Thermosiphons

Thermosiphons and Loop ThermosiphonsMRI machine, cryogenic magnet

Single phase and two phase loop thermosiphons (LTS) are used across multiple industries such as solar thermal hot water heating, electronics cooling, gas-fired heaters, and nuclear reactor cooling, and cryogenic magnet cooling. Since the operation of thermosiphons and loop thermosiphons is based on the natural circulation due to changes in fluid density in a gravity environment (buoyancy), the modeling of such systems can be challenging, especially in the presence of two-phase evaporation and condensation. With two-phase thermosiphons not only are density and gravity a factor, but you must also capture pool boiling and the falling water droplets as the fluid condenses.

A simple thermosiphon is a vertical pipe where liquid pools at the bottom and when heated, vapor rises in the middle of the pipe while liquid condenses near the top and fall down along the pipe walls. A higher-performing design is a loop thermosiphon (LTS), which separates the down-flowing liquid from up-flowing vapor (or two-phase) streams. Subcooling and superheating can occur more readily in such a loop. In some designs, liquid flows downward and two-phase fluid flows upward. In others, a two-phase mixture flows downward and vapor flows upward. As long as one line has a higher time-averaged density than the other, circulation will occur. An LTS self-determines both the pressure and the flow rate, and the flow rate is often unstable: intense, short time-scale oscillations and even temporary flow reversals are common.

Fortunately SINDA/FLUINT and FloCADĀ® provide the necessary tools required to capture all of these phenomena for both steady state and transient simulations. There are multiple approaches to modeling a thermosiphon depending on the design of the system and what data you need from the analysis.

To aid in demonstrating these options, CRTech has created the following sample models:

Unique features relevant for analyzing LTSsPostprocessed model of solar thermal collector panel with thermosyphons

  • Complete thermodynamics: phases appear and disappear as conditions warrant
  • 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
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.