Exhaust gas recirculation (EGR) cooiing devices are used in EGR systems to significantly reduce NOx emissions from diesel engines. However, the therma! and hydraulic performances of these devices change over time during operation due to the deposition of soot from the diesel exhaust gas in these devices. The objective of this work was to investigate in detail the heat transfer and the soot deposition characteristics in diesel engine EGR cooling devices.

Controlled soot deposition tests were performed on a series of generic single-tube and three-tube EGR cooling devices that were exposed to exhaust gas for different periods of times in the diesel exhaust test facility to investigate the time-dependent heat transfer and pressure drop characteristics of these devices. A non-destructive technique was developed to characterize the three-dimensional soot layers that occurred in the EGR cooling devices using the neutron radiography test facility of the McMaster Nuclear Reactor.

The results showed that the deposited soot layer in the single-tube devices was much larger for the higher flow rate which corresponded to turbulent flow, particularly in the entrance region. The thickness of the deposited soot in the entrance region of the tube was much larger than the remainder of the tube, and appeared to restrict the flow area by up to 75% over a length of 4 tube diameters. The variation in the deposited soot thickness throughout the rest of the tube is similar in magnitude to the layer itself indicating that the soot layer in the tube was three-dimensional. The results also showed that the profiles of the deposited soot had a wave-like structure in all cases. The thermal effectiveness of the devices decreased from approximately 82% to % over 5 hours for the laminar flow test case, and from 70% to 35% for the turbulent flow test. The pressure drop across these devices increased by up to 320% during the 5 hours of testing.

Measurements were also performed using three-tube EGR cooling devices with 45° and 60° expansion angle inlet headers exposed to diesel exhaust for 3 hours. The soot deposition was more evenly distributed in the tube bundle with the 60° expansion angle inlet header suggesting that the flow may be more evenly distributed in this device. There was more soot deposited in the center tube than outer tubes in the three-tube bundle with the 45° expansion angle inlet header suggesting that the flow was not evenly distributed. The thermal effectiveness in both cooling devices decreased during the 3 hours of testing, but the change in the effectiveness was larger for the device with the 45° expansion angle inlet header. The pressure change in the exhaust gas across the cooling devices increased more rapidly for the device with the 45° expansion inlet header, reaching a nearly constant value of nearly 5 times the initiai value after 3 hours.