Model of the formation of a dry band on an NaCl-polluted insulation

A model of the apperarance of a dry band on a flat insulator subjected to saline fog pollution has been experimentally and theoretically investigated with the following simple hypotheses: (a) the surface temparature of the insulator is uniform, (b) the mass of evaporated water is proportional to the temparature difference between the conducting pollution layer and the ambient atmosphere, (c) the product of electrical resistance into the residual mass of pollution remains constant at constant temparature, (d) a dry stripe appears when, after evaporation, the residual mass of pollution is lower than a critical threshold. After a theoretical evaluation of the different terms of the energy-balance equation, the temporal evolution of the current, of the mass of pollution and of the surface temparature, was numarically computed together with the delay for appearance of a dry band, the energy required and the average dissipated power. Qualitative and quantitative agreemet was found between the theoretical model and experimental data with a 0.23 ¿¿ 0.035 m2 plane insulator. A sheet blotting paper wetted by a NaCl solution simulated the pollution. This theoretical model predicts the time of formation of a dry band under conditions where formation is known to occur, not whether or not formation will occur.