Abstract Laminar flow condensation particle counters (CPCs) are uniquely sensitive detectors for aerosol particles in the nanometer size range (i.e. below 10 nm in size they can have single particle sensitivity). Their operation hinges upon the creation of supersaturation of a working fluid; particles exposed to supersaturated vapor grow by condensation to optically detectable sizes. The degree of supersaturation is fully controlled via differential rates of heat transfer and working fluid vapor mass transfer. Because of the Kelvin relationship governed vapor pressure of small particles, in all CPCs there is a critical size/cut-size (diameter), and particles smaller than this size do not grow and are not detected efficiently. While efforts have been made to control the CPC activation efficiency (i.e. the fraction of particles detected as a function of size), prior studies have not examined how differential heat and mass transfer in CPCs are governed by changes in gas composition. Here, we measure and model CPC activation efficiencies (with 1-butanol as the working fluid) in mixtures of gases of disparate thermophysical properties, namely helium and molecular nitrogen. Our experiments show that the activation efficiency of smaller particles (i.e. below 8 nm in the tested CPC) can be increased by adding a modest amount of helium to the aerosol (mole fractions near 0.20). This is expected based upon the increased Lewis number brought about by Helium addition, and supported by predictions of CPC activation efficiency based upon thermophysical property variable models of coupled heat, mass, and momentum transfer within the CPC condenser region. Interestingly, we find that when operating with a constant precision orifice diameter (choked flow), the activation efficiency for a given sub-10 nm particle diameter first increases with increasing Helium mole fraction and then decreases as the Helium mole fraction increases beyond 0.67. In comparison, experiments with constant mass transfer Peclet number (Pem = 77) show an increase in CPC activation efficiency up to a helium mole fraction of 0.67, but then the activation efficiency decreases more modestly beyond this helium mole fraction. We attribute these contrasting results to the increased flowrate through the instrument under constant orifice diameter conditions, which affects the performance of the CPC saturator. Finally, through modeling we show that the ability to enhance the activation efficiency of a CPC via a modest amount of helium addition is general, and can be applied with other heavy working fluids. The results presented in this study elucidate the importance of gas composition and Lewis number controlled differential heat and mass transfer rates on the performance of condensation based nanoparticle detectors.

中文翻译：

---

不同传热传质速率对层流冷凝粒子计数器活化效率的影响

摘要 层流冷凝粒子计数器 (CPC) 是纳米尺寸范围内（即尺寸低于 10 nm，它们可以具有单粒子灵敏度）的气溶胶粒子的独特灵敏检测器。它们的运行取决于工作流体过饱和的产生；暴露在过饱和蒸汽中的颗粒通过冷凝生长到光学可检测的尺寸。过饱和度完全通过热传递和工作流体蒸汽质量传递的不同速率来控制。由于开尔文关系控制着小颗粒的蒸气压，在所有 CPC 中都有一个临界尺寸/切割尺寸（直径），并且小于该尺寸的颗粒不会生长并且无法有效检测。虽然已经努力控制 CPC 激活效率（即 检测到的颗粒分数是尺寸的函数），但先前的研究并未研究 CPC 中的不同传热和传质如何受气体成分变化的控制。在这里，我们测量并模拟了具有不同热物理特性的气体混合物（即氦气和分子氮）中的 CPC 活化效率（以 1-丁醇作为工作流体）。我们的实验表明，通过向气溶胶中添加适量的氦气（摩尔分数接近 0.20）可以提高较小颗粒（即在测试的 CPC 中低于 8 nm）的活化效率。这是基于氦添加带来的路易斯数增加而预期的，并得到基于 CPC 冷凝器区域内耦合热、质量和动量传递的热物理特性变量模型的 CPC 活化效率预测的支持。有趣的是，我们发现当以恒定的精密孔口直径（阻塞流）运行时，给定亚 10 nm 粒径的活化效率首先随着氦摩尔分数的增加而增加，然后随着氦摩尔分数增加超过 0.67 而降低。相比之下，具有恒定传质 Peclet 数 (Pem = 77) 的实验表明，当氦摩尔分数为 0.67 时，CPC 活化效率会增加，但超过此氦摩尔分数时，活化效率会更温和地降低。我们将这些对比结果归因于在恒定孔口直径条件下通过仪器的流速增加，这会影响 CPC 饱和器的性能。最后，通过建模，我们表明通过添加适量的氦来提高 CPC 活化效率的能力是普遍的，并且可以与其他重质工作流体一起应用。本研究中的结果阐明了气体成分和路易斯数控制的微分传热和传质速率对基于冷凝的纳米粒子探测器性能的重要性。