In the electrostatic module, a ground (0 V) has been implemented on the outer electrode and the electric potential on the inner electrode (ranging from 0 to 45V). The zero-charge symmetry boundary condition was applied to the boundaries without walls, and is given by n$D¼0 where n is the outward normal from medium and D is the electric flux density. The flow inside the ion collector could be assumed to be steady, incompressible and laminar. Based on the principle of momentum conservation, the incompressible NaviereStokes