1. Department of Mechanical Engineering, Korea University, Anam-Dong, Sungbuk-Ku, Seoul 136-713, Korea
2. Center for Environmental Energy Engineering, University of Maryland, 4164 Glenn L. Martin Hall Bldg., College Park, MD 20742, USA
3. Department of Mechanical Engineering, Hanbat National University, Duckmyung-Dong, San 16-1, Daejeon 305-719, Korea