Abstract: In the chemical vapor deposition process of niobium (Nb) coating on fuel particles, the flow rate of the precursor NbCl5 vapor significantly influences the growth and properties of the thin films. However, measuring the vapor flow rate poses challenges due to the strong acidity of NbCl5, rendering the evaporation process a "black box" operation. In this study, a Volume of Fluid (VOF) method is developed to simulate the gas-liquid interface evaporation and wall boiling of NbCl5 in tank evaporator using argon (Ar) as carrier gas. The impact of various operating conditions, including liquid level height, flow rate of Ar carrier gas, and heating temperature, on the two-phase flow and the heat and mass transfer characteristics within the tank evaporator are investigated. Simulation results reveal that liquid level height primarily affects the evaporation area. When the liquid level resides in the near-bottom region of the tank, lowering the liquid level height significantly reduces the evaporation area, resulting in a decreased NbCl5 vapor flow rate at the evaporator outlet. When Ar gas is introduced into the evaporator, two distinct vortex structures are formed: a NbCl5 vapor vortex and an Ar gas vortex. Increasing the Ar gas velocity enables rapid sweeping of NbCl5 vapor from the liquid surface and lowers the saturation vapor temperature at the interface, thereby enhancing the evaporation rate. As the Ar gas velocity increases from 0 m/s to 2.1 m/s, the mass flow rate of NbCl5 vapor rises from 0.047 g/s to 1.095 g/s. Raising the heating temperature increases the liquid temperature at the evaporation interface, accelerating liquid evaporation. As the heating temperature increases from 533 K to 543 K, the NbCl5 vapor mass flow rate increases from 0.280 g/s to 0.359 g/s. These simulation results offer practical guidance for optimizing the operating conditions of the gas-carrying evaporation process and the evaporator structure.

Key words: gas-carrying evaporation, VOF model, two-phase flow, heat and mass transfer

摘要： 在流化床化学气相沉积制备铌金属层包覆颗粒的工艺中，前驱体NbCl5蒸气的流量对涂层质量的影响十分显著。由于NbCl5蒸气具有强酸性，蒸发罐出口蒸气流量难以有效测量，蒸发过程一直是黑箱操作，严重制约了该工艺的稳定运行与优化升级。本工作采用耦合气液界面蒸发模型和壁面沸腾模型的VOF方法，模拟了NbCl5-Ar体系载气蒸发过程，探究了液面高度、Ar气载气流量、加热温度等操作条件对蒸发罐中气液两相流动和热质传递特性的影响规律。模拟结果表明，液位高度主要影响蒸发面积。当液位处于罐底区时，液位高度降低使得蒸发面积减小，进而导致蒸发罐出口NbCl5蒸气流量显著减小。当蒸发罐中通入Ar气后，蒸发罐内部会形成NbCl5蒸气涡和Ar气涡两种不同的涡结构。提高Ar气速度能够实现液面上NbCl5蒸气的快速吹扫、降低液面饱和蒸气温度，进而增加蒸发速率；随Ar气速度由0 m/s增加至2.1 m/s，NbCl5蒸气质量流率由0.047 g/s增加至1.095 g/s。提高加热温度能够提高蒸发界面液体温度，进而加快液体蒸发；随加热温度由533 K增加至543 K，NbCl5蒸气质量流率由0.280 g/s增加至0.359 g/s。模拟结果对实际载气蒸发过程操作条件优化和蒸发罐的结构优化均具有指导意义。

关键词: 载气蒸发, VOF模型, 两相流动, 热质传递