Heat Pipe Modeling and Two-phase Loops

C&R tools are routinely used to model the complexities of two-phase transport devices such as loop heat pipes (LHP), capillary pumped loops (CPL), heat pipes, vapor chamber fins, thermosyphons, and loop thermosyphons (LTS). SINDA/FLUINT has been enhanced over the past 15 years specifically to handle the modeling of these complex devices. The code has been used for a variety of modeling tasks from capturing steady state system level effects of two phase devices to simulating detailed startup transients for component design and sizing. SINDA/FLUINT is unique in its ability to co-solve integrated thermal and fluid systems while providing the complete thermodynamics of two-phase flow necessary to model these devices accurately.

	Constant Conductance Heat Pipe	Gas Blocked Heat Pipe	Loop Heat Pipe

How to Model a Heat Pipe

Heat pipe routines built into SINDA/FLUINT provide fast system-level solutions to modeling heat pipes when a full two-phase solution is not required. Both constant conductance (CCHP, also called FCHP), with or without noncondensible gas (NCG), and variable conductance (VCHP) pipes can easily be simulated. Unlike other non-C&R heat pipe routines, this routine was written specifically to co-solve wall temperatures and gas-front locations, resulting in a more robust tool.

• Download a brief explanation on How to Model a Heat Pipe

How Not to Model a Heat Pipe - A common "trick" is to model a heatpipe as a bar of highly conductive material. However, that method does not simulate a heat pipe's length-independent resistance, cannot account for differences in film coefficients between vaporization and condensation, and cannot be extended to include NCG effects. Another misconception is that heat pipes, being two-phase capillary devices, require detailed two-phase thermohydraulic solutions. While codes capable of such details exist, such as C&R's SINDA/FLUINT, such an approach would represent computational overkill in almost all cases: even heat pipe vendors use simpler calculations when designing heat pipes.

FloCAD®, a Thermal Desktop® module, provides a unique tool for modeling heat pipes within a CAD based environment. Complex geometries, such as serpentine condensers or large networks of heat pipes, can easily be generated.

Flat Heat Pipe for Electronics Cooling	Two Phase Condenser

HEATPIPE and HEATPIPE2 features:

• Constant (fixed) conductance heat pipes (CCHP, FCHP) and vapor chamber fins
  • 1D or 2D thermal model (axial, axial and circumferential, rectangular)
  • Distinct vaporization and condensation coefficients for grooved designs
  • Prediction of QL_eff (power-length product)
  • Optional inclusion of noncondensible gas (NCG) degradation
    
    
    
    
  • Fast and easy geometric model generation using FloCAD, including bonding or contact to thermal surfaces and solids and even to other heat pipes
    
    

  Intel Xeon™ CPU Chip with Embedded Finned Heat Pipes

• Additional features for variable conductance heat pipes (VCHP)
  • Choose working fluid from library or define a new fluid
  • Perfect gas or real gas descriptions for control gas
  • Fast and stable 1D (flat front) gas blocking algorithm
  • Warnings for erroneous designs, gas charges, environments

Unique features relevant for analyzing LHPs, CPLs, and LTSs include:

• Complete thermodynamics: phases appear and disappear as conditions warrant
• Capillary modeling tools for static or vaporizing wicks
  • Vapor trapping (up to the bubble point) in capillary devices
  • Capillary flow regulators (constant back-pressure devices)
  • From top-level steady-state evaporator-pump modeling to
    detailed tracking of unsteady liquid/vapor interfaces within wicks
  • Full phasic nonequilibrium two-fluid modeling for unsteady hydrodynamics in heat pipes, LHP compensation chambers, etc.
• Two-phase heat transfer correlations built-in or user-defined
• Two-phase pressure drop correlations built-in or user-defined
• Automatic flow regime mapping
• Homogeneous and slip flow modeling, including countercurrent flow in the presence of gravity and other accelerations
• Conservation of total charge mass for accurate pressure predictions in transients or parametric studies
• Complex liquid/gas mixtures including optional dissolution of any gaseous solute into liquids
• Fast and easy geometric model generation of condensers (serpentine, manifolded, etc.), including bonding or contact to thermal surfaces and solids, using FloCAD

Sample applications include:

• Deployable two-phase radiator systems for aerospace applications
• Electronic cooling systems
• De-icing applications
• Isothermal furnace liners
• Heat pipe heat exchangers

Supporting Resources

• A free online presentation is available on Modeling Heatpipes in FloCAD

• CAD-based Methods for Thermal Modeling of Coolant Loops and Heat Pipes
  • Please visit our resource page to download LHP and CPL related technical papers.

Customization and Consulting

C&R also provides consulting and custom software solutions to specifically meet your needs.