In the realm of turbomachinery, seemingly minor design decisions can have a significant impact on performance, efficiency, and the overall lifespan of a machine. One component that often flies under the radar, but plays a crucial role in optimizing turbine efficiency, is the exhaust duct. For the radial turbine design process, the exhaust duct – especially the diffuser – plays a pivotal part in ensuring that the turbine operates at peak performance while minimizing losses. When designed well, a diffuser not only increases power output but also reduces the risk of operational inefficiencies.
Conical diffusers, which are widely used in radial turbine exhaust ducts, expand the flow area and enable a greater isentropic heat drop. This, in turn, improves turbine efficiency. However, designing the perfect diffuser isn’t as straightforward as simply enlarging the duct. Engineers must consider a range of parameters—such as cone angle and diffuser length—to maximize pressure recovery while minimizing hydraulic losses. This blog explores the complexities of diffuser design and the factors that need to be considered to ensure optimal performance in radial turbines.
The Challenge of Diffuser Design
Exhaust ducts, especially conical diffusers, serve as a crucial link between the turbine’s flow path and the downstream components of the turbine system. The main function of the diffuser is to increase the cross-sectional area of the duct. By doing so, it allows for a greater isoentropic heat drop within the turbine flow path, which boosts the generated power and reduces the working fluid mass flow rate [1]. This sounds relatively simple, but achieving the perfect design involves much more than just expanding the duct.
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Figure 1 – Radial-inflow turbine with Exhaust Duct in AxSTREAM
Designing an effective conical diffuser, which transitions from an annular to a circular cross-section, requires careful attention to flow dynamics. Engineers must strike a balance between optimizing the cone angle (α) and the diffuser length (L) to ensure maximum pressure recovery and minimize hydraulic losses [2]. An inefficient design can lead to a variety of problems, including excessive turbulence, boundary layer separation, and even structural damage.
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Figure 2 – Basic geometrical parameters of the conical diffuser
The graph below shows how turbine power depends on the axial length of the diffuser. This analysis helps determine the optimal geometric parameters of the conical diffuser to ensure efficient performance
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Figure 3 – Graph of power dependence on diffuser axial length in AxSTREAM (AxMAP)
Key Factors in Exhaust Duct Design
- Cone Angle (α) and Diffuser Length (L)
The cone angle and diffuser length are two of the most important factors in designing an efficient diffuser. The cone angle must be optimized to allow for smooth flow expansion while avoiding the formation of vortex structures that can lead to increased losses. The length of the diffuser also plays a crucial role in pressure recovery. Longer diffusers generally allow for better recovery of pressure but come with the trade-off of potentially higher hydraulic losses.
- Vortex Formation and Mixing Jets
The transition from an annular to a circular cross-section in a conical diffuser is where the flow undergoes a sudden expansion, creating jet mixing and vortex formation in the center of the diffuser channel. These vortices can disrupt smooth flow, resulting in uneven pressure distribution, turbulence, and potential performance loss.
- Hydraulic Losses and Performance Optimization
The goal of a diffuser design is to achieve the highest possible pressure recovery while minimizing hydraulic losses. These losses are often the result of non-optimized geometry, boundary layer separation, and vortex formation. Accurate CFD simulations are necessary to evaluate the flow structure and identify areas where losses can be minimized
- Flow Structure and Boundary Layer Separation
At high rotational speeds, the flow exiting the turbine often contains significant swirl. As the flow moves through the diffuser, the swirl causes the flow to interact with the diffuser walls, leading to the formation of vortices and potentially creating regions of reduced pressure. These effects can be amplified if the diffuser is not properly designed, causing boundary layer separation and increased losses.
A key characteristic of the flow structure in the channel is the presence of flow swirl at the rotor outlet. As the annular area transitions into a circular one behind a short spinner, the flow undergoes a sudden expansion, leading to jet mixing and the formation of local vortices in the central part of the diffuser channel. When the swirl is significant, most of the flow can be pushed against the diffuser walls, creating a low-pressure zone in the channel’s center, where the flow pattern resembles a tornado.
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Figure 4 – Radial-inflow turbine with Exhaust Duct in Ansys CFX
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Figure 5 – Static pressure along the Exhaust duct (Meridional section)
Real World Applications and Examples
A practical example of the impact of diffuser design can be found in the work SoftInWay engineers conducted on the T-100 radial turbine model. Using CFD simulations, engineers analyzed the diffuser’s performance and identified areas of concern. In particular, they found that at a distance of 10 outlet diameters from the diffuser, the flow remained non-uniform, with significant variations in the hydraulic loss coefficient. The most notable losses occurred in the first few sections of the diffuser, where boundary layer separation and vortex formation were most prominent.
This analysis highlights the importance of understanding the spatial flow structure and the need for 1D/2D calculations, validated with 3D CFD, to predict flow behavior accurately. It also emphasizes the need for engineers to consider the complex, non-uniform flow patterns at the outlet of the diffuser when designing downstream components.
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Figure 6 – Velocity graph along the axial length of the diffuser
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Figure 7 – Total pressure loss factor graph along the axial length of the diffuser
Conclusion: Optimizing Exhaust Duct Design for Radial Turbines
The design of an exhaust duct, particularly a conical diffuser, plays a pivotal role in optimizing the performance and efficiency of radial turbines. Engineers face the challenge of balancing several crucial factors—cone angle, diffuser length, flow structure, and vortex formation—when designing a diffuser that maximizes pressure recovery while minimizing losses. A thoughtfully engineered diffuser not only boosts turbine efficiency and power output but also mitigates operational issues like vibrations and instability.
To achieve this, utilizing CFD simulations is vital for evaluating and refining diffuser performance. These simulations allow engineers to predict flow behavior and hydraulic losses throughout the design stages, ensuring a more precise and efficient design. When paired with real-world testing, these tools enable engineers to fine-tune their designs, ensuring turbines operate at peak performance under all conditions.
Whether developing new turbines or optimizing existing systems, diffuser design is crucial in delivering reliable and efficient radial turbines that can withstand the demands of modern industrial applications. By embracing these principles, engineers can avoid common design pitfalls and create solutions that deliver lasting performance.
Curious about optimizing your own radial turbine designs? Check out our radial turbine design demo to see how the AxSTREAM software can help you achieve peak performance and efficiency.
REFERENCES
Jones A.C., (1996). Design and test of a small, high-pressure ratio radial turbine. J. Turbomach, 118, 362–370
Deitch M.E. Zaryankin A.E. – Gas dynamics of diffusers and exhaust duct of turbomachines, M., “Energy”, 1970.