1. Heat transfer in a liquid helium cooled vacuum tube following sudden vacuum loss
Ram C. Dhuley
Steven W. Van Sciver
National High Magnetic Field Laboratory,
Tallahassee, FL 32310
Mechanical Engineering Department,
FAMU-FSU College of Engineering,
Tallahassee, FL 32310
June 29, 2015 | Cryogenic Engineering Conference | Tucson, AZ

2. Overview Objective: To study heat transfer in a LHe cooled vacuum tube
resulting from accidental vacuum loss to atmosphere
The scenario resembles sudden vacuum loss in the beam-line of a SRF accelerator
air
vacuum
LHe
We have obtained from experiments and have analyzed:
Condensation heat transfer to the tube
Heat transfer to liquid helium

3. Experimental apparatus and procedure
gas tank
fast opening valve
Starting conditions
Valve closed;
N2 gas in the supply tank (295 K);
Copper vacuum tube (≈10-4 Pa)
immersed in LHe (4.2 K), He II (2.1 K)
Open the valve
Loss of vacuum, gas flows
and condenses in the cold
vacuum tube
Record data at four stations
Pressure and temperature rise in
the vacuum tube;
Duration of experiment = 5 s

4. Tube pressurizes uniformly after the front is stopped by the rigid end
Tube pressure profiles from the 4.2 K LHe experiment
uniform
pressurization
Tube pressurizes uniformly after the front is stopped by the rigid end
A pressure front propagates down the tube immediately after loss of vacuum

5. Tube pressure profiles from the 4.2 K LHe experiment
Tube pressurizes uniformly after the front is stopped by the rigid end
A pressure front propagates down the tube immediately after loss of vacuum
As more gas flows in, the tube pressurizes to atmosphere

6. Tube temperature profiles from the 4.2 K LHe experiment
The tube initially carries a temperature gradient,
but stabilizes to ≈50 K after the tube gets to atmospheric pressure
tube at atmospheric
pressure
gas reaches
the rigid end

7. Heat transfer processes in the tube
Energy conservation
over dx:
: rate of energy rise in the
tube wall
Calculated using the
tube temperature traces
: axial heat conduction
(the procedure is illustrated
using T2)
: heat transfer to LHe

8. Calculating condensation heat transfer
Calculating the RHS of energy conservation at station #2
Rate of energy rise in the tube wall
T(t)
c[1], d/dt
1NIST Cryogenic Material Properties
Database

9. Calculating condensation heat transfer
Calculating the RHS of energy conservation at station #2
Derivative of axial heat conduction
T(t)
k[1], d/dx
T-Δx(t)
TΔx(t)
1NIST Cryogenic Material Properties
Database

10. Calculating condensation heat transfer
Calculating the RHS of energy conservation at station #2
Heat transfer to LHe
T(t)
[2]
2S. W. Van Sciver, Helium Cryogenics, 2nd ed., Springer NY, 2012

11. Condensation heat transfer from a propagating gas front
uniform pressurization
results in spatially uniform qdep
station #1 #2 #3 #4
I) Rising pressure - > faster condensation
I >> II
I << II
Two competing processes
II) Rising tube temperature - > slower condensation
All the peaks in qdep occur when local Ttube = K; local ptube < 0.5 kPa

12. Comparing qdep with qLHe
tube at atmospheric
pressure
qdep
qLHe
qLHe is limited by film boiling!
qLHe ≈ 25 kW/m2
Cold tube, faster condensation: qdep >> qLHe (tube accumulates the incident heat)
Warm tube, slower condensation: qdep ≈ qLHe (LHe absorbs the incident heat)

13. Temperature profiles from the He II experiment
Tbath = 2.1 K at start, remains below 2.17 K for the entire duration (5 s)
station #1 not actively cooled by He II
Propagation -> temperature gradient
qdep shows similar behavior as in the
case of the 4.2 K LHe experiment *
He II heat transfer controlled by film boiling
No sure way to determine qHeII
- hydrostatic head varies along the tube
- mode of phase change
(He II -> vapor or He II -> He I -> vapor)
LHe I film boiling will onset when
Tbath will exceed 2.17 K
(this was not observed in our experiment)

14. Conclusions
A gas pressure front propagates in the tube following sudden vacuum loss
Condensation heat transfer to the tube is largely controlled by the
tube temperature
- highest when the tube temperature is in the K range
- rapidly drops as the tube warms above this temperature
High instantaneous heat fluxes (>200 kW/m2) are deposited on
to the tube by the propagating pressure front
Heat transfer to LHe is limited by film boiling

15. Acknowledgement Department of Energy Grant DE-FG02-96ER40952
Dr. Wei Guo and Dr. Ernesto Bosque of NHMFL-FSU
Colleagues at NHMFL Cryogenics lab
- Dr. Mark Vanderlaan, Jian Gao, Brian Mastracci, and Andrew Wray
NHMFL is supported by the US National Science Foundation and the State of Florida.

16. Extra slides
Instrumentation