Oscillating heat pipes, also known as pulsating heat pipes, are increasingly becoming a preferred high-performance thermal ground plane in a variety of heat spreading applications due to a number of advantages over traditional copper-water wicked heat pipes, including their lighter weight, thinner profiles, simpler fabrication, and greater variety of material and working fluid options. A major barrier to even wider adoption, however, is the lack of comprehensive analytical models to simulate their performance. A key input to first principles models simulating the fundamental physics of the devices is the initial condition of liquid and vapor segment lengths and their distribution throughout the device. To investigate the initial distribution of liquid and vapor segments in a representative system, water was charged into evacuated glass tubes with an inner diameter of 4 mm and the resulting distribution of liquid and vapor segment lengths recorded. The device was charged to three fill ratios, and the rate at which the working fluid is introduced was also varied. These variables all showed different results in the distribution of liquid segment lengths in the device, specifically noting more consistent and shorter average liquid lengths at slower fluid introduction rates and lower fill ratios, and conversely greater variation and longer average liquid lengths at faster fluid introduction rates and higher fill ratios. A critical fill ratio was also observed, where new liquid entering the channel began growing in average length once the first-most liquid-vapor segments had reached the far-end of the channels, referred here as the compression effect. These results lead to a better understanding of initial conditions in support of improved analytical models seeking to predict the device’s behavior more accurately. The initial conditions presented in this work and the methodology of collecting such initial conditions could lead to improved models for oscillating heat pipes and improved methods for charging these devices, saving time and cost during integration of the device by reducing the amount of experimental validation needed during the development of each new device.

Degree Date

Fall 2021

Document Type


Degree Name



Mechanical Engineering


Dr. Paul Krueger

Second Advisor

Dr. Minjun Kim

Third Advisor

Dr. David Willis

Fourth Advisor

Dr. Ali Beskok

Fifth Advisor

Dr. Mary Herndon

Subject Area

Mechanical Engineering

Number of Pages




Creative Commons License

Creative Commons Attribution-Noncommercial 4.0 License
This work is licensed under a Creative Commons Attribution-Noncommercial 4.0 License