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An important goal for CFD simulation in engine design is to be able to predict the combustion behavior as operating conditions are varied and as hardware is modified. Such predictive capability allows virtual prototyping and optimization of design parameters. For low-temperature combustion conditions, such as with high rates of exhaust-gas recirculation, reliable and accurate predictions have been elusive. Soot has been particularly difficult to predict, due to the dependence of soot formation on the fuel composition and the kinetics detail of the fuel combustion. Soot evolution in diesel engines is impacted by fuel and chemistry effects, as well as by spray dynamics and turbulence. In this work, we present a systematic approach to accurately simulate combustion and emissions in a high-performance BMW diesel engine. This approach has been tested and validated against experimental data for a wide range of operating conditions.

In exhaust systems with selective catalytic reduction (SCR) a fast conversion of liquid urea to gaseous ammonia and a uniform distribution of the ammonia vapor upstream of the SCR catalyst are essential to reduce the nitric oxides efficiently. For the prediction of the mixing process and the transport of ammonia vapor with the CFD method an accurate description of the turbulent flow field is a basic requirement. This paper presents the comparison of simulation results using three different turbulence models (high-Re kε-RNG model, low-Re kω-SST model, Reynolds stress model) with measurements of the turbulent velocity field using Laser Doppler Anemometry (LDA). The investigations were carried out for a SCR system with a swirl mixer on a cold flow test bench for two different volume flows. From the measured velocity signals different components of the Reynolds-tensor were derived.

The Selective Catalytic Reduction (SCR) is a promising approach to meet future legislation regarding the nitric oxide emissions of diesel engines. In automotive applications a liquid urea-water solution (UWS) is injected into the hot exhaust gas. It evaporates and decomposes to ammonia vapor acting as the reducing agent. Significant criteria for an efficient SCR system are a fast mixture preparation of the UWS and a high ammonia uniformity at the SCR catalyst. Multiphase CFD simulation is capable to support the development of this process. However, major challenges are the correct description of the liquid phase behavior and the simulation of the ammonia vapor mixing in the turbulent exhaust gas upstream of the SCR catalyst. This paper presents a systematic study of the impact of the turbulence model and the numerical spatial discretization scheme on the prediction of the turbulent mixing process of the gaseous ammonia.