Numerical investigation of dense condensing flows for next-generation power units

Metastable condensation is the phase transition from vapor to liquid that occurs in a fluid subjected to rapid temperature variations. Under these conditions, the nucleation process is triggered when the fluid is in a supersaturated thermodynamic state. The dispersed phase forming during the process of condensation is not in stable thermodynamic equilibrium with the surrounding vapor. As a consequence, models suitable for condensing flows under large temperature gradients, which are relevant to many scientific studies and industrial applications, are rather complex as they must correctly treat metastable thermodynamic states. Applications of metastable condensation flowmodels include improved climate models [1], biomedical treatments [2], heat transfer enhancement for industrial purposes [3], natural gas separation [4], power conversion [5] and many others. The scope of the research documented in this dissertation is the numerical investigation of metastable condensing flows in turbomachinery for propulsion and power applications. The flow inside turbomachinery components is highly compressible, with absolute temperature gradients that can reach values of the order of 1e6 K/s[6], in the case of supersonic expansions. In such extreme conditions, metastable phenomena impact severely on the component performance in terms of both thermodynamic and fluid dynamic losses and lifetime. The number of technologies for propulsion and power characterised by the presence of condensing mixtures in turbomachinery is increasing. Considerable research and development efforts are currently concerned with components of next-generation thermal power and refrigeration systems, in which the flow undergoes metastable condensation. The characterization of metastable condensing flows and the development of advanced fluid dynamic design tools capable of treating these complex flow phenomena are fundamental steps towards the commercial application fo such promising technologies.