Characterization and mitigation of permeability in 3D printed oscillating heat pipes and a conjugate heat transfer study at adiabatic section of OHP's
Date
2025
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Publisher
University of Delaware
Abstract
Oscillating Heat Pipes are a phase-change heat transfer device designed to move heat from a hot source to a cold source efficiently, exploiting oscillations of fluid slugs within tubes driven by vapor pressure differences generated between the hot/cold sides. There are two general geometries typically used to construct these devices: tube and flat-plate. This thesis addresses two different aspects of OHP’s. ☐ In the first part, this study examines the feasibility of utilizing 3D-printed sintered stainless steel parts, fabricated from Ultrafuse 17-4 PH (”Ultrafuse”) filament, as a cost-effective alternative OHP material. While additive manufacturing (AM) using fused filament fabrication (FFF) significantly reduces production costs and enables the creation of complex internal channels for enhanced thermal performance, OHPs are required to be vacuum-tight, and FFF made parts generally have some porosity. To gain understanding of the porosity of this material, measurement apparatuses were made to measure both the in-plane and through-plane Darcy permeability FFF samples. The apparatuses are based on compressed gas transport and appropriate compressible equations data extraction are developed. In addition to sintered Ultrafuse, it is also explore whether electroplating processes (both Nickel and Copper) can be used to improve/reduce the permeability of these materials. Microstructural analyses via scanning electron microscopy (SEM) is also employed to measure the pore density and sizes, both before and after plating. SEM confirmed that there are 50 micron pores in the final Ultrafuse parts that form in between adjacent nozzle paths, and these form an array that has spacing of 1 nozzle width x 1 layer height. These pores form in the print plane. In some cases, the pores connect to form cracks connect adjacent layers. It was found that electroless plating using Nickel, filled the surface cracks/pores of these samples and reduced permeability to an immeasurably-low value. Adhesion between electroplated copper and stainless steel was not strong and did not lead to parts with significantly lower permeability. ☐ In the second part, the effect of conjugate heat transfer in the adiabatic section of an OHP is explored experimentally. A modular OHP setup was designed to evaluate how different adiabatic section materials—copper, aluminum, and brass—which have significantly different thermal effustivity and diffusivity affect thermal behavior under controlled conditions. The system includes a uniform heat source using a polyimide heater, embedded thermocouples along the length of the pipe, and a water-cooled condenser. Importantly the evaporator and condenser sections are identical/re-used for each each experiment. The adiabatic section was made interchangeable to isolate the influence of wall conductivity. Acetone was used as the working fluid, filled to 65% of the internal volume, based on prior optimization studies. It was found that the conductance of the OHP with copper adiabatic section had performance ∼30% worse than either aluminum or brass, but otherwise was of similar scale considering the large differences in material properties.
Description
Keywords
Additive manufactruing, Conjugate heat transfer, Oscillating Heat Pipes, Permeability