An experimental investigation into the parameters affecting heat transport in two three-dimensional oscillating heat pipes (OHP) was implemented. A three-dimensional OHP is one in which the center axis of the circular channels containing the internal working fluid do not lie in the same plane. This novel design allows for more turns in a more compact size. The OHPs in the current investigation is made of copper tubing (3.175 mm OD, 1.65 mm ID) wrapped in a three-dimensional fashion around two copper spreaders that act as the evaporator and condenser. The two OHPs have 10 and 20 turns in both the evaporator and condenser. The 20 Turn OHP was filled to 50% of the total volume with high performance liquid chromatography (HPLC) grade water. Transient and steady state temperature data was recorded at different locations for various parameters. Parameters such as heat input, operating temperature, and filling ratio were varied to determine its effect on overall heat transport. Neutron radiography was simultaneously implemented to create images of the internal working fluid flow at a rate of 30 frames per second (fps). Results show the average temperature drop from the evaporator to condenser decreases at higher heat inputs due to an increase in temperature in the condenser region caused by greater oscillations. These large oscillations were visually observed using neutron radiography. As the operating temperature is increased, the thermal resistance is reduced due to increased fluid flow caused by changes in fluid properties. A decrease in filling ratio tends to create more steady fluid motion; however, the heat transfer performance is reduced.

The control of propellant boil-off is essential in long-term space missions. However, a clear understanding of propellant cryogenic condensation/evaporation in microgravity is lacking. One of the key factors in designing such systems is the location of liquid surfaces and the relation to wettability. The BT-2 Neutron Imaging Facility located at the National Institute of Standards and Technology (NIST), Gaithersburg, MD, is used to image evaporation and condensation of hydrogenated propellants inside of an aluminum 6061 container. Liquid hydrogen has larger neutron cross-section area than the aluminum, allowing the visualization of the liquid-vapor interface. The test cell has a conical section that enables determination of a contact angle with enhanced accuracy. If the contact angle is equal to the angle of the cone, a flat liquid-vapor interface is expected. The test cell has the cone angle of 10o and a flat interface was not observed. Using the Laplace-Young equation to fit the interface, the contact angle for hydrogen and aluminum was between 0° and 4°. The theoretical Laplace curves with contact angles of 2o and 10o are plotted on the liquid-vapor interface. The of 2o curve is a closer fit as compared to the 10o curve. The uncertainty arises from resolution limits of the neutron imaging setup and edge detection. More details on the neutron imaging mechanism and relevant physics can be found from the authors' other publication of Cryogenics, 74, pp131-137, 2016: doi:10.1016/j.cryogenics.2015.10.016.

One of the key limitations to long-term space missions is to avoid propellant boil-off in a microgravity space environment. Even with the use of active and passive controls of propellants, boil off is inevitable. Long-term CFD simulations on propellant behaviors depend on evaporation/condensation coefficients (known as accommodation coefficients) which are in turn dependent upon the wetting characteristics. Phase change experiments were conducted in the BT-2 neutron imaging facility at the National Institute of Standards and Technology (NIST) by introducing vapor H2 in 10 mm Al6061 and SS316L test cells placed inside the 70mm ‘orange’ cryostat. Condensation is achieved by lowering the cryostat temperature below the saturation point and vice versa for evaporation. The high neutron cross-section of liquid H2 in comparison to both the vapor and the test cell materials allows for visualization of a distinct liquid-vapor interface. Multiple images are stacked to increase the signal-to-noise ratio and the meniscus edge is obtained by detecting the pixels with largest gradients in intensities at the liquid meniscus. The contact angle is obtained by curve fitting of the Young-Laplace equation to the detected meniscus. The contact angle for Al6061 and SS316 is found to be between 0° and 4°. The uncertainty arises from edge detection, magnification, and resolution limits of the neutron imaging setup. The test was conducted at a saturation temperature of 21K (1.215 bar). The results from the neutron experiments will be then used in conjunction with FEA thermal models and kinetic phase change models to extract accommodation coefficients.

An experimental investigation into the parameters affecting heat transport in two three-dimensional oscillating heat pipes (OHPs) was implemented. A three-dimensional OHP is one in which the center axis of the circular channels containing the internal working fluid do not lie in the same plane. This novel design allows for more turns in a more compact size. The OHPs in the current investigation is made of copper tubings (3.175 mm outside diameter, 1.65 mm inside diameter) wrapped in a three-dimensional fashion around two copper spreaders that act as the evaporator and condenser. The two OHPs have 10 and 20 turns in both the evaporator and condenser. The 20-turn OHP was filled to 50% of the total volume with a high performance liquid chromatography grade water. Transient and steady state temperature data were recorded at different locations for various parameters. Parameters such as heat input, operating temperature, and filling ratio were varied to determine its effect on the overall heat transport. Neutron radiography was simultaneously implemented to create images of the internal working fluid flow at a rate of 30 frames per second. Results show the average temperature drop from the evaporator to condenser decreases at higher heat inputs due to an increase in temperature throughout the condenser region due to greater oscillations. These large oscillations were visually observed using neutron radiography. As the operating temperature is increased, the thermal resistance is reduced. A decrease in filling ratio tends to create more steady fluid motion; however, the heat transfer performance is reduced.

Demand for high purity hydrogen production using renewable energy sources is growing to meet the clean energy demands. Polymer electrolyte membrane water electrolyzer (PEMWE) is one of the viable options for H2 production, but its high capital cost and operational expenditures increase the cost of H2. Improving the interface between the catalyst layer (CL) and the porous transport layer (PTL) is critical to increasing the efficiency of PEMWEs and thereby lowering the cost of H2. Increased contact between the CL and PTL improves catalyst utilization, and the optimal structure of the PTL reduces the mass transport issues related to O2 bubble removal.(1-3) Improved understanding of the PTL microstructure is necessary to improve the performance and efficiency of the PEMWE. This work presents a systematic study to elucidate the effect of PTL properties (morphology, thickness, and porosity) and their impact on PEMWE performance under different operating conditions. Polarization curves with different anode PTL (felt, sinter and pore graded hierarchical PTL) are presented in Figure 1a. The separation of mass transport resistance and the contract resistance for the different PTLs will be elucidated to show the impact of water management and interfacial contact. Mass transport in an operating electrolyzer is also studied by estimating the water content using neutron imaging. Figure 1b shows water thickness across a membrane electrode assembly (MEA) with a pore graded hierarchical PTL in anode at different current densities. Water content across the MEA with different PTL is also studied. The cells with these PTLs were evaluated in operando using micro x-ray computed tomography (CT) and x-ray radiography. The x-ray techniques revealed oxygen distribution within the PTLs on the pore-scale at varied current densities, complementing neutron imaging water thickness studies and providing micro-scale insight into transport. Acknowledgment This research is supported by the U.S. Department of Energy (DOE) Hydrogen and Fuel Cell Technologies Office, through the H2NEW consortium. References J. K. Lee, C. Lee, K. F. Fahy, B. Zhao, J. M. LaManna, E. Baltic, D. L. Jacobson, D. S. Hussey and A. Bazylak, Cell Reports Physical Science, 1, 100147 (2020). T. Schuler, J. M. Ciccone, B. Krentscher, F. Marone, C. Peter, T. J. Schmidt and F. N. Büchi, Advanced Energy Materials, 10, 1903216 (2020). P. Lettenmeier, S. Kolb, F. Burggraf, A. S. Gago and K. A. Friedrich, Journal of Power Sources, 311, 153 (2016). Figure 1