Presented at the COMSOL Conference 2009 Milan COMSOL Conference 2009 Milan October 14-16 2009 Helium two-phase flow in a thermosiphon open loop Florian Visentin, Bertrand Baudouy CEA Saclay Accelerator, Cryogenics and Magnetism Division 91191 Gif-sur-Yvette Cedex, France bertrand.baudouy@cea.fr 1
Outline Cooling large superconducting magnet Thermosiphon open loops Experimental facility and ranges of the study COMSOL Multiphysics Modeling Results with COMSOL Multiphysics Comparison with experimental results BB, Comsol Conference 2009, Milan Wednesday 14 2009 2
Cooling large superconducting magnet Compact Muon Solenoid magnet 7 m diameter and 12.5 m long 4 T at the center Liquid helium temperature cooling (4.2 K) Unique magnet of large scale Reduction of the quantity of cryogen Lower the cost of operation Protection in case of quench Lv 2 10 4 J/Kg Phase change ρ l /ρ v 7 External cooling Two-phase thermosiphon cooling method BB, Comsol Conference 2009, Milan Wednesday 14 2009 3
Thermosiphon open loops (1/2) Open reservoir/phase separator No re-cooling of the warm liquid or re-condensation of the vapor Phase separator cryostat Power to be extracted Decrease in liquid density and/or vaporization Branch weight unbalance Flow induced Q Coil cryostat Suppression of any pressurization system Liquid level needs to be controlled to avoid dryout Minimum heat flux to start the flow Flow oscillations at low heat flux BB, Comsol Conference 2009, Milan Wednesday 14 2009 4
Thermosiphon open loops (2/2) In the downward branch, the flow is single phase Adiabatic branch and the liquid is sub-cooled Pressure and the temperature increase from to The upward branch is heated partially and above it is adiabatic (the riser) Flow is first in single phase from to Fluid reaches the saturation temperature at point Fluid temperature also increases up to the saturation line Saturation line Liquid Vapor Single phase flow Point is the onset of nucleate boiling Then the flow above is two-phase Saturation line Fluid temperature decreases following theliquid saturation line Vapor Single phase flow Two-phase flow BB, Comsol Conference 2009, Milan Wednesday 14 2009 5
Experimental facility and ranges To vapor cooled shield Test section Phase separator Pressure tap LHe 0.9 m 10 mm inner diameter ~1 m heated length Tsensor ± 2 mk Thermal anchor 0.76 m Ranges P: 1.004 ± 0.006 10 5 Pa Venturi flow-meter 0.53 m q: 0-25 kw/m 2 m: 0-12 g/s Downward tube x: 0 25% Heater 0.3 m Temperature sensors Pressure tap 0.07 m BB, Comsol Conference 2009, Milan Wednesday 14 2009 6
COMSOL Multiphysics Modeling (1/2) Homogeneous model implemented in Comsol Multiphysics Mass d du i 0 dz dz 1 x 1 x with i l or i m with m v l Momentum dp du dp iu ig cos dz dz dz f, j 0 dp f d u l dz f,d Dd ld 2 2 dp f u u lom dz f,u Du lu 2 2 f 0. 079 Re 0. 25 j lo l l 1 x 1 1 x 1 v v 1/ 4 Energy 2 q d u 4 iu hi gz cos Dj dz 2 with h CTor hc T L p p v BB, Comsol Conference 2009, Milan Wednesday 14 2009 7
COMSOL Multiphysics Modeling (2/2) 1 D model with downward, horizontal and upward branches PDE general form module was used Segregated mode with three groups First group : conservation of mass and momentum with u and p as variables Pressure fixed at the loop entrance and Neumann condition for other boundaries For velocity, only Neumann conditions are used Second group : Energy conservation equation and consider only one variable, T Temperature fixed at the loop entrance and Neumann conditions for other boundaries Third group : Energy conservation equation for the vapor quality, x Quality is set to zero until the saturation temperature is reached BB, Comsol Conference 2009, Milan Wednesday 14 2009 8
Results with COMSOL Multiphysics P (Pa) Case for 600 W/m 2 p=101325 Pa Convergence 10-7 960 elements Liquid Saturation line Vapor Single phase flow 4.240 0.10 Temperature (K) Donward + horizontal branch 4.236 4.232 4.228 4.224 4.220 0 2 length (m) Domain 2 : Upward branch 0.08 Liquid Saturation line 0.06 0.04 0.02 0.00 x Vapor Single phase flow Two-phase flow BB, Comsol Conference 2009, Milan Wednesday 14 2009 9
Comparison with experimental results At low heat flux, the flow is dominated by the gravity term At higher heat flux the friction term increases causing the slight decrease of the total mass flow rate 0.012 0.010 0.008 m t (kg/s) 0.006 0.004 0.002 0.000 0 1000 2000 q (W/m 2 ) BB, Comsol Conference 2009, Milan Wednesday 14 2009 10
Comparison with experimental results Vapor quality reproduced with good accuracy up to 1500 W/m 2 At 1500 W/m 2 film boiling appears No thermodynamic equilibrium between the two phases Homogeneous model no longer holds 0.40 0.35 0.30 0.25 x 0.20 0.15 0.10 0.05 0.00 0 1000 2000 q (W/m 2 ) Model sufficiently accurate to be used for designing cooling system BB, Comsol Conference 2009, Milan Wednesday 14 2009 11
Conclusions COMSOL Model sufficiently accurate to be used for designing cooling system with a thermosiphon loop in two-phase helium COMSOL easy to handle for non expert Easy implementation and modification of physics models Next is transient Pressure rise due to vaporization in liquid helium Mechanical constraints on the magnet structure Fluid management Adding multi-physics Interaction with the stability of superconductors Magnetic field interaction BB, Comsol Conference 2009, Milan Wednesday 14 2009 12