## 3 Experimental Approach  
A schematic diagram of the airflow channel of the present study is depicted in Figure 4, including mainly a centrifugal fan, two-stage flow straighteners, and a heat exchanger \begin{table}
\begin{tabular}{c c c c c c c c c} \hline
**Fin Surface Properties** & **Uncoated Aluminum Fin** & \multicolumn{4}{c}{**Hydrophilic Aluminum Fin**} & \multicolumn{4}{c}{**Hydrophobic Aluminum Fin**} \\ \hline Pitch (mm) & 1.5 & 2 & 2.5 & 1.5 & 2 & 2.5 & 1.5 & 2 & 2.5 \\ \hline Fin thickness (mm) & 0.1 & 0.1 & 0.1 & 0.1 & 0.1 & 0.1 & 0.1 & 0.1 \\ \hline Number of fins (piece) & 97 & 78 & 72 & 107 & 77 & 63 & 102 & 74 & 63 \\ \hline Number of fins per inch (FPI) & 16.3 & 13.1 & 12.1 & 18.0 & 13.0 & 10.6 & 17.3 & 12.4 & 10.7 \\ \hline Heat exchanger height (mm) & 154 & 154 & 154 & 154 & 154 & 154 & 154 & 154 \\ \hline Heat exchanger width (mm) & 170 & 170 & 170 & 170 & 170 & 170 & 170 & 170 \\ \hline Heat exchanger thickness (mm) & 38 & 38 & 38 & 38 & 38 & 38 & 38 & 38 \\ \hline \end{tabular}
\end{table} 
Table 1: Sizes of heat exchanger fins. Figure 3: Dimensions of heat exchangers with different fin spacing (unit: mm). section with a mirror water tray for collecting the water condensate. The flow channel was an open-type facility with a rectangular cross-sectional area made of acrylic plates. The dimensions of the channel were 1420 mm (length) by 170 mm (width) by 154 mm (height). The channel was designed in such a way that different heat exchangers could be installed for various tests. A fan of 135 W (TF 150, Shenzhen, China) was used to deliver the airflow. The temperatures of the cooling air and chilled water were measured by Rotronic HC2a-s (accuracy of \(\pm\)0.15 \({}^{\circ}\)C) and Sensor PT100a (accuracy of \(\pm\)0.15 \({}^{\circ}\)C), respectively. Air humidity was also measured by Rotronic HC2a-s (accuracy of \(\pm\)1%). The airspeed and pressures were measured by Sailors' tf100 (accuracy of \(\pm\)3%) and Nagono Keiki GC52 (accuracy of \(\pm\)1%), respectively. Industrial experience indicates that water splash could occur in the datacenters with the indoor temperature and humidity being 24.5 \({}^{\circ}\)C and 76%RH, respectively. Hence, the present test environment was kept at the condition of constant temperature and humidity of 24 \({}^{\circ}\)C and 80%RH, respectively, by using PID controllers to control the electric humidifier and heater while maintaining the chilled water inlet temperature at 7 \({}^{\circ}\)C.  
The air-side heat transfer rate was obtained using the air enthalpy difference deduced by the measured airspeed, the dry bulb temperature, and the relative humidity according to ASHRAE Handbook [20]. The dry bulb temperature and the relative humidity were used to determine the enthalpy and heat capacity properties of the air for the calculation. The amount of the splashed water was determined by averaging the water droplet weights which were collected continuously by the water tray (denoted by the symbol m in Figure 4) for five minutes. The experimental procedure is illustrated in Figure 5. In the beginning, the test chamber was warmed up for at least 30 min to reach the steady state environmental conditions of 24 \(\pm\) 0.1 \({}^{\circ}\)C/80 \(\pm\) 2%RH and the steady state inlet chilled water temperature of 7 \({}^{\circ}\)C. Then, both the chilled water and the centrifugal fan system were turned on. Afterwards, the airspeed in front of the heat exchanger in the test channel was adjusted to the targeted steady testing speed of 2.5, 3, 3.5, 4, 4.5, and 5 m/s, respectively, by controlling the rotational speeds of the fan. At these air speeds, the maximum turbulence intensity of  
Figure 4: A schematic diagram of the experimental facility (unit: mm). the flow determined by a hot wire (with an over-heat ratio of 0.7) was about 1.2%. Under these steady conditions, the weight of the condensate splashed and accumulated for five minutes into the water tray was measured. In the process, all the pressures, airspeeds, and temperatures were measured for further analysis.