Je-Hyeong Bahk

Fetal lower urinary tract obstruction (LUTO) and pleural effusion are conditions that can disrupt fetal growth and lead to fetal death. LUTO inhibits the formation of amniotic fluid, which is vital for lung development, while pleural effusions can compress the fetal heart, potentially causing fatal cardiac failure. To manage these conditions, a fetal shunt (vesico-amniotic shunt) is placed inside the fetal bladder. This paper presents a study on a new design incorporating a Tesla valve in the shunt. Six groups of Tesla valves with loop angles of 50 degrees and 60 degrees, and different end dimensions, are examined and evaluated in terms of the urine flow rate from the fetal bladder into the amniotic cavity, the pressure buildup between the two sides, and their potential in developing fetal bladder muscles. A mathematical method is used to compare diode characteristics, analyze flow rates, identify the Tesla valve angle, determine the Reynolds number, and assess diodicity. The Computational Fluid Dynamics (CFD) method is also employed to verify calculation results and simulate fluid behavior inside the Tesla valve. Combining the calculations and simulations, a 50-degree Tesla valve with specific dimensions showed the best performance and will be the optimal design for the fetal shunt.

Recent advances in additive manufacturing have enabled the use of binder solutions to selectively bond ceramic powders and create strong 3D ceramic parts. During this process, conducting nanocarbon materials can be dispersed in the binder solution during ink preparation, and then jetted into ceramic powder beds layer by layer to create hybrid hierarchical composites with embedded nanocarbon networks while preserving the powder microstructure. These secondary filler nanocarbons can form percolation networks in the space between ceramic particles to create electron and phonon conduction pathways throughout the composite. The resulting printed composites are mechanically strong, electrically conducting ceramic 3D structure that can be useful for various energy conversion and storage applications. Herein, we combine experimental and theoretical study to investigate the electron and phonon transport through the networks of nanocarbon materials such as carbon nanotubes embedded in binder-jetted silica ceramic composites. We present the broad tunability of transport properties such as electrical conductivity, thermal conductivity, and thermopower of the composites with varying filler concentration and printing process parameters. Electron tunneling at the junctions between nanomaterials and phonon boundary scatterings are investigated to understand the electron and phonon transport properties in the composites. Potential applications of these 3D-printed hierarchical ceramic composites for energy conversion and storage are also discussed.

The fundamental problem behind the numerical solutions of incompressible viscous flow equations lies in the lack of an update equation for the pressure field. In essence, there is no explicit relation that can be utilized to update the pressure field from a given velocity field. Following the principle of the artificial compressibility method and considering that it is the pressure gradient that drives incompressible flows, this study introduces a new primitive variables method to solve the incompressible Navier–Stokes equations. This method ensures that the continuity condition is rigorously satisfied at every grid point within the flow domain. Its principle lies in modifying the continuity equation to include a pseudo-transient pressure gradient term that vanishes upon convergence, which allows the incompressibility constraint to be satisfied to machine zero. This method eliminates the numerical difficulties associated with pressure-based solvers and improves the convergence of all dependent variables. Numerical solutions for three steady-state validation problems, namely fully developed channel flow, lid-driven cavity flow (2D and 3D), and flow in a constricted channel, are obtained to validate the proposed method. The results show good agreement with solutions available in the literature and demonstrate substantially improved convergence.

Thermoelectric (TE) heat pumps are a promising solid-state technology for space cooling and heating owing to their unique advantages such as environmental friendliness with no harmful refrigerants, small form factors, low noise, robustness with no moving parts, and demand-flexible operation. However, their low coefficient of performance (COP) presents a challenge towards broader adaptation of the technology at the market level. Built on our previous theoretical framework for a modular system design aimed at improving both COP and cooling capacity, this study focuses on experimental characterization and optimization of a unit air-to-air TE heat pump system. We employ double-sided heating/cooling and counterflow configuration for uniform heat transfer with TE modules and heat exchangers. System-level evaluations validate the theoretical model and show the variation of air temperature change and system COP with different electric current inputs and air flowrates. A maximum cooling COP of 4.4 and a maximum heating COP of 6.0 were obtained at optimized current levels. Further analysis shows that parasitic thermal resistances in the system significantly reduced both COP and air temperature change by over 50%.

