Modelling the condensation phenomena within the radial turbine of a fuel cell turbocharger

Authors

Tim Wittmann - Institute of Jet propulsion and Turbomachinery, Technische Universität Braunschweig
Sebastian Lück - Institute of Jet propulsion and Turbomachinery, Technische Universität Braunschweig
Christoph Bode - Institute of Jet propulsion and Turbomachinery, Technische Universität Braunschweig
Jens Friedrichs - Institute of Jet propulsion and Turbomachinery, Technische Universität Braunschweig

Abstract

Fuel cells are a promising power source for the automotive sector and, in the future, also for aircraft and a hydrogen-based industry. The feasibility and economic efficiency of such fuel cells depend on their chemical efficiency and power output, which increase for higher pressure levels. Therefore, the process gas for fuel cells needs to be compressed for an improved performance. The exhaust gas of the fuel cell still carries a pressure potential which may be utilised, for example by a radial turbine. However, the exhaust gas is enriched with water vapour. The gas expansion in the turbine decreases the saturation pressure for water vapour. As a result, droplets nucleate, which then grow by condensation. This leads to two effects: The thermodynamic cycle is influenced as condensation heat is released and the mass flow of gaseous medium decreases. As known from steam turbine research, the assumption of phase equilibrium is not valid for rapidly expanding flows. Droplet Nucleation occurs as a spontaneous change from sufficiently subcooled conditions towards equilibrium. Furthermore, the nucleation is closely coupled to the subsequent droplet growth due to condensation. This study aims to investigate the aforementioned phenomena of nucleation and condensation in the turbine of an automotive fuel cell turbocharger. For this purpose, the classical nucleation theory and Young´s growth law are incorporated into the Discrete Phase Model of ANSYS Fluent. The main advantages of this Euler-Lagrange approach over the more common monodispersed Euler-Euler approach are the calculation of individual trajectories for each droplet parcel and a full resolution of the droplet size distribution. The initial sections of the paper cover the requisite theory, methodology and a description of the turbine under investigation. Then, aspects of implementation and the influence of modeling parameters such as surface tension of water and the droplet forces will be discussed. The last part consists of a detailed analysis of various influences of the condensation phenomena at the turbine´s main operating point for different saturation levels. As the saturation level at the inlet varies from 0% to 110%, preliminary results indicate an onset of nucleation at the blade tip, then within the tip gap, followed by nucleation at the suction side and pressure side over the entire blade span. Furthermore, the condensing droplets show a strong interaction with secondary flow phenomena, especially with the tip gap vortex. For a saturated inlet, the outlet temperature nearly reaches the temperature at the inlet.