Modeling Liquid Film Evaporation in a Wetted Wall Bioaerosol Sampling Cyclone

Abstract

The wetted wall bioaerosol sampling cyclone (WWC) is a complex multiphase flow device which collects and concentrates dilute bioaerosols into liquid samples for biological analysis (McFarland et al., 2009, “Wetted Wall Cyclones for Bioaerosol Sampling,” Aerosol Sci. Technol., 44(4), pp. 241–252). Understanding heat and mass transfer processes occurring inside the WWC is the key to enhancing its performance through an effective coupling to lab-on-chip analysis platforms which require small volumes of liquid output. There exists a critical liquid input rate below which there is no sample collection since all liquid is lost to evaporative effects. The purpose of this study was to model critical film evaporation based on first principles and develop semi-empirical WWC performance correlations as an improvement to existing empirical correlations. A one-dimensional, coupled heat and mass transfer model was developed approximating WWC multiphase flow as cocurrent air-film flow. Governing equations were simplified and approximate solutions were used to optimize model parameters like the heat transfer coefficient based on empirical data from previous works. Optimized model parameters were then used in the full numerical solution to calculate liquid evaporation rates within the WWC over the full range of relative humidity and air temperature. Semi-empirical correlations developed in this study were compared to existing empirical models and showed much improvement: proper physical behavior at the extreme limits of temperature and relative humidity was observed, and the nonlinear dependence of evaporative effects on environmental conditions was also captured.