Compressible two-phase flows play a crucial role in many technical applications, where they can manifest as condensation and evaporation in unsteady flows, liquid accumulation in pipelines, boiling and cavitating flows. A relevant example is found in Organic Rankine cycle (ORC) systems, one of the most promising technologies for waste heat recovery. Unlike conventional Rankine cycles, ORCs use organic fluids, which makes them particularly suited for power generation from thermal sources below 300°C. These fluids may exhibit the so-called wet-to-dry expansion: starting within the two-phase region at sufficiently high pressure, an isentropic expansion can end in the dry-vapor region. This distinctive thermodynamic behavior exhorts the investigation of ORCs operating with two-phase expansion.
The overarching goal of the NI2PhORC project is to deliver a computational fluid dynamics (CFD) tool tailored to compressible unsteady non-equilibrium two-phase flows. In CFD, diffuse-interface methods are effective approaches for compressible multiphase flows, able to handle the complexities of evolving material interfaces that separate distinct fluids. At the core lies the Baer-Nunziato (BN) model, which describes compressible two-phase flows in full non-equilibrium, with each phase evolving according to its own pressure, velocity, temperature, and Gibbs free energy.
Various BN-type models have been proposed, incorporating different transfer terms for mass, momentum, and heat exchange between phases. Relaxation terms may also be added to model how the mechanical equilibrium is reached at the interfaces and to include heat and mass transfer. With suitable closures, BN-type models have proven effective in simulating a wide range of multiphase regimes, from dispersed to resolved-interface flows. Moreover, they are particularly suited for fluids described by different equations of state (EOSs), since each phase is treated as a separate continuum with its own thermodynamic model. This capability is essential when working near the saturation curve, where organic fluids often deviate strongly from ideal-gas behavior.
Nevertheless, open questions remain regarding the proper definition of finite relaxation parameters in BN-type models. While instantaneous mass transfer has been shown to be inadequate to match experimental data, an unambiguous formulation for finite relaxation times is still lacking. The NI2PhORC project will aim to extend modeling capabilities for compressible two-phase flows by developing a CFD tool with relaxation parameters that can be tuned based on physical insight or application-specific needs.
