## 4 Design of Stand Fan and Simulation Results
### Integrated Analysis of Stand Fan  
Integration of the optimal impeller and reference protective cover are denoted as the redesign fan #1. Also, the flow field and performance between the reference fan and the redesign fan #1 are  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Case**} & \multirow{2}{*}{**Setting Angle (\({}^{\circ}\))**} & \multicolumn{2}{c}{**Twist Angle (\({}^{\circ}\))**} & \multirow{2}{*}{**Flow Rate (CFM)**} & \multirow{2}{*}{**Torque (N-m)**} & \multirow{2}{*}{**Efficiency (CFM/N-m)**} \\ \cline{3-3} \cline{5-6}  & & & & & & \\ \hline A & 55 & 35 & 45 & 2074 & 0.238 & 8714 \\ B & 55 & 45 & 35 & 2174 & 0.267 & 8142 \\ \hline \hline \end{tabular}
\end{table}
Table 2: The simulation results of impellers with a setting angle of 55\({}^{\circ}\).  
Figure 4: The velocity distributions at the fan outlet for the reference impeller and Case A impeller. (**a**) References impeller; (**b**) impeller with 55\({}^{\circ}\) setting angle (Case A).  
simulated and analyzed carefully. The CFD outcomes show that the max flow rate of redesign fan #1 is 2037 CFM, which is equivalent to a 40% increase compared with the original reference fan (see Table 4). Furthermore, it is observed that the outlet flow field of the redesign fan #1 has a more uniform flow pattern with a broad distribution of high-speed air as shown in Figure 5.  
Besides, Figure 6 illustrates the reversed flow identified near the frame of the protective cover and the outlet center of the fan. These flaws can be improved after a series of design modifications on the protective cover. The numerical results indicate that the flow rate decreases with an increasing frame width. Furthermore, when the frame is moving closer to the blade tip, the flow rate is enlarging and the size of the reverse flow is becoming smaller. Therefore, for the case of the frame's width and spacing between the frame and blade tip assigned as 20 and 10 mm, the max flow rate of this fan would enlarge to 2067 CFM (about a 2.5% increase) while little reverse flow is identified in the associated flow filed.  
In order to improve the adverse flow field distribution in the above redesign fan and its performance, the ribs of the protective cover are considered for redesign later. The frame rib with a trapezoid cross section can be classified into three types including straight, counterclockwise and clockwise (see Figure 7). The simulation results demonstrate that the clockwise type delivers a better flow rate at 2075 CFM (about a 2.9% rise).  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Case** & **Flow Rate (CFM)** & **Percentage (\%)** & **Torque Needed (N-m)** & **Max. Air Speed (m/s)** \\ \hline Reference fan & 1436 & - & 0.099 & 2.72 \\ Redesign fan \#1 & 2017 & 40.5\% & 0.248 & 1.95 \\ \hline \hline \end{tabular}
\end{table}
Table 4: Simulation results for the reference fan and the redesign fan #1 with the reference cover.  
Figure 5: Simulated velocity distributions on the monitoring surface of the reference fan and redesign fan #1 with cover. (**a**) References fan; (**b**) redesign fan #1.  
Figure 6: The flow field between the protective cover and the impeller for the redesign fan #1.  
Besides, an appropriate trapezoid angle of the frame grid can be helpful in guiding the air stream to flow outward in a more normal direction. The simulation results find that the trapezoid angle in the range of 30\({}^{\circ}\)-35\({}^{\circ}\) is the optimized parameter of the protective cover with 61 ribs. This optimal fan constructed by integrating the optimal impeller and the new protective cover with an appropriate set of design parameters can generate a uniformly distributed outflow (see Figure 8) and deliver a maximum flow rate at 2126 CFM, which represents a 48% enhancement over the reference fan's performance.