1. Hitachi Research Laboratory, Hitachi, Ltd., 7‐1‐1 Omika, Hitachi, Ibaraki 319‐12, Japan
2. Hitachi Works, Hitachi, Ltd., 3‐1‐1 Saiwai, Hitachi, Ibaraki 316, Japan
3. Department of Energy Engineering and Science, Nagoya University, Nagoya 464‐01, Japan