High Flow Nasal Cannula (HFNC) oxygen therapy helps patients recover from respiratory distress by providing lower levels of CO₂ remaining in the airway at end exhale, which should be minimized for more effective patient recovery. Computational Fluid Dynamics (CFD) models were run with a generic HFNC geometry, one of two High Velocity Nasal Insufflation (HVNI) cannula geometries, and no cannula, each for open and closed mouth flush scenarios. With an open mouth, the CO₂ remaining in the airway at end exhale was 1.97 mg and 1.95 mg for the HVNI cannulas, 4.24 mg for the generic HFNC geometry, and 10.5 mg for the model with no cannula. For the closed mouth models, the distinction was smaller. Numerous internal flow mechanisms were responsible for CO₂ reduction. It was found that the higher velocity from the HVNI cannulas created a higher resistance against the infiltration of exhaled CO₂ into the upper airway. The HVNI cannulas also began flushing the airway (reducing total mass CO₂ in the airway) earlier in the exhalation cycle than both the HFNC and no cannula models. The higher resistance to expiratory flow entering the upper airway and earlier transition to flush led to the HVNI cannula geometries delivering more effective therapy than the HFNC or no therapy models.