Yasuyuki Takata

Development of a 22 kV/6.9 kV single-phase model for a 3 MVA HTS power transformer

We have developed a 22 kV/6.9 kV HTS single-phase transformer cooled by liquid nitrogen for field test, which is a practical model for the single-phase part of a 3 MVA HTS power transformer. First, we numerically simulated electromagnetic, mechanical and thermal conditions of the windings in accidental cases of short-circuit and lightning impulse, and considered the winding structure withstanding the severe loads. We constructed a small-sized model coil of Bi-2223 Ag/Mn-sheathed tapes and confirmed applicability of the design concept for the overcurrent and high-withstand-voltage tests. We designed and constructed a single-phase HTS transformer on the basis of the model-coil-test results. The primary and secondary windings are transposed parallel conductors of two and six Bi-2223 Ag/Mn tapes, respectively. The same tests for the HTS transformer as for usual oil-filled ones indicated the reliable operation and high performance. The field test in a distribution grid of Kyushu Electric Power Co. included in-rush-current test and long-term operation of the transformer cooled by a continuous supply system of subcooled liquid nitrogen with cryocoolers.

Boiling feature on a super water-repellent surface

The boiling feature on a super water-repellent (SWR) surface has been studied. The SWR surface has a coating layer of fine particles of nickel and PTFE. Its contact angle to water is 152°in room temperature. The heat transfer surface is facing upward, and the diameter of the heated section is 17 mm. The boiling feature of this surface is completely different from that of usual surfaces. The stable film boiling occurs in very small superheating, and there is no nucleate boiling region. The bubbles generated on the surface coalesce into a vapor film without departing from the surface. The stable vapor film exists even at a surface temperature below the saturation temperature.

Heat Transfer through a Condensate Droplet on Hydrophobic and Nanostructured Superhydrophobic Surfaces

Understanding the fundamental mechanisms governing vapor condensation on nonwetting surfaces is crucial to a wide range of energy and water applications. In this paper, we reconcile classical droplet growth modeling barriers by utilizing two-dimensional axisymmetric numerical simulations to study individual droplet heat transfer on nonwetting surfaces (90° < θa < 170°). Incorporation of an appropriate convective boundary condition at the liquid-vapor interface reveals that the majority of heat transfer occurs at the three phase contact line, where the local heat flux can be up to 4 orders of magnitude higher than at the droplet top. Droplet distribution theory is incorporated to show that previous modeling approaches underpredict the overall heat transfer by as much as 300% for dropwise and jumping-droplet condensation. To verify our simulation results, we study condensed water droplet growth using optical and environmental scanning electron microscopy on biphilic samples consisting of hydrophobic and nanostructured superhydrophobic regions, showing excellent agreement with the simulations for both constant base area and constant contact angle growth regimes. Our results demonstrate the importance of resolving local heat transfer effects for the fundamental understanding and high fidelity modeling of phase change heat transfer on nonwetting surfaces.

Droplet nucleation on a well-defined hydrophilic-hydrophobic surface of 10 nm order resolution

Water condensation on a hybrid hydrophilic-hydrophobic surface was investigated to reveal nucleation mechanisms at the microscale. Focused ion beam (FIB) irradiation was used to change the wettability of the hydrophobic surface with 10 nm order spatial resolution. Condensation experiments were conducted using environmental scanning electron microscopy; droplets, with a minimum diameter of 800 nm, lined up on the FIB-irradiated hydrophilic lines. The heterogeneous nucleation theory was extended to consider the water molecules attracted to the hydrophilic area, thereby enabling explanation of the nucleation mechanism under unsaturated conditions. Our results showed that the effective surface coverage of the water molecules on the hydrophilic region was 0.1-1.1 at 0.0 °C and 560 Pa and was dependent on the width of the FIB-irradiated hydrophilic lines and hydrophobic area. The droplet nucleation mechanism unveiled in this work would enable the design of new surfaces with enhanced dropwise condensation heat transfer.

EFFECT OF SURFACE WETTABILITY ON POOL BOILING: ENHANCEMENT BY HYDROPHOBIC COATING

Pool boiling from a super-water-repellent (SWR) and polytetrafluoroethylene (PTFE) surface with checkered and spotted patterns has been studied experimentally. The heat transfer surfaces are copper with the SWR coating of checkered and spotted patterns and TiO2-coated surface with PTFE spotted patterns. The domain of SWR and PTFE acts as nucleation sites and, therefore, bubble nucleation starts at very low superheating. In lower heat flux, bubbles with uniform size are generated on the SWR or PTFE domain of the heat transfer surface. These bubbles depart from the heat transfer surface when the contact line reaches the boundary of SWR or PTFE domain. Nucleate boiling with this surface was enhanced by seven times compared with the normal copper surface. The best was the spotted PTFE surface coated on TiO2 superhydrophilic surface.

