背景气体对金属原子二维平面蒸发过程的影响

Influence of background gas on two-dimensional metal evaporation

在原子蒸气法激光分离同位素中,金属原子蒸气宏观物理性质的空间分布会直接影响到分离过程的电离率和原料利用率.本文从分离过程的实际需求出发,建立了双组分气体的Bhatnagar-Gross-Krook模型方程组,并利用数值计算方法对方程进行求解,研究了背景气体对二维平面蒸发过程中原子蒸气宏观物理性质和蒸发速率的影响.研究结果表明:随着背景气体密度的增加,远离蒸发源位置处的金属原子蒸气密度增大,速度减小,温度升高,而近蒸发源位置处原子蒸气的性质则几乎不受影响,因而蒸发速率基本上不随背景气体密度发生变化.另外,随着尾料板温度的升高和对原子蒸气吸收率的增加,金属原子蒸气宏观物理性质受背景气体的影响逐渐下降.理论计算的结果对于分离装置的真空设计和光斑分布设计有较为重要的参考意义.

The spatial distributions of macroscopic parameters such as density, bulk velocity and temperature of the metal vapor have influences on the photo ionization yield of target isotope and the utilization ratio of material, which is related to the separation efficiency and the cost of atomic vapor laser isotope separation. To study this problem more practically,a system of binary gas Bhatnagar-Gross-Krook（BGK） model equations, which describe both the metal vapor and the background gas, is established. The physical characteristics are dealt with by dimensionless method for simplifying the calculations. The model equations are discretized by one-order upwind difference and then are solved by iteration method for obtaining stable results. The computational grids are adjusted to the flow field in order to acquire modest computational cost and accurate result simultaneously. Non-uniform grids in the phase space and in the velocity space are constructed to match the macroscopic parameter gradient and the form of the velocity distribution, respectively. The macroscopic parameters in the cases of different background gas densities, temperatures of tail plate and absorptivities are obtained for studying the influence of the background gas. The results show that with the increase of density of the background gas, the density and temperature of the metal vapor increase, the bulk velocities in the x and z direction decrease obviously in the domain far from the evaporation source, while the macroscopic parameters that are close to the evaporation source hardly change. As a result, the evaporation rate is not affected. Meanwhile, a circulation of the background gas is driven by the metal vapor, which in turn affects the diffusion of the metal vapor. Besides, as the temperature of tailing plate rises, the influence of the background gas on the macroscopic parameters of the metal vapor weakens. However, the rise of the temperature of tail plate leads the absorptivity of metal vapor to decrease,which enlarges the influence of the background gas. Therefore, it is appropriate to adjust the temperature of the tail plate based on the relationship between the absorptivity of metal vapor and the temperature. The results of theoretical calculation can serve as a reference for designing the vacuum and laser spot of the separation device.