Aerodynamic Characterization of Conical Diffusers for Radial Turboexpanders Under Realistic Inlet Conditions

Radial inflow expander units are increasingly employed in both oil and gas upstream and downstream markets for energy saving purposes. While prominent attention is given to the performance of the expansion wheel, the downstream diffuser is scarcely covered in literature. In modern expanders, the gas entering the diffuser is typically high-speed, non-uniformly distributed from hub to shroud. Recovery factor and pressure loss coefficient are then affected by both the inlet gas conditions and the geometry of the system, including shaft end and divergence angle of the diffuser. In the present work, the application of computational methodologies to expander diffusers is initially assessed. A sensitivity analysis is then performed with respect to the inlet flow conditions and the diffuser shape. Trends of variation of recovery factor and loss coefficient are provided as a function of selected geometrical parameters and boundary conditions. It is shown that, starting from a non-optimized diffuser, an overall machine efficiency gain can be achieved. That performance improvement can be appreciated by the current market of expanders as a non-negligible competitive advantage.