Thermal Patterns and Hydrothermal Waves (HTWs) in Volatile Drops

Experimental measurements of temperature and heat flux at the liquid-wall interface during the evaporation of sessile FC-72 droplets have been reported for the first time using infrared (IR) thermography. Simultaneous high-speed imaging of the evaporating drop was carried out to monitor the drop profile. The study demonstrates that recently evidenced hydrothermal waves are actually bulk waves that extend across the entire droplet volume. More importantly, thermal patterns occurring in the bulk of the drop affect the temperature and heat-flux distributions on the solid substrate and ultimately influence the droplet evaporation rate. These effects were found to be increasingly pronounced as the substrate temperature was raised. The implications for heat-transfer mechanisms and energy transport are discussed.

Localized synthesis of single-walled carbon nanotubes on silicon substrates by a laser heating catalytic CVD

Synthesis of high-purity single-walled carbon nanotubes (SWNTs) is demonstrated by a laser heating catalytic CVD method. This method makes it possible to produce SWNTs without the use of an electric furnace or hot filament. SWNTs were synthesized from alcohol using Fe/Co catalyst particles supported on zeolite and Mo/Co particles deposited directly on a substrate. Synthesis of high purity SWNTs was confirmed by in situ Raman scattering analysis and AFM and FE-SEM observations.

In-situ measurement of the heat transport in defect- engineered free-standing single-layer graphene

Utilizing nanomachining technologies, it is possible to manipulate the heat transport in graphene by introducing different defects. However, due to the difficulty in suspending large-area single-layer graphene (SLG) and limited temperature sensitivity of the present probing methods, the correlation between the defects and thermal conductivity of SLG is still unclear. In this work, we developed a new method for fabricating micro-sized suspended SLG. Subsequently, a focused ion beam (FIB) was used to create nanohole defects in SLG and tune the heat transport. The thermal conductivity of the same SLG before and after FIB radiation was measured using a novel T-type sensor method on site in a dual-beam system. The nanohole defects decreased the thermal conductivity by about 42%. It was found that the smaller width and edge scrolling also had significant restriction on the thermal conductivity of SLG. Based on the calculation results through a lattice dynamics theory, the increase of edge roughness and stronger scattering on long-wavelength acoustic phonons are the main reasons for the reduction in thermal conductivity. This work provides reliable data for understanding the heat transport in a defective SLG membrane, which could help on the future design of graphene-based electrothermal devices.

Effect of ambient temperature and relative humidity on interfacial temperature during early stages of drop evaporation

Understanding drop evaporation mechanisms is important for many industrial, biological, and other applications. Drops of organic solvents undergoing evaporation have been found to display distinct thermal patterns, which in turn depend on the physical properties of the liquid, the substrate, and ambient conditions. These patterns have been reported previously to be bulk patterns from the solid-liquid to the liquid-gas drop interface. In the present work the effect of ambient temperature and humidity during the first stage of evaporation, i.e., pinned contact line, is studied paying special attention to the thermal information retrieved at the liquid-gas interface through IR thermography. This is coupled with drop profile monitoring to experimentally investigate the effect of ambient temperature and relative humidity on the drop interfacial thermal patterns and the evaporation rate. Results indicate that self-generated thermal patterns are enhanced by an increase in ambient temperature and/or a decrease in humidity. The more active thermal patterns observed at high ambient temperatures are explained in light of a greater temperature difference generated between the apex and the edge of the drop due to greater evaporative cooling. On the other hand, the presence of water humidity in the atmosphere is found to decrease the temperature difference along the drop interface due to the heat of adsorption, absorption and/or that of condensation of water onto the ethanol drops. The control, i.e., enhancement or suppression, of these thermal patterns at the drop interface by means of ambient temperature and relative humidity is quantified and reported.

In-Situ ESEM and EELS Observation of Water Uptake and Ice Formation in Multilayer Graphene Oxide

Graphene oxide (GO) is hydrophilic and swells significantly when in contact with water. Here, we investigate the change in thickness of multilayer graphene oxide membranes due to intercalation of water, via humidity-controlled observation in an environmental scanning electron microscope (ESEM). The thickness increases reproducibly with increasing relative humidity. Electron energy loss spectroscopy (EELS) reveals the existence of water ice under cryogenic conditions, even in high vacuum environment. Additionally, we demonstrate that freezing then thawing water trapped in the multilayer graphene oxide membrane leads to the opening up of micron-scale inter-lamellar voids due to the expansion of ice crystals.