We modelled aerosol transport and deposition in a Valved Holding Chamber (VHC/HVC) used with metered‑dose inhalers, combining CFD with multiphysics particle tracking. The chamber geometry, valve behaviour and mouthpiece were represented to capture transient inhalation profiles and plume injection characteristics. Lagrangian parcels with realistic size distributions evaluated impaction and deposition on chamber walls and in the oropharyngeal region, while mixed or species transport resolved aerosol concentration available for inhalation. Study objectives including maximise delivery to the lower airways while limiting losses, were translated into measurable metrics: delivered dose fraction, residence times and regional deposition patterns. We assessed geometry variants (volume, taper, baffles) and valve timings across user breathing scenarios to understand sensitivity and tolerance. Mesh/solver settings balanced fidelity and turnaround; verification and parameter checks ensured robust comparisons. Outputs included dose‑response curves and annotated visuals that link physical intuition to product KPIs. The recommendations identified design levers that improve delivered dose without undue complexity, providing a clear path to prototype refinement and future validation.