In carbon dioxide (CO2) laser—gas metal arc hybrid welding, a shielding gas is supplied to isolate the molten metal from the ambient air, suppress the laser-induced plasma, remove the plume out of the keyhole, and stabilize the metal transfer. In this study, a shielding gas consisting of helium, argon, and CO2 was used, and its effects on the composition of the welding phenomena, such as behaviours of laser-induced plasma generation, molten pool flow, and droplet transfer in gas metal arc welding, were investigated. High-speed video observation was used to investigate the welding phenomena inside the arc regime. Consequently, helium was found to have a dominant role in suppressing laser-induced plasma; minimum helium content at a laser power of 8 kW was suggested for laser autogenous and hybrid welding. Argon and CO2 govern the droplet transfer and arc stability. A 12 per cent addition of CO2 stabilizes the metal transfer and eliminates undercut caused by insufficient wetting of molten metal.

This study investigated the influence of dissolved oxygen concentration and aeration time on nitrification and nitrite accumulation in an attempt to optimize the recently developed biological-partial-nitritation process for the treatment of strong nitrogen wastewaters. Investigation of dissolved oxygen concentration on ammonium and nitrite oxidation was carried out in a batch reactor. The dissolved oxygen concentration of 0.5 mg O 2 /L inhibited both ammonium as well as nitrite oxidation, while increase of dissolved oxygen concentration to ~1 mg O 2 /L increased the ammonium oxidation rate and was comparable to that at higher dissolved oxygen concentrations. Experiments were carried out in a sequencing batch reactor for more than 100 days to investigate the influence of aeration time on nitrite accumulation. The dissolved oxygen concentration was controlled at ~1.0 mg O 2 /L (in the range of 0.8-1.5 mg/L) during the aeration stage, and volatile suspended solid was maintained at 2.0 g/L while temperature and pH were 30±1ºC and 8.3±0.1, respectively. In a typical cycle, complete nitrification occurred at aeration time longer than 6 h. When the aeration time was reduced to 4 h., ~80 % of partial nitritation was achieved. With a further reduction in aeration time to 3 h., nearly 1:1 nitrite/ammonium ratio was yielded. This result revealed that for the reactor design, aeration time determined by feasibility experiments must be considered based on the nitrogen strength in wastewater and biomass concentration in the reactor with dissolved oxygen concentration of ~1.0 mg O 2 /L for satisfactory partial nitrification with subsequent processes such as anaerobic ammonium oxidation.

In the Francis turbine, there are gaps between the runner and the stator structure as the labyrinth seals. The flows inside gaps have the complicated flow phenomena; these complicated flows can affect the hydraulic performance and the downstream of the Francis turbine as the leakage flow. Therefore, it is important to investigate the flow characteristics induced by the runner gaps. However, normally the runner gap that has very complicated flow structure is often disregarded by considering the time and cost of the numerical analysis. But, it is necessary to apply all flow structures correctly to confirm the more accurate and reliable flow phenomena. In this study, the flow characteristics of gaps at the hub and shroud of the Francis turbine model runner were investigated with adding the leakage flow in the runner cone. In addition, the internal flow characteristics were compared by considering with and without the runner gaps application. For observing the influence of the leakage flow on the hydraulic performance and flow characteristics, the three-dimensional steady Reynolds-averaged Navier-Stokes analyses were performed using the shear stress transport turbulence model. The efficiency was decreased by applying the runner gaps; and the complicated flow phenomena were captured in the runner gaps.