The progressive replacement of traditional, fossil fuel-based energy transformation processes through a worldwide increase in the usage of alternative, renewable primary energy sources has boosted interest for thermodynamic engines operating with fluids other than steam, air and flue gases. Especially, the availability of low-quality (i.e. low temperature) heat sources, as it is the case concentrated solar systems, geothermal and biomass plants, makes use of classic working fluids, such as water, no longer viable. Similarly, efforts to move to delocalized, small-capacity (about 0.1–10 MW) power plants and mobile powertrains still running on C-based fuels but with a virtually vanishing C-footprint as backup for volatile renewable sources have brought renewed attention to waste heat recovery applications based on unconventional technologies involving non-ideal fluid flows mostly centred on the concept of the organic Rankine cycle (ORC) and of supercritical CO2 (sCO2) technology. Such systems operate with working fluids (e.g. siloxanes, refrigerants, CO2) and in thermodynamic states exhibiting thermo-physical behaviour largely departing from that of an ideal gas, often close to the critical point, either in vapour or in two-phase conditions. Though technologies exploiting the properties of non-ideal flows find already a widespread application in space propulsion (rocket engines) and in the oil and gas industry, their relevance and popularity are also growing in residential applications especially due to the advent of the next generation of heat pumps. Whilst representing an attractive evolution of energy transformation processes, the migration to trans- or supercritical and generally unsteady operations poses new challenges for the design, optimization and maintenance of systems and their components. The predictability of the performance and life cycle of individual components requires in particular an accurate description of the thermodynamic and transport properties of the working fluid, on one side, and a faithful modelling of all relevant flow features on the other side, especially in combination with turbulence, compressibility and possibly phase change effects, as it is the case in turbomachinery (compressors and expanders). @en