Numerical simulation on the heat transfer characteristics of two-phase loop thermosyphon with high filling ratios

Technological advances have allowed the integration of electronic devices with various types of equipment to enhance performance and functionality. However, the increased integration capacity comes with several drawbacks, the main one being the increase in the heat per unit area. For example, the high-performance processors used in computer processing units can generate high thermal power and heat flux exceeding 250 W and 10 W/cm², respectively, which is projected to increase in the future. Therefore, it is important to keep the operating temperature within the desired range in order to ensure efficient and stable operation of electronic equipment.

Heat dissipation of electronic devices is undoubtedly the main factor limiting further technological development in many fields, as the available heat dissipation technology fails to meet the required standards. Among the available heat transfer devices, the two-phase loop thermosyphon (TPLT) is a promising candidate for achieving effective heat management, equipment stability, and reliability. Its advantages include simple structure, low cost, and do not require external pumping devices. In fact, TPLT has been successfully used for thermal management in different fields like air conditioning and CPU cooling systems.

Heat transfer performance of TPLT is mainly influenced by the filling ratio (typically the volume of the working fluid relative to that of the evaporator). Although their heat transfer mechanism is extensively researched, most studies have mainly focused on TPLT with low filling ratios, which exhibits poor heat dissipation capabilities in high-power electronic equipment. On the other hand, TPLT with high filling ratios has better heat transfer performance and is thus a potential candidate for high heat flux dissipation application. However, their heat transfer mechanisms and phase change process characteristics remain elusive.

On this account, Dr. Kuiming Wang, Dr. Chengzhi Hu, Dr. Bo Jiang, Dr. Xianfeng Hu, and Professor Dawei Tang from the Dalian University of Technology investigated the heat transfer characteristics of TPLT with both low and high filling ratios using a newly established computational fluid dynamics (CFD) model. Specifically, the CFD model was used to simulate the condensation and boiling processes considering the relationship between the pressure and saturation temperature at both low and high filling ratios. Additionally, the effects of various parameters on the heat transfer performance of TPLT were studied. Their work is currently published in the International Journal of Heat and Mass Transfer.

The authors showed that the heat transfer capacity of TPLT at high filling ratios was remarkably more substantial than that at low filling ratios. Similarly, there was a significant difference in the heat transfer process at high and low filling ratios. Two flow patterns were reported: bubbly flow and slug flow. The former occurred under low filling ratios in the heat section dominated by the pool boiling heat transfer mechanism, while the latter was characterized by dropwise condensation in the condensation section.

At high filling ratios, a smoother circular flow than at low filling ratios was obtained because a higher filling ratio allowed easy transfer of the vapor bubble-induced driving force from the left to right tube. Additionally, the extremely higher clockwise circulation velocity at high filling ratios resulted in increased average flow velocity, decreased thermal resistance and improved heat transfer performance of TPLT with high filling ratios. An increase in the filling ratio initially led to a sharp increase in the circulation velocity before it decreased slowly. The optimum heat transfer performance and highest velocity were recorded at a 75% filling ratio.

In summary, Dalian University of Technology scientists studied and uncovered the mechanisms responsible for good heat performance of TPLT with high filling ratios. The results showed a strong coupling between flow characteristics and heat transfer performance of the loop thermosyphon. Regarding heat transfer performance, the results confirmed the superiority of TPLT with high filling ratios over their counterparts with low filling ratios. In a statement to Advances in Engineering, Professor Dawei Tang stated that their findings clarified the heat transfer mechanism of TPLT with high filling ratios, which would increase their practical applications, especially high-power electronic devices.