Axial or Radial? Selecting the Ideal Turbomachinery Configuration Considering Rotor Dynamic Constraints

In many industrial applications, from power generation to aviation, turbomachinery is essential. A crucial choice to be made during the design phase is whether to go with an axial or radial configuration. This decision significantly influences the performance, efficiency, and reliability of the turbomachine.

The primary distinction between axial and radial turbines lies in the way fluid flows through the components (compressor and turbine). These two types of turbine configurations are illustrated in Figure 1.

Figure 1 3D Image of a Radial and Axial Turbine Made in AxSTREAM, [1]
However, it is also important to remember that both axial and radial turbomachinery configurations have advantages and disadvantages, and they can be applied to various types of machines.

The fluid flow in an axial turbine is parallel to the rotational axis. Compared to certain other turbine configurations, the axial flow path allows for a progressive increase in fluid velocity, resulting in better energy extraction. Applications where a continuous, high-volume flow is required, such as gas turbines and jet engines, frequently use this design. Although the axial configuration is more efficient and has a more compact design, it presents difficulties in controlling the dynamics of the rotor, particularly in high-speed applications.

In contrast, a radial turbine uses a flow that is perpendicular to the axis of rotation. Applications like centrifugal compressors and pumps commonly use this design. However, when it comes to handling high-volume flows, they might not be as effective as axial designs.

To understand how to select the ideal turbomachinery configuration, considering rotor dynamic constraints, it should be examined in detail using the examples of two optimal flow path designs: axial and radial. The examples modeled using AxSTREAM RotorDynamics™ and AxSTREAM Bearing™ software [1] indicate that a comparative study of the rotor dynamic restrictions of an axial and radial turbine can be carried out.

Based on this analysis, it is possible to emphasize the following points:

  • The length between bearings in radial turbines substantially exceeds the length used in axial turbines due to the need to allocate space for the volutes.
  • The mass of the radial turbine is higher than the mass of the axial turbine due to the weighting in the form of impellers.
  • In the case of an axial turbine, the shaft diameter is not limited (it can be high), while the shaft diameter in a radial turbine has its maximum due to the lower hub diameter.

 

An axial rotor design is preferable, according to a comparison of the rotor dynamic restrictions of radial and axial turbines. The comparative study of the rotor dynamic restrictions of an axial and radial turbine is illustrated in Figure 2.

Figure 2 Axial vs Radial: Rotor Dynamic Restrictions, [1]
One can determine which turbine design (axial or radial) would be preferable based on the undamped critical speed analysis employing rigid bearings of an axial and radial turbine, as shown in Figure 3.

It can be seen that in a radial turbine, the first critical speed is below the operating speed, which makes the rotor flexible. In contrast, in an axial turbine, the first critical speed is much higher than the operating speed, making the rotor accordingly rigid.

From this observation, it follows that an axial rotor design is preferable because it helps avoid potential resonance conditions below the nominal operating speed of 12,000 rpm [1].

Figure 3 Axial vs Radial Undamped Critical Speeds
Figure 3 Axial vs Radial: Undamped Critical Speeds

The decision between axial and radial configurations in the field of turbomachinery design is complex and reliant on a number of variables. Engineers must carefully consider the advantages and downsides of each configuration when contemplating rotor dynamic restrictions. Ultimately, making an informed choice will result in the creation of turbomachinery that not only achieves performance objectives but also guarantees long-term dependability in a variety of industrial applications.

AxSTREAM RotorDynamics™ and AxSTREAM Bearing™ software can become an indispensable assistant in selecting the ideal turbomachinery configuration considering rotor dynamic constraints. To learn more about AxSTREAM RotorDynamics™ and AxSTREAM Bearing™, schedule a meeting with our team at Info@SoftInWay.com, or check out our webinar covering this very topic!

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