Based on stage inlet flow coefficients,?_0The two types of compressors axial-flow compressor and centrifugal compressor achieve their optimum performance at quite different dimensionless performance parameters. Expressed in term of specific speed or the characteristics stage inlet flow coefficients,?_0 (not flow coefficient ? )?_0=m ?/(?r_tip^2 U_tip ?_t0)The stage efficiency levels for centrifugal compressor stages is sufficiently high for the range of ?_0 less than 0.16. The operation at higher value of ?_0 requires a mixed-flow design. For Sufficiently high values of ?_0 , axial flow design is preferred.Flow coefficients ?_0and ? – approximately related using hub-to-shroud ratio, ??_0=1/2 (1-?^2 )(1+?)??=r_h/r_sFront stages of a compressor value of hub-to-shroud ratio, ? is in 0.4 to 0.6 ranges, which gives ?_0 in 0.25 to 0.3 range. In this case, optimized axial-flow stages is expected to be selected. However, for rear stages of a compressor, large Values of ? produce values of ?_0 values in the range normal to centrifugal. Centrifugal stage substituted for an axial flow stage will normally operate at a lower value of ?_0 than the axial-flow stage it replaced. This follows from the fact that r_tip and U_tip will typically be greater than axial for centrifugal stage. Higher stage pressure ratio offered by the centrifugal stage may permit replacement of several axial-flow stages to reduce cost.To decide for replacement of axial stages, compute in the rear stages and estimate r_tip of a replacement centrifugal compressor to compute for the centrifugal stage. If upstream axial-flow stages have reduced to a value suitable for a centrifugal stage, the replacement may be appropriate. If the axial length available is limited, in this case as well a final centrifugal stage may be opted.Based on specific speedSimilarity analysis offers a convenient method to characterize turbomachinery.567 From similarity considerations it has been shown that compressor efficiency is mainly a function of dimensionless similarity parameters specific speed (N_s), specific diameter (D_s), Reynolds number (Re) and Mach number ( M ). Figure 2.4 “Cordier line” in N_s D_s diagram 8N_s D_s diagram by Cordier 19558 for different types of turbomachines is obtained by plotting test data as a function of specific speed and specific diameter as shown in Figure 2.4. Solid line frequently referred as Cordier line is location of efficient compressor designs. The diagram indicates that axial machine more efficient at the high specific speed regime whereas radial machines are more preferred in the low specific speed regime. However, refining the geometry and flow field such as blade shape and meridional contour will be required to develop a design which is superior to current state of the art knowledge represented by the Cordier line.Studies on Axial-Centrifugal Configuration for Compressor Schweitzer and Fairbanks 19833 designed, fabricated, and tested a full-scale axial-centrifugal compressor test-rig. The compressor developed was utilizing six stages of highly loaded low-aspect-ratio axial combined with a centrifugal back end stage to achieve a pressure ratio of 18:1. The author explained the benefits of low-aspect-ratio blading (varied from 1.35 for the first rotor to 0.8 for the sixth-stage stator) to justify the consideration for high axial compressor loading levels (15% increase over their current production engine compressors) in terms of increased efficiency and stability. The combination of low-aspect-ratio and high loading resulted in fewer required stages and fewer airfoil per stage. They also suggested that the larger geometry low-aspect ratio blades can also provide improved durability and increased performance retention in extended operation. Whereas a centrifugal compressor as back end stage was justified as, it can replace a number of small axial stages, while the performance approaching that of axial compressors can be obtained especially in low-flow, low-pressure-ratio applications. Figure 2.5 Axial-centrifugal compressor flowpath 3 Figure 2.6 Schematic of axial-centrifugal compressor test rig 9Duc Vo and Trépanier 20159 and Michaud et al. 201610 of Ecole Polytechnique de Montreal, Canada has designed and developed an axial-centrifugal compressor test rig (see Figure 2.6 ) for research in compressor aerodynamics. They have designed and fabricated this rig by considering various aspects of design such as aerodynamics, structures, dynamics etc. They also have carried out experimentation by detailed flow measurements. These experimentations have been used for computational fluid dynamics (CFD) code validation. ?