Predicting Pulsating Heat Pipe (PHP) Design for High-Cooling Efficiency

Authors: Achref Rabhi, Amine Ben Hadj Ali, Samuel Talvy, Helmi Mlaouah

In the world of thermal management, the goal of an efficient heat transfer solution continues to drive rapid innovation. Typically, an efficient cooling system involves intricate physics and complex mechanics leading to many manufacturability challenges, including organic advanced shapes for cooling channels, wicked structures or even microstructure channels. These challenges can be overcome by 3D printing due to its flexibility and customization, however scalability of 3D printing to the production level is limited.?

One breakthrough heat transfer technology that has emerged in recent years is the Pulsating Heat Pipe (PHP) also known as Oscillating Heat Pipes (OHP). It belongs to the family of passive two-phase heat spreaders/transport solutions, which includes traditional heat-pipes, vapor-chambers and thermosiphons among others. PHPs are revolutionary devices, offering a promising avenue for addressing heat dissipation challenges across various industries at different scales, ranging from tiny electronics cooling to massive aerospace applications. In this blog, we will discuss the benefits of PHP and how they can be optimally designed using computational fluid dynamics (CFD).??

Why are PHPs a Powerful Cooling Solution?

Pulsating Heat Pipes are quite simple devices, consisting of a closed high-conductive tubing system, filled at vacuum with a certain amount of coolant. These tubes have a small hydraulic diameter and are generally serpentine. They are also lightweight and compact and can dissipate heat effectively and reliably over a long time. PHPs are mostly used in the high-tech industry currently for electronics cooling applications.

The most common PHP coolants are often water, alcohol, or refrigerants. Unlike traditional heat-pipes and vapor chambers, PHPs do not require a complex wicked structure to operate. PHPs utilize a combination of capillary actions and momentum differences (due to phase-change by evaporation and condensation) to allow coolant motion and therefore the transportation of heat. Simply, the operation of a PHP is a cycle of the following 4 stages:

1-????? Evaporation: Heat from the heat-source causes the working fluid at the evaporator section of the PHP to change phase from liquid state to vapor state in form of bubbles which might agglomerate / grow to vapor plugs.

2-????? Vapor migration: The generated vapor bubble and plugs travel towards the condenser section of the PHP, driven by pressure and momentum differences.

3-????? Condensation: When the bubbles and plugs reach the condenser, they release heat to the surrounding, condensing back into liquid.

4-????? Liquid return: Capillary forces pull the liquid back towards the evaporation section, completing the cycle.

This cyclic process continues, creating a pulsating motion within the PHP.

A Pulsating Heat Pipe has proven to be a powerful cooling solution with many success stories. They are considered highly reliable as they consist of a closed system with no moving parts. In addition, they are quite simple to construct as they do not require a wicked structure, and they provide more flexibility compared to a thermosiphon, which requires gravitational constraints.

PHP Design Challenges

However, PHP design challenges persist due to its involved complex phenomena and its intermittent transient behavior. Usually, a simulation-based design of PHPs relies on reduced order approaches such as “spring and mass” models. However, such kinds of methods are limited and do not provide the full picture required to understand the detailed PHP operation and to enhance the existing designs.

Ansys CFD Solutions for PHPs

?Ansys’ CFD tool, Ansys Fluent, enables thermal engineers and designers to visualize and investigate the influence of operating conditions on the PHP, including the heat load, the geometry, the coolant fluid, and fill ratio as well as the surrounding conditions and how they affect the thermal performance of the PHP.

Ansys Fluent allows flexible high-fidelity access to temporal and spatial varying quantities and fields which are almost impossible or overly complex and costly to measure during experiment. Bubbles, vapor plug and liquid shapes and dynamics, turbulence field and its influence on bubbles coalescence and break-up, evaporation and condensation rates, coolant natural flow rate variation can now be quantified. This provides more design insights and better assessment of the PHP process so that the primary goal of dissipating heat extremely efficiently and reducing the risk of failure of electronic components is achieved.

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How Does Ansys Fluent Predict PHP Thermal Cooling Efficiency?

?The simulation workflow in Fluent for PHP design analysis starts with user-friendly pre-processing steps which consists of creating the PHP geometry and meshing with the Ansys 3D CAD tool Ansys Discovery and Ansys Fluent Meshing, respectively.

Using the advanced multiphase framework in Fluent, this complex device is modeled while capturing all the flow and thermal details. Evaporation and condensation processes are incorporated through a robust temperature-driven phase change mechanism. The latter allows for user-friendly calibration of the phase change kinetic, and its formulation can be extended to include low Bond number effects. The thermal and transport properties are leveraged from REFPROP NIST Data based and deployed inside Fluent as Look-Up tables which are created via a simple Graphical User Interface (GUI).?

Watch a full demo of how to use Fluent to perform this workflow:

Conclusion

We are seeing a growing use of PHPs not only in the typical high-tech industries for advancing cooling electronic applications, but also in other industries such as automotive and aerospace due to the high reliability and low maintenance of PHPs. Being able to predict PHP performance and assess the ability to manage thermal loads becomes a prerequisite in times where power density in electronic components is rapidly increasing and affecting overall thermo-structural integrity. Ansys empowers engineers to inspect what is going on inside their pulsating heat pipes faster and more affordably and make the necessary design changes to improve performance.

Request a Free Trial of Ansys Fluent.