## 2 Experimental Method
### Wind Tunnel Tests  
The wind tunnel test table shown in Figure 3 was used to test the heat transfer and flow performance of three different flow channels. The test table is mainly composed of an air circulation system, water circulation system, test part, control unit and data acquisition system. The Reynolds number range during the test was 1200-5100, and the Reynolds number range is mainly suitable for heavy-duty vehicle use conditions. In order to prevent the change in the Reynolds number on the water side from affecting the experimental results, the water flow was kept constant during the experiment, and the Reynolds number on the water side was constant at \(Re=2432\). \begin{table}
\begin{tabular}{c c c c c} \hline
**NO.** & \(\mathbf{L_{f}/mm}\) & \(\mathbf{S_{f}/mm}\) & \(\mathbf{F_{h}/mm}\) & \(\mathbf{t_{f}/mm}\) \\ \hline
1 & 60 & 3.0 & 9.0 & 0.4 \\
2 & 60 & 3.5 & 9.0 & 0.4 \\
3 & 60 & 4.0 & 9.0 & 0.4 \\
4 & 55 & 3.0 & 9.0 & 0.4 \\
5 & 55 & 3.5 & 9.0 & 0.4 \\
6 & 55 & 4.0 & 9.0 & 0.4 \\
7 & 45 & 3.0 & 8.0 & 0.3 \\
8 & 45 & 3.5 & 8.0 & 0.3 \\
9 & 45 & 4.0 & 8.0 & 0.3 \\
10 & 45 & 3.0 & 9.0 & 0.3 \\
11 & 45 & 3.0 & 11.0 & 0.3 \\ \hline \end{tabular}
\end{table} 
Table 1: Parameter table for equal cross-section flow channels. \begin{table}
\begin{tabular}{c c c c c c} \hline
**NO.** & \(\mathbf{F_{h}/mm}\) & \(\mathbf{R/mm}\) & \(\mathbf{S_{f}/mm}\) & \(\mathbf{L_{f}/mm}\) & \(\mathbf{t_{f}/mm}\) \\ \hline
1 & 9.5 & 20 & 4.0 & 60 & 0.4 \\
2 & 10.5 & 20 & 4.0 & 60 & 0.4 \\
3 & 11.5 & 20 & 4.0 & 60 & 0.4 \\
4 & 9.5 & 30 & 4.0 & 60 & 0.4 \\
5 & 10.5 & 30 & 4.0 & 60 & 0.4 \\
6 & 11.5 & 30 & 4.0 & 60 & 0.4 \\
7 & 9.5 & 40 & 4.0 & 60 & 0.4 \\
8 & 10.5 & 40 & 4.0 & 60 & 0.4 \\
9 & 11.5 & 40 & 4.0 & 60 & 0.4 \\ \hline \end{tabular}
\end{table} 
Table 2: Parameter table for trapezoidal flow channels. \begin{table}
\begin{tabular}{c c c c c c} \hline
**NO.** & \(\mathbf{F_{h}/mm}\) & \(\mathbf{R/mm}\) & \(\mathbf{S_{f}/mm}\) & \(\mathbf{L_{f}/mm}\) & \(\mathbf{t_{f}/mm}\) \\ \hline
1 & 9.5 & 200 & 4.0 & 60 & 0.4 \\
2 & 10.5 & 200 & 4.0 & 60 & 0.4 \\
3 & 11.5 & 200 & 4.0 & 60 & 0.4 \\
4 & 9.5 & 250 & 4.0 & 60 & 0.4 \\
5 & 10.5 & 250 & 4.0 & 60 & 0.4 \\
6 & 11.5 & 250 & 4.0 & 60 & 0.4 \\
7 & 9.5 & 300 & 4.0 & 60 & 0.4 \\
8 & 10.5 & 300 & 4.0 & 60 & 0.4 \\
9 & 11.5 & 300 & 4.0 & 60 & 0.4 \\ \hline \end{tabular}
\end{table} 
Table 3: Parameter table for circular-arc-shaped flow channels. Water and air acted as a working medium for heat exchange. A T-type copper-constantan thermocouple was used to measure the air temperature of the inlet and outlet in the air circulation loop, and its calibration accuracy is 0.1 \({}^{\circ}\)C. The numbers of thermocouples at the inlet and outlet are 12 and 36, respectively, and the locations of thermocouples inside the rectangular duct were established following the method recommended by ASHRAE [23] standard. The temperature value at the inlet is the average value obtained by 12 thermocouples, and the temperature value at the outlet is the average value measured by 36 thermocouples. Air velocity was measured by an inclined pressure gauge and pitot tube assemblies. The model of the inclined pressure gauge is YYT-2000B and its measuring range is 0-2000 Pa. The pressure drops of the two heat transfer units were measured with a U-tube manometer. The equal power heating method described in [24] was adopted to heat the hot water bath to reduce the heat balance and experiment time. The water pipe, test channel and test unit were wrapped with 10 mm heat insulation materials for insulation. The mass flow rate on the water side was measured by a pulsed flow transducer with an accuracy of 0.0025 \(l/s\), which is a high-frequency liquid digital turbine flowmeter.