Exhaust gas of an automotive fuel cell is enriched with water vapour and has a pressure potential which can be utilized by a turbine. The gas expansion in the turbine leads to droplet nucleation and condensation. This results in a release of latent heat and a decrease of the gaseous mass flow which has a considerable influence on the turbine performance. This study aims to numerically investigate the influence of these phenomena on the performance map of the radial turbine of an automotive fuel cell turbocharger. For this purpose, the classical nucleation theory and Young’s droplet growth law are integrated into an Euler-Lagrange approach. The results show an almost linear relation between the pressure ratio and the condensation while the specific aerodynamics of an operating point has only a minor influence. At 80 % relative humidity of the inflow, the investigated turbine showed condensation above a total-to-static pressure ratio of 1.8. Condensation leads to thermal throttling of the turbine and to a temperature increase of the rotor outflow of up to 50 K. Increasing humidity of the inflow increases the power output, but condensation losses reduce the efficiency.

The ability to accurately model condensing flows is crucial for understanding such flows in many applications. Condensing flows of pure steam have been studied extensively in the past, and several droplet growth models have been derived. The rationale for the choice of growth models for condensation in humid air is less established. Furthermore, only a few validation cases for condensation in such flows exist. This paper aims to identify existing limitations of common droplet growth laws. The Hertz—Knudsen model is compared to heat-transfer-based models by Gyarmathy and Young, using an Euler—Lagrange approach in Ansys Fluent. For this, an adaption for Young’s growth law is introduced, allowing its application in condensation of different gas mixtures. The numerical model has been validated and applied to flows in nozzles and turbines in previous publications. The accuracy of the droplet growth models is investigated in transonic nozzle test cases. A case with pure steam and a case with humid air at two different humidity values are considered. Finally, the influence of humidity, Knudsen number, and droplet radius on the growth rate of each model is shown analytically. Flows at lower humidity values with longer condensation zones are shown to benefit from the higher sensitivity of the Hertz—Knudsen model to the mass fraction of water vapor in the flow. Heat-transfer-based models tend to overestimate condensation in such flows. However, the ability to empirically adapt the growth model by Young and its applicability in different Knudsen numbers results in good agreement with validation data over a wide range of cases.