This paper is intended to determine the space-charge-free field on the stressed discharge wires’ surface, the corona-onset voltage of wire-duct electrostatic precipitators (ESP) as influenced by the variation of the velocities of the incoming flow gases. The corona current-voltage (I-V) characteristics of wire-duct ESP is calculated under varying velocities of incoming flow gases. The calculation is made using the improvement of Deutsch’s Method. The method is endorsed by an iterative process to determine an estimate for the underlying dissemination of the charge density close to the surface of the stressed discharge wire(s). The electric potential, field, space-charge density in the interelectrode spacing, corona onset voltage and current-voltage characteristics of the precipitator are considered. Besides, the effect of gradually increase of the velocities of incoming flow gases, changing the number of stressed wires and changing the wire radius of wire-duct ESP are investigated. An experimental set-up has made in the Laboratory of High Voltage Engineering, Czech Technical University (CTU) in Prague, Czech Republic to investigate the accuracy of mathematical/simulation analyzed of the corona-onset voltage as well as the validation of the developed Deutsch’s Method in modeling the I-V characteristics of wire-duct ESPs. The experimental results reasonably agree with the theoretical analysis.