Voltage-current characteristics of a flame-assisted unipolar corona

The voltage-current characteristics (VCC) of flame-assisted electrical coro nas have been measured in an inhomogeneous electric field. The electric fie ld was created by applying a voltage less than 25 kV DC between a cylindric al metallic burner and a planar metallic grid, which was perpendicular to t he axis of the burner. Both positive and negative polarities of the applied voltage were studied in the presence of a pre-mixed methane-air flame. The results were compared with the VCC measured for the same discharge gap in the absence of a flame. Simultaneously, with measuring a VCC the behavior o f the flame was observed and its height was measured under a gradually incr eased voltage, which resulted in the flame shortening: For most cases compl ete extinction of the flame was achieved at a particular voltage. An exact spherically symmetric solution for a unipolar corona in the domain between two spherical electrodes, was used to model the discharge gap between the f lame boundary and the grid and also to calculate the VCC for comparison wit h the experimental results. It is shown that in the presence of a flame, th e experimental and theoretical VCCs can be matched if the radius of curvatu re of the flame tip increases with the total current. This agrees with the present experimental results, in which flattening of a flame was observed w hen the current was increased. (C) 2001 by The Combustion Institute.