Carbon nanostructures have attracted great attention in recent years as novel electrode materials in batteries with increased capacity and conductivity as well as enhanced material stability. In these nanostructures, nanocarbons such as carbon nanotubes and graphene nanoplatelets are interconnected to construct their three-dimensional networks around the host materials for Li deposition. This work focuses on the study of electron transport phenomena in these nanocarbon networks with two model material systems: (1) single-walled carbon nanotube (SWCNT) networks co-embedded with silica microparticles (Fig. (a)), and (2) free-standing three-dimensional graphene (3DG) structures with polymeric surface modifications (Fig. (b)). We characterize the materials for electrical conductivity and thermopower with varying material contents in the composites to reveal several key factors determining their energy-dependent carrier transport characteristics. It is found that in these three-dimensional nanocarbon networks, carrier tunneling at the junctions between nanocarbons along with the network morphology predominantly determines the transport properties. Simultaneous increase in electrical conductivity and thermopower is achieved for SWCNT networks co-embedded with silica microparticles by increasing silica content up to 40 w.t.% at a fixed CNT content. The enhanced electron transport is attributed to the reduced junction distances due to the intimate network formation on the surface of silica particles with a polymer binder. Our transport theory based on the Landauer formalism for junction tunneling reveals that the junction modification with polymer can alter the potential barrier heights for both electrons and holes at the junction to significantly change the transport properties of the composite. Doping of nanocarbons is also investigated for their impacts on the junction transport and the geometric factor of the networks. Finally, we also investigate the use of these carbon nanostructures in solid-state thermoelectric (TE) energy harvesters and ionic TE capacitors to harvest thermal energy from temperature gradients and fluctuations. Figure 1

Over the years, various processing techniques have been explored to synthesize three-dimensional graphene (3DG) composites with tunable properties for advanced applications. In this work, we have demonstrated a new procedure to join a 3D graphene sheet (3DGS) synthesized by chemical vapor deposition (CVD) with a commercially available carbon veil (CV) via cold rolling to create 3DGS-CV composites. Characterization techniques such as scanning electron microscopy (SEM), Raman mapping, X-ray diffraction (XRD), electrical resistance, tensile strength, and Seebeck coefficient measurements were performed to understand various properties of the 3DGS-CV composite. Extrusion of 3DGS into the pores of CV with multiple microinterfaces between 3DGS and the graphitic fibers of CV was observed, which was facilitated by cold rolling. The extruded 3D graphene revealed pristine-like behavior with no change in the shape of the Raman 2D peak and Seebeck coefficient. Thermoelectric (TE) power generation and photothermoelectric responses have been demonstrated with in-plane TE devices of various designs made of p-type 3DGS and n-type CV couples yielding a Seebeck coefficient of 32.5 μV K^-1. Unlike various TE materials, 3DGS, CV, and the 3DGS-CV composite were very stable at high relative humidity. The 3DGS-CV composite revealed a thin, flexible profile, good moisture and thermal stability, and scalability for fabrication. These qualities allowed it to be successfully tested for temperature monitoring of a Li-ion battery during charging cycles and for large-area temperature mapping.

Thermoelectric (TE) waste heat recovery has attracted significant attention over the past decades, owing to its direct heat-to-electricity conversion capability and reliable operation. However, methods for application-specific, system-level TE design have not been thoroughly investigated. This work provides detailed design optimization strategies and exergy analysis for TE waste heat recovery systems. To this end, we propose the use of TE system equipped on the exhaust of a gas turbine power plant for exhaust waste heat recovery and use it as a case study. A numerical tool has been developed to solve the coupled charge and heat current equations with temperature-dependent material properties and convective heat transfer at the interfaces with the exhaust gases at the hot side and with the ambient air at the heat sink side. Our calculations show that at the optimum design with 50% fill factor and 6 mm leg thickness made of state-of-the-art Bi₂Te₃ alloys, the proposed system can reach power output of 10.5 kW for the TE system attached on a 2 m-long, 0.5 × 0.5 m²-area exhaust duct with system efficiency of 5% and material cost per power of 0.23 $/W. Our extensive exergy analysis reveals that only 1% of the exergy content of the exhaust gas is exploited in this heat recovery process and the exergy efficiency of the TE system can reach 8% with improvement potential of 85%.

Thermoelectric (TE) waste heat recovery has attracted significant attention over the past decades, owing to its direct heat-to-electricity conversion capability and reliable operation. However, methods of system-level TE design optimization for specific application systems have not been thoroughly investigated. This work theoretically studies the performance optimization and exergy analysis of TE systems for recovering waste heat from the exhaust ducts of a gas turbine power plant. A numerical tool has been developed to solve the coupled charge and heat current equations with temperature-dependent material properties and convective heat transfer at the interfaces. Our calculations show that at the optimum design with 50 % fill factor and 6-mm leg thickness made of state-of-the-art Bi2Te3 alloys, the proposed system can reach power output of 10.5 kW for the TE system attached on a 2 m-long, 0.5 x 0.5 m2-area exhaust duct with system efficiency of 5 % and material cost per power of 0.23 $/W. Moreover, our extensive exergy analysis reveals that only 1 % of the exergy content of the exhaust gas is exploited in this heat recovery process and the exergy efficiency of the TE system can reach 8 % with improvement potential of 85 %.