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 than modeling heat pipes. Unlike a heat pipe, these devices require full thermohydraulic modeling of the fluid and containment system. In addition to supporting the modeling of single phased pumped loops, SINDA/FLUINT and FloCAD® have several features which make it uniquely capable of modeling two-phase pumped loops and capillary pumped pumps. Using SINDA/FLUINT and FloCAD, the analyst 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, reservoir quenching, partially primed wicks, and pressure 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 CPL's and LHP's during the late 1980s and early 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 modeling limitations. Such features include Interface elements for modeling compensation chambers and reservoirs along with the liquid/vapor interface in the wick, a capillary evaporator pump, and a capillary device to model wicks, grooves, tubules, etc. During this time period, the ability to model the dissolution and evolution of noncondensible gases was added to the code.

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)
    • 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

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