Thermal performance and mechanisms of soot deposition in foam structured exhaust gas recirculation coolers

Exhaust Gas Recirculation (EGR) coolers are commonly used in diesel engines to reduce NOx emissions. New diesel engine designs though require compact EGR coolers that can withstand severe deposition. This study aimed at determining the underlying mechanisms of soot deposition in metal foam structured EGR coolers. Various experiments were performed for 20 PPI aluminium foam at inlet gas temperatures of 250 and 400 degrees C and coolant temperatures of 25 and 90 degrees C to discern the importance of thermophoresis. The experimental results revealed that the thermophoresis would not be of great concern in the investigated metal foam as otherwise would have been expected in conventional flat EGR coolers. This has been attributed to the thermal equilibrium between fibers and the flue gas which was confirmed by visualization of fouled foams after experiments. In the absence of thermophoresis, isothermal soot deposition on the fibers, including interception, diffusion and inertial impaction were also investigated. For this purpose, a model is proposed to compare deposition velocities due to the aforementioned mechanisms. It is shown that interceptional deposition velocity was marginal, while inertial impaction was important for soot deposition on fibers, especially at higher inlet gas velocities and larger foam PPIs. Diffusion also made a contribution of about half of the total soot deposition on the foam ligaments at a gas velocity of 10 m/s. Finally, a comparison between foam structured and current EGR coolers showed that the higher pressure drop penalty of using foam as EGR cooler can be compensated by partial filling of the channel.