Niroh Nagai

Observation of boiling structures in high heat-flux boiling

Abstract This study is an attempt to attain a good understanding of boiling structures such as liquid-solid contact patterns and bubble structures near the boiling surface. This was accomplished by observing both the dynamic behavior of liquid-solid contact from below the surface and the sectional views of bubbles in a quasi-two-dimensional boiling space. Results of the observation indicate that the boiling structures at heat fluxes near the CHF point are different from the physical image given by the so-called macrolayer model. Based on this fact, we propose a new concept of the contactline-length density describing the contribution of evaporating thin liquid layers to high-heat-flux boiling heat transfer.

Experimental study of natural convection heat transfer in a semicircular enclosure

An experimental study of natural convection heat transfer in a differentially heated semicircular enclosure was carried out. The flat surface was heated and the radial surface was cooled isothermally. The effects of angle of enclosure inclination on the heat transfer across semicircular regions of several radii were measured for Rayleigh numbers RaR ranging from 6.72 × 106 to 2.33 × 108, using water as the working fluid. The angle of inclination varied from −90 degrees to 90 degrees with radii R of 50, 40, and 30 mm. The flow patterns were sketched from the results of a visualization experiment using aluminum powder. The temperature measurements in the enclosure were carried out using liquid crystals and thermocouples. The results indicate that different flow patterns were encountered as the angle of inclination varied, and the heat transfer rate was largely dependent on the flow pattern. In particular, enhanced heat transfer rates can be obtained when plume-like flow occurs along both hot and cold walls in the case of an upward-facing hot wall. Heat transfer for the inclined enclosure can be predicted using the equation for a vertical enclosure presented in this paper.

Natural convection heat transfer in a circular enclosure

Natural convection heat transfer in a circular enclosure, one half of which was heated and the other half of which was cooled, was investigated experimentally, focusing on the effect of the inclination angle. The experiments were carried out with water. Flow and temperature field were visualized by using the aluminum and liquid-crystal suspension method. The results show that with downward heating the heat transfer coefficient increased as the inclination angle of the boundary between the heating wall and the cooling wall approached the vertical. But with upward heating, the heat transfer coefficient showed minimal change, exhibiting a small peak value when the inclination angle was γ ˜ –45°. The heat transfer coefficient of a flat circular enclosure was estimated from the circular enclosures heat transfer coefficient. These results can be explained by the obtained flow and temperature fields.

Heat Transfer Control in Enclosure by using Rotating Plate and Stratified Fluids.

The effects of inner rotating plate with horizontal axis on the heat transfer between vertical two wails in a rectangular enclosure with stratified fluid layers were investigated experimentally. The aspect ratio of the enclosure height/width was 1 throughout the experiments. An acrylic plate with small thermal conductivity was installed horizontally at the center of the square enclosure, and was rotated at various speeds for normal and reverse rotations by using the motor attached outside of the enclosure. Purified water and silicon oil were used for the working fluid and were stratified in the enclosure. It is clarified here that the heat transfer rate of the enclosure with stratified fluid layers differs largely from that of the enclosure with single fluid layer. Namely, the heat transfer rate increases exceedingly at low rotating speed range, and keeps almost constant value at high rotating speed range.

Solidified-shell Formation Process during Immersion Process of a Cooled Solid Surface.

This paper investigates the solidified-shell formation process during the dipping process of a cooled solid surface into a melt pool. Since the shell formation process during this dipping process is related to the dynamic behavior of the meniscus contact line, the solidification process may be different from the usual solidification process on a stagnant cooled surface. The solidified shell obtained in this experiment has many surface defects on the surface. The results from visual observation of the shell formation process show that the shell does not grow continuously in the casting direction but is formed as a vertical pile of the horizontal shell units resulting from the following sequential process ; collapse of the meniscus, appearance of a locally solidified shell and propagation of the local shell along the contact line. Experimental results indicate that the formation of the surface defects is considered to relate to this dynamic process.