OF GEOMETRICAL PARAMETERS AND POSITION IN -D Parts Crosssection Area of Conductors ( ) Perimeter of channels (m) Cross-section Area of channels ( ) Length (m) No. of flow paths/parallel channels Linear Distance [x] (m) Inlet 0.26E-6 62.8E-3 3.0E-6 Flow divider/ -2 0.26E-6 62.8E-3 3.0E-6 manifold 2-Panels 50.0E-6 50.0E-6 50.0E-6 50.0E-6 50.0E-6 50.0E-6 98.0E-6 98.0E-6 98.0E-6 98.0E-6 98.0E-6 98.0E-6 2.05-3.05 (panel- ) 3.-. (panel-2).2-5.2 (panel-3) 5.3-6.3 (panel-) 6.-7. (panel-5) 7.5-8.5 (panel-6) Flow collector 0.26E-6 62.8E-3 3.0E-6 8.55-9.55 Outlet 0.26E-6 62.8E-3 3.0E-6 9.55-0.55.. 300K to 80K Cool Down 80K gaseous helium at 5 bar is supplied to the pump through the supply valve V2. Supply of gaseous helium cools the supply transfer line, then the manifolds and then consecutively each individual panel. In the Fig.3.A, mass flow rate variation over the -dimnsional distance is shown. A maximum mass flow of 0gm/s is observed within the differential pressure of 500 mbar across the system. During the event how the temperature variation is takes place is shown in Fig.3.B. It shows a required cool-down time of 90 seconds. Each panel will have approximately 5kg of weight; therefore, sensible cooling of panel is slow compared to the inlet and outlet connection pipes connected in each end of the panels. As the effective mass flow is high for the inlet and outlet connections of the panels, there is a sharp fall of temperature in transient mode (see Fig.3.B.) in locations where panel inlet and outlet connections are put. During the transient flow of the 80K gaseous helium, the velocity in the panel varies from 3 to 9 m/s.

SAMIRAN SHANTI MUKHERJEE et al. FIG. 3. A. Variations of mass flow rate across the linear distances during 300-80 K cool down, B. Temperature profile at different location of the cryopumps during the cool down of cryopump from 300-80K.2. 80K to K Cool Down During this event, forced flow.k super critical helium at bar pressure is supplied, considering the initial state of the panel assembly to be at 80K temperature. When, the geometry gets cooled, mass flow rate increases and reaches upto 00gm/s (as shown in the Fig..A). This is observed due to the pressure regulation at the output of the return line which was kept at 3.65 bar. This mass flow can be regulated either by outlet pressure regulation or optimizing the supply pressure to the cryopump. With a maximum flow rate of 00gm/s, all the panels gets cooled down to ~.5K temperature within 30 seconds. As shown in the Fig..B, each panel is gradually cools down and takes approximately 5 seconds. FIG.. A. Variations of mass flow rate across the linear distances during 80-K cool down, B. Temperature profile at different location of the cryopumps during the cool down of cryopump from 80-. K.3. K to 80K warming up During this event it is assumed that, the flow channels are filled with helium at. K and bar pressure and 80K gaseous helium pushes out the helium and starts warming up the cryopump system. As shown in Fig.5 maximum mass flow rate is 0gm/s and gets uniform at 0gm/s. Initial mass flow rate is slightly increased due to the flushing out of previously occupied helium and at the same time all the channels were cooled during the initial state, this leads to the increase in mass flow rate. 5

FIP/P3-50 FIG.5. Variations of mass flow rate across the linear distances during -80 K warm up/regeneration As seen in the Fig 6, panels initially warms up rapidly up to 0-50K within 0 second of time and after that it warms up slowly up to the ~80K temperatures. Here also all the inlet and outlet connection pipes at the end of each panels shows slightly higher temperature compares to the panel sections due to the increase in velocity and effective mass flow rates at those sections. During this event velocity is very less approximately.m/s at each channel of the panels whereas at the connection pipes it reaches up to 70-80 m/s. To warm up till ~80K temperature, the cryopump takes around 70 seconds. FIG. 6. Temperature profile at different location of the cryopumps during the warming up of cryopump from -80 K.. 80-300K warming up The cryopump usually operates at to 5K temperatures to pump helium and hydrogen isotopes effectively. After a certain pumping duration these cryopumps are regenerated and the duration of operation is decided based on the hydrogen safety limit and the gas load condition during the plasma operation. These details are not discussed here as the present system is stand-alone experimental test facility. After each operation cycles there is regeneration cycle and during this event all the trapped gases are removed. During a reactor relevant condition most of gas composition will be helium and hydrogen isotope. Therefore, 80K regeneration will be sufficient. During the cryopump operation there will always be a chance for reduction of the efficiency of cryopumping due to the water vapour, nitrogen and other gas entrapment in the charcoal beds. Those gases will fill up the

