Radial turbines find their application in different fields as in turbocharging and as organic Rankine cycles (ORCs). In these applications the turbine can operate at high pressure ratio leading to the presence of shock waves and choking inside the turbine.  For performance simulation of internal combustion engines or ORCs completed in a short time, fast one-dimensional models with wide turbine maps covering all possible working conditions are required. Pulsatile flow in turbocharger applications and low speeds of sounds of organic working fluids can lead to turbine operation at or close to the choking limit. Hence, information in this map region is needed to estimate the overall machine performance and to achieve an optimum engine-turbocharger matching or integration into an ORC.

In current physically based mass flow models, choking sub-models are typically built on simple assumptions or empirical correlations.  To get a deeper insight into the flow physics, the occurrence of choking in the stator passage, in the vaneless space and in the rotor passage is categorized and analyzed under different pressure ratio and turbine speeds using CFD simulation as a first approximation before doing complex experimental tests in a turbocharger gas stand. Map regions where those phenomena and further shock waves occur are defined. The impact on mass flow characteristics and efficiency losses are determined. Furthermore, the formation of shock waves results in an unsteady stator-rotor fluid dynamic interaction, which may compromise the integrity of the components and the performance, is described in detail.

The appearance of the shock wave depends strongly of the positon of the stator vanes. The stator passage shock wave was identified at closed rack positions causing the flow distortion at the impeller inlet and an increase in the entropy of the flow. Meanwhile, at open positions, the shock wave appears at the rotor throat with a different intensity. An additionally choking pattern was identified in the vaneless space. Furthermore, the results show the change in the behaviour of the leakage flow and pressure fluctuation in the vanes.

The findings of this paper will allow to develop physical reduced order models based on geometrical parameters and inlet flow conditions to extrapolate the turbine map towards high pressure ratios. In addition, this information will determine when the turbine can be affected by an aerodynamic phenomenon that has to be taken into account in the early phase of the design and during the running of the turbine to avoid operating in a region far from the point of maximum efficiency.


Numerical Simulation of a Radial Turbine at Off-Design Conditions in Presence of Choked Flow

Division / Sponsor:
International Gas Turbine Institute (IGTI)

Paper Proceedings: https://asmedigitalcollection.asme.org/GT/GT2020/volume/84058

Turbo Expo - Virtual Conference

Supporting the need among researchers worldwide for additional online and multimedia content, ASME now offers the ASME Conference Video Collection. This platform will collect and provide long-term access to conference presentation videos, encompassing all technology areas within ASME's technical conference and events program. About 30,000-40,000 engineers across all mechanical engineering and related disciplines participate in ASME conferences each year. This includes authors of approximately 6,000 to 8,000 technical papers published by ASME, the majority of whom also submit video presentations for our virtual conferences. The video collection will be available as an add-on to existing product packages or as a separate subscription.