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

Publications

 

dispersed vs. coalesced front

Tuesday, June 26, 2018, 1-2pm PT, 4-5pm ET

This webinar describes flat-front modeling, including where it is useful and how it works. A flat-front assumption is a specialized two-phase flow method that is particularly useful in the priming (filling or re-filling with liquid) of gas-filled or evacuated lines. It also finds use in simulating the gas purging of liquid-filled lines, and in modeling vertical large-diameter piping.

Prerequisites: It is helpful to have a background in two-phase flow, and to have some previous experience with FloCAD Pipes.

Register here for this webinar

FloCAD model of a loop heat pipe

Since a significant portion of LHPs consists of simple tubing, they are more flexible and easier to integrate into thermal structures than their traditional linear cousins: constant conductance and variable conductance heat pipes (CCHPs, VCHPs). LHPs are also less constrained by orientation and able to transport more power. LHPs have been used successfully in many applications, and have become a proven tool for spacecraft thermal control systems.

However, LHPs are not simple, neither in the details of their evaporator and compensation chamber (CC) structures nor in their surprising range of behaviors. Furthermore, there are uncertainties in their performance that must be treated with safety factors and bracketing methods for design verification.

Fortunately, some of the authors of CRTech fluid analysis tools also happened to have been involved in the early days of LHP technology development, so it is no accident that Thermal Desktop ("TD") and FloCAD have the unique capabilities necessary to model LHPs. Some features are useful at a system level analysis (including preliminary design), and others are necessary to achieve a detailed level of simulation (transients, off-design, condenser gradients).

CRTech is offering a four-part webinar series on LHPs and approaches to modeling them. Each webinar is designed to be attended in the order they were presented. While the first webinar presumes little knowledge of LHPs or their analysis, for the last three webinars you are presumed to have a basic knowledge TD/FloCAD two-phase modeling.

Part 1 provides an overview of LHP operation and unique characteristics
Part 2 introduces system-level modeling of LHPs using TD/FloCAD.
Part 3 covers an important aspect of getting the right answers: back-conduction and core state variability.
Part 4 covers detailed modeling of LHPs in TD/FloCAD such that transient operations such as start-up, gravity assist, and thermostatic control can be simulated.