Radial turbines have long occupied a unique niche in turbomachinery. Known for their compact size, mechanical robustness, and the ability to handle large enthalpy drops within a single stage, they’ve become the preferred solution for turbochargers, micro gas turbines, small power generation units, and other waste heat recovery systems.
One of their key advantages is their geometry, which enables high blade loading and substantial flow turning, allowing a single radial stage to achieve pressure ratios that would typically require multiple axial stages. As a result, the overwhelming majority of radial turbines designed over the past century have been single-stage machines. However, modern applications are beginning to require greater performance from these single-stage designs, creating opportunities for multi-stage configurations.
Fig 1. Common Examples of Radial Turbine Applications
As the landscape of energy conversion technologies rapidly evolves, emerging applications—particularly those involving high-density working fluids, elevated pressure ratios, and stricter efficiency requirements — are pushing single-stage radial turbines to their practical limits.
In response, designers are increasingly exploring multistage radial configurations to extend performance, improve efficiency, and meet geometric constraints. This shift is reflected in the recent introduction of multistage radial turbine design capabilities in AxSTREAM, giving engineers the tools needed to investigate these unconventional, but promising, architectures.
In this blog, we’ll look at why radial turbines are typically single stage, what is motivating the shift to multistage designs, and how they’re being developed using modern design tools.
When Does a Radial Turbine Need More Than One Stage?
The historical dominance of single-stage radial turbines stems from a few key strengths. Their ability to extract large amounts of work in a compact footprint often makes additional stages unnecessary, keeping the mechanical design simple and minimizing losses associated with interstage leakage, disk friction, and ducting.
Radial turbines also support strong flow turning and high blade loading, enabling a single radial stage to achieve pressure ratios that would normally require multiple axial stages—a fact well documented in classical turbomachinery literature and confirmed by experimental studies on small-scale radial machines [1, 6, 8].
A single rotor also reduces manufacturing costs and complexity, while the compact flow path helps maintain high efficiency across a wide range of operating conditions. For many applications, adding a second radial stage would introduce more aerodynamic and mechanical penalties than benefits.
Fig 2. Example of Single Stage In-flow Radial Turbine vs Multistage In-flow Radial Turbine in AxSTREAM
How High-Pressure Ratios Drive the Need for Multistage Design
Modern applications often demand performance beyond what a single radial stage can deliver. Systems with extremely high-pressure ratios—like supercritical CO₂ Brayton cycles, high-pressure Organic Rankine Cycles (ORC) [2, 7, 10], and advanced waste heat recovery units—often require expansion ratios that exceed the practical limits of a single rotor.
Early analyses of sCO₂ cycles [3], already recognized the potential benefits of compact, high efficiency turbomachinery capable of handling large expansion ratios. Attempting to force all the expansion into one stage can cause excessive Mach numbers, high losses, and unacceptable rotor stresses.
In these cases, dividing the expansion across two or more radial stages helps designers maintain reasonable blade speeds, reduce shock formation, and improve overall efficiency. Research on preliminary design methodologies, like the rotor-loss-based approach proposed by Tong et al. [4], supports the feasibility of multistage radial turbines by providing systematic tools for stage matching and loss prediction under variable operating conditions.
Fig 3. Typical ORC configuration with multistage turbine (top) and T-s diagram (bottom) [10]
How Mass Flow Rate Drives the Need for Multistage Design
Mass flow rate is another reason why designers turn to multistage radial turbines. As flow increases, the diameter of a single radial turbine must grow to accommodate it, but larger diameters increase centrifugal stresses, which could push materials to their limits.
Multistage configurations solve this by distributing the work across smaller rotors, each operating within acceptable stress and speed limits. This is particularly relevant for micro gas turbines in the 25–200 kW range, where single-stage radial turbines can become disproportionately large and difficult to manufacture [6, 8]. As mentioned above, for certain special cases staging not only improves efficiency, but also the manufacturability of small-scale systems, especially when silicon microfabrication techniques are used to produce compact, high-precision components[1].
How Geometric Constraints Drive the Need for Multistage Design
Geometric constraints can also motivate the use of multiple stages. In compact systems—such as distributed energy units, hybrid-electric propulsion modules, or integrated waste heat recovery systems—the maximum allowable turbine diameter may be limited by packaging requirements.
A multistage radial turbine can achieve the required expansion ratio within a smaller radial envelope, although this can come at the cost of increased axial length. Recent patent literature, like the multistage radial turbine design from Kawasaki Heavy Industries [5], reflects the industry’s interest in compact multistage configurations that can handle high-pressure working fluids while remaining manufacturable and reliable.
The Challenges of Designing Multistage Radial Turbines
Designing a multistage radial turbine comes with challenges that don’t exist in single-stage machines. One of the biggest challenges is stage matching, where each stage must operate at compatible mass flow rates, velocity triangles, and loading levels. Poor matching can lead to choking, stall, or excessive aerodynamic losses.
Interstage ducting is another major design challenge: the flow leaving a radial rotor is highly swirling and non-uniform, so the duct must guide it efficiently into the next stator without creating large pressure losses or secondary flow structures. Studies [7, 9] highlight how important interstage flow condition and loss minimization is to achieve high overall efficiency.
With multiple stages, the mechanical considerations also become more complex with additional rotors and stators increasing the number of seals, bearings, and potential leakage paths. Thermal gradients and rotor dynamics must be carefully evaluated to ensure reliability.
