Surge Prediction in a Single Sequential Turbocharger (SST) Compressor Using Computational Fluid Dynamics

The Single Sequential Turbocharger (SST) used in Ford’s 6.7L Scorpion Diesel is analyzed using Computational Fluid Dynamics (CFD) to draw conclusions about the compressor stability at low mass flows. The SST compressor concept consists of a double-sided wheel which flows in parallel fed by two separate inlets (front and rear), followed by a single vane-less diffuser, and a volute. CFD simulations for the full stage are performed at low mass flow rates Both, front and rear, sides have ported shroud casing-treatment (CT) in the inlet region. An objective of the analysis is to determine which side of the SST unit compressor (front or rear on the double-sided wheel) suffers flow break down first as the mass flow is reduced, and its impact on the overall stability of the SST compressor. Another objective is to better understand the interactions between the compressor inlet flow and the flow through the casing-treatment. It has been observed that these interactions reduce the effectiveness of the front ported shroud casing-treatment in the selected geometry. This leads to a breakdown of the flow field in the front wheel first and a subsequent overall system instability occurring at higher mass flows compared to a case where the rear wheel flow breaks down first. If the design is such that the rear compressor stalls first, then the SST compressor stage can remain stable to lower mass flow rates. The early instability (at higher mass flow rate) of the compressor due to surge in the front wheel is causing NVH and drivability issues in the vehicle. The utility of CFD to guide the design of the inlets and casing treatment for such type of stages has been demonstrated through comparisons of predicted results to test data.