When you open up the cooling system on a CPU or GPU, you might be surprised to find a heat pipe with a refrigerant. No matter where a heat pipe is deployed, it must be filled with a working fluid, which is the medium that is directly responsible for transporting heat away from a hot component. Refrigerants are one option, but many other fluids might be used depending on the system’s temperature range and expected heat flux into the cooling system.
Which Substances Can Be Used as a Working Fluid?
In theory, any fluid could be used as the working fluid in a heat pipe, but in practical cases of cooling systems used in electronics and industry, the range of useful fluids is somewhat limited. The chosen fluid must be able to vaporize within the operating range of the heat source, and it must condense at the cool end of the system where heat sinks. It must also be compatible with the heat pipe materials such that there is no degradation of the heat pipe during operation.
Helium (He), Nitrogen (N₂), and Neon (Ne): Used at cryogenic temperatures.
Methanol (CH₃OH): Used for temperatures ranging from approximately -40°C to 110°C.
Ethanol (C₂H₅OH): Suitable for temperatures from around -10°C to 120°C.
Ammonia (NH₃): Commonly used in space applications, with an operating range of approximately -70°C to 125°C.
Water (H₂O): This is probably the most common working fluid in heat pipes. It is typically used over very broad temperature ranges beyond boiling.
PFCs, HFCs, and CFCs: These are refrigerants that are long-term chemically stable.
Molten lithium (Li): Used in some specialized high-temperature applications.
Molten potassium (K): Operating from 400 °C to 1000 °C.
Molten sodium (Na): Operating from 600 °C to 1200 °C.
Mercury (Hg): Today this substance sees much less use due to health and environmental reasons, but it can be used from -40 °C to 600 °C.
The most important requirement for designing with a particular working fluid is to ensure the system allows vaporization and condensation at the hot and cold ends of the system within the recirculation time, respectively. This is the most important requirement because it is meant to prevent a condition known as dry-out, where fluid in the evaporator gets vaporized faster than it can be replenished with condensed fluid. At excessively high temperature and heat flux, all of the working fluid could be vaporized, and the system will not longer transfer heat.
Verify Heat Pipe Performance in CFD Simulations
When a certain material is selected as a working fluid, it should be qualified against the physical system design using CFD simulations. The use of CFD simulations allows a specific working fluid to be qualified to determine whether a heat transfer goal and temperature goal can be achieved. In the case where the design goals cannot be reached, it’s possible to adjust the material parameters of the candidate working fluid in a simulation until the design goals are reached.
One area that is important to simulate is the flow regime, namely whether the working fluid is flowing as laminar, transition, or turbulent flow. In a standard cylindrical heat pipe geometry, this is a straightforward simulation, and the flow regime could be identified in different regions of the pipe with several methods. While not all heat pipes require turbulent flow, when turbulent flow begins it could increase the heat transfer efficiency as the working fluid would experience mixing of hot and cold fluids during flow.
Example simulation results for a pulsating heat pipe illustrating the transient turn on time. [Source]
Whenever a different set of material parameters is needed to reach the design goal, it’s possible to adjust the material parameters of the working fluid by creating a mixture. By combining two or more similar liquids, it’s possible to adjust the working fluid’s material properties such that the mixture can satisfy the system design requirements. In other cases, an alternative material may be needed to hit the required design goals.
Whenever you plan to use a heat pipe in your electronic system, make sure you simulate and qualify your working fluid 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.