流动气溶胶动态光散射测量中的流速限制

Velocity Limitation in Dynamic Light Scattering Measurement for Flowing Aerosols

流速是流动气溶胶动态光散射(DLS)测量的重要制约因素。采用层流条件下的散射光强自相关函数(ACF)反演模拟和实测的流动气溶胶DLS数据,分析了流动对气溶胶颗粒粒度分布(PSD)测量的制约机制。结果表明,流速增加对气溶胶PSD反演结果的强烈影响不能通过流速的贡献在光强ACF模型中表达。导致粒度反演困难的原因是,流速的增加加剧了ACF所在的病态方程的病态性,表现为方程核矩阵条件数的增加。从信号分析的视角,是由于流速的增加降低了光强ACF中粒度信息的幅值。流速的影响与被测气溶胶的粒径有关,这种与粒径的关联性,可以通过表征布朗运动的扩散特征时间和表征气溶胶流动的平移特征时间之比进行评估。扩散与平移的特征时间比,既可表征DLS测量时流速对不同粒径气溶胶的不同影响,也可为实际测量时根据测量对象进行流速选择提供依据。

The flow velocity is an important limiting factor behind dynamic light scattering(DLS) measurements for flowing aerosols. In this paper, the autocorrelation function(ACF) of scattering intensity under the laminar flow condition is used to retrieve the simulated and measured DLS data of flowing aerosols, and the restriction of flow velocities on the measurement of the particle size distribution(PSD) for aerosols is analyzed. The results show that the strong influence of velocity increase on the PSD retrieval results cannot be expressed in the ACF model through the contribution of flow velocities. The reason for the difficulty in particle size inversion is that the velocity increase aggravates the ill condition of the ill-conditioned equation where the ACF is located, which is manifested as the increase in the condition number of the kernel matrix in the equation. From the perspective of signal analysis, the increase in flow velocities reduces the amplitude of particle size information in ACF. The influence of flow velocities is related to the particle size of the measured aerosols, and this relevance to particle sizes can be evaluated by comparing the diffusion characteristic time in Brownian motion with the translation characteristic time in aerosol flow. The characteristic time ratio of diffusion to translation can not only characterize the different effects of flow velocities on aerosols with different particle sizes in DLS measurements, but also provide a basis for velocity selection according to the measured objects in actual measurements.