Development of Boiling Prediction Method in LP-EGR Cooler and Shape Optimization for Suppressing Boiling using Boiling Index

An EGR system has been significantly used in order to cope with reinforced exhaust gas regulation and enhancement of fuel efficiency. For the well-designed EGR cooler, performance analysis is basically required. Furthermore, boiling prediction of the EGR cooler is especially essential to evaluate durability failure of abnormal operating conditions in DPF. However, due to intrinsic complexity of detailed 3-dimensional heat transfer tubes in the EGR cooler, no precise technique of boiling prediction has been developed. Therefore, this research had been performed in order to fulfill 3 goals: (1) development of 3-dimentional performance prediction technique including boiling occurrence, (2) generation and validation of a new evaluation index for boiling, (3) development of an optimized EGR cooler for suppressing boiling. In order to increase analysis accuracy and reduce analysis efforts at the same time, 3-dimensional single-phase flow analysis was developed. This technique was applied to LP-EGR cooler design of the next generation smart stream engine of HMC. Rig test results were used to validate this newly developed analysis method. Especially, in order to predict global system boiling, a new evaluation boiling index was introduced. It was defined as ratio of boiling heat transfer to total heat transfer and showed good behavior. For shape optimization, the boiling index was evaluated with 12 operating points. A base design showed poor distribution of coolant flow caused local boiling and system boiling when it gets worse. In order to achieve well distributed coolant flow and suppressing boiling, 43 different baffles were investigated using the boiling index. The shape optimized EGR cooler showed 32% reduction of boiling index and slight increment of cooling efficiency compared to base EGR cooler. Additional rig test of the optimized EGR cooler was performed to validate correlation and consistency and demonstrated good feasibility of the new method and the boiling index.