EFFICIENCY INCREASE IN CENTRIFUGAL COMPRESSOR WITH OPEN IMPELLER BY USING CURVILINEAR ELEMENT BLADE

The effect of curvilinear element blade on open-type centrifugal impeller is numerically examined in this research. We compared four different open-type centrifugal impellers. Two of them were linear element blade impellers: one has a positive tangential lean to the rotation at the outlet and the other has negative tangential lean. The other two were curvilinear element blade impellers having different tangential lean profiles. Numerical simulation of steady flow was conducted in one-blade path model with inlet, impeller and vane-less diffuser regions. The simulation confirm that impellers with negative tangential lean profiles improve the stage efficiency under the flow rate at the design point and the curvilinear element blade improve the stage efficiency not only at the design point but the lower flow rate regions also. The efficiency increase at the design point is obtained not in the vane-less diffuser region but in the impeller region since the negative lean profile reduce the tip-leakage loss near the impeller outlet region. The negative tangential lean profile changes the pressure distribution in the flow passage between two impeller blades. The pressure difference between the pressure surface and the suction surface of one blade near the tip region decrease because of the pressure distribution change. Hence, the mass flow rate of the tip leakage decreases and reduces the loss generation. In addition, the reason why the stage efficiency increases under the lower flow rate conditions by using curvilinear element blade is the suppression effect of the flow separation in the vane-less diffuser. This result indicates that there is a design criteria both tangential lean angle and the inflection point of the profile from positive to negative to suppress the flow separation at the hub side in the vane-less diffuser.