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

flow regimes

Introduction to Two-phase Flow

September 24, 2-3pm MDT

This webinar introduces basic concepts in two-phase flow modeling including quality, void fraction, flow regimes, slip flow, pressure drops and accelerations, and heat transfer.

No knowledge of CRTech software is required. However, references to the corresponding FloCAD features will be made to assist users of that product.

Click here to register

Introductory FloCAD Training

Class times: September 5, 10, and 12, 2019, 9:00 am to 12:00 pm MDT daily
Cost: no charge (attendees must have an active support contract)

CRTech will be hosting introductory training for FloCAD (Flow Modeling in Thermal Desktop). This is our standard FloCAD class previously hosted in a classroom environment and now restructured for an online teaching environment.

The class will introduce single-phase fluid modeling concepts and how to build fluid models within the FloCAD work environment. Topics covered include an introduction to fluid modeling components, geometric versus non-geometric modeling options, working with FloCAD Pipes, solution control, and an introduction to path and pipe libraries.

The class will be broken into three two- to three-hour sessions held over a 3 day period. The format will be online lecture and demonstration with opportunities to ask questions. Hands-on lab work will be provided to students to work on after each session. To gain the most from this class, students are encouraged to attend all three sessions.

Prerequisites: Attendees must have basic working knowledge of Thermal Desktop as many of its base features will not be covered in this class but their usage is required for FloCAD.

Eligibility Requirement: This class is a service to our customers. All attendees must have an active support contract. If you are unsure of your support status, please contact CRTech.

Click here to register