Abstract. The effects of cathode holder shape on a plasma cutting arc are investigated using a 2-D thermofluid model developed for arc plasma with consideration of hafnium cathode evaporation. The cathode holder shape is defined a convex structure which protrudes from the surface of the cathode holder. Results show that the gas flow near the hafnium cathode is attracted towards to cathode with effect of convex cathode holder. The temperature of cathode surface decreases markedly with the higher protrusion of convex cathode holder by a rapid clockwise rotation of gas flow velocity. The presence of gas flow vortex toward to cathode surface reduces the mass fraction of hafnium vapor on surface cathode. Thus, the amount of mass loss of hafnium evaporation was predicted to decreases significantly with effect of convex cathode holder.Submitted to: J. Phys. D: Appl. Phys.
Oxygen plasma arc cutting is widely employed in various industrial fields. In the case of cutting magnetized plates, the magnetic field is concentrated around the cutting front as cutting progresses, and the electromagnetic force induced by the leakage of magnetic field deflects the plasma jet. The deflected plasma jet leads to poor cutting quality and sometime causes damage to the electrode and nozzle because of double arc abnormal discharge. This phenomenon is called magnetic arc blow, and it is a critical issue when applying plasma cutting on magnetized plates. In this study, magnetic arc blow behavior is investigated, and a method to prevent it is devised. We examined the relationship between operating conditions and the double arc using external magnetic fields on a plasma jet. We found criteria regarding operation conditions that induce arc blow. In addition, we succeeded in suppressing the double arc caused by the leakage external magnetic field by using a magnetic shield cap composed of ferromagnetic material around the nozzle.
An experimental study of a torch for oxygen plasma arc cutting (PAC) was conducted. We made a nozzle equipped with a window and a port for measurement. We observed the hafnium (Hf) electrode during working, using a color high-speed video camera through the window. The color image consists of three images assign red (R), green (G) and blue (B). Each image is taken by different image sensors having different wavelength sensitivity. We estimated the temperature on the surface of the Hf electrode assuming the black body radiation of the surface. And the port of the nozzle is connected to a pressure sensor to measure the pressure in the nozzle. We could estimate the real-time flow rate of plasma gas from the difference between the pressure in the nozzle and the pressure of supply. Using the special nozzle for observation, we investigated the arc phenomena of PAC using an Hf electrode, during arc-start and arc-shut-off.
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