Improved Heat Transfer 2

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Improved Heat Transfer 2

• Enhanced heat transfer in SRF cavity through
• Improvement in helium heat transfer coefficient
  or Kapitza conductance
• Improvement in peak heat flux of super-fluid
  helium
• Reducing effective material thickness and
  increasing cooling area through embedded cooling
  channels or fins

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Improved Heat Transfer 3

• Surface roughness affecting Kapitza conductance
• Surface roughness increases Kapitza conductance
  by coupling phonons through surface or Rayleigh
  waves
• For T gt1K, nano-scale roughness that are
  comparable to the phonon wavelengths at the bath
  temperature, results in diffuse scattering of
  phonons. Hence causing an increase in Kapitza
  conductance
• For operation at 4K, the rough surface acts as
  preferential nucleation cites for the bubbles to
  form and hence increase in heat transfer
  coefficients

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Improved Heat Transfer 4
Surface roughness characterization

• Nano-scale surface roughness are superimposed
  over the higher-scale roughness
• Thereby, increasing micro-scale roughness
  statistically leads to more number of associated
  nano-scale roughness

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Improved Heat Transfer 5

• Affect of surface roughness on Kapitza resistance

Adopted from J. Amrit, et. al.
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Improved Heat Transfer 6

• Affect of surface roughness on Nucleate Boiling
  He-1

Surface roughness plays a vital role in reducing
the superheat (?T) Thereby increasing heat flux
Adopted from S.W. Van Sciver
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Improved Heat Transfer 7

• Enhanced peak heat flux (Q) in super-fluid
  helium
• Operation in pressurized helium state (Qpr gt
  Qsat by a factor of more than three at 1.8K)
• Exploiting fountain pressure effect (
  ) through porous coatings (depending upon
  porosity and pore size) on the cavity surface,
  Qsat can be raised to approach to Qpr while
  operating in saturated helium state

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Improved Heat Transfer 8
Adopted from I. Arend et. al.
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Improved Heat Transfer 12

• Parallel heat leaks
• Conduction Losses through G-10 material to the
  base plate and via support links to He are less
  than 0.5
• Conduction/Convection Losses through Stainless
  Steel Tube containing He are less than 0.2
• Radiation Losses through surrounding vacuum to He
  is less than .05

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Improved Heat Transfer 13
New Lid Design

• Existing lid is too clean to be used for
  experimental work
• New lid has the flexibility to carry out all sort
  of experimental work not requiring ultra clean
  conditions

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Improved Heat Transfer 14
Some Preliminary Efforts

• Two samples from same bulk Niobium rod (Ingot RRR
  232) of 3 cm diameter with each length of 3 cm
  were prepared for their exposed surface to liquid
  helium
• Sample 1 Chemical etched surface
• Sample 2 Un-prepared machined surface

March 2005 Test (At 4.2 K)
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Improved Heat Transfer 15
April 2005 Test 1 at T 2 K
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Improved Heat Transfer 16
End April Test 2 at 4.2 K
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Improved Heat Transfer 17
Possible Problem ?

• Presumably, the systematic error due to extra
  heat leaking into the sensor is caused by the
  sensor wires touching the heater or heater wires