Charge density enhancement due to recirculatory flow

This paper develops a system model for electrification in a flow loop and investigates volume charge accumulation due to re-circulatory flow between a charge source and several charge relaxation regions. Enhanced charge densities are predicted when the source is a filter and the residence time in the relaxation regions is small compared to the liquid dielectric relaxation time. Small scale flow loop experiments verified the enhanced charge densities. In these experiments, transformer oil was circulated at volume flow rates of 0.063-1.26 l/s between a 208 l reservoir and a paper cellulose filter. The filter and the reservoir were electrically isolated from the rest of the flow loop and either grounded or virtually grounded through current measuring electrometers. The charge densities of the fluid entering and exiting the filter and reservoir were measured using absolute charge sensors. Good agreement was obtained between the electrometer current and the calculated difference in convection currents. Bypassing the reservoir greatly reduced the residence time for charge relaxation and resulted in increased volume charge densities over the reservoir flow case. The charge density was found to have a strong dependence on temperature