Electrostatic charge generation during impeller mixing of used transformer oil

The purpose of this study was to examine the effects of varying conductivity and turbulent impeller mixing intensity on charge generation of used Shell Diala(R) transformer oil. The oil was mechanically agitated by a pitched six-bladed impeller in a 1.5 quart fully bah-led cylindrical vessel at impeller Reynolds numbers between 5 000 and 10 000. The electrical current generated from the mixing operation was measured on the wall of the mixing tank and also in the internal vapor space on a 1 cm(2) target disk positioned approximately 1.5" above the free liquid surface. Fluid electrical conductivity was varied using different concentrations of two types of commercially available antistatic additives (Shell ASA-3 and DuPont Stadis 450). A generalized dimensional analysis performed by using the Buckingham Pi theorem showed that a dependent dimensionless group, pi(1), which contains the measured wall current and other fluid dynamic and electrical variables was found to be dependent on six independent groups. Four of these groups were related to modified Reynolds numbers based on impeller diameter and width, and tank diameter and fluid depth or tank height. The two remaining independent groups were related to the mass transfer Schmidt number and the impeller rotational speed. The mean particle diameter from a Malvern ZetaSizer and pi(1) were plotted versus fluid conductivity to investigate the functional dependence of these parameters. The dimensionless group pi(1) was also plotted for different impeller Reynolds numbers. These different plots show results that appear to illustrate inverse dependencies. The current versus conductivity data for different impeller Reynolds numbers also show similar behavior to published data for both laminar and turbulent flow through capillaries (i.e., current first increases with increasing conductivity, hits a maximum and then decreases with further increases in conductivity). This apparently demonstrates that a single charge transport mechanism is responsible for the behavior in two different charge-generation situations for different geometrical conditions. (C) 2000 Elsevier Science B.V. All rights reserved.