A heat pipe is simply a device which, by combining principles of thermal conductivity and phase transition is able to efficiently transfer heat between two interfaces. As shown in figure 1, a heat pipe has two areas namely; a hot interface and a cold interface. As the liquid travels through the hot interface (1), it turns into vapor as a result of absorbing heat from the surface. This vapor travels along the pipe (2) towards the cold interface (3) and due to the drop in temperature, it condenses into a liquid and in the process releases latent heat. This liquid then returns to the hot interface (4) through various methods including gravity, capillary action or centrifugal force. This triggers a repeat of the action resulting in a series of endless cycles.

Figure 1: Heat pipe’s principle of operation
Heat pipe construction

The most important thing in the construction is to ensure that you pick the right materials based on the intended use of the hear pipe. For example, for water heat pipes, copper is mainly used owing to its excellent thermal conductivity whereas for ammonia heat pipes, aluminum is normally used to ensure that no reaction occurs between the fluid and the vessel. After using a vacuum to expel air from the pipe, the pipe is then filled partially with the fluid of choice. Design considerations should be taken to ensure that the pipe has both liquid and vapor over the desired working temperature.

Modelling for a working heat pipe

In order to come up with a suitable working model of the heat pipe, the following should be taken into consideration.
Operation temperatures of the heat pipe
This determines which fluid will be used. Some fluids require a very high temperature e.g. sodium (873-1473K) and indium (2000-3000) whereas others liquid helium (2-4K) are suitable for low temperatures.
A good knowledge of pressure losses (both liquid and vapor) in the different segments.

The heat transfer in a heat pipe involve four major categories;

  1. Flow of vapor in the main  (core) region
  2. Flow of liquid in the wick
  3. Liquid and vapor flows interaction
  4. Conduction of heat in the wall

Due to difficulties in describing liquid flow with an accurate theoretical model, most numerical and analytical research concentrate on the flow of vapor in the main region as well as conduction of heat in the wall. Also, owing to the wick’s structure, categories 2 and 3 demand empirical information which can only be obtained through experiments. Furthermore, unlike the dynamics of vapor flow which are closely related to the boundary and geometry specifications especially in the case of non-conventional heat pipes, the analysis of categories 2 and 3 are very similar across heat pipes of various shapes.

Modelling Techniques
  1. Basic 1D model (Axial model)
    • In this modelling method, a single node (vapor node) is used in the middle of the heat pipe to which all the wall nodes directly attach through either linear conductances or resistances. This vapor node, which is massless, corresponds to the pipe’s saturation condition. It is important to know that the wall nodes represent the liquid/vapor interface along every ‘ith’ axial segment with a small change in length.
    • Assumptions
      • The heat pipe is isothermal all around its circumference
  2. 2D models (Circumferential and Axial)
    These model cater for the limitations in the ‘isothermal’ assumption made by the 1D model which is not applicable for some cases i.e. an evaporator. In this method, all wall nodes will still connect to the same vapor node but with their resistances being inversely proportional to their area of heat transfer.
Advantages of heat pipes
  • High heat transfer efficiency
  • Large amounts of heat can be transferred via a small area of cross section over a long distance without the necessity of having an additional power input in the system.
  • Offer simplicity in design
  • Has the ability to transport and control high heat rates
  • Since they have no mechanical moving parts, they hardly require any maintenance.
  • However, impurities or breakdown of the working fluid need to be monitored as these can cause non condensable gases which reduce the heat pipe’s efficiency.
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References/ Important links
  1. https://www.thermalfluidscentral.org/encyclopedia/index.php/Heat_Pipe_Analysis_and_Simulation
  2. http://www.crtech.com/docs/papers/HowToModelHeatpipe.pdf
  3. https://www.thermalfluidscentral.org/encyclopedia/index.php/Historical_Development_of_Heat_Pipes
  4. https://en.wikipedia.org/wiki/Heat_pipe

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