Loop Heat Pipes (LHP)

Modeling Loop Heat Pipes and other Capillary Devices

TadSat4 Loop Heat PipeThe methods for modeling LHPs and capillary pumped loops (CPLs) are very different from those used to model heat pipes. Unlike a heat pipe, self-pumping loops require full thermohydraulic modeling of the fluid and the thermal-structural environment. In addition to supporting the modeling of single-phase pumped loops, SINDA/FLUINT and FloCAD® have several features which make them uniquely capable of modeling two-phase pumped loops and capillary pumped pumps. Using SINDA/FLUINT and FloCAD, you can capture everything from the system-level effects of the LHP all the way down to detailed thermodynamic and hydrodynamic transient events such as LHP start-up, compensation chamber (reservoir) quenching, partially primed wicks, and pressure and flow oscillations.

When it comes to modeling two-phase loops, SINDA/FLUINT has the unique advantage over other fluid flow codes in that a portion of the software development team (engineers by training) were actively involved in the development of CPLs and LHPs during the late 1980s and 1990s, while they were also working on the development of SINDA/FLUINT. Consequently, when obstacles were encountered in the process of modeling these two-phase loops, new features were added to the software to overcome previous simulation limitations. Such features include interface elements ("IFACEs") for modeling compensation chamber and the liquid/vapor interface within the wick, a capillary evaporator pump, and a capillary device to model wicks, grooves, tubules, and so forth. During this time period, the ability to model lack of thermal equilibrium within the compensation chamber was also added to better estimate start-up responses.

Unique features relevant for analyzing LHPs and CPLs

  • 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 used in parallel condensers)
    • 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 LHP compensation chambers
  • 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 Thermal Desktop.

LHP with Serpentine Condenser Line, postprocessed FloCAD model shown on the right

Resources

Please visit our support forum for a sample of how to model an LHP, or for an advanced example.

An example of a CPL system, used to illustrate automated tools to calibrate models to test data, is available here.
 

Publications

Recorded Training Webinars on LHPs and LHP Modeling

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