Guidelines for Modeling Capillary Two Phase Loops At the System Level

1
Guidelines for Modeling Capillary Two Phase Loops
At the System Level

• Aerospace Thermal Control Workshop 2003
• Jane Baumannjane.baumann_at_crtech.com

2
The Need for Analysis

• The users confidence in any technology is based
  in part on its predictability
• The ability to model predictable behavior
• The ability to bound unpredictable behavior
• Must have compatibility with industry standard
  thermal analysis tools, including
  radiation/orbital analyzers
• Should be able to integrate with concurrent
  engineering methods such as CAD and
  structural/FEM

3
LHP Modeling

• LHPs are not difficult to simulate provided the
  engineer
• has access to relevant performance metrics from
  the LHP vendor (wick properties, conductivity,
  etc.)
• possesses a basic understanding of the technology
• LHPs and CPLs require a sufficiently detailed
  two-phase thermohydraulic code
• Must contain at least rudimentary capillary
  modeling components
• Modeling of LHPs using thermal networks is
  inappropriate
• Accurate simulation of two-phase flow and
  condensation processes is critical to successful
  LHP performance predictions

4
The Unpredictable

• Analyst must capture predictable behavior and
  bound unpredictable behavior
• Bounding analyses may be necessary to capture
  effects of unpredictable behavior
• LHP core status (relative amount of
  back-conduction, etc.)
• Temperature spike associated with the start up
  transient and vapor line clearing
• NCG and evaporator mass effects
• Separate detailed loop model, not system level

5
The Predictable

• Evaporator and compensation chamber energy
  balance
• Capture wick back-conduction
• Axial wall conduction
• Fluid heat transfer
• Wall superheat
• Loop pressure drop (diameters, lengths,
  elevation, etc.)
• Detailed condenser modeling is necessary to
  accurately predict subcooling production
• Ability to capture the variable film coefficient
  along the length of the condenser, and flow
  splits in parallel legs (including static
  pressure recovery)
• Transport line environment parasitic losses/gains

6
LHP and CPL Modeling

• Must accurately predict seemingly minor heat
  gains or losses in the liquid line and the
  compensation chamber especially at low powers
• Must accurately predict condenser performance
  (specifically, the subcooling production)

Qback DTwick/Rwick Qsubcool

• Qsubcool mCp,liqDTsubcool where m is the
  loop mass flow rate

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Evaporator/CC Modeling
Model network representing the evaporator and
compensation chamber within SINDA/FLUINT
8
Wick Back-conduction

• Simplified back- conduction through a wet wick
• Treat wick as effective solid, using
• From Dunn Reay
• Sintered wicks, where g Kliq /Kwick
• Correcting for heat transfer with counter-flowing
  liquid in a tubular wick

G KA/ L or G 2p Keff L / ln( Ro /Ri )

• Keff Kwick 2 g 2 e( 1 g )

2 g 2 e( 1 g )

• Gcorrected (FRliq Cpliq ) / (Ro /Ri )
  FRliq Cp /( ln(Ro/Ri ) Guncorrected ) - 1

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Condenser Modeling

• Must accurately predict subcooling production
• Import CAD geometry for condenser layout
• Requires sufficient resolution to capture thermal
  gradients for accurate subcooling prediction
  (thermal cross-talk between condenser lines)
• Capture variable heat transfer coefficient in the
  condenser line based on flow regime

• Model flow splits in parallel leg condenser

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Condenser Modeling

• New tools easily convert CAD lines, arcs, or
  polylines to fluid pipes for quick model
  development

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LHP Modeling
Evaporator
Serpentine 1D Condenser
Compensation Chamber
12
CPL Modeling

• CPL GAS condenser temperature profile

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Hints and Tricks

• Keep fluid model simple, apply detail where
  necessary on the thermal side
• Use zero volume and time independent components
  (JUNCTIONS and STUBES in SINDA/FLUINT)
• The liquid side of the evaporator/cc should be a
  tank
• Possibly vapor side as tank with artificially
  high vapor volume for stability or in the
  presence of an IFACE
• Take advantage of symmetry to simplify models
  when feasible
• Fluid models require reasonable initial
  conditions
• Use PTEST logic and FASTIC (user control of
  solution) to create initial conditions for a
  STDSTL solution
• LHP Prebuilt available for CR Technologies

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Conclusions

• New CAD methods are available for modeling LHPs,
  CPLs, and heat pipes
• Focus LHP modeling on condenser/transport details
  and bracketing unknown behaviors
• Evaporator and compensation chamber energy
  balance
• Model condenser detail for subcooling production
• Bracket unpredictable behavior for core status,
  NCG, start up etc.

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References

• 1) D. Johnson et al, CAD-based Methods for
  Thermal Modeling of Coolant Loops and Heat
  Pipes, ITherm 2002
• 2) J. Ku, Operating Characteristics of Loop Heat
  Pipes, SAE 1999-01-2007, July 1999.
• 3) J. Baumann et al, Steady State and Transient
  Loop Heat Pipe Modeling, SAE 2000-ICES-105, July
  2000.
• 4) J. Baumann et al Noncondensible Gas, Mass,
  and Adverse Tilt Effects on the Start-up of Loop
  Heat Pipes, SAE 1999-01-2048.
• 5) J. Baumann et al, An Analytical Methodology
  for Evaluating Start-up of Loop Heat Pipes, AIAA
  2000-2285.