## 3 Experimental Results and Discussion  
### Electric Field and Temperature Field  
Regarding the proposed co-field treatment chamber configuration in Section 2.1, the treatment chamber configuration parameters of the grounded electrode (A), the elliptic insulator (B) and the high voltage electrode length (C), and the elliptic insulator height (h) had been optimized to maximize the electric field strength at the treatment zone from the FEM simulation using Elmer FEM, an open source multiphysical simulation software. The GiD\({}^{\text{TM}}\) commercial software was used to draw the electrode configuration and generate a mesh for the co-field treatment chamber. The FEM simulation was performed as follows. The finite element governing equation for the potential \(\Phi\) in this paper is \(\
abla^{2}\Phi=0\). The electric field normal to the boundary conditions surface of the inlet, outlet, insulator, and asymmetrical axis was set as zero, while the high voltage electrode and ground electrode boundary conditions were set as \(\Phi=7.5\) kV and \(\Phi=0\), respectively. The liquid flow was assumed to be laminar flow with an average inlet flow velocity of 0.05 m/s (Reynold number, Re = 499). The tangent velocity at the boundary condition of high voltage, insulator, and ground electrodes was zero. The material properties chosen from the Elmer library were applied to the fluid (_Escherichia coli_ (_E. coli_) suspension). However, the electrical conductivity of water was adjusted, followed by the actual solution, to 0.018 mS/cm. The temperature of the water at the inlet and the external temperature were set as room temperature (25 \({}^{\circ}\)C) with natural convection. The heat transfer coefficient at the metal electrodes and the heat flux of the outlet and insulator surface were 2000 W/m\({}^{2}\)K and 0 W/m\({}^{2}\), respectively. Regarding previous studies on the co-field treatment chamber designed in [26; 30], it was found that the electric field intensity at the treatment zone was affected by the high voltage electrode length and the insulator height. A highly non-uniform electric field was found at the junction between the elliptical insulator and the high-voltage (z = 20.5 mm)/grounded electrode (z = 26.5 mm), where z is the distance from the inlet to outlet along the asymmetrical axis, while the electric field distribution on the insulator surface was rather uniform, as shown in Figure 4a,b. However, the height of the insulator could affect the slight turbulent flow and accumulating heat in the flowing liquid [3; 14; 21; 28; 30], as shown in Figure 5. Since the effectiveness of microbial inactivation is strongly dependent on the electric field intensity, it could be difficult to avoid the accumulating heat in the liquid media resulting from the high pulsed electric field without an extra external cooling system. Our suggestion is that a serial connection of the treatment chamber would enable the reduction of the required intensity of the high-pulsed electric field and the maximum temperature of the liquid media compared with a single treatment chamber. This is since the increase in the number of the treatment chambers would increase  
Figure 3: The serial connection of 5 units of the treatment chamber. the residence treatment time and the probability of microorganism treatment. However, too high numbers of treatment chambers would also lessen the inactivation efficiency due to sharing of energy among the treatment chambers, where the capacity of the power supply is limited. Therefore, based on the aforementioned discussion, A, B, C, and h were optimized to be 20, 6, 5, and 0.8 mm, respectively, and the maximal number of the treatment chamber was 5 units. For this geometry, the volume of one treatment zone would be 83.13 mm\({}^{3}\), approximately. It could be seen in Figure 4b that the electric field intensity in the treatment zone was rather uniform near the center of the insulator, which occupies almost half of the length of the insulator.