カーボンフリーエネルギー変換に関連する気泡発生を伴う液相プロセスにおける熱・物質輸送の促進法に関する研究

概要

The pursuit of sustainable and carbon-free alternatives to replace the current carbon-based energy sources has become a goal of utmost importance. This work focused on aspects of two of the most important carbon-free technologies available today: improving the safety margin of invessel retention (IVR) in nuclear power plants; and increasing the efficiency of water electrolysis for hydrogen production. The manipulation of the liquid and vapor/gas phases was adopted as the main approach to improving boiling heat transfer in the former and mass transfer in the latter. To enhance IVR performance, the critical heat flux (CHF) of distilled water, 3.5 wt% artificial seawater and 7.0 wt% artificial seawater was investigated through saturated pool boiling experiments on a circular upward-facing plain copper surface with 30 mm in diameter. In order to diminish competition between the liquid and vapor phases near the heated surface, a honeycomb porous plate (HPP), attached to the surface, was used with all of the fluids and the results compared to those of the bare surface (BS). Although sea-salts deposition during boiling of artificial seawater increased thermal resistance and the superheat, it also enhanced surface wettability. As a result, the CHF of 3.5 wt% artificial seawater was 60 % higher than that of distilled water on the BS. However, on the BS, 7.0 wt% artificial seawater presented the same CHF of distilled water, but a lower boiling heat transfer coefficient (BHTC) due to intense seasalts deposition and a rapid increase of the superheat. When the HPP was employed, substantial improvements of 110 % and 31 % in the CHF of distilled water and 3.5 wt% artificial seawater, respectively, were obtained. The use of a HPP did not affect the CHF of 7.0 wt% artificial seawater due to clogging of its pores by sea-salts that compromised its capillary action.

The second step was quenching of a stainless-steel cylinder, with a HPP attached to its lower surface, in distilled water and 3.5 wt% artificial seawater. On the BS, artificial seawater presented a quenching time 13 times shorter than that of distilled water due to a much shorter film boiling regime. High-speed camera images showed that the vapor film in artificial seawater was unstable and several contacts between the coolant and the surface could be observed during film boiling. With the HPP attached to the surface, the quenching time of both fluids was substantially reduced. The best result was found with the combination of artificial seawater and a HPP. In this case, film boiling regime was absent, resulting in better performance and a quenching time 43 times shorter than that of distilled water on the BS.

Finally, the effect of an external magnetic field on oxygen evolution reaction (OER) in alkaline water electrolysis (AWE) was investigated. Nickel wires with 200 µm and 300 µm in diameter were used as working and counter electrodes, respectively. The electrolyte was 2 M KOH, which was kept at a constant temperature of 30 °C. The magnetic field was oriented perpendicular to the inherent electric field between the electrodes to induce shear forces in the electrolyte through Lorentz forces directed upward and downward, depending on the position of the magnetic poles. The objective was to promote the detachment of oxygen bubbles from the electrode’s surface using the electrolyte movement. Polarization curves and double layer capacitance (DLC) showed higher current densities and larger active surface areas when the Lorentz forces were present. The best results were found with a downward-oriented Lorentz force due to intense shear forces that promoted the early removal of bubbles from the electrode surface. These results were confirmed by high-speed camera images that revealed slower terminal velocities and average bubble diameters up to 47 % smaller, for a current density of 1.5 A/cm2 , when compared with the case in absence of the Lorentz forc