The implementation of exhaust gas recirculation (EGR) coolers has recently been a wide spread methodology for engine in-cylinder NOx reduction. A common problem with the use of EGR coolers is the tendency for a deposit, or fouling layer to form through thermophoresis. These deposit layers consist of soot and volatiles and reduce the effectiveness of heat exchangers at decreasing exhaust gas outlet temperatures, subsequently increasing engine out NOx emission. This paper presents results from a novel visualization rig that allows for the development of a deposit layer while providing optical and infrared access. A 24 h, 379-micron-thick deposit layer was developed and characterized with an optical microscope, an infrared camera, and a thermogravimetric analyzer. The in situ thermal conductivity of the deposit layer was calculated to be 0.047WlmK. Volatiles from the layer were then evaporated off and the layer reanalyzed. Results suggest that the removal o f volatile components affect the thermophysical properties of the deposit. Hypotheses supporting these results are presented.
Exhaust gas recirculation (EGR) is a major technology to reduce NOx from diesel engines for future emission standards. The implementation of EGR coolers has been a common methodology to provide engine in-cylinder NOx reduction. However, EGR cooler fouling is a common problem. The particulate matter in exhaust tends to form a deposit layer on the wall of the heat exchangers. This effect leads to a reduction of thermal effectiveness of the heat exchangers resulting in insufficient EGR cooling and subsequently higher engine NOx emission.
This paper utilized a unique test rig offering visible and infrared optical access to the deposit layer in a simulated diesel EGR cooler to study the evolution of the layer from fresh to heavy deposit. A 460μm thick deposit layer was built during a 37 hour exposure. Time lapse videos were taken provide visual in-situ evidence for the investigation of the layer thickness development and morphology change during the deposition. The layer growth tended to stabilize from about 22 hours of deposition. The shear force exerted by the gas flow moves surface particles of 20μm in diameter or larger. This could contribute to the stabilization phenomenon.
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