Characterizing flow of pressurized CO2 through micro-orifice for atomization applications: Experiments and CFD modelling

Improving therapeutic efficacy of newly developed drugs remains a major challenge for the pharmaceutical industry. Spray drying based on atomization using high pressure or supercritical CO₂ has been shown to be effective in improving therapeutic efficiency of drugs by forming smaller particles, owing to the distinct solvation power and enhanced mixing potential of supercritical CO₂. Several parameters contribute to the critical quality attributes of the final atomized pharmaceutical products resulting from CO₂-assisted atomization including high-pressure nozzle design, drying chamber geometry, and operating pressures and temperatures. In this context, the work is focused on a detailed analysis of supercritical CO₂ through micro-orifices used in a spray drying enhanced atomization process. We present a computational fluid dynamics model, developed using Ansys FLUENT, to describe the flow of pure, pressurized CO₂ through a micro-orifice undergoing trans-critical expansion. After establishing grid independence, the computational model was validated by comparing model predictions to measured mass flow rates and temperature distribution of the cooling effect of CO₂ free jet over an adiabatic surface. For supercritical inlet conditions and a nozzle orifice size of 80 µm, experimental results matched predictions reasonably well. The simulated results demonstrated the occurrence of shock waves, a prerequisite for fine droplets formation. Simulated results were critically analyzed to develop new insights into intricate fluid dynamics of flow of CO₂ through atomization orifices and an attempt is made to evolve specific guidelines. The presented model and results will be useful for researchers and engineers interested in understanding and optimizing CO₂-assisted spray atomization processes.