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
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.

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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