## 2 Experimental Setup
### Compressor Test Rig  
Water is a product of the PEMFC working process and is also necessary for humidity-ing air. In addition, the conversion efficiency of PEMFCs is about 50%, which means a large amount of heat will be generated during the working process. And the heat dissipation problem can be solved by the water management system conveniently. Therefore, there is a \begin{table}
\begin{tabular}{c c} \hline
**Parameters** & **Value** \\ \hline Lobes of the rotor (male/female) & 4/6 \\ Center distance between the rotors (mm) & 80 \\ Length of the rotor (mm) & 110 \\ Male wrap angle (\({}^{\circ}\)) & 220 \\ Outside diameter of the female rotor (mm) & 100 \\ Outside diameter of the male roto (mm) & 103 \\ Working voltage range (VDC) & 450–720 \\ Rated power (kW) & 8.9 \\ Ultimate pressure ratio & 2.2 \\ \hline \end{tabular}
\end{table} 
Table 1: Technical and design parameters of the compressor. Figure 1: Appearance diagram of the FCN155 fuel cell compressor. strong correlation between water and PEMFC. Therefore, it is more suitable for the cooling jacket of the dry screw compressor used in PEMFC to adopt water as the working medium than other liquids such as oil and ethylene glycol. Therefore, the test rig only uses water as the cooling liquid. The schematic diagram of the test rig used to test the cooling effect of the cooling water jacket of the dry twin-screw air compressor are shown in Figure 2. The test rig comprises a power supply system, control system, water cooling system and data acquisition system surrounding the integrated compressor, DC motor, controller, and intercooler mentioned above. The power supply system is relatively simple, mainly composed of a DC power supply for the screw compressor and a 24 V DC power supply for the data measurement system. The function of the former is to convert the input 380 V three-phase electricity into the output 450-720 V DC electricity, which is then input into the socket located on the controller to drive the DC motor. And the latter is responsible for supplying power to the sensors, flowmeters, and controller of the FCN155 fuel cell compressor. The control system mainly consists of a computer, CAN analyzer, and controller. The computer utilizes CANTest software (version 2.7) and sends control commands to the controller through the relay of the CAN analyzer to achieve the functions of switching and controlling speed. The water supply system is relatively complex, consisting of a water storage tank, a water pump, a turbine flowmeter, an air-cooled water cooler, and a water filter. Water is stored in a tank, and circulating power is provided by a water pump. The flow rate of cooling water is regulated by a ball valve in the water circuit and a bypass valve on the water pump and measured by a turbine flowmeter. After entering the water jacket of the compressor, cooling water will take away the heat generated by air compression and enter the water storage tank. Subsequently, cooling water is sent by the water pump to the air-cooled water cooler and is filtered by the water filter before returning to the compressor. The data measurement system consists of various instruments used to measure data such as temperature, pressure, flow rate, etc. Detailed information about these measurement instruments is listed in Table 2.  
Figure 2: Schematic diagram of the test rig. Figure 3 shows photos of the test rig. As shown in the photo, (a) is the exhaust pipeline of the compressor, which is mainly composed of a vortex flow meter, valves, and muffler. In traditional compressor experiments, the measurement of volumetric flow rate is usually completed by a nozzle flowmeter. However, the volumetric flow measurement involving fuel cell systems usually uses vortex flow meters, so the test rig in this paper adopted it instead of a nozzle flowmeter. The pressure-regulating valve is used to adjust the discharge pressure, and the ball value can change the air pressure in the vortex flowmeter to increase its measurement upper limit to a certain extent.