1. Department of Mechanical Engineering, Hanyang University Sa1-dong Ansan, Gyunggi-do, 426-791 Korea,
2. Department of Mechanical Engineering, Hanyang University Sa1-dong Ansan, Gyunggi-do, 426-791 Korea
3. Korea Electric Power Research Institute 103-16 Munji-dong Yusong-gu Daejon, 305-380 Korea