## 3 Experimental Results and Discussion
### Electric Field and Temperature Field
For the proposed configuration, the maximum and average electric field intensities at the treatment zone were as high as 12.66 and 12.21 kV/cm at the supplied pulsed voltage of 7.5 kV, respectively. It should be noted that all the treatment zones have the same distribution of electric field intensity. Joule heating could be generated in the conductive liquid media, since an amount of current could flow through the media once the high voltage was applied to the treatment chamber. It not only causes a thermal rise in the liquid media but also influences the profile of the flowing liquid's viscosity and density [13; 28; 30]. Figure 5a presents the simulation results for the temperature rise in the treatment system. It could be noticed that the most accumulating thermal spots were found at the locations behind the elliptical insulators indicated by the letters A and B, which are the locations with the highest temperatures at the first and second insulators of each treatment unit. This issue comes from the combination of many factors, such as the fluid flow velocity, electric field distribution, and joule heating. Figure 4: Simulation results for (**a**) Electric field intensity, and (**b**) electric field intensity distribution along the insulator surface and symmetrical line in the first treatment zone. Figure 5: Simulation results for the liquid’s (**a**) temperature, (**b**) velocity, and (**c**) joule heating along the \(z\)-axis at the first treatment chamber (\(N_{HV1}\)). During the treatment process, the liquid heated in the treatment zone continued flowing along the \(z\)-axis. However, the fluid velocity near the insulator surface is close to zero (a blue vortex-like region in Figure 5b) This could imply that the temperature rise, generated owing to joule heating in the treatment zone (Figure 5c), could slowly diffuse to the tube wall. This results in thermal accumulation in the regions A and B, which are slightly behind the maximal joule heating region. Since the region A is in the first treatment zone, there was no thermal accumulation in the liquid before. The treated liquid from the first treatment zone would flow to the second treatment zone. During the flow, some amount of thermal energy would diffuse to the metal ground electrode, resulting in the reduction of the treated liquid medium temperature. However, the distance of the metal electrode is not long enough to diffuse the entire generated heat. Therefore, the heated liquid medium from the first treatment zone will be heated up again, owing to the joule heating in the second treatment zone [3; 14; 21; 28; 30] This reason causes the temperature to rise at region B higher than that at region A. However, the PEF system containing a single unit of a high voltage electrode (\(N_{HV1}\)) in the non-recirculated treatment system practically requires a high pulsed voltage to efficiently inactivate the microorganism, since there is only one short treatment period for one round of treatment. The high-pulsed voltage, in this case, would cause the more serious problem of a temperature rise and troublesome electrochemical reactions in the liquid media [3; 14; 21; 28; 30; 31]. Therefore, a series of multiple treatment units would be key to enable the multi-treatment process, which could improve microbial inactivation efficiency in the non-recirculated treatment system. Moreover, it decreases the temperature rise since the lower pulsed voltage magnitude is applicable. Figure 6 presents the simulation results for the liquid temperature along the flowing axis (\(z\)-axis) of five treatment units. It can be seen that the temperature evolution at positions A and B of every treatment unit has the same trend as that illustrated in Figure 6a. This reason could be explained in the same way as discussed for one treatment unit. However, the magnitude of the temperature rise increases corresponding to the order of the treatment units, owing to the accumulated thermal energy from the previous treatment unit.