Self-generated electrical fields in flames

Abstract

When a spherical electrostatic probe is immersed in ionized flame gases, the probe floatingelectrical potential, measured relatively to a fixed electrical earth potential, varies with position. Results are presented for a stoichiometric premixed methane-air flame at a pressure of 38 torr in an axial traverse. The range of variation of floating potential was 1.3 volts and cannot be explained in terms of the calorelectric effect. It is proposed that the generated voltage is a consequence of the diffusion of positive ions and electrons from the region of ion formation. A theoretical expression is presented for the spatial variation of plasma potential. This variation is derived from experimental values of positive ion concentration. Floating potentials are in turn obtained from these values of plasma potential using electrostatic probe theory. Theoretical and experimental values of floating potential are in fair agreement. Differences in floating potential can yield an open-circuit voltage that is able to supply current through an external resistance. A generator working on these principles is termed a diffusion generator. It is shown how current-voltage characteristics for both diffusion and calorelectric generators may be obtained from theoretical dimensionless probe characteristics and a knowledge of the differences of plasma potential and temperature between electrodes. Experimental diffusion generator characteristics are in fair agreement with those of theory.