Development of a multi-stage cloud water collector Part 2: Numerical and experimental calibration

Results from a numerical and laboratory experimental evaluation of the new Colorado State University (CSU) 5-Stage cloud water collector are presented. The CSU 5-Stage collector is a cascade inertial impactor developed to obtain samples of cloud water in five independent size fractions for chemical analysis. To determine the populations of drops that are collected during operation, numerically predicted and experimentally measured 50% cut diameters and collection efficiency curves were generated. Laboratory calibration of the CSU 5-Stage collector, using a fluorometric technique, provided an accurate assessment of collector performance that could be used to evaluate the numerical predictions. The experimentally determined 50% cut diameter for the first stage was 25.5 μm, while the second stage had a slightly higher 50% cut diameter of 29 μm. Stages three, four, and five had 50% cut diameters of 17.5, 10.5, and 4.5 μm, respectively. These are comparable to the design 50% cut diameters of 30, 25, 15, 10, and 4 μm. The numerical analysis of the collector was performed with the computational fluid dynamics software package FLUENT. Simulations of the continuous phase (air) flow field and discrete phase (cloud drop) trajectories were performed. For all stages, the efficiency curves derived from trajectories based on mean continuous phase velocities overestimate the 50% cut diameters and have unrealistically steep slopes when compared to the experimental calibration results. The curves based on trajectories that include turbulent velocity fluctuations provide a better representation of the experimentally determined 50% cut diameters and the overall shape of the efficiency curves for drop diameters >50% cut diameter. For drops <50% cut diameter, however, the numerical simulations overpredict collection efficiency when compared to the experimental measurements, which show drop collection falling to nearly zero with decreasing drop size.