## 3 Results and Discussion
### Flow Characteristics of a Conventional RFB Frame Cell  
The flow characteristics of the basic RFB designed by [PERSON] et al. [17] were analyzed using CFD (Figure 9). Figure 9a shows the velocity distribution of the whole RFB. The maximum flow velocity in the channels was 2.73 m/s, and the mean velocity in the electrode was 0.016 m/s. When the diverging section from the inlet channel to the electrode and the converging section from the electrode to the outlet channel were magnified, the electrode inlet section (Figure 9b) exhibited a higher flow nonuniformity than the electrode outlet section (Figure 9c). When the variability range coefficient (\(R_{i}\)) was calculated for this section, the electrode inlet (y = \(-\)148 mm) exhibited the highest value (\(R_{i}=0.189\) m/s). As the measurement position moves up, the flow nonuniformity decreases, owing to the porous medium characteristics of the electrode (Figure 10).  
Figure 8: Volume of a fraction of the Zn/Br solution after 5 s (computational fluid dynamics (CFD) result): (**a**) inflow from the top and (**b**) inflow from the bottom.  
Figure 9: _Cont._Figure 10: Velocity profiles at the electrode inlet region.  
Figure 9: Velocity magnitude contours (**a**) in the conventional flow battery, (**b**) in the electrode inlet, and (**c**) in the electrode outlet.  
The conventional RFB uses 32 outlets from the electrode, and the flow nonuniformity can be identified by the mass flux difference at each outlet (Figure 11). The mass flux difference at each bifurcation was obtained using the symmetry coefficient (\(C_{sym}\)), and the difference between the maximum and minimum flow rates for the 32 outlets was analyzed through the maximum flow rate deviation (\(D_{m}\)). The maximum flow rate deviation (\(D_{m}\)) of the electrode inlet was calculated as 17.9%, indicating a high difference in flow rate. For the symmetry coefficient (\(C_{sym}\)), the bifurcations in the five stages from the channel inlet to the electrode exhibited average values of 0.4%, 1.35%, 0.55%, 3.85%, and 2.9% (Figure 11). This suggests that flow nonuniformity occurs throughout the entire channel, and the fourth and fifth bifurcations exhibit the severest flow rate differences.  
In a conventional RFB, the top and bottom surfaces contain v-shaped chevrons in an alternating pattern, as shown in Figure 12. These chevrons are installed to crush the oily bromide complex and mix it properly with Zn/Br. The CFD analysis shows that the vortices were generated in the cross-section of the flow path with the chevrons (Figure 13); this structure increased the pressure drop. While the pressure drop without chevrons was 0.683 bars, that with chevrons was 0.993 bars, indicating an  
Figure 11: Mass fluxes at five bifurcation locations between the inlet channel and the electrode.  
approximately 45% increase. Figure 14 shows the pressure drops at the several locations of the inlet channel. The first and longest section before the beginning of the of bifurcation exhibited the largest pressure drop.  
Figure 12: Chevron geometry in the inlet channel (marked with a red box).  
Figure 13: Flow characteristics in the several cross-sections of the inlet.