1. Department of Thermal Engineering, Tsinghua University, Beijing 100084, China;
2. Saint Anthony Falls Laboratory, Department of Mechanical Engineering, University of Minnesota, Minneapolis, MN 55414
3. Department of Mechanical Engineering, College of Engineering Roorkee, Roorkee 247667, Uttarakhand, India
4. Saint Anthony Falls Laboratory, University of Minnesota, Minneapolis, MN 55414
5. Department of Thermal Engineering, Tsinghua University, Beijing 100084, China
6. Saint Anthony Falls Laboratory, Department of Mechanical Engineering, University of Minnesota, Minneapolis, MN 55414 e-mail: