Factors to Consider in Heat Pipe Design Heat pipes are frequently used in current thermal management designs, including in common devices such as laptops and mobile phones. When designing heat pipes, the following factors need to be considered:
Thermal load or heat to be transferred
Operating temperature
Tube material
Working fluid
Wick structure
Length and diameter of the heat pipe
Contact length of the evaporator zone
Contact length of the compensation zone
Orientation
Impact of bending and flattening of the heat pipe
What Materials Can Be Used to Construct Heat Pipes?
Specific working fluids can only operate within certain temperature ranges. Moreover, specific working fluids require compatible container materials to prevent corrosion or chemical reactions between the fluid and the container, as corrosion can damage the container and chemical reactions can produce non-condensable gases. Table 1 illustrates the typical operating characteristics of heat pipes from past research, experiments, and commercial production. For example, liquid ammonia heat pipes operate within a temperature range of -70 to +60˚C and are compatible with aluminum, nickel, and stainless steel. Liquid ammonia heat pipes have been widely used in space applications and use only aluminum containers due to their lighter weight. Water heat pipes operate within a temperature range of 5 to 230°C and are most effective for electronic cooling applications, with copper containers being compatible with water. When the temperature of the heat pipe is below the freezing point of the working fluid, the heat pipe will not function. Freezing and thawing are design issues that can potentially damage the heat pipe's sealing joints when placed vertically. Proper engineering and design can overcome this limitation.
What is a Wick Structure? How Does It Affect Heat Pipe Performance?
Heat pipes have four common wick structures, including grooves, meshes, sintered powder metals, and fibers. The wick structure lines the inner wall of the heat pipe container and allows liquid to flow from one end of the heat pipe to the other through capillary action. Each wick structure has its own advantages and disadvantages. There is no perfect wick structure. Each wick structure has its own limits.
Grooved Wick Structure
· Has the lowest capillary limit but works best in gravity-assisted orientations where the condenser is above the evaporator.
Mesh Wick Structure
· Has the most uniform wick and works on the principle of gravity when the evaporator is above the condenser.
Sintered Powder Metal Wick Structure
· Works best in gravity-assisted orientations. Since the sintered powder metal wick is bonded to the tube wall through metal, it has the best conductive heat transfer from the tube wall to the wick or vice versa among the four common wicks.
Fiber Wick Structure
· Best suited for small-radius bends.
Figure 1 depicts the performance of the four wick structures. It can be seen that the grooved heat pipe has the lowest capillary limit among the four but works best under gravity-assisted conditions.
How Do Length and Diameter Affect Heat Pipe Performance?
The vapor pressure difference between the condenser and evaporator determines the rate of vapor propagation between them. Additionally, the diameter and length of the heat pipe affect the vapor propagation speed and must be considered when designing heat pipes. A larger cross-sectional area of the heat pipe (i.e., a larger diameter) will allow a larger amount of vapor to be transported from the evaporator to the condenser. The cross-sectional area of the heat pipe is a direct function of the heat pipe's acoustic limit and entrainment limit. However, the operating temperature of the heat pipe also affects the acoustic limit of the heat pipe. Figure 2 compares the heat transfer of heat pipes with different diameters. It can be seen that heat pipes transfer more heat at higher operating temperatures.
The rate of working fluid returning from the condenser to the evaporator is controlled by the capillary limit and is an inverse function of the length of the heat pipe. Longer heat pipes transfer less heat than shorter ones.
Figure 3 represents the heat transferred by a copper water sintered powder metal wick heat pipe with a diameter of 6mm across various lengths and orientations.
How Does Orientation Affect Heat Pipe Performance?
Structures with higher capillary limits can overcome gravity to transport more working fluid from the condenser to the evaporator. However, as mentioned earlier, sintered powder metal wick heat pipes, which have the highest capillary limit, work best in gravity-assisted conditions (where the evaporator is above the condenser). Figure 3 shows the impact of gravity on sintered powder metal heat pipes.
How Does Bending of Heat Pipes Affect Performance?
If a heat pipe is bent, the acoustic limit and entrainment limit can be reduced relative to the bending radius and each bending angle. If the bending radius is too tight, the wick may crack (powder metal sintering) or collapse and get pinched (mesh). Therefore, bends in heat pipes may reduce the heat that can be transferred. Figure 4 illustrates the experimental results of the temperature difference between the evaporator and condenser of a 6mm diameter x 300mm long heat pipe bent from straight to a 180° U-bend at 30° bending intervals. The bending radius is the minimum bending radius recommended by Enertron, which is 3 times the pipe diameter or 18mm. The experimental results demonstrate that bending does not affect performance if the bending radius is equal to or greater than 3 times the diameter.
How Does Flattening of Heat Pipes Affect Performance?
If a heat pipe is flattened, the acoustic limit and entrainment limit will decrease relative to the flattened thickness. Therefore, excessive flattening of the heat pipe will reduce the heat that can be transferred or even completely block vapor passage. The right figure shows a sintered powder metal wick heat pipe that is overly flattened and has its vapor channel blocked. Figure 5 illustrates the experimental results of the temperature difference between the evaporator and condenser of a 6mm diameter x 300mm long heat pipe that is round, flattened to 3.5mm thickness, and flattened to 2.5mm thickness. The experimental results demonstrate that appropriate flattening does not affect performance, but excessive flattening does. If the steam channel thickness after flattening is greater than 2mm, there will be no performance degradation compared to a round tube.
How does the average operating temperature of a heat pipe affect its performance?
The average operating temperature of a heat pipe impacts its performance. The higher the average temperature, the better the performance. This is because the working fluid has lower viscosity at higher temperatures, allowing more working fluid to flow from the evaporator through the condenser to the wick structure. At higher temperatures, the working fluid also more easily transitions into a gaseous state. Figure 6 displays experimental results from testing a heat pipe with a 6mm diameter and 250mm length, featuring a copper-water sintered powder metal wick structure. By varying the coolant temperature in the condenser and changing the orientation from -90 degrees (against gravity) to +90 degrees (assisted by gravity), a heat input of 30 watts was applied to the evaporator.
Are heat pipes reliable?
Heat pipes are highly reliable due to the absence of moving parts. However, caution must be exercised during their design and manufacturing. Two manufacturing factors can reduce the reliability of heat pipes: sealing and cleanliness. Any leaks in a heat pipe will ultimately cause it to fail. If the internal chamber is not thoroughly cleaned, residues will produce non-condensable gases when the heat pipe is heated, degrading its performance. Improper bending and flattening of the tubes can also lead to leaks in the tube seals. There are external factors that may also shorten the lifespan of heat pipes, such as impact, vibration, force shock, thermal shock, and corrosive environments.
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