Geometric Description of the Soot Cake in a One-Dimensional Model of an Octo-Square Asymmetric Particulate Filter

摘要

Asymmetric particulate filters (PF), where the inlet channel is wider than the outlet channel, are commonly used because of their greater ash capacity. Surprisingly, very few models for asymmetric PFs have been published. This paper considers how to model the soot cake in octo-square asymmetric PFs. Some previous studies have neglected the octahedral shape of the inlet channel and instead assumed that the inlet channels were square. As the correct approach for modelling the soot cake is not obvious, three options are considered. The calculation of soot-loaded channel perimeter and hydraulic diameter (which are important for heat and mass transfer), soot thickness and backpressure as a function of soot loading are given for each geometry. In option 1, the shape of the soot-loaded channel is assumed to be geometrically similar to the soot-free channel. This is the simplest option mathematically, but results in the soot cake being thicker over the slanted sides of the inlet channel than over the straight sides (for the normal case where the straight sides are longer than the slanted sides), resulting in non-uniform flow which complicates calculation of backpressure and soot oxidation rate. Options 2 and 3 avoid such complications. Option 2 assumes that gas only passes through the straight sides of the inlet channel, resulting in the soot building up along the straight sides only. With option 3, the thickness of the soot cake is the same over the straight and slanted sides with gas leaving the channels through both straight and slanted sides. Predictions for option 2 are significantly different from those of options 1 and 3, which are quite similar. Literature data (CFD, microscopy), suggest against option 2. Therefore, option 1 is favoured as it gives comparable results to the seemingly more correct option 3, but is much simpler to code.