Heat Pipes have been called Heat Superconductors! In this two part series we’ll talk about what a heat pipe, how they are made, compare them with heat sinks, and talk about performance in various thermal management applications.
Figure 1 Schematic View of a Heat Pipe [1]
Heat pipes are transport mechanisms that can carry heat fluxes ranging from 10 W/cm2 to 20 KW/cm2 at a very fast speed. Essentially, they can be considered as heat super conductors. Heat pipes can be used either as a means to transport heat from one location to another, or as a means to isothermalize the temperature distribution.
The first heat pipe was tested at Los Alamos National Laboratory in 1963. Since then, heat pipes have been used in such diverse applications as laptop computers, spacecraft, plastic injection molders, medical devices, and lighting systems. The operation of a heat pipe is described in Figure 1.
A heat pipe has three sections: the evaporator, the adiabatic, and the condenser. The interior of the pipe is covered with a wick, and the pipe is partially filled with a liquid such as water. When the evaporator section (Le) is exposed to a heat source, the liquid inside vaporizes and the pressure in that section increases. The increased pressure causes the vapor to flow at a fast speed toward the condenser section of the heat pipe (Lc). The vapor in the condenser section loses heat to the integral heat sink and is converted back to liquid by the transfer of the latent heat of vaporization to the condenser. The liquid is then pumped back to the evaporator through the wick capillary action. The middle section of the heat pipe (La), the adiabatic portion, has a very small temperature difference.
Figure 2 Pressure Drop Distribution in a Heat Pipe [1]
Figure 2 shows the pressure drop distribution inside a heat pipe. In order for the capillary force to drive the vapor, the capillary pressure of the wick should exceed the pressure difference between the vapor and the liquid at the evaporator. The graph also shows that if the heat pipe is operated against the force of gravity, the liquid undergoes a larger pressure drop. The result is less pumping of the wick with reduced heat transfer. The amount of heat transfer decrease depends on the particular heat pipe.
A typical heat pipe is made of the following:
- Metallic pipe: The metal can be aluminum, copper or stainless steel. It must be compatible with the working fluid to prevent chemical reactions, such as oxidation.
- Working fluid: Several types of fluids have been used to date. These include methane, water, ammonia, and sodium. Choice of fluid also depends on the operating temperature range.
- Wick: The wick structure comes in different shapes and materials. Figure 3 shows the profiles of common wick types: axial groove, fine fiber, screen mesh, and sintering. Each wick has its own characteristics. For example, the axial groove has good conductivity, poor flow against gravity, and low thermal resistance.
Conversely, a sintering wick has excellent flow in the opposite direction of gravity, but has high thermal resistance.
Figure 3 Different Wick Structures
That ends part 1 and in part 2 we’ll address factors that can limit a heat pipes effectiveness, differences in the thermal performance of various heat pipe types, and the spreading resistance of different materials.
Have you got a question on heat pipes or their application? How about an interest in bringing ATS’s team of experienced thermal engineers into one of your projects? You can reach us by visiting http://www.qats.com Purchase heat sinks through our Heat Sink eShop or email us at ats-hq@qats.com or give us a call at 781-769-2800
References:
1. Faghri, A. Heat Pipe Science and Technology Taylor & Francis, 1995.
2. Thermacore Internation, Inc., www.thermacore.com.
3. Xiong, D., Azar, K., Tavossoli, B., Experimental Study on a Hybrid
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