# Experimental Study on the Heat Transfer Performance of Pump-Assisted Capillary Phase-Change Loop
## 2 Experimental System  
### Experimental Apparatus  
Figure 1 presents a system sketch of a pump-assisted capillary phase-change loop. The phase-change loop consists of an evaporator, liquid tank, diaphragm pump, vapor/liquid pipelines, simulated heating system, valve, temperature/pressure sensors, data acquisition system, etc. The working medium methanol was stored in the liquid tank, and its temperature controlled by an industrial cooler. The diaphragm pump delivered the liquid working medium through the liquid line to the upper compensation chamber of the evaporator. A portion of the liquid entered the baseplate of the evaporator for heat dissipation through the capillary wick, while the remaining liquid flowed out the other end of the compensation chamber to the liquid tank. After entering the baseplate of the evaporator, the liquid absorbed heat and phase-change occurred, and then it flowed out of the gas pipeline to the liquid tank for condensation. The remaining liquid and vapor flowed into the upper space of the liquid tank. Pressure measuring points were set at the outlets of both the compensation chamber and the vapor collector to monitor both the pressure of subcooled liquid \(p_{\mathrm{L}}\) and the pressure of hot fluid \(p_{\mathrm{V}}\). High precision regulating valves were installed on the liquid lines at both the inlet and outlet of the compensation chamber to regulate the liquid pressure of the compensation chamber, which made it possible to control the liquid-vapor pressure difference between both sides of the capillary wick to \(\Delta p_{\mathrm{LV}}=p_{\mathrm{L}}-p_{\mathrm{V}}\). The thermophysical properties of the working medium methanol are shown in Table 1.  
During the experimental procedure, the cooler was started first and the pump was started as soon as the liquid tank temperature reached 10 \({}^{\circ}\)C. Secondly, the liquid pressure \(\Delta p_{\mathrm{LV}}\) was adjusted to a certain value by regulating valves and the heating system was started to generate joule heat on the heating plate. Finally, the heating power was gradually increased when the baseplate temperature reached a steady state. When the heating plate \begin{table}
\begin{tabular}{c c c} \hline \hline
**Parameter** & **Unit** & **Value** \\ \hline Boiling point & \({}^{\circ}\)C & 64.7 \\ Freezing & \({}^{\circ}\)C & \(-97.8\) \\ Kinematic viscosity & mPa-s & 0.59 \\ Latent heat & kJ/kg & 1109 \\ Surface tension & mN/m & 22.6 \\ Specific heat capacity & kJ/kg\(\cdot\)K & 2.51 \\ Density & kg/m\({}^{3}\) & 792 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Thermophysical properties of methanol at room temperature and pressure. Figure 1: Schematic diagram of the pump-assisted capillary phase-change loop. temperature reached the limit of the material (180 \({}^{\circ}\)C in the present experiment), one group of experiments ended. The next group of experiments was started by adjusting the liquid pressure \(p_{\text{L}}\) to another certain value. The evaporator is a core part of the pump-assisted capillary phase-change loop, including an upper cover plate, capillary wick, copper baseplate, heating plate, and support plate reaching from top to bottom, as shown in Figure 2. The compensation chamber, liquid inlet/outlet channels, and vapor collector were set on the upper baseplate, while pressure measuring points were set on the sides of the compensation chamber to monitor fluid pressure. The capillary wick was located between the upper cover plate and copper baseplate and embedded directly below the compensation chamber. The copper baseplate was close to the capillary wick, with a rubber ring used to achieve a fluid seal. The heating plate was attached to the copper baseplate using thermally conductive silicone grease. The main dimensions of the evaporator are shown in Table 2.  
The capillary wick was rectangular, with dimensions of 23 \(\times\) 23 \(\times\) 5 mm\({}^{3}\).