Transcript No Slide Title

FDM/FEM System-level Analysis of Heat Pipes
and LHPs in Modern CAD Environments
Aerospace Thermal Control
Workshop 2005
Brent Cullimore, Jane Baumann
[email protected]
C&R Technologies, Inc.
www.crtech.com
Phone 303.971.0292
Fax 303.971.0035
The Need for Analysis
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The user’s confidence in any technology is based in
part on its predictability
 The ability to model predictable behavior
 The ability to bound unpredictable behavior
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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
How Not to Model a Heat Pipe:
Common Misconceptions

“Full two-phase thermohydraulic modeling is required”
 Overkill with respect to heat pipe modeling at the system level
 Applicable thermohydraulic solvers are available for detailed modeling,
but uncertainties in inputs can be quite large

“Heat pipes can be represented by solid bars with an artificially
high thermal conductivity”
 Disruptive to the numerical solution (especially in transient analyses)
 Unlike a highly conductive bar, a heat pipe’s axial resistance is
independent of transport length: not even anisotropic materials
approximate this behavior
 No information is gleaned regarding limits, design margin
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“Heat pipes can be modeled as a large conductor”
 Analyst shouldn’t assume which sections will absorb heat and which
will reject it
 Heat pipes can exhibit up to a two-fold difference in convection
coefficients between evaporation and condensation
Typical System-Level
Approach

Targeted toward users (vs. developers) of heat pipes:
 Given simple vendor-supplied or test-correlated data …
 How will the heat pipe behave? (Predict temps accurately)
 How far is it operating from design limits?
 From this perspective, no need to model what happens past these limits!!
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Network-style “Vapor node, conductor fan” approach:
Gi = 1/Ri = Hi*P*DLi
where:
Hi = Hevap (Ti > Tvapor)
Hi = Hcond (Ti < Tvapor)
Next Level: QLeff

Checking Power-Length Product Limits
 Sum energies along pipe, looking for peak capacity:
QLeff = maxi | [ Si( Qi/2 + Sj=0,i-1Qj ) DLi ] |
 Can be compared with vendor-supplied QLeff as a function of
temperature, tilt

What matters is verifying margin, not modeling deprime
 Exception: start-up of liquid metal pipes (methods available)
Noncondensible Gas

Gas Front Modeling (VCHP or gas-blocked CCHP)
 Amount of gas (in gmol, kmol, or lbmol) must be known or
guessed (can be a variable for automated correlation)
 Gas front modeled in 1D: “flat front”
 Iteratively find the location of the gas front
 Sum gas masses from reservoir end (or cold end). For a perfect
gas:*
mgas = Si {(P-Psat,i)*DLi*Apipe/(Rgas*Ti)}
 Block condensation in proportion to the gas content for each
section
 Provides sizing verification for VCHP, degradation for CCHP
____________
* Real gases may be used with full FLUINT FPROP blocks
Gas Blockage in CCHPs
Parametric Study of
Heat Pipe Degradation
from Zero NCG (left)
to 8.5e-9 kg-mole (right)
VCHP Modeling

Requires reservoir volume
and gas charge (sized by
heat pipe vender)
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Model axial conduction along
pipe to capture heat leak
through adiabatic section of
pipe
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Accurately capture reservoir
parasitics through system
model

Easy to integrate 1D or 2D
Peltier device (TEC),
proportional heater, etc. for
reservoir (or remote payload)
temperature control
VCHP rejecting heat
through a remote
radiator
2D Wall Models

Relatively straightforward
to extend methods to 2D
walls
 Example: top half can
condense while bottom
half evaporates
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However:
 QLeff remains a 1D
concept
 Gas blockage remains
flat front (1D, across
cross-section)
 This can complicate
vapor chamber fin
modeling
Condenser Section
The Old Meets the New
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Proven Heat Pipe Routines
 VCHPDA SINDA subroutine
 1D Modeling of VCHP gas front
 Vapor node as boundary node for stability
 SINDA/FLUINT Heat Pipe routines (HEATPIPE, HEATPIPE2)
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Modeling of CCHP with or w/out NCG present
Modeling of VCHP gas front
1D or 2D wall models available
QLeff reported
Vapor node as boundary node optionally
 Implicit within-SINDA solution used for improved stability
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New CAD-based methods
 CAD based model generation
 New 1D piping methods within 2D/3D CAD models
New CAD Methods

Modeling heat pipes in FloCAD
 Import CAD geometry
 Quickly convert CAD lines and polylines to “pipes”
 Generates HEATPIPE and HEATPIPE2 calls automatically
without heat pipes
Heat Pipes Embedded in a Honeycomb Panel
with heat pipes
Heat Pipe Data Input
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User-defined heat pipe options and inputs
CAD-based Centerlines and
Arbitrary Cross Sections
Attach to 2D/3D Objects
(contact), radiate off walls …
What’s Missing?
Future Heat Pipe Modeling Efforts
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Currently heat pipe walls are limited to 1D or 2D finite difference
modeling (FDM)
 Other FloCAD objects (like LHP condenser lines) allow walls to be
unstructured FEM meshes, collections of other surfaces, etc.
 But a detailed model can conflict with common assumptions such as
heat transfer at the “vapor core diameter”
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Vapor Chamber Fins
 2D “power-length” capacity checks
 2D gas front modeling (not currently a user concern)
A little about Loop Heat Pipes
(LHPs)

CCHPs and VCHPs are “SINDA only” (thermal networks)
 Can access complex fluid properties, but FLUINT is not required
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LHPs require more complex solutions (two-phase
thermohydraulics: fluid networks)
Condenser can be quickly
modeled using FloCAD’s
pipe component.
 Walls can be FEM meshes,
Thermal Desktop surfaces,
or plain tubes (piping schedule
available)
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Easy to connect or
disconnect pipes
 Manifolds, etc.
LHP Condenser Modeling
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Must accurately predict subcooling production and
minor liquid line heat leaks
 Import CAD geometry for condenser layout
 Requires sufficient resolution to capture thermal gradients
 Capture variable heat transfer
coefficient in the condenser
line based on flow regime
 Model flow splits in parallel
leg condenser
 Model flow regulators
Conclusions
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Heat pipes and LHPs are can be easily modeled at the
system-level
 Heat pipes: using modern incarnations of “trusted” methods
 LHPs: using off-the-shelf, validated thermohydraulic solutions

New CAD methods permit models to be developed in a
fraction of the time compared with traditional techniques