In this study, a simulation-based flow optimization of the diesel particulate filter (DPF) system is performed. The geometry and the swirl component of the inlet flow is optimized to improve flow uniformity upstream of the filter and to decrease overall pressure drop. The flow through the system is simulated with Fluent computational fluid dynamics (CFD) software from Fluent Inc. The wall-flow filter is modeled with an equivalent porous material. This study only investigates the clean flow.
The DPF system is composed of three parts: the inlet diffuser, the filter and the outlet nozzle. In the original system a linear cone joins the inlet and outlet pipes to the cylindrical filter. Due to the large opening angle of this cone, flow separates and creates a recirculation zone between the inlet and the filter. The flow pattern reveals that a large area of the filter is not used: More than 88% of the air flow passes through less that 53% of the area. The filter, as well as the inlet and the outlet pipes, have standard diameters, therefore only the geometry of the cone can be optimized to increase the use of the filter. A unipolar sigmoid function (USF) is used to generate a streamlined diffuser and nozzle therefore avoiding any separation zone. This function is controlled by one parameter which is optimized. The optimum USF shape considerably improves the flow uniformity in front of the filter.
To further improve the flow uniformity in the actual system, a swirl velocity is added. Two cases are investigated; a swirl flow with different angular velocities and a swirl flow with constant velocity applied only on a small annular area on the outer diameter of the inlet pipe. All results indicate that the addition of a swirl component to the flow improves flow uniformity, nevertheless, a constant swirl on the outside of 20 m/s is ideal. This effect, combined with the optimum USF shape, greatly increases flow uniformity and reduces the overall pressure drop.