New challenges facing thermal management include an increasing need for remote heat dissipation as well as more efficient thermal solutions with low weight, low cost, and high reliability. Heat pipes are increasingly filling this role. To keep up with today's thermal solution challenges, heat pipes must improve efficiency and integrate remote heat transfer into thermal management solutions.
Heat Pipe Characteristics
A heat pipe is a vacuumed vessel charged with working fluid (Fig. 1). The walls of the heat pipe are lined with a wick structure that allows liquid to travel from one end of the heat pipe to the other via capillary action. Heat added to one end of the heat pipe, called the evaporator end, causes the working fluid to evaporate or change phase from liquid to vapor. The vapor travels through the center of the pipe to the other end — the condenser end — where a heatsink or other means remove the heat energy. The release of heat causes the vapor to condense back to liquid for the wick to absorb. The liquid working fluid is carried in the wick by capillary action back to the evaporator zone.
Factors such as wick, working fluid, bending, flattening, and orientation can influence heat pipe performance. The four common, commercially produced heat pipe wick structures are groove, wire mesh, powder metal, and fiber/spring. Different wicks have varying capillary limits, the rate at which the working fluid travels from condenser to evaporator influence the overall heat pipe performance.
The orientation of a heat pipe also plays an important role in its performance. The performance of a heat pipe under specific orientations is directly related to its wick structure. For example, the groove wick has the lowest capillary limit but works best under gravity-assisted conditions, where the evaporator is located below the condenser.
The type of working fluid also influences heat pipe performance. A heat pipe is not functional when its temperature is below the freezing point or above the vapor condensation point of the working fluid. When the temperature is above the vapor condensation point of the working fluid, the vapor will not condense back to liquid phase. This causes the heat pipe to stall, because no fluid circulation occurs. Flattening or bending the heat pipe may reduce the amount of heat that can be transported and is another important design issue.
In conventional use, heat pipes are integrated into a total thermal subsystem to transport heat from the heat source to remote areas. The heat pipes' ability to act as a primary heat conductive path allows engineers to solve thermal problems in applications with space constraints or other limitations. Thus, you can use heat pipes to carry heat away from the heat-sensitive components to the finned array or a heatsink located in an area where more space for heat dissipation is allowed — leaving room for electronics layout flexibility.
A high-capacity power electronics cooler (Fig. 2) is an example of a thermal solution where no sufficient space was available to directly mount a finned heatsink to the heat source. Eight high-capacity heat pipes form an efficient thermal path to the fin array, which dissipates heat using forced convection. The cooler can dissipate 800W.
In addition to acting as a heat conductive path and aiding in remote heat transfer, heat pipes can improve thermal solution efficiency. You can accomplish this by embedding heat pipes into the heatsink base or passing the heat pipes through the fins. In most cases, embedding heat pipes into the conventional thermal solution results in size or weight reduction.
The most appropriate application for heat pipe integration into the heatsink base is when the base is large compared with the heat source. In such applications, the heat source location produces the highest temperature. The smaller the heat source, the more spreading has to occur over the heatsink base, resulting in a greater temperature rise in the center of the base. Integrating heat pipes into the base of the heatsink decreases the temperature gradient across the base, yielding a more efficient solution.
You can also improve heatsink fin efficiency with heat pipe integration. Fin efficiency is related to the rate at which the fin can dissipate heat energy. The maximum rate at which the fin can dissipate energy is the rate that would exist if the fin were at base temperature. Therefore, the efficiency of the fin can be improved by passing a heat pipe through the fin (Fig. 3). Compared with the traditional finned heatsink, the use of a heat pipe configuration in Fig. 3 reduces footprint area of the power heat sink and improves heat dissipation capability.
While external factors such as shock, vibration, force impact, thermal shock, and corrosive environment can affect heat pipe life, its integration into a thermal solution also delivers many benefits. If manufactured and designed properly, heat pipes are highly reliable and have no moving parts. Also, heat pipes are economical, having little effect on the overall cost of the total thermal solution.
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