Fuel cell vehicle (FCV) technology is gaining traction as a promising alternative to both traditional internal combustion engines and battery-powered electric vehicles. To enhance performance and improve efficiency, turbocharging can be integrated into the system by passing the fuel cell exhaust air—containing water vapor—through a turbine to recover some of the otherwise wasted energy. However, the introduction of humid air into the turbine can lead to flow condensation during expansion. This is a particular concern for Mitsubishi Heavy Industries Engine & Turbocharger Co., Ltd. (MHIET), since steam condensation may alter the flow through the turbine by releasing latent heat, ultimately compromising the efficiency. In addition, the water droplets formed can cause mechanical damage to blade surfaces due to erosion.

The present study conducts a numerical investigation of this process using Computational Fluid Dynamics (CFD). Previous computational research on condensation phenomena in nozzles and steam turbines was surveyed, and relevant insights were extended to the current case. The simulations were performed in ANSYS Fluent using an Euler–Euler multiphase framework to evaluate the two-phase flow field and to identify the location and intensity of the onset of condensation. A User-Defined Function (UDF) was employed to trigger phase change, and appropriate turbulence and multiphase models were selected so that the interphase exchange of heat and mass could be captured.

The model was first validated against a benchmark nozzle case and subsequently applied to the turbine configuration. Several surfaces of analysis were carefully selected and illustrated to clarify the results and highlight key flow features. The simulations revealed that the release of latent heat had a pronounced influence on the flow, and the wet case results were compared against a hypothetical dry case under the same conditions in order to isolate and quantify the impact of liquid generation. The results showed good qualitative agreement with previous studies reported in the literature.

A series of parametric studies was carried out to investigate the effects of inlet relative humidity, inlet pressure and rotational speed. Higher inlet relative humidity caused stronger subcooling of the flow, leading to enhanced liquid formation. The resulting heat release raised the flow temperature and therefore increased available power, which led to a marginal rise in efficiency as the air became more humid. However, when compared to the dry (non-condensing) cases at the same relative humidity, the wet cases exhibited a drop in efficiency due to the loss of gaseous mass flow.

The impact of inlet pressure was also assessed. The phenomenon of thermal throttling was discussed, wherein the pressure ratio across the turbine adjusts in order to maintain a fixed mass flow rate. The amount of liquid produced was found to depend on a balance between the degree of expansion and the subsequent reheating of the flow at the turbine outlet. In addition, increasing rotational speed initially led to a rise in liquid generation and reached a peak, after which further increases in rotational speed resulted in a decline in the amount of condensate.

The study provides valuable insight into the flow behaviour in humidified fuel-cell turbocharging systems and discusses the associated implications for the air-management system. Finally, recommendations are proposed to improve the current numerical model and to make it more comprehensive.