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
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 will last 60 minutes and are designed to be attended in the order they were presented. If you miss one in the series, please check out our video page for a recording, or contact us before the next webinar starts. 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.

May 31, 2018, 1-2pm (PT), 4-5pm (ET)

This webinar provides an overview of LHP design and operation, from a basic understand of components to a review of important performance considerations and limitations.

Many topics will be covered, from start-up issues to the purpose of the evaporator bayonet to capillary flow regulators to load balancing in parallel LHP units. However, we will cover these topics only in enough depth that you will be able to understand the reasons for various modeling approaches that will be covered in later webinars. In other words, this webinar will survey the various ways in which LHPs require a specialized approach to design analysis and simulation.

This webinar is one of a four-part webinar series on LHPs and approaches to modeling them. Each webinar will last 60 minutes and is designed to be attended in the order they were presented. If you miss one in the series, please check out our video page for a recording, or contact us before the next webinar starts.

Prerequisites: Basic understanding of two-phase thermodynamics and heat transfer.
Please register for Part 1 here

June 5, 2018, 8-9am (PT), 11am-noon (ET)

This webinar explains how the toolbox approach of Thermal Desktop and FloCAD can be used to design and simulate LHPs at a system level, where the focus is on predicting conductance of nominally operating LHP, including thermostatic control (variable conductance).

This webinar is one of a four-part webinar series on LHPs and approaches to modeling them. Each webinar will last 60 minutes and is designed to be attended in the order they were presented. If you miss one in the series, please check out our video page for a recording, or contact us before the next webinar starts.

Prerequisites: Basic understanding of Thermal Desktop and FloCAD operation as applied to two-phase systems. Basic familiarity with LHP components and operation (see Part 1).
Please register for Part 2 here

June 7, 2018, 8-9am (PT), 11am-noon (ET)

Modeling wick back-conduction in an LHP is critical to accurate prediction of the overall loop conductance and operating point. This prediction can't be separated from an understanding of what is happening in the wick core. This webinar presents time-honored methods of dealing with these complex topics in a relatively simple (if abstract) thermal/fluid network.Prerequisites:

This webinar is one of a four-part webinar series on LHPs and approaches to modeling them. Each webinar will last 60 minutes and is designed to be attended in the order they were presented. If you miss one in the series, please check out our video page for a recording, or contact us before the next webinar starts.

Basic understanding of Thermal Desktop and FloCAD operation as applied to LHP modeling (see Part 1 and Part 2).
Please register for Part 3 here

June 12, 2018, 1-2pm (PT), 4-5pm (ET)

This webinar explains how Thermal Desktop and FloCAD can be applied to simulate complex and transient phenomena in LHPs, including condenser design, start-up, thermostatic control, and gravity assist (evaporator below condenser). The design of an actual LHP will be used to demonstrate concepts; the implications of attaching large masses to the evaporators (cooled electronics and support structures) will become clear as a result.

This webinar is one of a four-part webinar series on LHPs and approaches to modeling them. Each webinar will last 60 minutes and is designed to be attended in the order they were presented. If you miss one in the series, please check out our video page for a recording, or contact us before the next webinar starts.

Prerequisites: Familiarity with LHP modeling approaches in TD/FloCAD (see Part 1Part 2 and Part 3).
Please register for Part 4 here