Impact of working fluid properties on heat transfer and flow characteristics of two-phase loop thermosyphon with high filling ratios

The two-phase loop thermosyphon (TPLT), known for its excellent heat transfer performance, simple and compact structure, and lack of need for a pump, effectively addresses heat transfer challenges in confined spaces under high heat loads.

Compared to TPLT with low filling ratio, high-filling-ratio TPLT not only exhibit a higher maximum heat transfer capacity but also has a more complex heat and mass transfer process, leading to increased sensitivity to the working fluid's properties. Therefore, studying the impact of the working fluid on the operational state of high-filling-ratio TPLTs is crucial for understanding their heat transfer mechanisms. In this paper, comprehensive experiments were conducted on TPLT filled with H₂O and R134a as working fluids in a wide filling ratio range (30 % -90 %), and their heat transfer performance and flow characteristics were compared. Heat transfer diagram, two-phase flow pattern diagram, and the distribution of gas-liquid two-phase of the TPLT was established with different filling ratio and heat input. Due to differences in latent heat of vaporization, the maximum heat transfer capacity of the H₂O-TPLT (390 W/cm²) is greater than that of the R134a-TPLT (270 W/cm²). In the H₂O-TPLT, the predominant large-volume slug flow leads to significant flow resistance. Whereas in the R134a-TPLT, the flow pattern is primarily dominated by small-volume bubbly flow and churn flow, resulting in low resistance. High viscosity and flow pattern in the H₂O-TPLT cause oscillation phenomena, leading to significant temperature and pressure fluctuations. Under high filling ratio, both types of TPLT experiences geyser boiling phenomena causing periodic temperature and pressure fluctuations and flow pattern changes. In summary, R134a is the preferred working fluid when heat transfer requirements are met, as it effectively reduces temperature fluctuations while dissipating heat. When exceeding the R134a-TPLT's maximum heat transfer capacity and less stringent temperature control is acceptable, H₂O may serve as the working fluid.