Neutron Radiography Study of Diesel Engine Exhaust Soot Depositions in a Exhaust Pipe With and Without Water Coolant

An investigation was performed to study the soot deposition and its effect on heat transfer in a cooled cylindrical section. The soot layer thickness was measured using a non-destructive neutron radiography technique. Experiments were performed for a diesel exhaust mass flow rate of 20kg/hr or Reynolds number of approximately 9,000, initial inlet coolant temperatures of approximately 22 and 40°C, and exposure times from 1 to 3 hours. The results show that the nominal soot layer thickness was approximately uniform in the flow direction, hence, the thicker soot layer observed near the entrance by Ismail et al. [8] and de la Cruz et al. [9] was due to entrance effects. The deposited soot layer shows evidence of long wavy thickness variations that appears to be due to a soot re-entrainment and re-deposition moving bed type mechanism. The soot thickness increased and the long wavy variations persisted for larger soot thicknesses when the coolant temperature or wall temperature was lower. There was also evidence of larger soot thickness layer roughness for the lower wall temperature. These results could be due to an increase in the condensation of the vapour components at the lower wall temperature that may increase the soot adhesion. The heat transfer performance decreased faster for the lower coolant temperature, which is consistent with the soot deposition thickness measurement.