The conventional concept of soot and ash wall deposits (i.e. cake-layers) gradually building up along the channels of a ceramic honeycomb and then periodically or continuously being swept downstream toward the end-plugs of the channels may not always occur in practice. When deposits irregularly form on or detach from the walls, causing premature clogging usually around the mid-sections of the channels (also known as Mid-Channel Collapse), and the particulate filter is prone to experiencing significantly elevated back pressure, resulting in the need for earlier repair or replacement than desired. Here we describe related experiments that were performed, accompanied by analysis and simulation, in order to investigate the factors that contribute to the patterns of wall deposits that form-particularly of ash-and the effects of these irregular patterns.
Experiments attempting to duplicate conditions in which irregular deposits occur were carried out with a combination of actual engine and diesel-burner exhaust to achieve a variety of exhaust conditions, such as changing soot and ash concentrations and their relative ratios, flow rates (space velocities), and exhaust temperatures. In this study, the particulate filter was loaded with different soot ash ratios in the exhaust and periodically regenerated. X-Ray Computed Tomography (CT) was used to scan the spatial distributions of the ash deposits, radially and axially along the channels for the various conditions. In addition, HRSEM (High Resolution Scanning Electron Microscopy) and FIB (Focused Ion Beam) were employed to monitor the filter wall surface and deep-bed area respectively, and then related images were analyzed to study the detailed morphology of the ash deposits and their interactions with the substrate.
Computer simulations were also conducted, in conjunction with the image analyses, to correlate the results of the physical plugging and deposit distribution information, and to decipher the plausible connections to the soot/ash deposit conditions. The ultimate objective is to isolate the causes of Mid-Channel Collapse and control the stability and progression of the deposits.
Experiments have explored only a relatively small portion of the full test matrix we have generated that was intended to elucidate all potentially contributing factors. However, interesting data have already begun to emerge. Results thus far indicate that the observed patterns of irregular deposits are consistent with the corresponding measured pressure-drop data, and that the exhaust and soot/ash accumulation parameters affecting local temperatures (peak, duration, frequency) do appear to affect the properties of the cake layer and DPF performance as well.