Research and development of vaneless counter-rotating turbomachinery (VCRT) is briefly reviewed. Then basic technical challenges on inflow angle and off-design operation are analyzed regardless of extrinsic differences among any kinds of VCRT (either compressor or turbine or lift fan, either highly or moderately loaded). A somewhat new approach based on velocity triangle analysis played the most important role in obtaining useful results. It is shown that the range of relative inflow angle of the second rotor is narrowed by counter-rotation. And as compared with conventional turbomachinery, varying back-pressure and varying rotation speed off-design operation of VCRT are more complicated. For some rotation speed ratios and flow coefficients, VCRT leads to better operation of varying back pressure type than conventional one, whereas some not. In varying rotation speed operation, VCRT needs variable rotor to get a comparable performance. Counter-measures to overcome these challenges are also given. These analyses will be directly used to judge the applicability of VCRT and determine basic rules and ranges in selecting design parameters of VCRT.

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