Convection, Diffusion, Thermophoresis and Electric Field Effects on Diesel Soot Deposition in a Cooled Exhaust Channel

Abstract

New demands and tighter government legislations on greenhouse gases and pollutants, especially for those produced by diesel engines, there has been much focus on developing more efficient diesel engine designs and pollution control devices. There are several pollution control devices currently being implemented in diesel engines such as, diesel particulate filters, selective reduction catalyst, electrostatic filters, exhaust gas recirculation systems, etc. Diesel particulate matter is of particular importance especially when deposited because of its corrosive and thermal insulating nature. There are many complex mechanisms involved in fine particle deposition. This study will focus on the main deposition mechanisms such as convection, diffusion, thermophoresis and electric field effects. The objective of this study was to evaluate experimentally the mechanisms of diesel soot deposition in a rectangular (RWCS) and cylindrical (CWCS) wall cooled sections to evaluate thermophoretic effects. In additional, the coaxial cylindrical wall cooled section with additional with coaxial wire electrode was used to study applied electric field effect (CCWCSE) on soot deposition. A non-destructive Real-Time Neutron Radiography technique was used to evaluate the soot deposition thickness profiles inside the cooled sections. The experiments were conducted using diesel engine exhaust from a single cylinder diesel engine operated at fixed 2.4kW, at a exhaust gas mass flow rate of 20 kg/hr with exposure times ranging 0 to 3hrs, coolant temperatures from 20 to 40°C and exhaust gas temperatures from 250 and 270°C. The resulting Reynolds Number based on the mass flow rate per cross-sectional area times the hydraulic diameter was 6300 for the RWCS and 9000 for the CWCS and CCWCSE. The results show that for the RWCS, the soot deposition pattern qualitatively matched the cooling water channel outer wall surface temperature profile along with thicker deposition at the entrance region due to convection effects. For the ewes, the deposition was more uniformly distributed throughout the device. It was observed for both devices that as the mean soot deposition thickness increases with increasing exhaust gas exposure time and decreasing wall cooling temperature. Finally the experimental results for the CCWCSE shows that the soot deposition was enhanced by a positive or negative applied electric field.