## 3 Results and Discussion
### Numerical Software Validation
#### 3.1.2 Comparison of Flow Pattern through Flow Visualization  
Through the flow visualization experiment on the single RFB cell, we investigated the flow characteristics according to the direction of gravity, and compared the results with the numerical analysis. In the case of an inflow from the top, the electrolytic solution entered from the channel at the top of the RFB, flowed through the electrode, and was discharged into the outlet channel at the bottom  
Figure 5: Experimental setup for a single cell RFB.  
Figure 6: Comparison of pressure drops between the numerical and experimental results.  
of the RFB (Figure 7a); however, the air in the electrode remained inside. Because of the difference in density between the solution and air, light air cannot escape upward and becomes trapped.  
In addition, the oily bromide complex settled in the bifurcations of the channel, clogging the flow paths; this reduced the active surface area by hindering the flow of the electrolyte, and forming dead zones. However, in the case of the flow from the bottom, dead zones were not generated as the electrolyte fills the electrode (Figure 7b). The orange lines shown in Figure 7b are the traces left by the experiment, shown in Figure 7a. In this case, a dead zone did not form, because the solution was filled from the bottom and the internal air was pushed out. However, the oily bromide complex settled at the bottom and interfered with the flow.  
A numerical analysis was conducted under the same conditions as those of the experiment, so as to verify these flow characteristics. The VOF model [16] was used to analyze the flow characteristics, including the influence of the air layer, and an unsteady flow analysis was performed considering gravity; the VOF model was mainly used for tracking the phases between the phases for an incompressible multiphase flow. The numerical results show that dead zones were generated, owing to the influence of the air layer in the case of the inflow from the top (Figure 8a); this result agrees with Figure 7a. In the case of an inflow from the bottom, the calculated flow characteristics matched the experimental results (Figures 7b and 8b).  
Additionally, we studied the horizontal inflow to characterize the settlement of the bromide complex due to the gravitational field in the experiments performed under the top and bottom inflow. The flow characteristics resembled the bottom inflow experiment. However, the charge/discharge experiment showed that the energy efficiency increased by up to 3% compared with that of the bottom inflow case (Table 4). Based on these results, the horizontal inflow, which was less affected by gravity, increased the system efficiency.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Inflow Type.** & \begin{tabular}{c} **Energy** \\ **Efficiency [\%]** \\ \end{tabular} & \
\begin{tabular}{c} **Voltage** \\ **Efficiency [\%]** \\ \end{tabular} &
\begin{tabular}{c} **Coulomb** \\ **Efficiency [\%]** \\ \end{tabular} \\ \hline Bottom inflow & 71.4 & 79.1 & 90.2 \\ Horizontal inflow & 74.4 & 80.5 & 92.5 \\ \hline \hline \end{tabular}
\end{table}
Table 4: Battery efficiencies from the charge/discharge experiments of eight cells per stack.  
Figure 7: Results of the flow visualization experiment: (**a**) inflow from the top part and (**b**) inflow from the bottom part (the orange lines are traces left by the experiment shown in Figure 6a).