被覆面における飽和液体ヘリウムのプール沸騰熱伝達

抄録

The maximum operating current in a superconducting composite of a superconducting magnet, can not be increased up to the critical current of the superconducting filaments, because of the need to have a stable performance of the magnet. In relation to this, some researchers have reported that an insulating coating on a superconductor is very effective in raising the recovery current and that there is an optimum thickness for a coating material to increase the recovery current, Such an effect seems to be of importance in thermal design of superconducting magnet, but only a few related works seem to have been reported on this problem. In particular, the real effects of surface coating on boiling heat transfer to liquid helium have not been clarified because the boiling characteristics have not been discussed based on the real surface temperature of coated surface. Thus, in the present study, boiling experiments on a circular flat plate were conducted to study the real influence of surface coatings on the pool boiling heat transfer to saturated liquid helium at atmospheric pressure. The coating material, its thickness and surface inclination were the parameters of interest in these experiments. Polytetrafluorethylene and stainless steel were chosen as the coating materials because their thermal properties are known, Experimental data obtained in the present study show the following results. A low thermal conducting coating shifts the nucleate boiling curve to a higher region of superheat. Its degree depends not only on surface inclination but also on coating material. On the other hand, film boiling curve is not affected by surface coating and is determined only by surface inclination. Critical heat flux (CHF), which decreases with increasing the angle of inclination from facing upward to facing downward orientation, is slightly increased as the coating thickness increases. As the coating thickness increases, the minimum-heat-flux (MHF) and the degrees of superheat at the CHF and MHF points become larger respectively, and they approach the respective asymptotic values determined by the thermal properties of the coating material. Using the experimental data obtained in this study, a nondimensional relation, which describes universally the influence of coating thickness on the MHF and the degrees of superheat at the CHF and MHF points, was derived. Further, the aforementioned asymptotic values of the degree of superheat at the MHF point was correlated to the thermal properties of the liquid and the surface material. Based on the above experimental results and the concept of cold-end recovery, the influence of the coating thickness on the maximum operating current was estimated. The results show that the maximum operating current can be increased by a thin insulating coating. They also show, that the current gained reaches the maxima at a certain coating thickness, thus it will be necessary to enhance the nucleate boiling heat transfer on a coated surface to realize a large improvement in the maximum operating current.