What Exactly is a Mixed-Flow Turbomachine?

A review of the literature on mixed-flow turbomachinery reveals a surprising variation in the ways engineers use the term ‘mixed-flow turbomachine’. In particular, two main conceptions of the term stand out. On the first conception, ‘mixed-flow’ refers to the union of axial and radial flow directions into one diagonal flow, resulting in what some call diagonal turbomachines. In the second conception, ‘mixed-flow’ refers to the combination of distinct elements where the flow is axial in some and radial in others. This kind of machine is sometimes called an axial-radial combined turbomachine.

Diagonal turbomachines utilize a flow angle that is between axial and radial and may be considered mere variants of radial machines. The diagonal flow angle allows these machines to enjoy some benefits from both axial and radial flows. In contrast, axial-radial combined turbomachines represent a strategic integration of both axial and centrifugal designs.

In light of the ambiguous use of the term, one might reasonably wonder what truly defines a mixed-flow turbomachine. Is it the convergence of flow directions, the strategic blending of different machine types, or a fusion of design elements?

Figure 1. 3D view of a mixed-flow (diagonal) compressor in AxSTREAM

Diagonal turbomachines are characterized by their flow’s meridional exit angle, which ranges between 0 and 90 degrees. This geometry enables the flow to exit closer to an axial direction, with the exit mean radius greater than that of the inlet. In mixed-flow compressors, this design facilitates higher efficiencies within a constrained cross-sectional area. This advantageous setup addresses a critical need in various applications like unmanned aerial vehicles (UAVs), where the integration of gas turbines demands superior performance within limited spatial constraints as well as a high thrust-to-weight ratio. Read More

Turbomachinery Design Strategies & Tips: How to Choose Between an Axial or Radial Configuration

From the electricity that charges our phones to the jet engines that propel airplanes across the sky, turbines can be found powering our modern world in various forms and configurations.  These mighty machines are the silent heroes of our energy infrastructure, found in everything from locomotives and power plants to industrial machinery and rocket engines. But what distinguishes one turbine from another? How do engineers decide on the design and configuration of these mechanical marvels? This intricate task requires an understanding of turbomachinery design, including axial and radial configurations. So, let’s dive into the differences between an axial and radial configuration.

Fig.1 Example of an axial turbine used in a jet engine. Source

In an axial turbine, the fluid (such as steam, gas, or water) flows along the rotation axis, similar to a windmill where the fluid enters and exits in the same direction. The turbine blades are arranged in stages along the rotor, with each stage converting the fluid’s energy into mechanical energy. Read More