Influence of Working Pressure and Pressure Difference on Static Droplet Evaporation Characteristics

The function of steam separator is to remove the small droplets carried by the vapor stream and to provide qualified saturated vapor for the steam turbine in the nuclear power station. The separating characteristics of the steam-water separation plant are of vital importance to the safe operation, economy as well as reliability of the power station. In order to satisfy the requirement of power increase of large nuclear power station as well as space compaction of the vessel power plant, the steam vapor quality must be improved, which requires that the steam-water separator has better separating function to make sure that it can provide the qualified steam on the condition of higher steam pressure, power load as well as circulating ratio. There are many complex phenomena when the droplet moves in the steam-water separating plant, including the droplet emergence, the droplet moving with steam vapor, the collision between droplets and with solid wall, evaporation. It is a good way to study the steam-water separating characteristics for the microcosmic behavior of the droplet. Thus, in order to know the droplet evaporation characteristics in the steam-water separator, the static droplets phase transformation model under the pressure variation condition is built according to the physical phenomenon description and mechanism comprehension when the droplet moves with the steam vapor in the steam-water separation plants. This model is solved by the typical four steps Runge-Kutta method and validated by comparing with the experimental results. Then, the influence of working pressure as well as pressure difference between the droplet surface and the environment on the static droplet evaporation characteristics is conducted. The simulation results show that the working pressure and pressure difference have great impact on the static droplet evaporation characteristics. With the increase of the working pressure, the droplet evaporation rate becomes slower, that is because the physical property parameters of the water vapor and water become closer to each other and the self-diffusion coefficient of the water vapor as well as the evaporation condensation coefficient become smaller, which results in the droplet evaporation rate becomes slower. When the pressure difference between the droplet surface and the environment rises, the droplet evaporates faster, that is because the vapor velocity around the droplet becomes larger and the droplet evaporates faster. These results of the simulation can lay the foundation for subsequent study of the droplet evaporation characteristics when the droplet moving in the separating plants and for the droplet fast evaporation characteristics when the environment pressure changes fast.