Investigations on thermal interface and its impact on heat transfer performance in a large-scale water tank

Limited by space height in the underwater nuclear-power device, the passive residual heat removal (PRHR) heat exchanger has to be arranged horizontally at the middle height of the water tank. This unconventional spatial arrangement and complex heat release process coupled with a low-temperature water environment make the thermal-hydraulic characteristics and heat transfer performance of the water tank specific compared with the regular PRHR water tank studied before. Therefore, a large hemispherical water tank with a horizontal C-shape heat exchanger and a cooling water jacket has been set up. A special visible thermal interface with a steep temperature gradient is observed under the heat exchanger, which harms the residual heat removal ability and inherent safety of the reactor core. To study the forming mechanism and movement feature of the thermal interface, numerical simulations and experiments on the effects of the cold source, heat load power, and initial water temperature are conducted. The results show that a particular flow field that a large natural convection circulation shearing above a stagnant zone induces the thermal interface. The stable interface restrains the cross heat and mass transfer, so the inner wall heat transfer coefficient (HTC) of the upper wall above the thermal interface is 5 times larger than that of the bottom part. A passive heat transfer enhancement structure equipped to the inner wall is designed and evaluated, the experimental data show that the thermal stratification degree is impaired apparently and the overall inner-wall-averaged HTC is improved by 41.6 %, ensuring nuclear system safety.