Aerodynamic Design and Performance Evaluation of Pipe Diffuser for Centrifugal Compressor of Micro Gas Turbine

Core Concepts

The paper presents a methodology for designing the pipe diffuser of a centrifugal compressor in a 100 kW Micro Gas Turbine (MGT) and evaluates its performance through 3D-RANS CFD simulations, comparing it to an optimized airfoil diffuser.

Abstract

The paper focuses on the aerodynamic design and performance evaluation of a pipe diffuser for the centrifugal compressor of a 100 kW Micro Gas Turbine (MGT). The key highlights are:

The design of the pipe diffuser was influenced by the parameters of an available optimized airfoil diffuser, as well as considerations on the topological constraints of the pipe diffuser geometry. Diffuser maps were also used to guide the selection of some parameters.

3D-RANS CFD simulations were performed using ANSYS CFX to evaluate the performance of the baseline pipe diffuser design (P1) and compare it to the optimized airfoil diffuser. At the design mass flow rate of 0.806 kg/s and 100% rotating speed, the pipe diffuser showed a 2.89% lower isentropic efficiency and 2.04% lower total-to-total pressure ratio compared to the airfoil diffuser.

The authors attribute the inferior performance of the pipe diffuser to the generation of two counter-rotating vortices at the leading edge, which have a beneficial effect in the pseudo- and semi-vaneless space but a destabilizing effect in the channel space, leading to large separation on the pressure side.

The authors investigated the span-wise characteristics of the vorticity-induced flow separation in the pipe diffuser and found that the vortices have a dual effect - helping to reattach the flow near the shroud region but dominating the separation near the hub region.

The performance of the pipe diffuser was also evaluated at off-design operating points, and the authors found that the flow separation mechanism changes from vorticity-induced pressure side separation at the design point to boundary layer-induced suction side separation at the near-surge point.

A parametric study was conducted on four different pipe diffuser designs to investigate the effect of various geometric parameters, but no significant improvement in performance over the baseline design was achieved.

Overall, the paper provides valuable insights into the design and performance of pipe diffusers for micro gas turbine applications, highlighting the trade-offs between the beneficial and detrimental effects of the leading edge vortices.

Stats

The following key metrics are provided in the paper:

Design mass flow rate: 0.806 kg/s
Design rotating speed: 70,000 rpm
Impeller outlet diameter: 145 mm
Optimized airfoil diffuser parameters:

Number of vanes: 23
Area ratio: 4
Channel length: 57 mm
Divergence angle: 10°
Throat area: 817.85 mm^2
Radius ratio (R3/R2): 1.03
Outlet diameter: 255 mm

Pipe diffuser design parameters (P1):

Number of pipes: 23
Area ratio: 4
Channel length: 57 mm
Divergence angle: 6°
Throat area: 1213.7 mm^2
Radius ratio (R3a/R3): 1.03
Outlet diameter: 255 mm

Quotes

"The leading ridges of pipe diffuser generates two counter rotating vortices of unequal strength. Which offers advantage in pseudo- and semi-vanless space. But destabilize the flow in Channel space leading to PS separation."
"Interaction and transport of two vortices in Channel space help to reattach the flow at PS near Shroud region. But the separation at middle and near Hub region outweighs this effect, leading to inferior overall performance."

Key Insights Distilled From

Aerodynamic Design and Performance Evaluation of Pipe Diffuser for Centrifugal Compressor of Micro Gas Turbine

by Sujal Bhavsa... at arxiv.org 04-19-2024

https://arxiv.org/pdf/2404.11828.pdf

Deeper Inquiries

What modifications to the pipe diffuser geometry or flow control techniques could be explored to mitigate the detrimental effects of the leading edge vortices and improve the overall performance

To mitigate the detrimental effects of the leading edge vortices in the pipe diffuser and improve overall performance, several modifications and flow control techniques could be explored:

Leading Edge Redesign: Altering the shape or profile of the leading edge of the pipe diffuser to reduce the generation of counter-rotating vortices. A smoother or more streamlined leading edge design could help in minimizing the vorticity-induced flow separation.

Vortex Control Devices: Introducing vortex control devices within the diffuser geometry to manipulate the strength and behavior of the vortices. These devices could help in stabilizing the flow and reducing the negative impact of the vortices on performance.

Flow Conditioning: Implementing flow conditioning techniques such as boundary layer control or flow straightening mechanisms to ensure a more uniform and controlled flow distribution entering the diffuser. This could help in reducing the likelihood of flow separation and improving overall efficiency.

Throat Geometry Optimization: Fine-tuning the geometry of the throat region to enhance flow attachment and minimize blockage. Adjusting the throat length or shape could help in optimizing the flow conditions and reducing separation effects.

How would the performance of the pipe diffuser compare to the airfoil diffuser under off-design operating conditions, such as part-load or high-speed operation, where the flow separation mechanisms may change

Under off-design operating conditions, such as part-load or high-speed operation, the performance of the pipe diffuser may vary compared to the airfoil diffuser due to changes in flow separation mechanisms.

Part-Load Operation: At part-load conditions, where the mass flow rate is reduced, the pipe diffuser may experience increased flow separation and pressure losses due to lower flow velocities and altered flow patterns. This could lead to a decrease in efficiency compared to the airfoil diffuser, which may be more resilient to flow disturbances at lower flow rates.

High-Speed Operation: During high-speed operation, the pipe diffuser may encounter challenges with managing the increased inlet flow angles and higher velocities, potentially leading to more pronounced flow separation and performance degradation. In contrast, the airfoil diffuser's design may be better suited to handle the higher flow velocities and maintain efficiency under such conditions.

Overall, the pipe diffuser's performance under off-design conditions would depend on its ability to adapt to varying flow conditions and mitigate separation effects, which may differ from the airfoil diffuser's performance characteristics.

Given the potential advantages of the pipe diffuser in terms of compactness and lower throat blockage, are there any specific applications or operating regimes where the pipe diffuser may be better suited than the airfoil diffuser for micro gas turbine compressors

The pipe diffuser's potential advantages in terms of compactness and lower throat blockage make it well-suited for specific applications and operating regimes where these factors are critical:

Space-Constrained Environments: In applications where space is limited, such as small-scale gas turbine systems or microturbines, the compact design of the pipe diffuser can offer significant advantages. The reduced frontal area and lower throat blockage make it ideal for installations with space restrictions.

Variable Operating Conditions: The pipe diffuser's performance may excel in certain operating regimes where flow control and separation mechanisms play a crucial role. For applications with fluctuating load demands or varying operating speeds, the pipe diffuser's unique flow characteristics and vorticity control may provide benefits over the airfoil diffuser.

Supersonic Flow Conditions: In scenarios where transonic or supersonic flow conditions are prevalent, the pipe diffuser's ability to handle high inlet flow angles and manage flow separation could make it a preferred choice over the airfoil diffuser. Its design features may be better suited to accommodate and optimize performance in such challenging flow regimes.