Radial turbines are quite popular for turbochargers and micro-gas turbines. They can also be found in compact power sources like in auxiliary power units of aircrafts. In short, they are suitable in power generation applications where expansion ratios are high and mass flow rates are relatively small. In a radial turbine, the flow enters radially and exits either axially or radially depending on whether it is an inflow or outflow type radial turbine. The most commonly used type of radial turbine is a radial-inflow turbine, in which the working fluid flows from a larger radius to a smaller radius. A centripetal turbine is very similar in appearance to the centrifugal compressor, but the flow direction is reverse. Figure 1 shows the radial-inflow turbine on the left and radial-outflow turbine on the right.
Nowadays, the popularity of radial-outflow turbines, in which the flow moves in the opposite direction (from the center to the periphery), is growing. With recent advancement in waste heat recovery applications, there has been a renewed interest in this type of turbines. These radial-outflow turbines are most commonly used in applications based on organic Rankine cycles (ORC).
The radial-outflow turbine design was first invented by the Ljungström brothers in 1912, however it was rarely used for a number of reasons. One of which was related to the decrease of turbine-specific work due to the increase of the peripheral velocity from inlet to outlet while expanding the vapor. Another reason was the usage of steam as a working fluid. It is known from thermodynamics that the expansion of steam is characterized by high enthalpy drops, high volumetric flows and high volumetric ratios. Thus, a significant number of stages are needed to convert the enthalpy drop of the fluid into mechanical energy.
Organic Rankine cycles solve this limitation. Organic fluids have high molecular weights, which leads to significantly lower enthalpy drops, volumetric flows and volumetric ratios than steam. This makes it possible to rethink radial outflow turbine technology, and develop a product which has several valuable advantages. Moreover, application of a non-standard design approach, which is based on non-equal enthalpy drop distribution between stages, allows for the design of radial-outflow turbines with high efficiency – even higher than axial and radial-inflow turbines. Schematic view of a radial-outflow turbine is shown in Figure 2.
Radial-Inflow vs Radial-Outflow Turbines
In contrast with radial-inflow turbines, multi-stage configurations are used for radial-outflow turbines, therefore, the overall heat drop is unevenly divided between stages. The work output increases in each stage due to significant change in the number of rotor blades. It is not possible to use uniform blade profiles since geometrical parameters of turbine stages have significant difference from the first stage to the last stage due to an increase in diameter from stage one to the last stage.
Benefits of Radial-Inflow
The benefit of using radial-inflow turbines is the increase of specific work per stage, which means large pressure and temperature drops are achieved in a single stage. Hence, the turbine can extract more work in a single stage. Turbines of this type are suitable for low mass flow, high pressure drop and low power applications. It has a wide range of applications ranging from hydroelectric power plants to small gas turbines. They are robust and resistant to corrosion and erosion. At equal mass flow values, these turbines have lower blade heights in comparison to radial-outflow turbines. The major limitation of radial-inflow turbines is in terms of the maximum number of stages. A maximum of two stages can be used, however, typically single stage configurations are applied. As a result, such configurations are not suitable in high power applications and there is a significant efficiency reduction at high mass flows.
Benefits of Radial-Outflow
Now let’s talk about the benefits of using radial-outflow turbines. Their blades are always prismatic which reduces the cost of manufacturing. They operate more efficiently at lower rotational speeds with equal or higher performance and they are often better for ORC applications in comparison to radial-inflow turbines. In most cases, only a single rotor disk is required to carry the number of required blade rows so the vibrations are lower. This produces a lightweight rotor arrangement, reducing the bearing and seal requirements. Due to the compact arrangement, thermal losses are minimized and they do not require partial admissions. Small clearances are applied which means there are less leakages. They are also easily scalable and simple to construct. The major shortcomings of ROT (Radial Outflow Turbines) are the decreased specific work per stage and limited applicability in terms of working fluids.
Designing Radial Turbines
Radial turbine design approaches are always evolving for improved aerodynamic performance while shortening the design period, though this is a classical problem. The goal of engineers is to obtain the best design with high aerodynamic performance under given constraints and reduce the dependency on human expertise through optimization algorithms when designing radial turbines. This is why it’s important to find the right tools to use in radial turbine design.
For designing a radial turbine (both inflow and outflow) or to analyze an existing radial turbine, SoftInWay offers a complete turbomachinery design and analysis tool called AxSTREAM® which can generate optimized designs with less time and effort starting from the specifications. Using this tool, designers are provided the opportunity to create hundreds or even thousands of designs from scratch with minimal data available. The flexibility of this software enables a person with basic knowledge of turbine design and use of design tools to perform a turbine design and analysis task.
In AxSTREAM® the design process starts from the preliminary design, then progresses to the streamline analysis and then to the profiling and blade design. It finally concludes with 3D analysis.
- Maksiuta D., Moroz L., Burlaka M., Govoruschenko Y. Study on applicability of radial-outflow turbine type for 3MW WHR Organic Rankine Cycle. 4th International Seminar on ORC Power Systems; September 13-15, 2017; Milano, Italy.