# Experimental Study on the Heat Transfer Performance of Pump-Assisted Capillary Phase-Change Loop
## 4 Conclusions  
In order to solve the problem encountered by the traditional boiling heat dissipation technology, i.e., two-phase flow instability, a pump-assisted capillary phase-change loop with methanol as the working medium was established. The effect of liquid-vapor pressure difference and heating power on the start-up characteristics and steady-state heat transfer characteristics was studied. The conclusions are as follows:  
1. There was no temperature fluctuation in the starting process under various liquid-vapor pressure differences and powers, indicating that the pump-assisted capillary phase-change loop overcame the two-phase flow instability. There were four modes  
Figure 6: Two-dimensional heat transfer mode distribution diagram.  
Figure 7: Variation in thermal resistance under different liquid-vapor pressure differences and heating powers.  
of heat transfer, including flooded, partially flooded, thin-film evaporation, and overheating. It took the shortest time (i.e., within 100 s) to start in thin-film evaporation mode, while it took more than 500 s in the other three modes.
2. Both liquid-vapor pressure difference and power were not only important to the heat transfer modes, but also determined the steady-state heat transfer performance of the phase-change loop. The heat transfer coefficient could be significantly improved and the thermal resistance reduced by increasing liquid-vapor pressure as long as it did not exceed 8 kPa. For instance, if the liquid-vapor pressure difference increased from 0 to 8 kPa while the heating power remained at 30 W, the heat transfer coefficient could be improved by nearly 27 times and the thermal resistance reduced by 37%. When the liquid-vapor pressure difference exceeded 8 kPa, the temperature power curve and thermal resistance power curve fairly coincided under high heating power. At that time, if the liquid-vapor pressure differential continued to increase, the heat transfer enhancement effect decreased and more pumping power resulted.
3. The thin-film evaporation mode with the maximum heat transfer efficiency was the most desirable heat transfer state in engineering applications. A two-dimensional heat transfer mode distribution diagram depicting both liquid-vapor pressure difference and heating power was created after a number of experiments. The thin-film evaporation mode was between flooded/partially flooded and overheating in the shape of a horn. During engineering applications, the difference in liquid-vapor pressure could be controlled to maintain efficient thin-film evaporation at the baseplate of the evaporator to achieve the optimum heat dissipation effect.
4. The capillary wick is a core part of the evaporator. Therefore, capillary wick material and its primary performance parameters (pore size, porosity, and permeability), as well as the type of its adhesion to the baseplate--which has an important effect on the heat transfer performance of the phase change loop--deserve in-depth and systematic research in the future.  
Writing--original draft preparation, X.Y. and G.W.; writing--review and editing, C.Z.; investigation, data curation, J.L.; conceptualization, project administration, funding acquisition, X.Y. and J.W. All authors have read and agreed to the published version of the manuscript.  
The research was supported by the National Natural Science Foundation of China, grant numbers 52006166 and 51961135102; and the Fundamental Research Funds for the Central Universities, grant number XJH012020040.  
We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, there is no conflict of interest in the manuscript entitled \"Experimental study on the heat transfer performance of pump-assisted capillary phase-change loop\".