CFD and Measurements of a VNT-CCC Configuration: Is it Feasible to Replace Exhaust Pulsations with Constant Flow?

Velocity and temperature distributions in a Close Coupled Catalyst (CCC) were analyzed using two otherwise identical Computational Fluid Dynamics (CFD) setups: one with pulsating inflow boundary conditions and the other with constant inflow. The objective was to assess the validity of the common simplification of assuming constant inflow. Simulated temperature fields were also compared with experimental measurements. While several experimental and CFD studies have addressed this question, they have primarily focused on naturally aspirated engines. This study examines a CFD domain featuring a turbine without a wastegate, resulting in fundamentally different flow conditions reaching the CCC. The results indicate that the assumption of constant flow in CCC CFD analysis should not be taken for granted, but rather critically evaluated. Before adopting this simplification, it is important to assess the specific EATS configuration under pulsating flow conditions to determine whether the associated loss in accuracy is acceptable. This study also shows that assuming constant flow leads to higher temperatures entering the catalyst, likely due to the enhanced convective heat transfer associated with pulsating flows, as reported in the literature. Consequently, for CFD applications such as catalyst light-off prediction, assuming constant inflow may result in overly optimistic estimates of light-off times.