Theoretical and experimental analysis of the core sampling method: Reducing diffusional losses in aerosol sampling line

Core sampling method (extracting a portion of a flow from the core of the flow) will reduce diffusional losses of highly diffusive species (e.g., aerosol nanoparticles, ions, and gases) when transporting them through a sampling tube. Revealing parameters governing the sampling efficiency of a core sampling system, ηsam, helps to design the apparatus and to optimize its performance. In this study, we report an analytical solution for quantifying the ηsam by solving the convection diffusion equation of laminar flow field. The analytical results were experimentally evaluated using 1–5 nm tungsten oxide nanoparticles. ηsam is governed by a dimensionless loss parameter and the transport-to-sample flow ratio. Theoretically predicted values for ηsam agree with experimental results, e.g., the relative deviation is within 5% when the value for the loss parameter is less than 0.1. The core sampling method is recommended to work at the loss parameter less than 0.1 such that ηsam is equal or close to the maximum value of unity and is also insensitive to variations in sampling conditions. In this study, how to apply the findings in designing and optimizing a core sampling system was discussed. A core sampling apparatus was then designed and experimentally evaluated. Its sampling efficiency was shown to be significantly higher than those of a tee, a cross fitting, and a Y fitting when the same sampling conditions were used.
Copyright © 2019 American Association for Aerosol Research

核心采样法（从流场核心区域提取部分气流）在通过采样管输送高扩散性物质（例如气溶胶纳米颗粒、离子与气体）时，可有效降低此类物质的扩散损失。明确核心采样系统采样效率ηsam的控制参数，可为装置设计与性能优化提供重要依据。本研究通过求解层流流场的对流扩散方程，推导出用于量化ηsam的解析解，并采用1~5 nm氧化钨纳米颗粒对解析结果开展了实验验证。ηsam由无量纲损失参数与输送-采样流量比共同决定，其理论预测值与实验结果吻合良好：当损失参数小于0.1时，相对偏差不超过5%。研究建议核心采样法在损失参数小于0.1的条件下运行，此时ηsam等于或接近最大值1，且对采样工况的变化不敏感。本研究还探讨了如何将研究结论应用于核心采样系统的设计与优化，并设计了一套核心采样装置进行实验评估。结果表明，在相同采样工况下，该装置的采样效率显著高于三通、四通和Y型接头采样装置。
© 2019 美国气溶胶研究协会