SAMIRAN SHANTI MUKHERJEE et al. charcoal pores, therefore effectively reduces the pumping efficiency. During such an event, once should goahead for rapid regeneration up to the 300K or higher instead of natural warm up of the system. Considering this scenario, warm up to 300 K is being studied. Here, warm room temperature helium at a pressure 5 bar is supplied to the system which effectively delivers up to 80gm/s mass flow to the system as shown in Fig.7. FIG. 7. Variations of mass flow rate across the linear distances during 80-300 K warm up As seen the Fig 8, with the mass flow of approximately 55 to 75 gm/s all the panels can be warmed up in 280 second time. FIG.8. Temperature profile at different location of the cryopumps during the warm up of cryopump from 80-300 K 5. CONCLUSION In the present work, complete operational cycle of the cryopump has been analysed. Based on the cryogen supply system availability, major flow conditions are put in the simulation and the cool down and warm up details are found for the operation of the cryopump. The results shows required cool down for 300K to 80K is 90 seconds and for 80K to. K cool down it is 30 second for helium mass flow of 0 gm/s and 00 gm/s (max.). Whereas for the warm up or regeneration for.k to 80K it takes 70 seconds and for 80K to 300K warm up it takes 280 seconds when the mass flow is considered 0 gm/s and ~75 gm/s respectively. During the analysis pressure domain are regulated by applying a pressure control valve before the outlet collector. Based on 7

FIP/P3-50 the studies carried out all the cryogen supply system will be modulated to offer the best suitable flow conditions. Actual operational conditions will be monitored during the experimental testing of the cryopump system. ACKNOWLEDGEMENTS The author s would like to thank Department of Atomic Energy, Govt. of India for supporting this work. For carrying out the simulation Venecia software package is used which is supplied by M/s Alphysica, St. Petersburg, Russia. Authors would like to thank Ms. Reena Sayani for her contribution to the initial software installation and benchmark analysis. REFERENCES [] V.KALININE et al., Cryogenic Subsystem to Provide for Nominal Operation and Fast regeneration of the ITER Primary Cryosorption Vacuum Pumps, AIP Conf. Proc., vol. 8, no. 70, 200. [2] C. DAY. et Al., Validated Design of the ITER Main Vacuum Pumping Systems, IAEA fusion energy conference, 200, IT/P3 7,. [3] M. DREMEL et al., The new build to print design of the ITER Torus Cryopump, Fusion Eng. Des., vol. 88, no. 6 8, pp. 760 763, 203. [] Press Releases from FE, Europe delivers to ITER the first cryopump, 29th August 207, weblink: http://fusionforenergy.europa.eu/mediacorner/newsview.aspx?content=66 [5] S. DESHPANDE AND P. KAW, Fusion research programme in India, Sadhana, vol. 38, October, pp. 839 88, 203. [6] R. SRINIVASAN et al. Role of Fusion Energy in India, Journal of Plasma Fusion Res. SERIES. vol. 9, pp. 630 63, 200. [7] R. GANGRADEY et al., Progress towards achieving large pumping speed for exhaust from fusion grade machines, IAEA fusion energy conference, 206, FIP/P7-2. [8] R.GANGRADEY et al. Design and indigenous development and fabrication of cryopanels for cryopump applications IEEE/NPSS 2th Symposium on Fusion Engineering 20, SP-0. [9] R. GANGRADEY, Pumping speed offered by activated carbon at liquid helium temperatures by sorbents adhered to indigenously developed hydroformed cryopanel. IOP Conf. Series: Materials Science and Engineering 0 (205) 020 doi:0.088/757-899x/0//020 [0] Http://www.alphysica.com/index.php/introduction.html, VENECIA : Venecia software package for thermal hydraulic analysis of forced flow cooled superconducting equipment and their primary cryogenic subsystems Introduction, 208. [] V. ARP, Thermodynamics of single-phase one-dimensional fluid flow Cryogenics, P 285 289, May 975. [2] VENECIA User s Guide. by M/s Aplphysica, St. Petersburg, Russia [3] NICKOLAY A. SHATIL Quench Simulation of VLHC Transmission Line Magnet. Report No: TD-0-025, St.- Petersburg, Russia, February 999