CFD analysis of the effect of tip clearance on the turbocharger tandem-bladed centrifugal compressor stage

In order to explore the benefits of tandem-bladed design, numerical investigation of the effect of tip clearance on the performance of an unshrouded tandem-bladed centrifugal compressor is carried out. Conventional impeller was modified to tandem design with no other modifications in geometry. Tandem bladed impeller with nine clearance levels was investigated at various mass flow rates. It has been observed that at the mass flow rate of 0.40 kg/s, a loss of about 15% in stage pressure ratio is observed as the tip clearance is increased from 0 to 1.0 mm whereas the peak efficiency has also been reduced by 12%. The impeller efficiency is found maximum at clearance of 0, however minimum diffuser effectiveness is observed at 0.4 mm. The simulations reveal no performance improvement with tandem design except the increase in choke flow. Contours for entropy and normalized relative velocity at impeller exit for 0, 0.1, 0.4 and 0.6 mm clearances are presented, which indicate the loss production and verify the classical jet-wake theory. The blade loading diagrams reveal that in order to achieve improvement through tandem design, either the shape of inducer or/and exducer or blade clocking or axial gap need to be modified so that the blade loading could be improved.