the thermosiphon principle (example of two phase ) self sustained natural boiling convection

Symposium for the Inauguration of the LHC Cryogenics, CERN, 31/05 & 01/06 2007

Challenges and limitations of

thermosiphon cooling What we learned from CMS…

Philippe BrédyCEA Saclay

DSM/Dapnia/SACM/LCSE

2PhB, Symposium for the inauguration of the LHC Cryogenics, CERN, 31/05 & 01/06/2007

•The thermosiphon principle (example of two phase )

– Self sustained natural boiling convection • Heating applied (Heat to be removed)

– Boiling– Weight unbalance between legs of a loop– Induced flow limited by friction

Q

Single channel Loop


•Rising heat exchangers with various geometry •A mass flow and a flow quality deduced from geometry and heat load•An upper tank –the well-named phase separator- is needed

– to recover gas and to supply supply or condensation of gas (cryocooler)– for separation of the two phases

P flow (m,x,geometry)P (x, hi)

Hyp: homogenous model(x << 10%)


Advantages vs Inconveniences

•A passive creation of flow– no pumps or regulation

valves

•Autonomy in case of external cryogenic failure (volume of liquid in the phase separator)

•Minimization of the liquid volume

– use of the phase separator as a back-up volume for liquid

•A quasi-isothermal loop- mainly function of height

•Need a minimum height– Pressure head to create

flow

•Circuit geometry must avoid any high point or strong singularities

– separation of the phases– risk of vapor lock

•No possible external action

– and a possible frustration for the operator !

•Pre-cooling before starting the ThS effect


Examples of applications on large superconducting magnets with LHe/GHe loop at Tsat

ALEPH (LEP)

ATLAS CS (LHC)

CMS (LHC)

PANDA, R3B (GSI)…

CLOE (CORNELL)

SMS G0 (JLAB)

…


CMS

X 2 X 5

• 220 tons at 4.5 K

• 174 to 500 W at 4,5 K

• 5 modules

• 86 parallel exchangers

Phase separator cryostat

Coil cryostat


Phase separator cryostat (892 l GHe+LHe)

Top

Bottom

Cryogenicchimney

CB-2 CB+1CB0CB-1

LHe supply pipe 60.3 x 1.65 LHe/GHe return pipes 48.3 x 1.6

Inlet manifolds( 45 x 2.5)

Outlet manifolds( 45 x 2.5)

CB+2

Heat exchangers(i 14)

Thermosiphon loop (350 l)

CMS thermosiphon

First global calculation : x = 3,5 % ( 6,5% in SD)m = 200 g/s ( 400 g/s in SD)


CMS R&D

eAltitude 0

Venturi flowmeter

Collecting manifold ( 0.04)

Distributing manifold ( 0.04)

7 heatexchanger tubes (5.00 )

Upstreamline (4.56)

Height between outflow and liquid level

LHe tank

(3.04)

Hydrostatic head 9.22(level at 50 %)

Experimental test loop scaling CMS design


CMS R&D

•A strong mass transfer in comparison with heat deposit

•A large margin•Homogenous model

correct still above 14%•h > 1600 W.m2.K• Pr/ps 1• T height ~ 60 mK•x < 3% (CMS

nominal)

0

20

40

60

80

100

120

140

160

180

200

0 50 100 150 200 250 300 350 400 450

Mas

s fl

ow (

g/s)

Total deposited power Q (Watt)

0,00

0,02

0,04

0,06

0,08

0,10

0,12

0,14

0 50 100 150 200 250 300 350 400 450

Vap

or Q

ual

ity

Total deposited power Q (Watt)

(on experimental test loop 1/10 CMS)

CM

S st

atic

hea

t loa

d

CM

S dy

nam

ic h

eat l

oad

Experimental and homogenous model

(scaling values)


CMS R&D

4.72 K

4.80 K

4.64 K

4.56 K

4.48 K

4.40 K

total mass flow vs different heat loads

0 W

25 W

50 W

75 W

100 W

125 W

150 W

•Self-sustained circulation with the heat deposited

(on experimental test loop)

temperatures

time


CMS on site•Self-sustained circulation with the heat deposited

(on site, at the beginning of a slow dump with dynamic heat load)

current

mass flow


CMS on site•Stabilisation and increase of the mass flow by heating the return lines

- Increasing and stabilization of mass flow rate

Effect on the mass flow

Effect on the coil temperatures

- 0.01 K temperature decrease on coil

T

t

Mass flow


CMS R&D

•Smooth sensitivity near and above the critical point

Pc


Limitations and Extensions-Circuit geometry :

- A minimum height- Risk of high point or

singularities where gas could be separated from the liquid

- Minimization of pressure drop (singularities)

-A quasi-adiabatic supply line

-Good separation phase in the upper tank

(what could be its minimum size?)

-Feeding of several parallel circuits with different heat deposits is possible (in CMS design, in ratio of 5)

(feeding and collecting pipes must be designed to be as isobaric)

-Starting the natural circulation is achievable before the liquid presence (between 15 and 20 K for CMS design)

-Heaters on the return pipes could be used to limit instabilities due to gas/liquid separation at low velocity

(low heat load or over-sizing)

-Flow quality could be chosen well higher than the traditional limit of 5 %


Conclusion

Confirmed by these tests and operation measurements, thermosiphon loop stays a convenient way to insure the indirect cooling of large equipment and must be taken into account during a design study without preconceived fears.


References•1. L. Benkheira, B. Baudouy, and M. Souhar, Heat transfer characteristics of two-phase He I

(4.2 K) thermosiphon flow, International Journal of Heat and Mass Transfer, (2007) 50 3534-3544

•2. L. Benkheira, B. Baudouy, and M. Souhar, Flow boiling regimes and CHF prediction for He I thermosiphon loop, Proceedings of the 21th International Cryogenic Engineering Conference, to be published, Ed. G. Gistau, (2006)

•3. P. Brédy, F.-P. Juster, B. Baudouy, L. Benkheira, and M. Cazanou, Experimental and Theoretical study of a two phase helioum high circulation loop, Advances in Cryogenics Engineering 51, AIP, Ed. J. G. Weisend, (2005) 496-503

•4. L. Benkheira, M. Souhar, and B. Baudouy, Heat and mass transfer in nucleate boiling regime of He I in a natural circulation loop, Advances in Cryogenics Engineering 51, AIP, Ed. J. G. Weisend, (2005) 871-878

•5. B. Baudouy, Heat transfer near critical condition in two-phase He I thermosiphon flow at low vapor quality, Advances in Cryogenic Engineering 49, AIP, Ed. S. Breon, (2003) 1107-1114

•6. B. Baudouy, Pressure drop in two-phase He I natural circulation loop at low vapor quality, International Cryogenic Engineering Conference proceedings 19, Ed. P. S. G. Gistau Baguer, (2002) 817-820

•7. B. Baudouy, Heat and mass transfer in two-phase He I thermosiphon flow, Advances in Cryogenic Engineering 47 B, AIP, Ed. S. Breon, (2001) 1514-1521

the thermosiphon principle (example of two phase ) self sustained natural boiling convection

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