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A comparative study of DC and AC vortex stabilized arcs

University of British Columbia

A comparative study of high intensity DC and AC vortex stabilized arcs operating in argon (at pressures of one to five atmospheres) has been conducted. The energy balance for both the AC and DC arcs has been determined calorimetrically. From these measurements the radiative efficiency (radiation losses/input power) has been calculated. It was found over the current range examined (150-450 amperes) that the radiative efficiency of the AC vortex stabilized arc was comparable to the DC arc. Since DC vortex stabilized arcs have been used as a high intensity radiation source, these results indicate that the AC vortex stabilized arc shows promise for use as a high intensity radiation source. From the energy balance results the heat transfer to the wall was surprisingly found to scale linearly with the radiation losses. The wall loading is not due to absorption of radiation and is much larger than that expected from laminar radial heat transfer. To investigate this further a simple channel model was developed for the luminous DC arc core. From this model the radius and temperature of the luminous arc core was determined as a function of current. The predicted radii were in good agreement with time integrated photographs of the luminous core of the arc. At high current (I>350 amperes) the DC arc radius was essentially constant. The wall heat transfer continued to increase when the arc radius was essentially constant, so that highly efficient heat transfer processes must be taking place outside the central luminous arc core. It is believed that turbulent mixing might be present in this region and be responsible for the large wall heat transfer. The heat transfer processes to the arc electrodes have been measured calorimetrically and the electrode surface temperature has been measured spectroscopically. For both AC and DC electrodes the heat transfer scaled linearly with the arc current. The electrode voltage drop is strongly dependent on gas flow direction with the voltage drop always larger for flow towards the electrode than for flow away from the electrode. These results are not due to convective heat transfer effects. The geometry of the electrode arc attachment region changes when the flow direction is reversed. It is believed that both the anode and cathode fall potentials are altered when the flow direction is reversed, and this is responsible for the difference in electrode voltage drop when the flow direction is reversed. From the electrode surface temperature measurements the heat transfer to the arc electrodes was shown to be essentially one-dimensional in nature. A model of the AC electrode heat transfer was developed using the DC heat transfer results which predicts results for the electrode voltage drop that are in good agreement with the experimental results. The AC electrode heat transfer was found to be <50% of the anode heat transfer in a DC arc at the same current. In the DC arc the anode heat transfer is much larger than the cathode heat transfer. For a practical DC vortex stabilized arc radiation source anode failure is a serious problem, so that the results for the AC electrode heat transfer is of considerable practical importance.

Text

2010-03-23

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http://hdl.handle.net/2429/22407

Physics

University of British Columbia

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