Exhaust Diffuser with Local Convergence for Gas Turbine Pressure Recovery
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Solution Overview
Problem
Gas turbines face challenges in achieving high pressure recovery and low exhaust loss across a wide range of operational conditions due to limitations in exhaust diffuser geometry and inflow conditions, particularly in flexible operation modes.
Innovation Solution
An exhaust diffuser design featuring an annular duct with a cross-sectional area that locally converges downstream of the trailing edges of struts, stabilizing the boundary layer and increasing pressure recovery, with a preferred axial location of the local minimum area between 2-10 times the strut thickness and a reduction of 3-18% in cross-sectional area, followed by a continuous increase to exceed the original area.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Stress or pressure
If the cross-sectional area of the annular duct continuously increases from inlet to outlet, then the diffuser acts as a pressure recovery device, but the boundary layer becomes unstable downstream of the struts, reducing pressure recovery efficiency
Solution Approach 1:
The diffuser duct is segmented into multiple axial regions with different area variation characteristics: a first region with continuous area increase, a second region with local area decrease (converging section), and a third region with continuous area increase. This segmentation allows the boundary layer to be stabilized in the second region while maintaining pressure recovery in the first and third regions.
Solution Approach 2:
The area variation follows a periodic pattern of expansion-contraction-expansion along the axial direction. The converging section (second region) acts as a periodic intervention to stabilize the boundary layer, followed by another expansion phase to continue pressure recovery, creating a rhythmic flow control mechanism.
2Loss of energy
If the diffuser geometry is optimized for high pressure recovery, then exhaust loss decreases, but the performance degrades when operating conditions change, limiting flexibility
Solution Approach 1:
The multi-region area variation geometry serves multiple functions simultaneously: the first region provides initial pressure recovery, the second region stabilizes the boundary layer across various flow conditions, and the third region provides final pressure recovery. This multi-functional design allows the diffuser to maintain high performance across a wide range of operating conditions including different mass flow rates and inlet total pressures.
Solution Approach 2:
The area variation ratio and axial lengths of different regions are optimized as geometric parameters to achieve robust performance. By carefully selecting these parameters, the diffuser maintains effective boundary layer control and pressure recovery across varying operational parameters such as mass flow rate and inlet conditions.
3Stress or pressure
If a simple continuous expansion geometry is used, then the device complexity is low, but the pressure recovery coefficient is reduced due to boundary layer instability
Solution Approach 1:
The duct geometry is divided into three distinct axial regions with specific area variation characteristics. This segmentation introduces only moderate complexity while achieving significant improvement in pressure recovery coefficient by stabilizing the boundary layer in the converging section.
Solution Approach 2:
The area variation in each region is achieved through smooth curved transitions rather than sharp angles or discontinuities. The inner and outer wall contours use continuous curved profiles, which reduces flow separation and maintains attached flow, thereby improving pressure recovery without excessive complexity.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This design enhances pressure recovery and reduces exhaust loss for a wide range of operating conditions, stabilizing the boundary layer and improving gas turbine efficiency and power output.
Implementation Method 1
By having the cross-sectional area of the annular duct locally converge downstream of the trailing edges of the first struts, gas flow is locally accelerated downstream of the first struts. This can be shown to stabilize the boundary layer of the flow in this region.
Data Source
AI summary
An exhaust diffuser for a gas turbine includes an annular duct. A row of struts is arranged in the duct. In a region downstream of the trailing edges of the struts, the cross-sectional area of the duct decreases to a local minimum and then increases again towards the outlet end of the duct. Thereby the gas flow is locally accelerated downstream of the struts. This stabilizes the boundary layer of the flow in this region and leads to a marked increase in pressure recovery for a wide range of operating conditions.