DescriptionIntroduction: Condensation nuclei counters (CNC) are widely used for measuring particle number (PN) concentrations. To both minimise the effort of aerosol preconditioning and eliminate possible condensation artefacts, CNCs operating at “high temperature” > 150 °C (HT-CNC) are of high interest for PN measurements, in particular in engine exhausts. Targeted research shows that key elements to realising such a long-term stably operable sensor are i) a suitable selection of wick and working fluid and ii) a special operation mode to prevent thermal degradation of the working fluid. Experimental: An experimental mixed-flow geometry HT-CNC setup was designed for application on raw automotive exhaust gas. To enable a loss-free long-term operation that is immune to thermal degradation of the working fluid, a special operation mode was developed that mixes a controlled fraction of the concentrated exhaust gas with an inert gas stream pre-saturated with the working fluid, a long-chained aliphatic hydrocarbon. This prototype was used for both, a fundamental proof of principle of an HT-CNC and various detailed studies. These studies include material tests of wick materials and a detailed examination of nucleation at HT-operation, investigating particles from ambient air, a GRIMM 7860 WOx-generator and a Jing 5201 miniCAST. The grown droplets were counted using specially developed fibre-optically coupled counting optics and sized using a dedicated static light scattering (SLS) setup. Finally, a calibration was performed and the HT-CNC was compared to a commercially available CNC (TSI 3775) using soot from the miniCAST. Results: Tests with particles from ambient air and the WOx-generator proved heterogeneous nucleation in all tested wicking materials. The behaviour shows a strong dependence to the wick used since the (super-)saturation differs. Further it turned out that under normal operation the speed of degradation of the working fluid differs also from one used wick to another. Full prevention of the degradation of the working fluid was obtained by a special operation mode taking advantage of the mixed-flow geometry for all used material combinations. The examination of the grown droplets by SLS yielded reproducible droplet sizes at defined temperature gradients between saturator and condenser also at long-term operations. The comparison of the particle count to a TSI 3775 using soot from a Jing 5201 miniCAST showed a good agreement.
|6 Sept 2018
|International Aerosol Conference 2018
|St. Louis, United States, Missouri
|Degree of Recognition