Fundamentals of heat pipes

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Principle of heat pipe with wick
Principle of heat pipe with wick

Heat pipes are a very promising technology for achieving high local heat removal rates and uniform temperatures in computer chips. Miniature and micro heat pipes have been and are being used in electronic cooling. For example, a majority of laptop computers use heat pipes to get rid of heat produced in chip processors.

The wick heat pipe can operate in any orientation because it uses a wick to distribute the liquid. The principle of wick heat pipe operation is illustrated with the aid of figure on the right. Heat is applied to the evaporator section and is conducted through the wick and liquid. Liquid evaporates at its interface with vapor as it absorbs the applied heat. In the condenser section, the vapor releases heat to its cooler interface with liquid as it condenses. In the wick, the menisci are increasingly pronounced approaching the evaporator end, due to the growing pressure drop required to draw the liquid through the increasing length of wick. There are additional contributions to pressure drop, such as friction of the vapor flow, adverse orientation against gravity, or other acceleration sources. Subsequently, the vapor pressure drops as it flows from the evaporator to the condenser. As stated above, the friction of the liquid flow through the wick causes the liquid pressure to drop from the condenser to the evaporator. If the heat pipe is to function, all pressure drop sources must be balanced by the capillary pressure differential provided at the menisci in the capillary wick.

In addition to the limitations on maximum chip temperature, further constraints may be imposed on the level of temperature uniformity in electronic components. The micro and miniature heat pipes are very promising technologies for achieving high local heat removal rates and uniform temperatures in computer chips (Faghri, 1995). Micro heat pipe structures can be fabricated on the substrate surfaces of electronic chips by using the same technology that forms the circuitry. These thermal structures can be an integral part of the electronic chip and remove heat directly from the area where the maximum dissipation occurs.

The micro heat pipe is defined as a heat pipe in which the mean curvature of the liquid vapor interface is comparable in magnitude to the reciprocal of the hydraulic radius of the total flow channel. Typically, micro heat pipes have convex but cusped cross-sections (for example, a polygon), with hydraulic diameters ranging from 10 to 500 μm. Khrustalev and Faghri (1994) analyzed a triangular micro heat pipe in which the corners act as a wick. The heat load is uniformly distributed among all corners. Near the evaporator end cap, if the heat load is sufficiently high, the liquid meniscus is depressed in the corner, and its cross-sectional area, as well as the radius of curvature of the free surface, is extremely small. Most of the wall is dry or is covered by a nonevaporating liquid film. In the adiabatic section, the liquid cross-sectional area is comparatively larger. The inner wall surface may be covered with a thin liquid film due to the disjoining pressure. At the beginning of the condenser, a film of condensate is present on the wick, and surface tension drives the liquid flows through this film toward the meniscus region. The possibility of liquid blocking the end of the condenser is also shown in Fig. 1.24. Attempts were also made to etch micro heat pipes directly into silicon and use them as thermal spreaders (Peterson, 1996; Benson et al., 1998). These heat pipes have hydraulic diameters on the order of 10 μm, which are classified as “true” micro heat pipes. The performance characteristics of the micro heat pipes are different from those of conventional heat pipes.


References

Benson, D.A., Mitchell R.T., Tuck M.R., Palmer, D.W., and Peterson G.P., 1998, “Ultrahigh-Capacity Micromachined Heat Spreaders”, Microscale Thermo-physical Engineering, Vol. 2, pp. 21-30.

Faghri, A., 1995, Heat Pipe Science and Technology, Taylor & Francis, New York.

Faghri, A., and Zhang, Y., 2006, Transport Phenomena in Multiphase Systems, Elsevier, Burlington, MA.

Faghri, A., Zhang, Y., and Howell, J. R., 2010, Advanced Heat and Mass Transfer, Global Digital Press, Columbia, MO.

Khrustalev, D., and Faghri, A., 1994, “Thermal Analysis of a Micro Heat Pipe,” ASME Journal of Heat Transfer, Vol. 116, pp. 189-198.

Peterson, G.P., 1996, “Modeling, Fabrication, and Testing of Micro Heat Pipes: An Update,” Applied Mechanics Review, Vol. 49, Part 2, pp. 175-183.

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