Numerical and Experimental Performance Evaluation of the 3-Stage FROSTY Supercooled Cloud Collector

Abstract

An evaluation of the collection characteristics of a new multistage cascade inertial impactor designed for size-resolved cloud drop collection has been performed. The FROSTY supercooled cloud collector is intended for the collection of supercooled cloud drops in a winter environment in three independent size fractions with stage 50% cut diameters of 15 μm, 10 μm, and 4 μm. Two approaches were selected for the evaluation of the collector. Numerical simulations provided a visualization of the airflow patterns and drop trajectories through the collector while experimental laboratory calibration provided a quantitative analysis of true collection performance. For each of these methods, 50% cut diameters, efficiency curves, and wall losses for each stage of the collector were determined. Collection characteristics were determined experimentally by introducing fluorescein-tagged monodisperse drops into the collector and analyzing collection patterns by fluorescence. The experimental measurements at laboratory conditions indicated 50% cut diameters of 19.0, 11.5, and 5.0 μm for the three stages. Adjusted for operation at collector design conditions, the 50% cut diameters were 17.0, 10.5, and 4.5 μm. Numerical modeling of the airflow patterns and drop trajectories through the collector was performed with the commercially available Computational Fluid Dynamics (CFD) software package FLUENT. Trajectory simulations based on the average continuous phase (air) velocity field as well as trajectory simulations which included the effects of statistically derived turbulent velocity fluctuations on drop motion were performed. Comparisons between the numerical and experimental work indicated that the inclusion of turbulent fluctuation effects on drop motion provided much better agreement with experimental observations than trajectories based solely on average flow field velocities. However, the use of continuous phase velocity fluctuations also produced unrealistic losses to wall surfaces for small drop sizes. Despite this shortcoming, numerically derived 50% cut diameters and overall collection efficiency curve shapes for drop trajectories including turbulent velocity fluctuations agreed reasonably well with experimental observations.