Flow boiling heat transfer mechanisms and flow characteristics of pump-driven two-phase flow systems used in data center cooling

• 1.For 11 mm scale and little dryness area, flow boiling mechanism was nucleate boiling. • 2.Threaded tube had little strengthening effect in our experiment condition. • 3.The inside tube boiling and flow characters of R410A were better than R134a. Pump-driven two-phase flow systems are commonly used in data center cooling owing to their outstanding heat-transfer performance. The evaporator is a key component of the cooling system, and its design significantly affects the overall cooling efficiency of the system. The boiling heat transfer process in the evaporator is primarily affected by the boiling heat transfer mechanism. Such a mechanism is mainly influenced by the type of coolant, inner wall structure of the tube (threaded tube or smooth tube), heat flux, and mass flow rate. In this study, we experimentally investigated the heat transfer mechanisms, capacity, and flow characteristics of the boiling process in a pump-driven two-phase flow system commonly used in data center cooling. The heat transfer capacity involved in this experiment was 140–1150 W, and the mass flow rate was 40–270 kg/h. Different coolants (R134a and R410A) and tube structures (threaded and smooth) were tested. The main novelty of this study is as follows. First, under the working conditions in this experiment, the inside-tube flow boiling mechanism was nucleate boiling for both the smooth and threaded tubes, and the boiling heat transfer coefficient remained constant as the mass flow rate decreased and vapor quality increased. Second, although it is commonly believed that a threaded pipe can significantly enhance the heat transfer ability, our study found that under data center cooling applications, the threaded tube had little strengthening effect on the heat transfer performance. Third, commonly used coolants, including R134a and R410A, were tested and the above conclusions were verified.