Date of Award
12-24-2025
Date Published
January 2026
Degree Type
Dissertation
Degree Name
Doctor of Philosophy (PhD)
Department
Mechanical and Aerospace Engineering
Advisor(s)
Shalabh Maroo
Keywords
Design of oscillating heat pipe;Electronic Cooling;Equivalent thermal conductivity;Operation failure;Oscillating heat pipe;Thermal Management
Subject Categories
Engineering | Mechanical Engineering
Abstract
Oscillating heat pipes (OHPs) have garnered significant attention as passive thermal management devices owing to their high heat transport capability, ease of fabrication, and design flexibility. In this work, the equivalent thermal conductivity of OHPs using four different working fluids is experimentally measured, followed by the development of a novel predictive model to capture OHP operational failure using a tubular configuration. Furthermore, an embedded OHP designed for easy integration into practical systems is investigated to examine the effects of filling ratio and working fluid on thermal performance and their interdependence with flow regimes. A computationally efficient methodology for OHP design is also proposed, enabling low-cost optimization and performance prediction. In addition, zeolite is explored to modify surface wettability, a material not previously studied for heat transfer or OHP applications. First, a tubular OHP with a true adiabatic section made of 14 glass tubes was fabricated, ensuring that heat transfer occurs solely through the oscillatory motion of the working fluid. Both the heat input at the evaporator and the heat output at the condenser were measured to quantify the heat transferred across the OHP. The device was tested with four different working fluids – DI water, ethanol, methanol, and FC-72 – at a constant filling ratio. Due to the presence of the true adiabatic section, two distinct equivalent thermal conductivities which capture the oscillatory heat transfer behavior of the working fluid were estimated from experimental data. The first was calculated across the entire OHP using the temperature difference between the evaporator and condenser, while the second was determined solely across the adiabatic section using local temperature measurements. Remarkably high maximum thermal conductivities were attained for the four fluids, ranging from 3336.79 W/mK to 6452.34 W/mK for the full OHP, and from 8573.51 W/mK to 67048.62 W/mK for the adiabatic section. All of these values exceed the conductivity of any known solid at room temperature, including graphene and diamond. The maximum thermal conductivity of fluid achieved is ~ 20 times that of diamond, the highest known solid thermal conductivity material in earth. Furthermore, the thermal conductivity of all working fluids used in this study exceeded that of copper and silver at heat inputs as low as 50 W, highlighting the potential of OHPs for efficient heat dissipation in various applications. In the next study, detailed characterization of a single tube (the eighth) within the tubular OHP was performed using synchronized pressure and temperature measurements of three working fluids. Analysis across four distinct regions revealed that different parts of the OHP operate at distinct thermodynamic states, unlike the conventional assumption of uniform thermodynamic states used in many existing models. OHP failure is directly observed during experiments conducted with water as the working fluid. Based on experimentally observed trends and fundamental thermodynamic insights, two novel criteria the thermal convergence criterion and the thermodynamic saturation limit criterion were proposed to explain the underlying mechanisms and predict the onset of OHP operation failure- an unsolved issue in OHP research community. The predictive capability of the two criteria is validated using the experimental results for water, and subsequently applied to methanol and ethanol, enabling failure prediction without operating the OHP to the point of failure. Next, an embedded OHP consisting of fourteen channels machined into a copper block and capable of experimentally measuring condenser heat output was fabricated and tested. The device was tested in a vertical bottom-heating configuration over fifty four experimental runs. Four working fluids: deionized (DI) water, methanol, ethanol and FC-72 were investigated at constant FR of 0.62, with DI water exhibiting the best overall thermal performance. For DI water, the FR was varied from 0 to 1 and the heat input from 60 W to 250 W, while startup behavior was examined across all filling ratios. The optimal FR was identified as 0.48 resulting in a minimum thermal resistance of 0.097 K/W, a 56% reduction relative to the copper block baseline (0.22 K/W). Flow regimes of DI water within the channels were analyzed using MATLAB-based image processing, revealing that smaller liquid slugs and vapor plugs strongly correlated with higher heat output and improved thermal performance. These results suggest that intentionally inducing smaller slug/plug distributions can effectively reduce thermal resistance. In the later part of the work, a novel methodology for designing embedded OHPs based on an the tubular OHP through which heat passes exclusively through the working fluid is presented and its relationship with the embedded OHP is established using variables dependent on the capillary number. The proposed design model is tested with two working fluids of distinct thermophysical properties: DI water and FC-72 on an embedded OHP with copper, and the results are validated experimentally. The mean and standard deviation of the difference of thermal resistance between simulation and experiment for water at all heat inputs are 4.6% and 3.39%, respectively, and for FC-72 are 9.64% and 3.76%, demonstrating the reliability and robustness of the model. This approach enables accurate prediction of OHP performance, providing a framework for systematic design strategies and supporting the development of theoretical and numerical models for OHPs. Furthermore, zeolite films are synthesized on substrates using a microwave-assisted in-situ method, which drastically reduced the crystallization time from days in conventional hydrothermal synthesis to just a few minutes. The resulting ZSM-5-coated surfaces exhibited vigorous wicking behavior and superhydrophilicity, highlighting the potential of microwave-assisted synthesis for rapid fabrication of functional zeolite film for enhanced heat transfer.
Access
Open Access
Recommended Citation
Thapa, Ashok, "THERMAL MANAGEMENT USING PASSIVE OSCILLATING HEAT PIPES: PERFORMANCE, FAILURE LIMITS, AND DESIGN METHODOLOGY" (2025). Dissertations - ALL. 2232.
https://surface.syr.edu/etd/2232
