Phase transition modeling of supercritical CO₂ in cryogenic milling: CFD analysis with extended real gas property integration

Supercritical carbon dioxide (scCO₂) is an environmentally friendly coolant in machining, addressing concerns associated with traditional metalworking fluids. In medical component manufacturing, selecting an appropriate coolant is crucial for ensuring contamination-free parts, which scCO₂ achieves by leaving no residue upon sublimation. Computational Fluid Dynamics (CFD) simulations offer valuable insights into the hydraulic efficiency of coolant delivery systems, essential for optimizing nozzle design and cooling effectiveness in internal cooling milling tools. This study investigates scCO₂ discharge and dispersion through internal milling tool nozzles in cryogenic machining, emphasizing CO₂ solidification and phase transitions from supercritical to solid CO₂ and gas phases. Utilizing ANSYS CFX software and extended real gas property (RGP) tables, the research examines scCO₂ behavior under various operational conditions. The extension of RGP tables accommodates phase transitions during cooling, addressing challenges in determining CO₂ properties in supercritical and sub-triple-point states, where solid CO₂ forms under atmospheric pressure. Experimental validation shows good agreement between simulated and observed mass flow rates and flow characteristics, with relative percentage errors of 7.7% for mass flow rate and 0.7% for minimum temperature upon expansion. Additionally, the simulation predicted a solid CO₂ mass fraction of up to 0.28 in the expansion zone, confirming the model’s ability to capture complex phase transitions. These simulations provide accurate and computationally efficient insights for tool design and optimization, overcoming challenges in determining CO₂ properties under extreme conditions.