In vitro exposure systems are increasingly used in modern toxicology assessment studies. An understanding of the functioning of these complex systems and their characterization ensures well-controlled delivery of substances to cell cultures and reliable biological response of cells to the given external conditions. In this work, we present the flow-mixing efficiency of the dilution unit in the Vitrocell® 24/48 (VC24/48) exposure system, together with the role of thermal coalescence. We adopted a computational fluid dynamics approach with integrated aerosol dynamics to analyze the dilution system using high-resolution simulations. We performed simulations for three total flow rates through the system: 3, 6, and 12 L/min, to investigate the influence of turbulent mixing and thermal coalescence on the aerosol characteristics. We found that for all simulated flow rates, instantaneous and localized turbulent mixing is induced by the two opposing dilution jets. We found the aerosol to be uniformly mixed and the flow to be laminarized upstream of the location of the first aerosol sampling trumpet. In summary, the design of the double-tee junction, serving as the dilution unit of the VC24/48 system, guarantees efficient and localized mixing/dilution of the aerosol, while still delivering laminar flow toward deposition wells, when operated under these high flow-rate conditions. In addition, we showed that thermal coalescence effects do not influence the aerosol droplet size distribution during the dilution process for the studied simulation conditions.