Numerical Simulation of Entrained Bubbles Flow in the Shell-Tube Heat Exchanger of MSRs Based on Population Balance Model

In molten salt reactors (MSRs), a small amount of inert gas could be entrained from the free liquid surface to the primary loop, which may have obvious impacts on the heat transfer performance of the heat exchanger, the reactivity of the core, and the migration of insoluble fission products. It is necessary to understand how the bubbles flow into the reactor core, and what kinds of size distribution should they be. Meanwhile, the heat exchanger is an important and complicated place before the gas enters the core, in which the bubbles may exhibit complex behavior, such as coalescence, breakup, or retention. This research employs a coupling method between the Eulerian two-phase flow model (ETFM) and the population balance model (PBM) to simulate the two-phase flow of bubbles entrained in molten salt on the shell side of a vertical U-tube heat exchanger. The gas fraction and poly-dispersed bubble size distribution are analyzed under different calculation methods and input conditions. The results show that the distribution of gas volume fraction and bubble size are significantly influenced by the characteristics of the flow field, and the salt flow can also be affected by the bubble distribution. The bubbles exhibit obvious non-uniformity distribution, especially in the center of the separation and the backflow vortexes, and a significant accumulation of gas attachment occurs behind the baffles. The interfacial area concentration and surface heat transfer coefficient are also discussed with or without the bubble distribution. All indicate that a precise bubble spatial and size distribution is necessary when in the simulation of multiphase flow in MSRs.