Niro Nagai

Liquid-Solid Contact at Evaporation of Water Liquid Droplet Emulsified With Lubricant and Its Adhesion Situation

In aluminum die casting process, water-soluble lubricant which is a kind of emulsion and consists of oil, surfactant and water are sprayed onto metal die surfaces, where the surface temperature is relatively high, over 300°C. When a lot of small liquid droplets of the water-soluble lubricant impact on a superheated surface, it is important to assure that liquid-solid contacts occur and oil component adhere onto the surface. However, many parameters such as liquid temperature, concentration, diameter and impact speed, affect this phenomena, so the phenomena themselves has not been totally revealed and it has not been fully established how to assure the liquid-solid contact and adhesion of oil component. Therefore, as a fundamental and experimental research, this paper aims to obtain essential information and data of liquid-solid contact of water-soluble lubricant and its adhesion situation in order for improving aluminum die-casting process. Firstly, evaporation curves, especially Leidenfrost temperature, of water droplets including water-soluble lubricant on metal surface were measured. Experimental parameters were as follows: temperature of droplet, 20 ∼ 80°C, concentration, 0 ∼ 50% corresponding to surface tension 20 ∼ 70mN/m, falling height of droplet, 1 ∼ 9cm, and two kinds of lubricant. The metal surface was made of SKD61, usually used as materials of die casting. Experimental results show that Leidenfrost temperature is not strongly affected by droplet velocity, but significantly affected by concentration and liquid temperature. It is worthy to note that there was found to be optimum temperature and concentration of droplet from view points of aluminum die casting. These results can be explained by effects of surface tension and interfacial temperature between solid and liquid. Secondly, visual observations of these phenomena by high-speed camera and measurements of electric resistance between droplet and metal surface were conducted. Surface chemical analysis was also performed by SEM/EDX after evaporation experiments for detecting carbon and silicone components on the surface to check adhesion situations of lubricant. The obtained results show that liquid-solid contact becomes gradually localized and limited as initial surface temperature increases. However, liquid-solid contact surely occurred at higher surface temperature than Leidenfrost temperature, 210 ∼ 270°C, and lubricant adhesion was clearly confirmed even if liquid-solid contact duration was limited to very small time scale. This result implies that oil component can adhere on die surface of higher temperature than Leidenfrost temperature.Copyright

Experimental Study on Free-Convection Condensation Heat Transfer From Moist Air

Characteristics of free-convection condensation heat transfer from moist air under atmospheric pressure were experimentally investigated, for further improvement of physical modeling on heat and mass transfer of solar distillation device. The cooled metal surface was 50mm width × 100mm height. The experimental conditions were as follows. Moist air temperature range was 40∼100°C for saturated moist air, and 50∼70°C for non-saturated moist air. Relative humidity range was 50∼90%. Inclination angle of cooled surface was 0° (downward facing) ∼ 180° (upward facing). All experimental results of heat transfer characteristics for vertical surface (angle 90°) were well correlated into a single equation with partial air pressure ratio using classical Nusselt’s liquid-film theory. The experimental results for the effects of inclination angle show that heat transfer coefficients for angle 0°∼105° were almost constant with slight peak value at angle 45°, followed by rapid decreasing of heat transfer coefficient over angle 120°.Copyright

Temperature Control Inside Steel Cabinet in Open Air Using Underground Heat Source

This research aims to develop temperature control system for a cabinet which is set up in outfield and has heat source inside. Underground heat source was utilized for this system with steel pipes embedded into underground and water as working fluid. The cabinet was cooled the in summer season and warmed in winter season. The experimental results show that this system can favorably control the inside air temperature of the cabinet in summer and winter season with relatively high COP. The results of numerical simulation using only meteorological data show good agreements with experimental data in summer and winter season. The results of numerical simulation where some parameters such as pipe distance, diameter, and length were changed show the effect of these parameters on performance of the system.Copyright

Snow Melting System Around Steel Top of Underground Fire Cistern Using Heat Pipe BACH

This research aims to develop snow melting system around steel top of underground fire cistern by using Bubble Actuated Circulating Heat pipe (BACH). Water in afire cistern installed underground is heated by underground heat source, 10∼15 °C Heat is transported by BACH from water to snow melting panel near the surface, which contains the steel top, resulting in snow melting around the top. The experimental results show that this system can melt the snow on the snow melting panel in winter season preferably, and has enough anti-freezing ability around the steel top. The numerical simulation program was found to predict temperature variations of the whole system preferably. From a thermal point of view, BACH showed better results compared to those of a conventional thermosiphon heat pipe.Copyright

Experimental Demonstrations and Optimal Design Conditions of Snow-Melting System Using Geothermal and Solar Energy

The authors have been proposed and developed snow-melting system using geothermal and solar energy. In summer, solar heat is stored into underground from road surface to underground piles. In winter, the underground heat is utilized to melt snow on the road surface. This system was applied to parking lots and bridges of relatively small scale (less than 1000 m2 ). Numerical simulation program was also developed to predict temperature field of the system and to evaluate system performance. This program was verified by experimental data only for relatively small scale test area. In addition, appropriate design conditions, such as pile diameter, length and number, can not be easily estimated when road surface area and ability (average heat flux) of snow-melting are given. This paper aims to demonstrate the system for relatively large scale (larger than 1000 m2 ), and to obtain optimal design conditions of the system at given road surface area and ability. The snow-melting system using geothermal and solar energy was applied to a parking lot and a bridge of large scale. Both sites were under practical use which means cars were sometimes parked and run over the bridge. Obtained experimental data of temperature field of the system and snow melting situation show that numerical simulation program predicted system performance and temperature field adequately even though the program contains several simplifications. To discuss the optimal design conditions, numerical simulation was conducted by changing the following parameters: diameter, length, number and pitch of piles, pitch and diameter of heat dissipation pipe, flow rate of circulating water, road surface area. All these parameters are considered to affect system performance. The simulation results revealed that pile surface area determined by diameter, length and number of piles is the dominant parameter for deciding snow-melting ability. Namely, when road surface and snow-melting ability are given, necessary pile surface area can be obtained from the simulation results, and system design of piles becomes possible with considering cost for embedding piles.Copyright

Image Measurement of Hydrogen Bubble Generation in Water Electrolysis

Water electrolysis is one of the methods for hydrogen gas production. The objective of this study is to investigate the effect of the electrode surface roughness on the bubble generation from a viewpoint of the multiphase-flow engineering. We vertically set two electrodes into water tank and capture the images of hydrogen bubbles generated from cathode surface using the high-speed video camera. As a result, we found that bubbles are generated and released from fixed positions of the electrode surface. The size and velocity of released bubbles were measured.