A quick measurement method of heat transfer coefficient and friction factor for finned tube bundle of the air cooler under large temperature difference

The annular finned tube bundle of air cooler serves as an essential heat exchanger to perform passive decay heat removal from the reactor core to the ultimate heat sink in the NHR200-II small modular nuclear reactors. The performance of heat transfer and flow resistance is crucial for nuclear safety as a finned tube bundle operates under natural circulation involving a significant temperature difference between the inlet and outlet. Therefore, a new quick method of experimental measurement is proposed in this article to determine and verify the heat transfer coefficient and friction factor of the finned tube bundle, and the theoretical true mean temperature difference is established from the exact energy conservation relation. The new measurement method is universal to various tube bundle types, eliminating the need for time-consuming preparation of steady heat state and expensive devices of high-pressure and high-temperature water. Additionally, it allows for the acquisition of abundant data within a relatively short period. This work has conducted 131 tests across an extensive variety of Reynolds numbers (Re: 1176 - 4813), Prandtl numbers, inlet temperatures, and powers to verify the applicability of the empirical correlations used in design. Furthermore, this article proposes a suitable empirical correlation of heat transfer coefficient based on experimental data for the crucial natural circulation range (Re: 1176 - 2500). Considering the large inlet-outlet temperature differences in natural circulation range, a new empirical correlation of friction factor is provided to address the limitations of existing correlations derived under isothermal conditions. This quick measurement method shows excellent agreement with the classic Briggs-Young correlation within a valid range, confirming its correctness and accuracy.