Flow Maldistribution in Exhaust Aftertreatment Systems - Numerical Simulations

Licentiate thesis, 2022

To comply with the stringent emission norms under every driving condition, including the coldstart conditions and Real Driving Emissions (RDE) tests, accurate models that capture the spatial and temporal variation of flow distribution information in Exhaust AfterTreatment Systems (EATS) are required. This study is aimed at characterizing and quantifying flow distribution in EATS under transient conditions with realistic geometry that illustrates the complex nature of the flow conditions. Catalytic converters are employed to control emissions. Space limitation in the exhaust line creates non-uniform flow in terms of flow through bends and dead volumes. This limits the performance of the EATS. The flow from the engine does not proceed uniformly to the EATS, creating flow maldistribution at the inlet of EATS. Due to the non-uniform flow, the velocity, temperature and concentrations at the exit are functions of the flow distribution at the inlet. Reactor models that predict conversion of emissions require this information for accurate prediction of conversion of emission gases at the tail pipe.

 

To this end, transient Computational Fluid Dynamics (CFD) simulations are performed to understand the evolution of the flow profiles in the catalytic converter, both under non-reactive and reactive conditions. The flow from the engine is turbulent. Hence, Unsteady Reynolds Averaged Navier Stokes (URANS) equation is used solve for the transport equations to obtain the profiles of velocity, temperature, and concentration in the catalytic converter. The turbulence closure is achieved by using k - w model. The catalyst is modeled as porous media.

 

This thesis presents the results of both non-reactive and reactive simulations, to illustrate the importance of flow and temperature non-uniformity and pulsations respectively. Flow uniformity index is used to characterize the extent of variation of the flow parameters in any catalyst plane.  In addition to the uniformity index, contours and histograms are employed to demonstrate the non-uniform flow field. The specification of inlet fluctuations is also required to emulate real time flow to a catalytic converter. 

 

The contours of velocity and histograms show that there is significant distribution of the flow variables at the catalyst outlet. This illustrates that a single channel model is very ideal, and it requires flow distribution information to predict tail pipe concentrations accurately. The inclusion of flow distribution information in EATS model can make the model more accurate. These models can be then used in control and monitoring of emissions from automobiles.  This will take a step closer towards Zero Emissions.