## 4 Results of Flow Characteristics Analysis
### Experimental Results Using the Flow Visualization Method
Airflow rises through the slope of the coal pile, and rotational flow occurs. In both the simulation results and the visualization experiment results, a recirculation area occurs in A(6,2), A(6,3), and A(6,5), respectively. However, in A(4,3) above the recirculation area, the two results do not align. At point A(4,3), the flow direction changes rapidly, and at this point, the error rate is high. Therefore, it becomes necessary to further divide the area. Both the stagnant and recirculating areas at the top of the central wall are shown in Figure 14c with a relatively low velocity. After carrying out both simulation and visualization experiments, the same recirculation area with low velocity is formed in both areas A(5,6), A(5,5), A(4,5), and A(3,6). However, in area A(4,6), an error occurs between the two results due to the slow flow velocity. Therefore, a thorough summary of the findings proved that the stagnant area existed at the top of the central wall, and the recirculation area also formed identically there. The error rates of the computational analysis results and flow visualization experiments derived from this study are as follows. In this study, the Reynolds number for analysis and experiment is over 20,000, so trend analysis is possible. Therefore, assuming a flow field of the same size, the size and direction components of the velocity vector are examined. Figure 15 below shows an analysis of the flow field. Direction and magnitude components are output from the velocity of each segment. Below (Table 3), a refers to the x component of the velocity vector, b refers to the y component, and c refers to the size. The direction is expressed as an angle from the origin. In both results, the error rate in predicting the directional component of the flow field is about 10.45%. Large errors occurred in relatively slow flow fields. Therefore, as mentioned earlier, it is necessary to create a smaller grid in the recirculation area and areas with slow flow velocity. Figure 11: The average velocity vector at A(4,6) derived from the velocity vectors of the two flow objects. Figure 12: G-value and velocity vector in the partitioned area: (**a**) G-value information in the internal flow field of shad-type CSS; (**b**) velocity vector using flow information derived through visualization experiments. \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline  & \multicolumn{4}{c}{**CFD Results**} & \multicolumn{4}{c}{**Flow Visualization Experiment**} \\ \hline  & \multicolumn{4}{c}{**Length of Vector**} & \multicolumn{4}{c}{**Direction of Vector**} & \multicolumn{4}{c}{**Length of Vector**} & \multicolumn{4}{c}{**Direction of Vector**} \\ \hline  & **a** & **b** & **c** & & **a** & **b** & **c** & & \\ \hline \multirow{6}{*}{High velocity area} & A(6,3) & 0.85 & 0.83 & 1.18 & 45.68 & 1.22 & 0.52 & 1.32 & 66.91 \\ \cline{2-9}  & A(5,3) & 0.77 & 0.59 & 0.97 & 52.54 & 1.02 & 0.07 & 1.02 & 86.07 \\ \cline{2-9}  & A(5,4) & 0.90 & 0.60 & 1.08 & 56.31 & 1.02 & 0.48 & 1.13 & 64.79 \\ \cline{2-9}  & A(4,4) & 0.77 & 0.46 & 0.89 & 59.14 & 0.77 & 0.74 & 1.06 & 46.14 \\ \cline{2-9}  & A(3,4) & 0.51 & 0.38 & 0.63 & 53.31 & 0.78 & 0.79 & 1.11 & 44.63 \\ \cline{2-9}  & A(3,5) & 0.66 & 0.59 & 0.88 & 48.20 & 0.83 & 0.79 & 1.14 & 46.41 \\ \cline{2-9}  & A(2,5) & 0.52 & 0.66 & 0.84 & 38.23 & 0.66 & 0.99 & 1.19 & 33.69 \\ \cline{2-9}  & A(2,6) & 0.61 & 0.76 & 0.97 & 38.75 & 0.48 & 1.00 & 1.11 & 25.64 \\ \cline{2-9}  & A(1,6) & 0.43 & 0.66 & 0.78 & 33.08 & 0.30 & 1.09 & 1.13 & 15.38 \\ \hline \end{tabular}
\end{table} 
Table 3: Analysis of flow characteristics using CFD and flow visualization experiment results. Figure 14: Comparison of results according to flow characteristics: (**a**) area of high-velocity flow rising through the slope of a coal pile; (**b**) recirculation area adjacent to the inlet; (**c**) fluid stagnation region at the top of the coal pile.