Numerical simulation of saturated steam condensation heat exchange in a vertical channel

[Objective] Condensing heat exchange is a crucial process in the primary circuit of small modular reactors and passive safety systems that rely on natural circulation as the driving force. With the higher requirements for heat exchange efficiency and reactor safety, in-depth research and an understanding of the condensation heat exchange process are needed. Therefore, numerical simulations of the condensing heat exchange process have attracted increasingly more interest. However, due to the complex phase change, the condensing heat exchange process is difficult to model using analytical equations. Traditional numerical simulation methods use the empirical equations summarized in experiments, and their universalities are controversial. In contrast, the lattice Boltzmann method is a mesoscopic-level numerical simulation method that tracks particle clusters and uses probability density functions to describe their distribution, resulting in a simple and clear structure that appropriately ignores the details of molecular motion. Moreover, it allows direct iterative solving of the probability density distribution function without relying on empirical equations. In previous studies, the feasibility of using the lattice Boltzmann pseudopotential model in condensation process simulation was verified. Subsequently, numerous researchers have used this model to analyze the condensation mechanism. [Methods] This study is based on the lattice Boltzmann method and uses a dual distribution function to simulate the condensation process of stationary saturated vapor within a vertical channel. To analyze the fluid flow characteristics, a pseudopotential model is used to simulate the density field variations during the vapor condensation. Additionally, a temperature distribution function is employed to simulate the temperature field changes during the vapor condensation, allowing for an examination of heat transfer efficiency. Throughout the simulation, we analyze the effects of channel width and the hydrophilicity and hydrophobicity of wall conditions on the condensate flow and heat transfer rate. [Results] The results showed thattl) When saturated vapor encountered a hydrophilic wall, it first condensed to form a thin liquid film covering the entire wall surface and then formed a steady liquid film from the top of the vertical channel, gradually expanding downward. Due to the pressure difference caused by the vapor condensation, the saturated vapor flowed down into the channel from the inlet at the top of the channel. 2) Under hydrophilic wall conditions, decreasing the channel width from 500 to 150 decreased the steady-state average mass flow rate at the inlet by approximately 20% and decreased the steady-state average heat flux density on the wall by approximately 6. 5%. 3) The simulation results under different hydrophobic and hydrophilic characteristics were consistent with the theoretical analysis, indicating that the stronger the wall hydrophobicity was, the later the starting time of droplet nucleation and the lower the starting point of the vertical liquid film. On the ordinary hydrophobic wall surface, the droplet condensation was difficult to sustain, and after this surface was covered by a liquid film, the heat transfer rate was slower compared to the hydrophilic wall surface. Before the liquid film slipped out of the computational domain, the maximum average wall heat flux at an angle of 127° was approximately 75. 8% of that at an angle of 51°. [Conclusions] The lattice Boltzmann pseudopotential model can simulate the condensation process of stationary saturated vapor within a vertical channel. During the simulation process, the effects of channel width and the hydrophilicity and hydrophobicity of wall conditions on condensate flow and heat flux density are important. In general, wider channel widths lead to higher wall heat flux density, and a higher inlet mass flow rate of steam is achieved at a steady state for saturated vapor initially in a stationary state within the channel. However, ordinary hydrophobic wall surfaces cannot sustain droplet condensation and do not demonstrate enhanced heat transfer. These findings have certain reference values for designing and optimizing heat transfer systems involving condensation in vertical channels. © 2023 Press of Tsinghua University. All rights reserved.