AAAR 31st Annual Conference
October 8-12, 2012
Hyatt Regency Minneapolis
Minneapolis, Minnesota, USA
Abstract View
Visualization of Filter Pore Bridging with Diesel Particles from Two Different Size Distributions
SIMON PAYNE, Nick Collings, University of Cambridge
Abstract Number: 715 Working Group: Combustion
Abstract Particulate bridging over filter pores is responsible for the steep initial increase in engine exhaust back pressure as a clean Diesel Particle Filter (DPF) is loaded. It is of significant advantage to engine operation to mitigate this pressure drop penalty by minimising deposition of particles deep within the pores and forming a cake as rapidly as possible. Understanding of the pore bridging process is immensely aided by visualization with scanning electron microscopy (SEM). The same pores of a silicon carbide (SiC) filter wall were loaded successively with diesel particles from two different size distributions. The first aerosol was sampled from the exhaust of the Cambustion Diesel Particulate Generator (DPG) and the second from a medium-duty diesel engine via a catalytic stripper; the modal electrical mobility diameters were 120 and 60 nanometers respectively.
While initial filtration efficiency in the SiC wall was higher with the smaller engine aerosol, the transition from deep-bed to cake filtration occurred at a higher pressure drop and SEM images show a greater amount of particulate matter from the smaller aerosol was deposited deep within the walls. Smaller particles appear to deposit uniformly along the necks of pores giving the appearance of gradually shrinking pores. It was found that aggregates several hundred nanometers in diameter contribute to the growth of dendrites that extend over pores up to 20 microns across, which is particularly evident with the DPG aerosol. If particles several microns in size are present, these can cause sudden plugging of smaller pores which substantially reduces the pre-cake pressure drop penalty. The nature of pore-filling is best characterised by a locally defined interception parameter; a threshold value of 0.05 for the ratio of particle to pore size was estimated for the transition between shrinking pore behaviour and the dominant action of dendrites.