When heat loads get very high in an electronic system, designers typically start to look at switching from passive to active cooling. This typically involves switching to a fan, or in extreme cases water/refrigerant cooling may be used. This has become progressively more common in data centers, high-performance PCs, aerospace, and industrial systems. What if there was a passive cooling system that can provide comparable heat dissipation as a large active cooling system?
Heat pipes are one solution that use fluid flow to remove heat from an electronic system, but without requiring power to drive fluid flow. This guide will cover the types of heat pipes that are typically found in electronic systems.
Types of Heat Pipes
All heat pipes operate on similar principles; a fluid is used as a heat transfer medium by flowing from a hot region of a system to a cooler region. The fluid first absorbs heat from the high temperature region, and it begins to flow towards the region of cooler temperature. Once the fluid reaches the cool region, the fluid loses its heat and its temperature drops. The cooled fluid then flows back towards the hot end of the system, completing a closed loop so that the process can repeat.
There are different types of heat pipes that use different physical mechanisms to drive the fluid flow and thereby remove heat from a hot system. In electronics cooling, heat pipes are used in areas like laptop cooling, data center cooling systems, and even in spacecraft. The three main types of heat pipes are: wicking, thermosiphon, and pulsating.
Conventional Wicking Heat Pipe
Wicking heat pipes drive fluid flow through capillary action. There are two main regions in a conventional wicking heat pipe:
- Evaporator, which is where heat is absorbed and the working fluid evaporates
- Condenser, which is where the working fluid loses heat and condenses back to a liquid
The evaporator is where flow begins, and the fluid captures heat from a source via latent heat of vaporization. This is then released in the condenser via latent heat of condensation. The fluid flows from the condenser back to the evaporator through a wick or mesh structure, and this is driven by capillary action.
Wicking heat pipe used for cooling a CPU on a motherboard.
Because these systems require capillary action to drive fluid flow back to the evaporator, the system can have trouble working against gravity over long vertical distances. However, they can be very effective in horizontal flow, which is a typical case used in CPU cooling as shown above.
Thermosiphon Heat Pipe
A thermosiphon heat pipe operates via buoyancy in 2-phase fluid flow. When the fluid is heated at the evaporator, the evaporated fluid has lower density and thus rises to the top of the heat pipe assembly. At the top of the thermosiphon assembly is a condenser, where latent heat is removed from the fluid via condensation. A wick or mesh system can assist fluid flow back into the evaporator and the process can repeat again.
Thermosiphon heat pipe cross section. [Source]
Pulsating Heat Pipe
A pulsating heat pipe, better known as an oscillating heat pipe, also uses two-phase fluid flow to enable heat transfer. The working fluid quickly changes phases between liquid and vapor during the flow process, which creates a pulsating pressure differential that drives fluid flow. The difference between a pulsating heat pipe and the other two types of heat pipes is in the type of motion used to drive fluid flow.
Pulsating heat pipes use oscillatory motion in the working fluid to drive flow from hot to cold regions. The oscillatory motion is driven passively via pressure differential along the flow direction; there is no need for an external power source to drive fluid flow. These systems are a clear improvement over wicking heat pipes as they can provide higher heat flux to a sink, and they can transport over much larger distances (including large vertical distances).
Oscillating heat pipe cross section. [Source]
Pulsating heat pipes have a particular problem that is not observed in other heat pipes, where the oscillation can become very unstable at low thermal loading. When the heat flux into the heat pipe is below some threshold, the oscillation can become chaotic and it will not effectively force fluid flow along the system’s thermal gradient. Because these heat pipes must maintain single-phase flow with some minimum input heat, they can have limited operating range.
Whenever you plan to use a heat pipe in your electronic system, make sure you simulate and qualify heat transport with the complete set of system analysis tools from Cadence. Only Cadence offers a comprehensive set of circuit, IC, and PCB design tools for any application and any level of complexity. Cadence PCB design products also integrate with a multiphysics field solver for thermal analysis, including verification of thermally sensitive chip and package designs.