Neutron Radiography of Convective and Thermophoretic Diesel Engine Exhaust Soot Depositions in a Cooled Rectangular Chamber

An investigation was performed to study the effects of convection, diffusion and thermophoresis on diesel exhaust soot deposition inside a plate-type rectangular cooling section for recirculation (EGR) applications since deposited soot can be detrimental to the heat transfer efficiency of EGR cooling devices. A non-destructive neutron radiography technique was used to measure the soot deposition thickness distribution on the plate surface inside the cooling chamber. The chamber cooled with an inlet water coolant of 20 and 40°C, was installed in a modified exhaust system of a 2.4kW diesel engine and subjected to a mass flow rate of 20kg/hr of diesel exhaust ranging 0 to 3 hours with the exhaust gas temperature at 260°C upstream of the cooling chamber. In this work, the effect of cooling temperature and operation time on thermophoretic deposition was investigated. Results show the mean soot deposition thickness increases with increasing engine operating time and decreases with increasing inlet coolant temperature. Further based on the soot deposition thickness distribution profile and outside wall surface temperature profile, the thermophoretic soot deposition was dominant since the soot deposition pattern qualitatively matched the temperature profile of the cooling wall. Mechanism of soot deposition will be discussed based on the observed soot deposition.