The Influence of Turbulence on Current Interruption
Optical measurements have convincingly established the existence of turbulence in a gas blast arc downstream of the nozzle throat, during the current zero period. The turbulence appears to be produced in the shear layer separating the high velocity plasma flow from the surrounding, low velocity, imposed flow field. These vortex shear layers have similar properties to those of free jets and wakes, about which much information exists in the literature.
Recent measurements of local voltages along the axial extent of a gas blast arc have shown a rapid electrical decay before current zero along the arc region downstream of the nozzle throat where pronounced turbulence occurs. The plasma decay rate in this region is not explicable in terms of convection and radial thermal diffusion, suggesting that turbulent diffusion has a significant effect. Approximate calculations indicate that the properties of the observed turbulence are consistent with the enhanced recovery measured electrically. However, it is also clear that other power loss processes occurring upstream of the nozzle throat are important even close to current zero in conditioning the arc column for final extinction.
A direct theoretical prediction of the arc electrical behaviour from measured turbulent properties would clearly provide indisputable evidence of the importance of turbulence for arc quenching. Unfortunately such an approach is still plagued by uncertainties in the values of the turbulent parameters required theoretically. Nonetheless encouraging progress is currently being made in correctly predicting electrical recovery characteristics with a semi-empirical arc model involving turbulence.
KeywordsNozzle Throat Turbulent Heat Transfer Current Zero Vortex Shear Layer Axisymmetric Disturbance
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