Overview of Oscillating Heat Pipes

When heat flux from components becomes very large, airflow from fans simply stops providing the necessary cooling needed to prevent component overheating. In these cases, liquid or refrigerant cooling is needed to keep components operating within acceptable limits. One type of system that complements forced airflow is a heat pipe, which uses phase changes to facilitate heat transport away from hot components. These are used in many laptops to help keep the CPU cool.

One type of heat pipe is an oscillating heat pipe, which takes advantage of pulsations in the fluid flow to move heat. These heat pipes are still the subject of research and development, but systems designers are exploring these heat pipes in some niche application areas that require very high heat flux in any orientation.

What is an Oscillating Heat Pipe?

In general, heat pipes take advantage of phase change in a fluid to absorb heat from a hot object. After changing its phase (vaporization), the vapor expands and flows to a cooler region of the pipe, where it loses heat to the surroundings and condenses. The liquid then flows back to the evaporator region of the pipe to repeat this process. Taking advantage of latent heat of phase changes provides high heat flux in a closed system.

Oscillating heat pipes operate on a similar principle, but without large flows of vapor to provide heat transport. Instead, fluid vaporizes and condenses quickly inside the pipe, which creates regions of vapor and liquid that expand and contract along the flow direction. This pulsation drives a pressure gradient that forces fluid flow. Eventually, the working fluid reaches a cool region, and heat is lost to the surroundings.

The typical structure in an oscillating heat pipe is shown below. During the flow, the liquid and vapor regions are continuously pulsating, thus giving the system its distinctive name. You can read more about this structure and get a comparison with conventional heat pipes in this article.

Oscillating heat pipe cross section. [Source]

The structure shown above requires its serpentine bends to sustain the pulsating action that drives fluid flow, and the design of these bends is an important factor that determines heat dissipation in the cold region of the system. During the flow, the working fluid can exhibit turbulent flow that encourages mixing and gives the system a high heat transfer rate.

Why Use Oscillating Heat Pipes?

There are several reasons oscillating heat pipes might be used for electronics cooling.

Size: Although the overall amount of piping required in an oscillating heat pipe might be quite large, the serpentine bends can allow the system to be small if needed. In a standard heat pipe, the overall size of the system is equal to the transfer distance, so it is not as easy to make the system small if needed.

Varying thermal loads: Heat being driven into an oscillating heat pipe will not always be constant in time. As long as the heat flow into an oscillating heat pipe is above a minimum threshold, the flow can remain stable despite varying thermal loads.

Flow against gravity: Flow in an oscillating heat has the capability of being vertical and flowing over long distances against gravity. This is because oscillating heat pipes do not rely on capillary action or buoyancy to move fluid. As long as the expansion/contraction action exerts large enough oscillations, the fluid can overcome the gravitational force.

Transient start-up: Because an oscillating heat pipe can be designed to have a significant amount of its volume in the hot region, it can reach its stable operating conditions much faster than a conventional heat pipe.

Design Difficulty

Because oscillating heat pipes are driven coupled nonlinear systems that require a stable oscillation, there must always be some minimum input heat in order to ensure stability. By “stability,” we are referring to the case of a steady state pseudo-sinusoidal pulsation in the flow rate.

• Below threshold - Below the stability threshold, any excited oscillation can become chaotic and it will decay to zero farther away from the heat source.
• Above threshold - Above some minimum heat input, the system converges to a stable oscillation, with the decay rate increasing at higher input heat.

These stability conditions are characteristic of relaxation oscillations, where the decay rate in an underdamped transient response actually depends on the input energy (in this case, input heat). If the oscillation does decay (threshold is too high), then there will not be an oscillating flow and the system will fail to move heat away from a heat source.

How to Qualify an Oscillating Heat Pipe Design

Oscillating heat pipes are still the subject of engineering research due to the difficulty in developing analytical models describing the fluid’s temperature distribution and flow rate in an arbitrary periodic geometry. Without a simpler analytical model and formula for predicting heat transfer in an oscillating heat pipe, CFD simulations must be used to design oscillating heat pipes. The various points to simulate in an oscillating heat pipe are turbulence, fluid temperature at each end of the heat pipe system, and the amount of fluid mass converted to vapor vs. liquid.

Whenever you plan to use an oscillating heat pipe for electronics cooling, make sure you simulate 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.

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