Physicochemical Characteristics of Soot Deposits in EGR Coolers

Physicochemical characteristics of the soot deposits in a fouled EGR cooler are studied in this paper. It is found that a three-layer model for the soot deposited in the EGR cooler may well describe the behavior of the depositing process: a dense base layer with micro pores (≺ 5 nm), a randomly packed intermediate layer with meso pores (5-50 nm) and a loose surface layer with macro pores (≻ 50 nm). The surface layer is thick and highly porous, formed by mechanical interlocking of the agglomerated primary soot particles or soot clusters. The soot particles in the surface layer may be removed by a high shear EGR flow. Condensates in the deposit, especially water, can have a significant influence on the structure of the deposit. Capillary forces on the wetted soot particles could be comparable to the contact forces holding the particles together. It is found that the hydroscopicity of the soot particles vary with their content of soluble organic fraction (SOF). The dry soot particles themselves are hydrophobic. However, when the soot particles are covered with a thin film of the (SOF) hydrocarbon condensate they may become hydrophilic. The soot particles can be wetted with water vapor or condensed water, causing soot hydration. Soot hydration can generate strong inter-particle forces, which could break the original structure and lead to restructuring. The restructured soot deposit has a denser packing and thus a higher thermal conductivity. Some of the soot particles that result from this restructuring are removable by a high EGR flow. Soot hydration may be applied as a method for the cooler refreshment in the real-world engine operations.