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A centrifugal compressor usually operates with low isentropic efficiency and a terrible stable operating range, resulting from the complex impeller flow structure companied with the intense interaction among the impeller and the diffuser downstream. In many studies, the potential of centrifugal compressor tandem-impeller configurations for improving the compressor has been demonstrated. Whereas, compared with the convincing results on the tandem-designed axial compressors, the results on tandem impellers are limited and contradictory. Very little insight has been provided into the flow mechanisms inside tandem impellers, which is considered to be the primary reason for the confusion in tandem impeller design and application. Tandem impellers are expected to exhibit a totally different behavior due to the intense aerodynamic interaction between the inducer and the exducer, which substantially contributes to the flow structure and the compressor performance change. In the present study, a numerical study of a highly-loaded centrifugal compressor with various tandem designs was conducted to explore the inducer/exducer matching characteristics and the underlying flow mechanism inside tandem impellers. Two tandem impeller design parameters, namely, the inducer/exducer clocking fraction and the axial gap (overlap), were considered in the tandem impeller design process. The tandem impeller was also compared to the existing conventional impeller which the tandem impeller was redesigned for. The results demonstrated that the tandem-designed impeller can improve the centrifugal compressor stage performance and intense inducer/exducer interaction can be observed with changes in the clocking fraction and the axial gap (overlap). The tandem impeller performance is sensitive to changes in axial gap (overlap) when the suction side of the exducer blade is circumferentially close to the inducer blade. The fundamental reason for the performance variation in the inducer and the exducer lies in the inducer pressure change in the blade trailing edge that is determined by the Kutta condition. Additionally, the correlation between the tandem impeller slip effect and the discharge flow quality should be emphasized in the inducer/exducer gap jet analysis, in which the jet injection angle and the Coanda effect of the exducer suction surface critically affect the discharge flow characteristics.

In typical small engines, the cooling air for high pressure turbine (HPT) in a gas turbine engine is commonly bled off from the main flow at the tip of the centrifugal impeller. The pressurized air flow is drawn radially inwards through the impeller rear cavity. The centripetal air flow creates a strong vortex because of high inlet tangential velocity, which results in significant pressure losses. This not only restricts the mass flow rate, but also reduces the cooling air pressure for down-stream hot components. The present study is devoted to the numerical modeling of flow in an impeller rear cavity. The simulations are carried out with axisymmetric and 3-D sector models for various inlet swirl ratio β0(0–0.6), turbulent flow parameter λT(0.028–0.280) with and without baffle. The baffle is a thin plate attached to the stationary wall of the cavity, and is proved to be useful in reducing the pressure loss of centripetal flow in the impeller rear cavity in the current paper. Further flow details in impeller rear cavity with and without baffle are displayed using CFD techniques. The CFD results show that for any specified geometry, the outlet pressure coefficient of impeller rear cavity with or without baffle depends only on the inlet swirl ratio and turbulent flow parameter. Meanwhile, the outlet pressure coefficient of the cavity with baffle is indeed smaller than that of cavity without baffle, especially for the cases with high inlet swirl ratio. The suppression of the effect of centrifugal pumping and the mixing beween the main air which is downstream of the baffle and the recirculating flow of the vortex in the stationary cavity, which are caused by the use of baffle, are the underlying reasons that lead to the reduction of outlet pressure loss.