# Experimental Study on the Heat Transfer Performance of Pump-Assisted Capillary Phase-Change Loop
###### Abstract  
To overcome the two-phase flow instability of traditional boiling heat dissipation technologies, a porous wick was used for liquid-vapor isolation, achieving efficient and stable boiling heat dissipation. A pump-assisted capillary phase-change loop with methanol as the working medium was established to study the effect of liquid-vapor pressure difference and heating power on its start-up and steady-state characteristics. The results indicated that the evaporator undergoes four heat transfer modes, including flooded, partially flooded, thin-film evaporation, and overheating. The thin-film evaporation mode was the most efficient with the shortest start-up period. In addition, heat transfer modes were determined by the liquid-vapor pressure difference and power. The heat transfer coefficient significantly improved and the thermal resistance was reduced by increasing liquid-vapor pressure as long as it did not exceed 8 kPa. However, when the liquid-vapor pressure exceeded 8 kPa, its influence on the heat transfer coefficient weakened. In addition, a two-dimensional heat transfer mode distribution diagram concerning both liquid-vapor pressure difference and power was drawn after a large number of experiments. During an engineering application, the liquid-vapor pressure difference can be controlled to maintain efficient thin-film evaporation in order to achieve the optimum heat dissipation effect. liquid cooling; phase-change loop; pressure difference; heat transfer enhancement +
Footnote †: journal: Applied Sciences  
in the fields of high heat flux chip cooling and battery thermal management, with the pump-driven micro/mini-channel cooling plate as its main form [6; 7; 8; 9; 10; 11]. However, the phase-change cooling system may become unstable in pressure, temperature, and flow during operation, which may not only reduces critical heat flux but also causes problems such as vibration, noise, and local hot spots [12; 13]. Therefore, overcoming two-phase flow instability in the phase-change cooling system has become a focus of research. For years, many scholars have carried out research on the mechanism of two-phase flow instability in a phase-change cooling system from both macro-scale [14; 15] and micro-scale channels [16; 17; 18; 19]. [PERSON] et al. [14] carried out an experimental study on the pressure pulsation caused by flow boiling in a horizontal pipe with a diameter of 10.9 mm and found three types of pulsation: pressure drop type, density wave type, and thermal type. According to the dynamic pressure test results, the pressure pulsation amplitude of pressure-drop type instability may reach 160 kPa with a frequency of less than 3 Hz, which may easily cause pipeline resonance. With significant advantages such as strong heat transfer capacity, compact structure, and easy integration, micro-channel flow boiling is promising in the thermal management of high heat flux electronic devices [20; 21]. However, the applications of two-phase flow are limited due to its instability. [PERSON] and [PERSON] [16] carried out synchronous visualization research. They found that the flow patterns of micro-channel flow boiling are closely related to temperature, pressure, and flow pulsation. Specifically, the alternate flow of liquid/vapor/liquid-vapor under high heat flux and low mass flux may cause strong pulsation, with a pressure fluctuation amplitude of up to 44 kPa and a temperature fluctuation of up to 229 \({}^{\circ}\)C. This shows there are many types of two-phase flow instability in the phase-change cooling system, which are closely related to the transition of flow patterns. Flow pattern is a special physical phenomenon of vapor phase dispersion in the liquid phase. The compressibility of liquid-vapor two-phase flow and the evolution of vapor volume fraction may cause fluctuations in both upstream and downstream flow field parameters (pressure, temperature, and flow rate).