Numerical Modeling of Centrifugal Compressors With Heterogeneous Incoming Flow due to Low Pressure Exhaust Gas Recirculation

Low pressure EGR (LP-EGR) has become recently in one of the most investigated technique in the field of the internal combustion engines. When LP-EGR is active, two streams with different psychrometric conditions are mixed at the compressor inlet, i.e. fresh air (considered as cold and dry) and EGR (humid and warm).  The main objective of the present work is to assess the impact of the inlet flow heterogeneity, associated with LP-EGR systems, in the numerical configuration and performance of compressor 3D CFD simulations. Two intake configurations will be studied, namely a standard straight duct providing the compressor with a homogeneous flow and T junction with a mixing of air and EGR streams. The results of the considered cases will be analyzed, after the corresponding unsteady RANS simulations, in terms of global variables and local differences. A number of operating conditions will be studied in order to assess the heterogeneous flow field at different working points, focusing on the overtemperatures that could limit the EGR rates to prevent compressor damage. After that, the possibility of reducing the computational effort by performing steady RANS simulations will be explored in both configurations, showing the evidences of how this approach is not enough accurate for LP-EGR applications, and giving some possible guidelines to improve this methodology. Later, the sensitivity of considering the conjugate heat transfer through the walls on the temperature flow field will be studied, which depends on the type of inlet configuration. In this way, the risk of local excessive temperatures for the LP-EGR case at certain operating points can be accurately addressed. Finally, some general conclusions of this work will be summarized, giving some advices to improve this model in future works.