3D Numerical Study of Pressure Loss Characteristics and Soot Leakage Through a Damaged DPF

Diesel Particulate Filters (DPF) are widely used to meet 2007 and beyond EPA Particulate Matter (PM) emissions requirements. During the soot loading process, soot is collected inside a porous wall and eventually forms a soot cake layer on the surface of the DPF inlet channel walls. A densely packaged soot layer and reduced pore size due to Particulate Matter (PM) deposition will reduce overall DPF wall permeability which results in increasing pressure drop across the DPF substrate. A regeneration process needs to be enacted to burn out all the soot collected inside the DPF. Soot mass is not always evenly distributed as the distribution is affected by the flow and temperature distribution at the DPF inlet. As a result, the heat release which is determined by the burn rate is locally dependent. High temperature gradients are often found inside DPF substrate as a result of these locally dependent burn rates. DPF substrate failures such as a Ring-Off Crack (ROC) can be observed due to high thermal stresses during regeneration events.

The main objectives of the current paper were to understand the effects of a ROC failure on flow distribution and pressure loss, as well as soot leakage characteristics utilizing a 3-D Computational Fluid Dynamics (CFD) tool. Detailed 3-D CFD simulations were performed for a realistic sized DPF inlet and outlet channel geometry with a small crack in the transverse direction. Initially, the general flow and pressure distribution characteristics inside the inlet and outlet channels, as well as inside filter wall, are studied in baseline case where a ring-off crack failure is assumed at the center of DPF. Then the effects of filter wall permeability, crack location and exhaust mass flow rate on flow and pressure distributions are investigated. The total filter efficiency was defined in the current study to quantify overall filter filtration efficiency. The total filtration efficiency combines the effects of wall flow efficiency for both the upstream and downstream half substrates and the flow through efficiency for only the downstream half substrate because of exhaust gas leakage through a crack. Additionally, the effects of filter wall permeability, exhaust mass flow rate, ROC location on pressure drop changes, percentage of gas leakage and total filtration efficiency are investigated. These numerical results can potentially be utilized to assist in improving On Board Diagnostic (OBD) accuracy.