The Benefits of Multistage Radial Turbines
Despite these challenges, the potential benefits of multistage radial turbines are significant. In sCO₂ cycles, for example, the high density of working fluid and extreme pressure ratios make multistage radial turbines an attractive alternative to axial machines—especially at small scales.
ORC expanders, which often operate with heavy organic fluids and large enthalpy drops, can also benefit from staging to boost efficiency and reduce rotor size.
Industrial waste heat recovery systems, which must operate reliably under variable load conditions, can also use multistage radial turbines to achieve higher efficiency across a broader operating range.
Designing Multistage Radial Turbines in AxSTREAM
The addition of multistage radial turbine capabilities in AxSTREAM provides engineers with a comprehensive environment to explore these new design possibilities. The platform supports the full workflow, from preliminary sizing and work split optimization to detailed aerodynamic analysis and 3D geometry generation.
Designers can rapidly evaluate different stage configurations, assess interstage duct performance, and refine velocity triangles to achieve optimal matching. With AxCFD and AxSTRESS integrated into the platform, engineers can evaluate flow fields and structural stresses in detail, ensuring that both aerodynamic and mechanical requirements are met.
Integrated optimization tools, like these, enable multi-objective trade studies, helping engineers balance efficiency, size, stress, manufacturability, and cost.
Fig 4. Multistage radial turbine flow path domains in AxCFD
The Future of Multistage Radial Turbines
While radial turbines have historically been single-stage machines, modern energy systems are pushing them to handle higher-pressure ratios, greater efficiency, and more compact architectures, driving the need for multi-stage designs. By combining the robustness of radial flow with the flexibility of staged expansion, multistage radial turbines can meet these modern demands.
And now, with AxSTREAM’s new multistage radial turbine capabilities, engineers have the tools to explore, design, optimize, and validate these advanced configurations faster, and with greater confidence. This opens the door to new innovations in sCO₂ cycles, ORC systems, micro gas turbines, and industrial waste heat recovery—pushing the boundaries of what radial turbines can achieve. If you’re interested in testing AxSTREAM’s new multistage radial turbine capabilities, request a software trial here
References
- Fréchette, L. G., Jacobson, S. A., Breuer, K. S., Ehrich, F. F., Ghodssi, R., Khanna, R., and Schmidt, M. A., 2005, “High-speed microfabricated silicon turbomachinery and fluid film bearings,” Journal of Microelectromechanical Systems, 14(1), pp. 141–152.
- Witanowski, Ł 2024, “Numerical Investigation of Multi-Stage Radial Turbine Performance Under Variable Waste Heat Conditions for ORC Systems,” Applied Sciences, Vol. 14, no. 24 p. 11600. https://doi.org/10.3390/app142411600
- Angelino, Carbon Dioxide Condensation Cycles for Power Production, – Journal of Engineering for Power, Jul 1968, 90(3): 287-295 (9 pages) https://doi.org/10.1115/1.3609190 (Carbon Dioxide Condensation Cycles For Power Production: G. Angelino | PDF | Gas Turbine | Temperature)
- Tong, Z. Cheng., and S. Tong., “Preliminary Design of Multistage Radial Turbines Based on Rotor Loss Characteristics under Variable Operating Conditions,- Energies 15, pp. 2019, 12, 2125, https://doi:10.3390/en12112125
- Kawasaki Heavy Industries, 2017, “Multistage Radial Turbine,” Patent US 2012/0134797 A1 . Patents Assigned to Kawasaki Heavy Industries, Ltd. – Justia Patents Search
- Listed under IPC classification F02C (Gas-turbine plants) , 2019 “Multistage radial compressor and turbine” Patent Number ID: US20190178159A1 Field of Invention: Small gas turbine engines (microturbines) designed to convert heat energy into mechanical rotational energy US20190178159A1 – Multistage radial compressor and turbine – Google Patents, S. Patent Application for MULTISTAGE RADIAL COMPRESSOR AND TURBINE Patent Application (Application #20190178159 issued June 13, 2019) – Justia Patents Search
- Xu, G.; Zhao, G.; Quan, Y.; Liang, R.; Li, T.; Dong, B.; Fu, J. Design and Optimization of a Radial-Axial Two-Stage Coaxial Turbine for High-Temperature Supercritical Organic Rankine Cycle. Appl. Therm. Eng. 2023, 227, 120365. https://doi.org/10.1016/j.applthermaleng.2023.120365
- P. Shah, S. A. Channiwala, D. B. Kulshreshtha1 and G. C. Chaudhari,- Design, Numerical Simulation and Experimental Investigation of Radial Inflow Micro Gas Turbine. – Journal of Applied Fluid Mechanics, Vol. 12, No. 6, pp. 1905-1917, 2019. Available online at www.jafmonline.net, ISSN 1735-3572, EISSN 1735-3645.DOI: 10.29252/jafm.12.06.29782
- Li, Q. Zheng Numerical Simulation of a Multistage Radial Inflow Turbine. – ASME Turbo Expo 2006: Power for Land, Sea and Air May 8-11, 2006, Barcelona, Spain GT2006-91307
- Alshammari, A. S. Alshammari, A. Alzamil. – Advanced energy conversion strategies using multistage radial turbines in Organic Rankine Cycles for low-grade heat recovery.- Case Studies in Thermal Engineering.– 2025.-19pp. https://doi.org/10.1016/j.csite.2025.106034
