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Heat Transfer performance for high Prandtl
and high temperature molten salt flow
in sphere-packed pipes
FUSION HIGH POWER DENSITY COMPONENTS AND SYSTEM
and
HEAT REMOVAL AND PLASMA-MATERIALS INTERACTIONS
FOR FUSION
Inn on the Alameda, Nov. 15-17, 2006
Tomoaki Satoh1, Kazuhisa Yuki1,
Hidetoshi Hashizume1, Akio Sagara2
1 Advanced
Fusion Reactor Eng. Lab.
Dept. of Quantum Science and Energy Eng., Tohoku University, Japan
2
National Institute for Fusion Science, Japan
1. Background
1-1. Fusion blanket
D-T Fusion Reaction
D(2H)+T(3H) → 4He + n + 17.06MeV
Roles of a blanket in a fusion reactor
• Generating and transporting heat energy
• Shielding nuclear radiation
• Producing and recovering Tritium
Fig. 1. 3D view of FFHR
In the design of a force-free helical reactor (FFHR),
molten salt Flibe (LiF : BeF2 = 66 : 34) is recommended
as a blanket material..
1. Background
1-2. Flibe blanket system
Advantages
• MHD pressure drop is low in comparison with Li flow.
• Stable in high temperature and vapor pressure is low, etc.
Disadvantages
• High Prandtl number fluid ⇒ Heat transfer performance is low
• Electrolysis can occur due to induced current
It is necessary to enhance heat transfer performance
with relatively low flow velocity.
To investigate heat transfer performance of Flibe, Tohoku-NIFS
Thermofluid loop (TNT loop) was built in 1998.
1. Background
1-3. Features of HTS
Flibe contains toxic Be ⇒ Using alternative molten salt, HTS
Air cooler
Comparison of features
Circulation pump
between Flibe and HTS
Flibe
Flibe (LiF - BeF2 : 66 - 34)
HTS
Dump Tank
Melting point : 459 C
Pr : 35.6 @500 C
Difficult to treat
HTS (KNO3,NaNO2,NaNO3 : 53-40-7)
Melting point : 142 C
Test section
Pr :27.7 @200C
No toxic substances
Fig. 1 Tohoku-NIFS Thermofluid loop
(Before
modification)
Fig.2
Temperature
dependance of Pr
1. Background
1-4. Early studies
450 mm
Fig. 3 Overview of previous test section
Fig. 4 Comparison between Re and Nu
Causes of disagreements
• Entrance region was too short to develop turbulence flow.
• Heating method was not suitable for heat transfer experiment.
1. Background
1-5. Aim of this study
TNT loop was modified.
Entrance region : nearly 50D
Direct electrical heating method
Aim of this study
Fig. 5 Tohoku-NIFS Thermofluid loop
(After modification)
• To investigate heat-transfer performance in a circular pipe using
modified TNT loop, and to get more accurate data.
• To quantitatively evaluate heat transfer enhancement of SPP by
comparing with the performances of other heat transfer promoter.
2. Experimental
2-1. Experimental apparatus
Air cooler
Pump
Dump tank
Test section
Fig. 5 Tohoku-NIFS Thermofluid loop
(After modification)
2. Experimental
2-1. Experimental apparatus
450mm
About 1800mm
(a) Previous test section
(b) Modified test section
Fig. 6 Comparison between modified test section and previous test section
2. Experimental
2-1. Experimental apparatus
Inner tube diameter：D=19mm
Material：SUS304
T.C. for measuring
inlet bulk temp.
Entrance region : 30D
FlowLength
straightener
to develop boundary layer : lv
Stainless mesh
lv = 0.693 ×
Re0.25
×D
Electrodes
( Re : Reynolds number, D : Inner tube diameter )
When Re = 15000,
Flexible tubes
1000 mm
1000 mm
T.C. for measuring
outlet bulk temp.
Test section：
About 50D
lv= 0.693 × 150000.25 × D ≈ 8D << Entrance region
To alleviate an effect of thermal stress
Fig.7 Schematic view of the modified section
3. Results and Discussion
3-1. Chemical effect
(a) Inlet
(b) Center
(c) Outlet
Fig. 8 Observations of test-section inner surfaces
HTS is thermally decomposed above 450C. Main reaction is as follows,
5NaNO2  3NaNO3  Na2O N2
The present study is carried out under the condition of
HTS temperature up to 350C. ⇒ No chemical deposition

No effect for heat transfer experiment.
3. Results and Discussion
3-2. Heat transfer of circular pipe (CP)
Tin=200 [°C], Pr ≈
27
Re=4610
q’’=30.6 [kW/m2]
Developed
Transition
Entrance
Fig. 9 Typical temperature profile
along the test section
Thermal boundary layer is fully
developed from 500 mm position.
Temperatures measured in this
region are used for evaluating
heat transfer performance.
3. Results and Discussion
3-2. Heat transfer of circular pipe (CP)
Modified Hausen equation
 2
 1  0.14
Nu  0.116Re 3 125Pr 3  b 

 s 
(3500≤Re≤10000)
Sieder-Tate equation
0.14



Nu  0.027Re0.8 Pr 3  b 
 s 
(10000≤Re)
1
Good agreement with above Eqs.
Maximum error : 10%
Fig. 10 Comparison between acquired
Nu and the empirical Eqs.
3. Results and Discussion
3-2. Heat transfer of circular pipe (CP)
Modified Petukhov equation
 f /8Re1000Pr
0.11
 Pr 
Nu =
 
1 12.7  f /8Pr 2 3 1Prw 
(104 ≤ Re ≤ 5x106, 0.5 ≤ Pr ≤ 2000)
Also good agreement with above Eq.
Maximum error : 15%
Fig. 11 Comparison between acquired heat transfer
coefficient and the modified Petukhov Eq.
Conclusion
Heat transfer correlations for general fluid in CP can be used to
evaluate heat transfer characteristics for high temp. molten salt.
3. Results and Discussion
3-2. Heat transfer of sphere-packed pipes (SPP)
Manglik equation for swerl tube
0.8
0.2
 
   2  2 d  b 
Nuy  0.023Re Pr 
 
  
  4 d     4 d  w 
0.8
0.4
Nu Nu y=   1 0.769 y
Fand equation for SPP

Nu  C Rem Pr p  fw Rew arctan D / d
q

4 times higher
n r
m = 0.25, n = 1, p = 0.4054, q = 0.5260, r = -0.6511.
• SPP performance is higher than that of
other heat transfer promoter.
• It significant, especially, at low flow
velocity condition.
Fig. 12 Results of the heat transfer with
packed bed under same velocity condition.
3. Results and Discussion
3-2. Heat transfer of sphere-packed pipes (SPP)
In the case of using D/2 spheres, pressure
drop gives good agreements with drag
model. (by M. OKUMURA et. al)
Evaluating pumping power of SPP
Fig. 12 Results of the heat transfer with
packed bed under same pumping power.
Is there any future for Flibe ?
Allowable temperature of structural material
1) =500C -> No solution
2) =550C ->
Outlet temp. can be raised to 650C.
Other coolant is necessary for first wall cooling.
(or should we change the composition of Flibe to
decrease melting temperature ?)
3) =650C -> Some possibility