SAMIRAN SHANTI MUKHERJEE et al.

1

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 ~4K and warm up from ~4K 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 1.5mm. Hydroformed panel of size 1000mm (l) x 200mm (w) with the 3mm

thickness connected by inlet and outlet tubes are used as 4K cryopanel. Thermohydraulic analyses are carried out in Venecia

software developed by Alphysica for the 24 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.

1. 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 [1] . 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 10 years,

cryopump design for ITER scenario was completed [3] and the first delivery of cryopump to ITER site,

Cadarache © F4E was completed during August 2017[4].

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 1) 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 4.5K 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 4.5K by liquid helium and warm up by gaseous helium up to 80K. Pre-

estimating 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 24-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.1 shown the stand alone test scheme of the cryopump integrated with 4K liquid helium

plant supply system and 80K gaseous helium supply system. Super critical helium at 4.4K and 4 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

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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 4K components. The internal

distribution of the 80K system is not shown in the Fig.1. At actual it will have a distribution line, through which

80K gaseous helium will be supplied to the 4K system as well as thermal shields. Present study will help to

understand the mass flow requirement for the 4K panel assembly and the required cool down and 80K warm up

time. The 80K plant available at IPR is capable of delivering 50-210 gm/s variable mass flow rate at 80K and

delivers at 15 bar pressure.

FIG. 1. Lay-out diagram of the cryopump assembly with the 4K liquid helium and 80K gaseous helium supply system.

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 24 panels with flow connectors and headers

For estimating the cool down and warm up time for the 24 panel cryopanel assembly thermohydraulic analysis

is being carried out in Venecia Code developed by Alphysica. 24 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

A B

C

SAMIRAN SHANTI MUKHERJEE et al.

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have four parallel channels through which fluid will be passed through. Each cryopanel is 1000 mm in length

and 200 mm in width with hydroformed sheet thickness of 1.5 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 10 years of experience in thermal hydraulic

simulations for large magnet systems (ITER, KSTAR, JT-60) this code was developed and was validated [10].

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

4.2K 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 [11]. 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 1D compressible fluid flow involving mass, momentum and energy conservation

[12]. 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 4th

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 1D 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 [13] and how to define the numerical

models for the simulations are elaborated in the Venecia user’s guide version 1[12].

Simulations are carried out for 4 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 V1 and V3 is kept

closed. 80K gaseous helium passes through the cryopump and cools down the system close to the 80K

temperatures.

— 80K to 4.4K Cool Down: In this process 4.4K helium is supplied to the cryopanel assembly. During this

mode, valve V3 (see Fig.2.A) is kept open and other fluid supply valve V1 and V2 is kept closed. 4.4K

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.

— 4.4K 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 V1 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 4.4K cool-down and 4.4K 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 V1 (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.

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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 140kg 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.

4. 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 4 parallel channels of same hydraulic dimension. The main supply of

fluid initially divides into 4 parallel sections in the manifolds, thereafter supply fluid is fed to the panel section

having 6 panels in series. A total of 24 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 1m in length. Details of the

geometrical arrangements are put in table.1.

TABLE 1.VALUES OF GEOMETRICAL PARAMETERS AND POSITION IN 1-D

Parts

Cross-

section

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 101.26E-6 62.8E-3 314.0E-6 1 1 1

Flow divider/

manifold 101.26E-6 62.8E-3 314.0E-6 1

1

1-2

24-Panels

150.0E-6

150.0E-6

150.0E-6

150.0E-6

150.0E-6

150.0E-6

94.584E-3

94.584E-3

94.584E-3

94.584E-3

94.584E-3

94.584E-3

198.0E-6

198.0E-6

198.0E-6

198.0E-6

198.0E-6

198.0E-6

1

1

1

1

1

1

4

4

4

4

4

4

2.05-3.05 (panel-

1)

3.1-4.1 (panel-2)

4.2-5.2 (panel-3)

5.3-6.3 (panel-4)

6.4-7.4 (panel-5)

7.5-8.5 (panel-6)

Flow collector 101.26E-6 62.8E-3 314.0E-6 1 1 8.55-9.55

Outlet 101.26E-6 62.8E-3 314.0E-6 1 1 9.55-10.55

4.1. 300K to 80K Cool Down

80K gaseous helium at 15 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 1-dimnsional distance is shown. A maximum mass flow of 110gm/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 190 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.

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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

4.2. 80K to 4K Cool Down

During this event, forced flow 4.4K super critical helium at 4 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 400gm/s (as shown in the Fig.4.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 400gm/s, all the

panels gets cooled down to ~4.5K temperature within 30 seconds. As shown in the Fig.4.B, each panel is

gradually cools down and takes approximately 5 seconds.

FIG.4. A. Variations of mass flow rate across the linear distances during 80-4K cool down, B. Temperature profile at

different location of the cryopumps during the cool down of cryopump from 80-4.4 K

4.3. 4K to 80K warming up

During this event it is assumed that, the flow channels are filled with helium at 4.4 K and 4 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 140gm/s and gets uniform at 110gm/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.

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FIG.5. Variations of mass flow rate across the linear distances during 4-80 K warm up/regeneration

As seen in the Fig 6, panels initially warms up rapidly up to 40-50K within 10 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 4.4m/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 4-80 K

4.4. 80-300K warming up

The cryopump usually operates at 4 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

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charcoal pores, therefore effectively reduces the pumping efficiency. During such an event, once should go-

ahead 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 15 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

190 seconds and for 80K to 4.4 K cool down it is 30 second for helium mass flow of 110 gm/s and 400 gm/s

(max.). Whereas for the warm up or regeneration for 4.4K to 80K it takes 70 seconds and for 80K to 300K

warm up it takes 280 seconds when the mass flow is considered 140 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

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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

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[10] Http://www.alphysica.com/index.php/introduction.html, ―VENECIA : Venecia software package for thermal hydraulic

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[12] ―VENECIA User ’ s Guide.‖ by M/s Aplphysica, St. Petersburg, Russia

[13] NICKOLAY A. SHATIL ―Quench Simulation of VLHC Transmission Line Magnet.‖ Report No: TD-01-025, St.-

Petersburg, Russia, February 1999