Heat Pipes

Variable and Fixed Conductance Heat Pipes

Heatpie for Electronics CoolingCRTech's tools have been validated many times for the modeling of constant conductance (CCHP or FCHP) and variable conductance heat pipes (VCHP), along with other specialized pipes such as diode heatpipes. The methods used are fully capable of accurately capturing the effects of a heat pipe on the host system without allowing the model to get bogged down in the hydrodynamics internal to the heat pipe. These methods have been considered an industry standard since the 1970 and have been validated many times over.

How Not to Model a Heat Pipe

A common “trick” is to model a heat pipe as a bar of highly conductive material. However, that method has many drawbacks.

  • It does not simulate a heat pipe’s length-independent resistance
  • It does no account for differences in film coefficients between vaporization and condensation
  • It can be disruptive to numerical solutions an potentially cause instabilities in a model
  • It does not provide information on power-length product (QLeff) for comparison against vendor-supplied heat pipe capacity
  • It cannot be extended to include the effects of noncondensible gas (NCG)

Another misconception is that heat pipes, being two-phase capillary devices, require detailed two-phase thermohydraulic solutions. While methods exist to model such details, such an approach would represent computational overkill in almost all cases: even heat pipe vendors use simpler calculations when designing heat pipes.

How to Model a Heat PipePostprocessed model of a vapor blocked VCHP

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. These routines were written specifically to co-solve wall temperatures and gas-front locations, resulting in a more robust tool. The methods used in the built-in subroutines are based on the following recommended modeling methods.

Download a brief explanation on heat pipes are modeled in CRTech products.

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

Features for Modeling Heat PipesChart showing NCG Effects on a CCHP

  • 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 QLeff (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
  • Additional features for variable conductance heatpipes (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

Sample Applications

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

Supporting Resources

A free online webinar on this topic is available: Modeling Heatpipes in FloCAD



Choking and High-speed Flow

Tuesday December 17th, 2pm MST

When flow velocities get big, things gets interesting. Above Mach=0.1, the bulk fluid "sees" a wall that is warmer than the structural temperature due to deceleration within the boundary layer. Above Mach=0.3, kinetic energy changes cease to be negligible. And of course, nothing moves faster than Mach=1.0 for internal flow. When you also add in changes in flow area, or changes in phase ... well, let's just say that doesn't simplify anything.

This webinar will introduce you to the phenomena involved, with a focus on the FloCAD modeling parameters available and their associated correlations and assumptions.

Click here to register

Turbomachinery and Rotating Passages (Secondary Flows)

Thursday December 19th, 2pm MST

Are turbomachines a component in your system, and you'd like to treat them as a "black box"?

Or are they the focus of your work, and the cycle is just a boundary condition to you?

Either way, this webinar will have something to offer you. Each type of turbomachine will be covered: pumps and fans, positive and variable displacement compressors, and turbines (whether gas or hydraulic). Methods for modeling systems like turbochargers and turbopumps will be introduced. Tools for handling spinning flow passages and rotating cavities will be presented.

Click here to register

Starting in 2020, we will begin offering Introduction to Thermal Desktop and Introduction to RadCAD as either in-person training or online training, alternating between online and in-person every three months. The training uses lectures and demonstrations to introduce you to basic Thermal Desktop and RadCAD usage. Hands-on tutorials provide practice building models and interpreting results (tutorials are completed by students outside of the online class time).
The next training class will be an online format in January 2020:
  • Introduction to Thermal Desktop (and SINDA) - A three-part series on January 14, 16, and 21 from 9am to 12pm, Mountain time
  • Introduction to RadCAD - January 23 from 9am to 12pm, Mountain time
For up-to-date schedules, fees, and policies, visit our Product Training page. To register for the class above, complete our registration form and select "Online" for the Training Format.
If you are interested in product training for your company based on your schedule, please contact us to obtain a quote for training between 8-12 attendees. We can come to your facility or the lectures can be presented online. Descriptions of the available classes can be found in our course catalog.
To keep up with our training opportunities, take a look at our new Events and Training Calendar.