A CHEMICAL REACTION-BASED BOUNDARY-CONDITION FOR FLOW ELECTRIFICATION

A physical model is developed for the charge transfer boundary conditi on in semi-insulating liquids. The boundary condition is based upon in terfacial chemical reactions and extends established relations for the interface by including the effects of interfacial surface charge and charge desorption at the interface. A steady state model for now elect rification in a rotating cylindrical electrode apparatus incorporated this boundary condition and described polarity changes in the open-cir cuit voltage and short-circuit current as a function of the fluid velo city, the volume charge density dependence on the terminal constraints , and the charge density dependence on applied de voltages. Previously used boundary conditions are shown to be special cases of the chemica l reaction rate boundary condition. A general methodology is developed for combining the volume charge density and voltage/current terminal measurements to estimate the parameters describing the interfacial cha rge transfer kinetics. Volume charge densities rho(omega) on the liqui d side of the interfaces of 1 to 20 mC/m(3) were estimated from the op en-circuited electrode measurements,with the stainless steel rho(omega ) typically larger than that of copper but smaller than that of transf ormer pressboard. Activation energies for an Arrhenius temperature dep endence of similar to 0.16 eV for pressboard, 0.25 eV for stainless st eel and 0.28 eV for copper were obtained. Interfacial adsorption react ion velocities, estimated to be 10(-5) mis, were not large enough to m ake the terminal current transport limited which contradicts the often used assumption that the reaction velocities can be considered 'infin ite'. Estimated surface reaction rates at a 70 degrees C stainless ste el/oil interface of similar to 20 mu m/s for adsorption and similar to 0.5 s(-1) for desorption were obtained. The additive BTA reduced the rho(omega) for pressboard and stainless steel at concentrations > 8 pp m in transformer oil.