A. James Clark School of Engineering
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Item Simulation of polymeric drop dynamics: Effect of photopolymerization, impact velocity, and multi-material coalescence(2023) Sivasankar, Vishal Sankar; Das, Siddhartha; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Over the past couple of decades, additive manufacturing has emerged as one of the most promising manufacturing tools and has rightfully garnered the attention of researchers across various fields ranging from biochemistry and medicine to energy and infrastructure. Especially, direct-ink-writing methods (e.g., inkjet printing, aerosol jet printing, or AJ printing, etc.) have been widely studied because of their ability to print highly complex geometries with finer resolution. In order to design a more efficient droplet-based direct ink writing system, it is essential to understand the deposition process and the post-deposition dynamics of the drop. The post-deposition drop dynamics dictate the spreading radius of the drop and hence the print resolution. Such an understanding is even more critical when there are multiple drops interacting with each other, given the fact that such interactions determine the presence/absence of surface defects in addition to determining the print resolution. Moreover, to have a holistic understanding of the post-deposition process, it is essential to further account for the droplet solidification mechanisms (for example, through effects such as in-situ curing) that might interplay with multiple drop dynamics events (such as drop spreading, drop coalescence, drop impact, etc.). In this dissertation, computation fluid dynamics (CFD) frameworks have been developed to investigate the facets dictating the post-deposition dynamics of one (or several) solidifying polymer drops, with these dynamics show-casing the different post-deposition events that are intrinsic to the droplet-based additive manufacturing processes. First, we considered a situation where the polymeric drop undergoes simultaneous spreading and photopolymerization, with the timescales of the spreading and photopolymerization events being τ? and τ? respectively. The findings from this work confirmed the significant impact of the ratio of timescales (τ? and τ?) on the thermo-fluidic-solutal dynamics of the polymeric drops. Moreover, the evolution of the curing front showed distinct behaviors as a function of the timescale ratio.Subsequently, the effect of the interaction of multiple polymeric drops during the post-deposition event, as seen in the typical printing process, was investigated. Specifically, we studied the effect of drop impact on the coalescence dynamics of two polymeric drops of identical and different sizes. The study revealed the presence of two distinct stages of coalescence. The early-stage coalescence was found to be enhanced with an increase in the impact velocity; however, the late-stage coalescence behavior remained unaffected by the impact velocity. Further, the coalescence dynamics of polymeric drops of different materials, as witnessed in multi-jet printing, was probed. This study shed light on the mechanisms that drive the mixing process at different stages of drop coalescence. Finally, we evaluated the effects of the in-situ photopolymerization on the coalescence dynamics of multiple polymeric drops deposited on a substrate. Here too the comparative values of the drop dynamics timescale and the photopolymerization became important. Our results show three-distinct regimes characterizing the bridge growth which was further validated through physics-based theoretical scaling. This study would provide key insights into the direct-ink writing process and would aid in designing parameters for polymer-based additive manufacturing and product repair.Item COMPARISON OF HIGH STRAIN RATE PROPERTIES OF ADDITIVELY MANUFACTURED AND WROUGHT INCONEL 625 VIA KOLSKY BAR TESTING(2019) Morin, Jason; Fourney, William; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Additive manufacturing is becoming an important part of modern manufacturing technology. Before additively manufactured parts gain widespread adoption, the material properties of the additively manufactured material itself must be accurately quantified. Stress strain curves must be produced over a wide variety of test conditions so that accurate modeling of material behavior can be done. Materials that may undergo dynamic loading must therefore be tested under dynamic conditions. In this study the tensile and high strain rate compressive material properties of additively manufactured Inconel 625 are compared to conventionally formed wrought material. The results of testing showed that there is a clear difference in material properties between wrought and additively manufactured Inconel 625 in tension and compression. The additively manufactured tensile samples showed anisotropy between print directions of approximately ±10%. The printed samples had a 35% higher yield strength, a similar ultimate strength, and 20-40% the elongation when compared to wrought. There was also a significant difference in properties between the additive and wrought materials during the compressive tests. The additive material showed little anisotropy and had a 30% higher yield stress than wrought. Additionally, the additive material had a higher strain hardening rate than the wrought samples. No significant strain rate effects were noted.Item DESIGN AND PERFORMANCE CHARACTERIZATION OF AN ADDITIVELY-MANUFACTURED HEAT EXCHANGER FOR HIGH TEMPERATURE APPLICATIONS(2018) Zhang, Xiang; Ohadi, Michael; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)In its early stages of development, additive manufacturing was used chiefly for prototyping, but over the last decade, its use has evolved to include mass production of certain products for numerous industries in general, and speciality industries such as biomedical and aerospace industries in particular. Additive manufacturing can be used to fabricate unconventional/complex designs that are difficult and time-consuming through conventional fabrication methods, but offer significant performance advantage over state of the art. One such example is high temperature heat exchangers with complex novel geometries that can help improve the heat transfer density and provide better flow distribution, resulting in more compact and efficient designs and thereby also reducing materials costs considering fabrication of these heat exchangers from the suitable super alloys with the conventional manufacturing techniques is very difficult and laborious. This dissertation presents the results of the first high-temperature gas-to-gas manifold-microchannel heat exchanger successfully fabricated using additive manufacturing. Although the application selected for this dissertation focuses on an aerospace pre-cooling heat exchanger application, the results of this study can still directly and indirectly benefit other industrial sectors as heat exchangers are key components of most power conversion systems. In this work, optimization and numerical modelling were performed to obtain the optimal design, which show 30% weight reduction compared to the design baseline. Thereafter, the heat exchanger was scaled down to 66 × 74 × 27 mm3 and fabricated as a single piece using direct metal laser sintering (DMLS). A minimum microchannel fin thickness of 165 μm was achieved. Next, the additively manufactured headers were welded to the heat exchanger core and the conventionally manufactured flanges. A high-temperature experimental loop was next built, and the additively manufactured heat exchanger was successfully tested at 600°C with ~ 450 kPa inlet pressure. A maximum heat duty of 2.78 kW and a heat transfer density close to 10 kW/kg were achieved with cold-side inlet temperature of 38°C during the experiments. A good agreement between the experimental and numerical results demonstrates the validity of the numerical models used for heat transfer and pressure drop predictions of the additively manufactured heat exchanger. Compared to conventional plate-fin heat exchangers, up to 25% improvement in heat transfer density was achieved. This work shows that additive manufacturing can be used to fabricate compact and lightweight high temperature heat exchangers, which benefit applications where space and weight are constrained.