Differential heat and mass transfer rate influences on the activation efficiency of laminar flow condensation particle counters
Differential heat and mass transfer rate influences on the activation efficiency of laminar flow condensation particle counters
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DOI：
10.1016/j.ijheatmasstransfer.2018.07.002
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发表时间：
2018-12
影响因子：
5.2
通讯作者：
Jikku M. Thomas;Xiaoshuang Chen;A. Maisser;Christopher J. Hogan
中科院分区：
文献类型：
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作者：
Jikku M. Thomas;Xiaoshuang Chen;A. Maisser;Christopher J. Hogan
层流冷凝颗粒计数器（CPC）是纳米尺寸范围内的气溶胶颗粒的独特灵敏度检测器（即，尺寸小于10 nm，它们可以具有单颗粒灵敏度）。它们的工作取决于工作流体的过饱和度的产生;暴露于过饱和蒸汽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。由于开尔文关系支配小颗粒的蒸气压，在所有CPC中存在临界尺寸/切割尺寸（直径），并且小于该尺寸的颗粒不会生长并且不能有效地检测。虽然已经努力控制CPC活化效率（即，作为尺寸的函数检测到的颗粒的分数），但是先前的研究没有检查CPC中的差热和质量传递如何受气体组成的变化的支配。在这里，我们测量和模型CPC激活效率（1-丁醇作为工作流体）在不同的热物理性质的气体，即氦和分子氮的混合物。我们的实验表明，较小颗粒（即在测试的CPC中低于8 nm）的活化效率可以通过向气溶胶中加入适量的氦气（摩尔分数接近0.20）来提高。这是预期的基础上增加的刘易斯数所带来的氦气添加，并支持CPC激活效率的预测的基础上耦合的热，质量，和CPC冷凝器区域内的动量传递的热物理性质变量模型。有趣的是，我们发现，当以恒定的精确孔直径（扼流）操作时，对于给定的亚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，具有恒定质量传递Peclet数（Pem= 77）的实验示出CPC活化效率增加到氦摩尔分数为0.67，但是随后活化效率降低得更适度地超过该氦摩尔分数。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量增加，这影响CPC饱和器的性能。最后，通过建模，我们表明，通过适量的氦添加提高CPC的活化效率的能力是一般的，并且可以应用于其他重质工作流体。在这项研究中提出的结果阐明了气体成分和刘易斯数控制的微分传热和传质速率的冷凝为基础的纳米粒子检测器的性能的重要性。
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.