# Experimental Study on the Heat Transfer Performance of Pump-Assisted Capillary Phase-Change Loop
## 3 Results
### Steady-State Heat Transfer Characteristics
#### 3.2.3 Regime Map of Different Heat Transfer Modes  
The heat transfer modes of the baseplate fundamentally depended on the balance between evaporation and liquid replenishment. The evaporation was related to heating power, while the liquid replenishment was related to the driving force (liquid-vapor pressure difference and capillary force). When the liquid replenishment was much greater than the evaporation, the heat transfer process was dominated by convection, leading to flooded or partially flooded states. However, when the liquid replenishment slightly exceeded the evaporation, a stable evaporating meniscus could be maintained within the vapor channel and the liquid could completely wet the vapor channel, leading to thin-film evaporation mode. When the liquid replenishment was slightly lower than the evaporation, the liquid bridge between the liquid and capillary wick in the vapor channel would disappear, and a part of the vapor channel would expose and overheat the generated vapor. At that time, the heat transfer mode was overheating.  
From the above discussion, thin-film evaporation mode is superior to the other three heat transfer modes in heat transfer efficiency and start-up speed, and is also the most desirable state in engineering applications. As for the copper baseplate, its heat transfer mode is closely related to the liquid-vapor pressure difference and heating power. A two-dimensional heat transfer mode distribution diagram recording both liquid-vapor pressure variation and heating power was drawn after a number of experiments, as shown in Figure 6. Under a high liquid-vapor pressure difference or low heating power, the liquid replenishment capacity was high, the heat transfer was dominated by single-phase convection, and the heat transfer modes were mainly flooded and partially flooded. Under low liquid-vapor pressure or high heating power, the liquid replenishment capacity was low while the evaporation rate was high and the heat transfer mode would switch to overheating. The thin-film evaporation mode lay between the two, with its distribution area in the shape of a horn. In engineering applications, the liquid-vapor pressure difference could be regulated dynamically according to the heat transfer mode distribution diagram in order to maintain an efficient thin-film evaporation mode at a certain heating power.  
Figure 5: Variation in heat transfer coefficient under different liquid-vapor pressure difference and heating powers.  
The variation of in resistance of the phase-change loop with liquid-vapor pressure and heating power differences is shown in Figure 7. The thermal resistance decreased with the increase in heating power under various pressure differences. When the power was below 40 W, thermal resistance constantly decreased with the increase in liquid-vapor pressure. For example, when the liquid-vapor resistance decreased from 6.0 K/W to 1.8 K/W, a decrease of 70%, which indicates that heat transfer could be significantly enhanced by increasing the liquid-vapor pressure difference at lower power. When the power was higher than 40 W and the liquid-vapor pressure difference exceeded 8 kPa and the thermal resistance curves fairly coincided, indicating that the thermal resistance could not be reduced at all times by increasing the liquid-vapor pressure under high power.