Oscillating Heat Pipes

Oscillating heat pipes (OHPs), also called pulsating heat pipes (PHPs), use pressure-driven, two-phase fluid flow in a channel to rapidly transfer heat between heat sources (e.g. microchips, diodes, waste heat streams, etc.) and heat sinks (air, liquid, deep space, etc.).

OHPs can be built in materials, shapes, and sizes that are not possible with conventional capillary-driven heat pipes or vapor chambers.

As a result, the OHP technology can be embedded into structures as drop-in replacements for their existing solid-state materials and in doing so create multi-functional ones that provide exceptional thermal performance without compromising their mechanical properties.

Oscillating Heat Pipes are passive heat transfer devices typically embedded into a material with capillary-sized microchannel pattern (or volume). This volume is partially filled with a saturated working fluid, and its pattern is such that it crosses between the material’s heat source areas and sink areas multiple times. When the microchannel’s cross-sectional shape is properly designed, the saturated working fluid’s liquid “slugs” span its full diameter. Because the volume is only partially filled, the liquid slugs are separated by saturated vapor “plugs” (or bubbles) along the microchannel’s length.

When heat is applied at a source, the internal volume’s local vapor expands, liquid evaporates, and even boiling may occur - all increasing the pressure and moving fluid away from the source toward the cooler, lower pressure areas near the material’s sink. There, relatively warm, incoming vapor contracts and condenses at the sink’s temperature and fluid’s saturated pressure. These pressure instabilities cause the fluid to rapidly move along the channel length with random changes in direction. The result is highly efficient sensible heat transfer from the liquid slugs movement as well latent heat transfer at the phase change sites. By routing the channel between the heat sources and sinks a sufficient number of times, these random pulsations result in stable operation.

As for their mechanical properties, the array of internal microchannels with walls between such microchannels results in lightweight “i-beam” structures. This results in a high compressive strength, similar to the base material itself. If needed, the configuration of microchannels can be routed in flat, 2D or more complex, conformal 3D geometries so that heat can be spread or transferred in multiple directions.

Oscillating heat pipes are similar to conventional heat pipes, vapor chambers (VCs) or constant conductance heat pipes (CCHPs), and loop heat pipes (LHPs):

• Passive heat transfer devices: all heat pipes utilize a hermetically sealed saturated working fluid to transfer heat between heat sources and sinks without the aid of power-consuming pumps, fans, etc.

• Working fluid limits: all heat pipes have fluid-based constraints - e.g., the fluid must be compatible with the base material, its operation is limited by lower temperature (freezing point) and upper temperature (critical point), and for a given design the power and performance varies by temperature..

Oscillating heat pipes are different from CCHPs and LHPs in several ways:

• High power capability: OHPs can operate with (10s to 100s W/K) across a wide range of heat fluxes (<1 W/cm2 to >300 W/cm2) and heat loads (<1 W to >1000 W) not possible with other heat pipes

• Thin form-factor: The small channel size of the OHP enables designs as thin as <1 to <4mm without sacrificing performance

• Greater structural integrity: The channel size of an OHP is smaller than the cavity size in other heat pipes resulting in a shorter unsupported length and therefore greater structural integrity than other heat pipes

• Gravity independence: OHPs can be designed to work independent of gravitational orientation and in high adverse acceleration environments

• No wick: The OHP does not require a wicking structure which reduces manufacturing complexity

Oscillating Heat Pipes (OHPs) are versatile thermal management devices used for efficient heat transfer. They find applications in cooling opto-electronic, RF and power-energy components, such as microprocessors, Li-ion cells, lasers, inverters/converters, and power amplifiers, where maintaining optimal operating temperatures is crucial, but the size weight and power (SWaP) of the thermal management technology must be limited.

OHPs are also often used when power density (W/cm2 or W/cm3) must be maximized with minimal heat pipe thickness.

OHPs are particularly valuable in spacecraft, as they can passively dissipate heat in the vacuum of space. They're also used in various terrestrial and airborne applications, including in electronics cooling, battery systems, and photonics systems. OHPs provide a compact, reliable, and energy-efficient solution for cooling and thermal management in a wide range of industrial and technological settings.

In the world of heat transfer technology, Pulsating Heat Pipes (PHPs) and Oscillating Heat Pipes (OHPs) often seem to be distinct entities. However, they are the same technology.

In practice, both terms are interchangeable, and engineers, researchers, and manufacturers may use one or the other to refer to this efficient heat transfer solution. What's most crucial is understanding the concept and applications of this technology, regardless of whether it's called a PHP or OHP.

The thermal performance of an oscillating heat pipe refers to its ability to efficiently transfer heat. It depends on factors like the working fluid, geometry, orientation, and operating conditions. Generally, OHPs can operate with conductances of 10s to 100s W/K, across a wide range of heat fluxes from <1 W/cm2 to >300 W/cm2, and with a wide range of heat loads (<1 W to >1000 W). This makes them effective for cooling applications in various industries, such as electronics and aerospace, where efficient heat transfer is crucial for maintaining optimal temperature levels.