Anti-fouling interfaces for energy applications
Fouling and accretion have negative impacts on a plethora of industrial processes. This doctoral thesis focuses on the theoretical and experimental exploration of mitigation strategies for heterogeneous nucleation of foulants. The dissertation covers low-temperature precipitation fouling, corrosion fouling, fuel fouling, and high-temperature scale fouling. We begin the thesis by providing a literature review of previously conducted work and understanding that has been established about multiple fouling mechanisms and conditions for a plethora of applications. We first study precipitation fouling. Classical nucleation theory dictates that in order to reduce the heterogeneous nucleation rate of a foulant, the surface should have a low surface energy and be as smooth as possible. Past approaches have focused on lowering surface energy via the use of hydrophobic coatings, or creating atomically smooth interfaces to eliminate nucleation sites, or both, via the infusion of low surface energy lubricants into rough superhydrophobic substrates. Although lubricant-based surfaces are promising candidates for anti-scaling, lubricant drainage inhibits their utilization. Here, we develop methodologies to deposit slippery omniphobic covalently attached liquids (SOCAL) on arbitrary substrates. Similar to lubricant-based surfaces, SOCAL has ultra-low roughness and surface energy, enabling low nucleation rates, and eliminating the need to replenish the lubricant. To enable SOCAL coating on metals, we investigated the surface chemistry required to ensure high quality functionalization as measured by ultra-low contact angle hysteresis (< 2). Using a multi-layer deposition approach, we first electrophoretically deposit (EPD) silicon dioxide (SiO2) as an intermediate layer between the metallic substrate and SOCAL. The necessity of EPD SiO2 is smoothen (< 10 nm roughness) as well as to enable the proper surface chemistry for SOCAL bonding. To characterize anti-scaling performance, we utilized calcium sulfate (CaSO4) and calcium carbonate (CaCO3) scale tests, showing a 20X reduction in scale deposition rate when compared to untreated metallic substrates. Descaling tests revealed that SOCAL dramatically decreases scale adhesion, resulting in rapid removal of scale buildup. This chapter not only demonstrates a robust methodology for depositing anti-scaling SOCAL coatings on metals, it also develops design guidelines for the creation of anti-fouling coatings for alternate applications such as fuel-fouling and high temperature scaling. An improvement of the EPD SiO2 methodology was made to better incorporate thinner coatings that are more suitable for mass production. The silicon dioxide (SiO2) coating is made with dip coating into a sol-gel solution, which ensures an atomically smooth (< 1 nm) interface. To demonstrate performance and scalability of our coating method, we apply our coating to the internal walls of aluminum (Al) tubing and test fouling performance in a flow-fouling setup with single-phase flow of synthetic seawater. The seawater consists of saturated calcium sulfide (CaSO4), and fouling characterized in both laminar, transient and turbulent flow regimes (Reynolds numbers 1,030 to 9,300). Our coating demonstrates a reduction in salt scale fouling by 95% when compared to uncoated Al tubes. Furthermore, we show our coating to easily withstand turbulent flow for durations much longer than lubricant based surface. This chapter demonstrates a coating methodology applicable to a variety of metal substrates and complex internal passages to achieve anti-fouling in single-phase flows. Next, we study the fuel fouling process. In modern aircraft, kerosene-based jet fuel is widely used not only to power the aircraft but also as a heat sink. However, the decomposition of jet fuel at elevated temperature leads to fuel fouling. Decomposed kerosene forms deposit onto the walls of the heat exchanger which lowers the heat transfer efficiency and shortens the lifetime of the heat exchanger. Furthermore, the well-known tendency of jet fuel to decompose imposes strict limits on the maximum allowable welted wall temperature of the heat exchanger, resulting in lower overall conductance and thus necessity for much higher area for heat exchange. The larger required area directly translates to added mass and oversizing of the heat exchanger, a harsh penalty for aviation applications. The hybrid SOCAL coating was used to mitigate fuel fouling behavior. Our coating has low in surface roughness (<1 nm) and surface energy (<12 mJ/m2), thus resulting in reduction of the heterogeneous nucleation rate during deposition. The sol-gel SiO2, which forms the base coating, further eliminates exposure of metal catalysts to the fuel, which can dramatically increase the reaction rate. To characterize the efficacy of our coatings and to benchmark performance with both bare (copper, stainless steel, Inconel 600) and commercially-coated substrates (alumina, SilcoTek), we tested the fouling behavior in a custom built fuel fouling test loop based on the Jet Fuel Thermal Oxidation Test (JFTOT). Our coated samples demonstrate reduced fouling rates of 96% when compared to uncoated bare samples. Our hybrid coating shows comparable results with commercially available coatings with promise for reduced cost based on manufacturing methods. Our work develops an innovative strategy for fuel fouling mitigation in aircraft and jet fuel applications, enabling the use of highly compact heat exchangers with aviation applications and enhancing performance of the aviation platform. Finally, we study high temperature fouling for steam generation applications. The proposed hybrid SOCAL coating has good thermal stability up to 230C. This property makes it a potentially good candidate for high temperature fouling mitigation. We explored the possibility of using the hybrid SOCAL coating in high temperature fouling conditions through the fabrication of two customized boiling tests. The coated metal samples showed 3X lower fouling rates when compared to uncoated metallic samples. Although further testing is needed to achieve a more comprehensive understanding of fouling dynamics in boiling conditions, our results indicate that our hybrid SOCAL coating provides a promising future direction for coating development. We end the thesis by outlining promising future directions for research related to fouling mitigations strategies based on the mechanistic understanding and mitigation strategies developed within this thesis.
fouling, heat transfer, sol-gel, scaling
University of Illinois - Urbana-Champaign
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University of Illinois at Urbana-Champaign