# Experimental Study on the Heat Transfer Performance of Pump-Assisted Capillary Phase-Change Loop
## 3 Results  
### Start-Up Characteristics  
The start-up characteristics of a pump-assisted capillary phase-change loop under liquid-vapor pressure differences \(\Delta p_{\text{LV}}\) = 0, 2, 4, 6, 8, and 16 kPa are shown in Figure 2(a)-_f_, respectively. By comparison, it was found that when \(\Delta p_{\text{LV}}\) = 0 kPa (Figure 2(a)), the start-up characteristics  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Parameter** & **Unit** & **Value** \\ \hline Inlet liquid temperature, \(T_{\text{cc}}\) & \({}^{\circ}\)C & 10 \\ Heat sink temperature, \(T_{\text{sink}}\) & \({}^{\circ}\)C & 5 \\ Heating power, \(Q\) & W & 10–80 \\ Heat flux, \(q\) & W/cm\({}^{2}\) & 2.5–20 \\ Liquid-vapor pressure & kPa & 0–22 \\ difference, \(\Delta p_{\text{LV}}\) & & \\ \hline \hline \end{tabular}
\end{table}
Table 3: Test conditions.  
of the phase-change loop are different from those at \(\Delta p_{\rm LV}>0\) kPa. With the increase in heating power, the outlet temperature \(T_{\rm v}\) of the vapor collector increased first, was maintained at 65 \({}^{\circ}\)C (saturation state), and finally increased to about 80 \({}^{\circ}\)C (overheating state). The heat transfer modes of the copper baseplate can be divided into four types: flooded, partially flooded, thin-film evaporation, and overheating according to the change in \(T_{\rm v}\). From the perspective of stable temperature (horizontal section), the heat transfer process was dominated by single-phase convection under low heating power, and the heat transfer efficiency was low with a relatively higher temperature of the copper baseplate \(T_{\rm eva}\). At that time, the heat transfer modes were flooded and partially flooded. With the increase in heating power, the heat transfer mode changed to efficient thin-film evaporation. With the increase in heating power, the increased magnitude of \(T_{\rm eva}\) decreased. When the heating power exceeded 38 \(W\), the liquid could not completely infiltrate the copper baseplate, part of the vapor channel began to be exposed, and the vapor at the outlet of the vapor collector became overheated. At that point, the heat transfer mode was overheating. As for the time required to reach stable temperature (rise section), it took more than 500 s under flooded and partially flooded modes, but less than 100 s under thin-film evaporation mode.  
When the liquid-vapor pressure difference increased to above 2 kPa, the liquid replenishment capacity was enhanced and overheating mode was avoided. Meanwhile, with the increase in liquid-vapor pressure the copper baseplate temperature \(T_{\rm eva}\) decreased under the same heating power. In addition, the difference between \(T_{\rm eva}\) and the vapor collector temperature \(T_{\rm v}\) under high power decreased, indicating that the liquid-vapor pressure difference could enhance the heat dissipation capacity of the phase-change loop.  
Additionally, according to the start-up characteristics of the phase-change loop under different liquid-vapor pressures, the copper baseplate temperature \(T_{\rm eva}\) was relatively stable under both low and high power, without temperature fluctuation or temperature overshoot, indicating that the phase-change loop could eliminate two-phase flow instability.