CFD Investigation of Unsteady Interactions in a Radial-Axial Turboexpander for Geothermal Power Plant Applications

Global Warming is one of the main concerns in modern society. To address this issue, the European Union has launched the Green Deal, whose main objective is to achieve carbon neutrality by 2050. Zero-carbon sources, such as geothermal energy, can therefore play a key role. To cope with the variability of the primary source, the energy conversion needs to be handled by a system which can provide high efficiency in a wide range of operating conditions. Thus, the aerodynamic behavior of such machines needs to be carefully investigated. This paper describes the aerodynamic behavior of an innovative radial-axial expander architecture operating in transonic conditions for geothermal applications. Steady-state calculations are typically used in the design phase of turbomachines due to their low computational cost, but they cannot consider the unsteady interactions between the rows. In highly loaded expanders, when shock waves and large wakes occur, transient calculations are required to understand their impact on performance. In this activity, the unsteady analyses of the machine are carried out at design and off-design conditions to better evaluate the turbine performance in the whole operating range. The CFD analyses are performed using the TRAF code with a real gas model to properly describe the behavior of the process gas. The results show that if unsteady effects are not considered during the design phase, the overall machine performance may be overlooked.