Heat pipes optimize cooling in telecom

John Thayer

August 30, 2001

John Thayer

Heat pipes will boost the performance of heat sinks above and beyond standard cast or extruded-metal types. As phase-change heat-transfer devices, heat pipes effectively transfer heat over their length with only a small temperature gradient. They are widely available in two form factors: cylindrical types, which transfer heat in a single dimension, and vapor chambers (flat heat pipes), which transfer heat in two dimensions. When used in high heat-flux applications where conduction losses are significant, heat pipes can reduce component temperatures by 10 degrees C to 20 degrees C.

Consider the following heat-pipe designs to achieve these results for a high-power, forced-convection telecom application. The design options include an aluminum base with three cylindrical heat pipes embedded in machined grooves, a vapor-chamber base and a plain aluminum base for comparison. All options use the same folded-fin stack. The heat sink has a base of 150 mm in the flow direction and 215 mm across the flow (design airflow is 30 m3/hour at 30-pascals pressure drop, with the heat sink mounted on a circuit board and installed in a card cage at a 30-mm board pitch). The heat sink base is 8-mm thick. The fins in the stack have a thickness of 0.6 mm, a pitch of 1.6 mm and a height of 18 mm. They extend to a length of 76 mm.

The base is alloy 6063, and the fins are alloy 1100. Both are nickel-plated and soldered together. Two heat sources are bolted to the base, with a layer of thermal-interface material in between. One heat source, measuring 25 x 9.5 mm, delivers 84 W (heat flux is 35 W/m2). The other source-19 x 6.4 mm-delivers 11 W (flux is 9 W/cm2).

The spreading resistance in the base is the dominant portion of the overall thermal resistance. Such sinks are an excellent candidate for heat pipes. The embedded heat pipes are 0.25-inch-diameter copper/water heat pipes of varying length, formed into curved shapes that fit within the restricted areas of the base but extend to spread heat to the far corners of the sink. The pipes are pressed into round bottom grooves and are flattened on top to be coplanar with the base surface. The copper vapor chamber is mounted in an aluminum frame.

With these designs, the vapor-chamber type provides the best thermal resistance (0.24 degrees C/W, heat sink to ambient). The embedded-heat-pipe type is next, at 0.3 degrees C/W. The plain base has a thermal resistance of 0.39 degrees C/W.

Trio of design options
Flotherm models were constructed of all three designs to evaluate their performance. The heat pipes were modeled with four elements:

• Copper walls, with a cuboid with the external dimension of heat pipe, and where the conductivity, k, is 380 W/mK;
• Vapor space, with a cuboid having the internal dimension of the hollow space, and where k = 50,000 W/mK;
• Wick, with a collapsed cuboid at the interface between the copper wall and the vapor space, thickness of 1 mm and k = 40 W/mK;
• Interface, with a collapsed cuboid at the interface between the copper wall and the surrounding solid, and where k is appropriate for the contact resistance.

This modeling technique captures the main aspects of heat-pipe heat transfer. In particular, the high-conductivity vapor space allows heat to flow with virtually no temperature gradient along the entire length of the pipe. It captures the wick resistance as the primary component of overall thermal resistance of the pipe. It allows the possibility of spreading in the copper walls.

Also, any thermal interface between the heat pipe and other model elements can be included. The modeling scheme was simplified for the embedded-heat-pipe model by lumping the copper wall, wick and interface into one collapsed cuboid element with one-dimensional heat flow.

John Thayer is Product-Engineering Group Leader at Thermacore International (Lancaster, Pa.). He can be reached at john.thayer@thermacore.com.