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Multi-Dimensional Modeling of the Soot Deposition Mechanism in Diesel Particulate Filters
Abstract:
A computational, three-dimensional approach to investigate the behavior of diesel soot particles in the micro-channels of wall-flow Diesel Particulate Filters is presented. The KIVA3V CFD code, already extended to solve the 2D conservation equations for porous media materials [1], has been enhanced to solve in 2-D and 3-D the governing equations for reacting and compressible flows through porous media in non axes-symmetric geometries.
With respect to previous work [1], a different mathematical approach has been followed in the implementation of the numerical solver for porous media, in order to achieve a faster convergency as source terms were added to the governing equations. The Darcy pressure drop has been included in the Navier-Stokes equations and the energy equation has been extended to account for the thermal exchange between the gas flow and the porous wall. The mesh generator K3PREP and the code have been extended to define geometries having an arbitrary number of symmetry axis, in order to perform simulations of 3D sectors of the filter, where a sector represents a group of DPF channels.
Also, Lagrangian particles in the flow were used to represent the diesel soot particles. The increase of the soot cake layer thickness on the porous wall and its spatial distribution as a function of time have been determined by the fully coupled particle-hydrodynamics in the gaseous flow, since KIVA3V calculates in the computational domain the rates of mass, momentum, and energy exchange between the gas and particles. The influence of gas molecules-particle interaction on overall particle behavior is examined by including Brownian motion and partial slip in particle equation of motion. Simulations help to highlight three-dimensional non-uniform particle deposition, mainly due to flow distribution in the micro-channel; the soot packing density and thickness on each face of the computational porous cells are calculated.
A set of unsteady simulations was run to steady state for wall-flow Diesel Particulate Filters having substrates with different geometrical and physical properties. Distinct types of diesel particulate matter (PM) in terms of chemical composition, concentration, and size distribution, known by the experiments carried out on a single-cylinder 2.3-liter D.I. heavy-duty diesel engine, were used in the simulations. The experimental distribution of particle sizes and density was injected through the inlet boundary of the computational domain and its deposition along the filter porous surface was monitored. A validation of the code has been carried out by the comparison with pressure drop measurements across the different DPFs studied.