Flow mechanisms suppressing the flow separation in two diffusers, a low-solidity cascade diffuser and a vaned diffuser with additional small vanes near the inlet, were compared mainly by numerical simulation. As the superiority of the low-solidity cascade diffuser was expected, a series of experiments was conducted using a transonic centrifugal compressor with a maximum pressure ratio of 7. The performance of the compressor with the vaned diffuser was comparable to that of the low-solidity cascade diffuser only between the surge point and the design flow rate at a pressure ratio of 3.5. The maximum flow rate of the vaned diffuser was lower than that of the low-solidity cascade diffuser. At higher rotational speeds, the pressure ratio at the surge point, the efficiency, and the flow range of the low-solidity cascade diffuser exceded those of a vaned diffuser at a pressure ratio of 3.5. Keywords Compressor, Diffuser, Numerical analysis, Stall In many radial compressors, vaned diffusers are widely used to obtain high efficiency. However, this causes the problem of a Laboratory of Hitachi Industries Co., Ltd., for the execution of the numerical simulations. The authors are also grateful to Hideo Sasaki and Hidenori Hasegawa, who were graduate students while this paper was being composed, and Nobumasa Kawaguchi of Kyushu University who assisted in running the experiments.Address correspondence to Koji Nakagawa, Department of Mechanical Engineering, Hakodate National College of Technology, 14-1 Tokura-cho, Hokkaido, 042-8501, Japan. E-mail: nakagako@ hakodate-ct.ac.jp narrow flow range. A low-solidity circular cascade diffuser was proposed so as to avoid this problem in regions of subsonic flow (Senoo et al., 1983) and transonic flow (Hayami et al., 1990). The special feature is that this type of diffuser does not have a throat.Diffusers with throats, such as channel-type diffusers, are still in wide use, despite the problem of choking, because of the accumulation of data concerning them. To extend the flow range of these diffusers, modification of the vane shape is practical because of its simplicity. The V-shaped inlet (Dallenbach and Van Le, 1960) and the pipe diffuser (Morris and Kenny, 1971) are representative techniques. This V shape was effective even in subsonic cases (Yoshinaga et al., 1980). The three-dimensionally twisted vane (Jansen and Rautenberg, 1982) extended the available operation range by a factor of 2. A common feature of these diffusers is that the vane inlet matches the real flow. As a flow distortion is stronger on the shroud side than on the hub side, small vanes were added near the diffuser's vane inlet on the shroud side. This diffuser had a remarkable effect in a highsubsonic, centrifugal compressor with a pressure ratio of 2, with a suction damper (Nakagawa et al., 1995). When the damper opening was 40%, the surge flow rate decreased by 35% and the maximum head rise increased by 15%. At full damper opening, the flow range and the maximum head rise increased only a little. In addition, a...

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