CFD-Based Numerical Modeling and Multi-Objective Optimization of Three-Way Catalytic Converters in Bangladesh: Flow-Direction Transformation, Thermal Characteristics, Pressure Drop, and Pollutant Conversion Efficiency

Abstract

The increasing use of internal combustion engines and natural-gas/diesel dual-fuel technologies has created a continuing need for effective exhaust after-treatment systems capable of maintaining high pollutant conversion under highly transient operating conditions. This study develops a computational fluid dynamics (CFD)-based numerical framework for evaluating and optimizing a three-way catalytic converter (TWC) incorporating periodic flow-direction transformation for low-load and variable-load engine operation in Bangladesh. The developed model considers the coupled effects of flow reversal, catalyst temperature, inlet velocity, residence time, pressure drop, air–fuel equivalence ratio, and catalytic conversion of hydrocarbons (HC), carbon monoxide (CO), and nitrogen oxides (NOx). A two-dimensional catalytic-channel model was constructed using a ceramic honeycomb substrate with a Pt/Rh catalytic washcoat and porous-medium representation. The computational framework incorporates transient flow, heat transfer, species transport, porous resistance, heterogeneous catalytic reactions, and oxygen-storage behavior. The computational configuration consisted of a 200 mm catalytic region with 50 mm non-catalytic sections on both sides, a 1 mm channel scale, a 0.2 mm porous catalytic layer, and a porosity of 0.41. The model was evaluated under flow-reversal periods of 5–30 s and inlet velocities of 1–6 m/s. Under the low-load reference condition, flow-direction transformation increased HC conversion from 84.6% under one-way flow to approximately 88.5–91.0%, while CO conversion increased from 94.7% to approximately 96.7–97.4%. The influence of reversal frequency demonstrated that excessively short reversal intervals generate repeated transient deterioration, whereas intervals around 10–15 s provide a practical operating window. Increasing inlet velocity from 1 to 6 m/s reduced the reported maximum reactor temperature from approximately 820 to 740 K and reduced HC conversion from 99.5% to 71.3%. CO conversion decreased from 99.9% to 85.5%, while NOx conversion changed from 85.7% to 81.0%. Under rich and lean conditions, HC/CO/NOx conversion efficiencies were 83.9/95.0/96.3% and 91.5/97.6/53.8%, respectively. The results demonstrate that flow-direction transformation can enhance HC and CO conversion by improving thermal regeneration and catalyst utilization, while inlet velocity, residence time, and air–fuel conditions govern the trade-off between conversion efficiency, thermal behavior, and pressure losses. The study provides a computational basis for optimizing TWC operation for Bangladesh-oriented natural-gas/diesel exhaust-treatment applications.