Two-Phase Closed Thermosyphon

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 Related Topics Catalog
Types of Heat Pipes
  1. Two-Phase Closed Thermosyphon
  1. Capillary-Driven Heat Pipe
  1. Annular Heat Pipe
  1. Vapor Chamber
  1. Rotating Heat Pipe
  1. Gas-Loaded Heat Pipe
  1. Loop Heat Pipe
  1. Capillary Pumped Loop Heat Pipe
  1. Pulsating Heat Pipe
  1. Monogroove Heat Pipe
  1. Micro and Miniature Heat Pipes
  1. Inverted Meniscus Heat Pipe
  1. Nonconventional Heat Pipes
Gravity-assisted wickless heat pipe (two-phase closed thermosyphon).
Figure 1: Gravity-assisted wickless heat pipe (two-phase closed thermosyphon).

A two-phase closed thermosyphon [1] is a gravity-assisted wickless heat pipe (Fig. 1) [2]. The condenser section is located above the evaporator so that the condensate is returned by gravity. The sonic and vapor pressure limits are constraints to the operation of the thermosyphon as with capillary-driven heat pipes. The entrainment limit is more profound in the thermosyphon than in capillary-driven heat pipes due to the free liquid surface. The counterpart of the entrainment limit in thermosyphons is called flooding [2], which is the most severe limitation in the operation of these systems. A sudden oscillating wall temperature and vapor pressure rise will occur at the flooding limit. The boiling limit in thermosyphons is due to film boiling, rather than nucleate boiling as in capillary-driven heat pipes. The boiling limit in thermosyphons occurs when a vapor film forms between the pipe wall and the liquid in the evaporator section of the heat pipe. For small liquid fill volumes, the dryout limit may be reached, where all of the working fluid is held in the liquid film, and no liquid pool exists. In this case, any further increase in the input heat will cause a severe temperature increase at the bottom of the evaporator section.

The operation of the thermosyphon is sensitive to the working fluid fill volume. For thermosyphons without wicks, it has been shown experimentally that the maximum rate of heat transfer increases with the amount of the working fluid up to a certain value. A wick structure is sometimes included in the design of thermosyphons to postpone flooding and improve the contact between the wall and the liquid. The capillary limit is generally of no concern in the operation of the thermosyphon due to the fact that gravity is the major driving force for condensate return.


  1. Faghri, A., 1995, Heat Pipe Science and Technology, 1st ed., Taylor & Francis, Washington, D.C.
  2. 2.0 2.1 Faghri, A., 1989, "Performance Characteristics of a Concentric Annular Heat Pipe: Part II-Vapor Flow Analysis," Journal of Heat Transfer, 111(4), 851-857.