Important and unique metal powders are made industrially by a variety of vapor condensation processes in tube reactors. Often, however, the fundamental mechanisms for particle formation and growth are still not well understood. In this article, a computational fluid dynamics (CFD) model was developed to examine a tube reactor's internal flow characteristics. The model identified a massive zone of fluid recirculation in the top half of the reactor. In‐situ sampling from an experimental reactor under the same conditions revealed a large increase of aerosol particle size corresponding to the region of recirculation. A first principles mass balance model based on chemical kinetics and aerosol physics was developed for this system which showed that the average particle size grew monotonically with time in the reactor. On the basis of this firmly established link between residence time and particle size, a new reactor geometry was proposed to produce a “plug‐flow” velocity profile with a narrower particle size distribution. A CFD model was used to prototype the new configuration, and then this new reactor design was tested experimentally to confirm that the design objective was achieved. This work shows the potential synergies between first principles models for process understanding and CFD models for process prototyping and optimization. © 2007 American Institute of Chemical Engineers AIChE J, 2007
Objective: High flow nasal cannula (HFNC) is an evolving respiratory therapy whereby high flow rates of conditioned breathing gas are delivered into the nasal cavity to purge anatomical dead space of CO 2 rich expired gas. The aim of this project was to create a computational fluid dynamics (CFD) model to evaluate the fluid patterns in the human nasal and pharyngeal cavities with HFNC application, and quantify time to purge for two cannula configurations.Methods: Three-dimensional geometry of the human airway was used to define the extrathoracic dead space and the two cannula designs tested incorporate large vs small bore nasal prong configurations (Vapotherm, Exeter, NH, USA). The fluid flow simulations were performed using FLOW-3D software, set up for a cannula flow rate of 20 L•min -1 and run until steady state.Results: Basic flow patterns were similar between cannulae, creating vortices around a central inward flow path. Flow velocity around the vortices was greater with the small prong cannula, resulting in a lower pressure in each region of the nasal and nasopharyngeal space. The calculation of purge time revealed that the small prong nasal cannula was able to clear the nasal, pharyngeal and oral cavities in 2.2 seconds, whereas the large bore cannula required 3.6 seconds (64% longer). Conclusion:The current CFD data validate that a smaller bore nasal prong facilitates the purge action, which is related to velocity and dynamic energy induced by the tighter prong nozzle as opposed to the lesser occlusion of the nares.
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