Influence of Unsteady Turbine Flow on the Performance of an Exhaust Diffuser

Kuschel, Marcus; Seume, Joerg R.

doi:10.1115/gt2011-45673

Cited by 9 publications

(7 citation statements)

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“…The findings were experimentally validated by Sieker and Seume (2008). Kuschel and Seume (2011) conducted additional experiments using a NACA profile instead of cylindrical spokes. A detailed analysis of these experimental results can be found in Kuschel et al (2015) and Drechsel et al (2015).…”

Section: Stabilisation Numbermentioning

confidence: 80%

Correlation between total pressure losses of highly loaded annular diffusers and integral stage design parameters

Mimic

Jätz

Herbst

2018

J. Glob. Power Propuls. Soc.

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Diffusers convert kinetic flow energy into a rise in static pressure. This pressure recovery is the primary aerodynamic design objective for exhaust gas diffusers in power-generating steam and gas turbines. The total pressure loss is an equally important diffuser design parameter. It is strongly linked to the pressure recovery and the residual kinetic energy of the diffuser outlet flow. A reduction benefits the overall thermodynamic cycle, which requires the adjacent components of a diffuser to be included in the design process. This paper focuses on the total pressure losses in the boundary layer of a highly loaded annular diffuser. Due to its large opening angle the diffuser is susceptible to flow separation under uniform inlet conditions, which is a major source for total pressure losses. However, the unsteady tip leakage vortices of the upstream rotor, which are a source of losses, stabilise the boundary layer and prevent separation. Experiments and unsteady numerical simulation conducted show that the total pressure loss reduction caused by the delayed boundary layer separation exceed the vortex-induced losses by far. This flow interaction between the rotor and diffuser consequently decreases the overall total pressure losses. The intensity of the tip leakage vortex is linked to three rotor design parameters, namely work coefficient, flow coefficient and reduced blade-passing frequency. Based on these parameters, we propose a semi-empiric correlation to predict and evaluate the change in total pressure losses with regards to design operating conditions.

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Section: Stabilisation Numbermentioning

confidence: 80%

Correlation between total pressure losses of highly loaded annular diffusers and integral stage design parameters

Mimic

Jätz

Herbst

2018

J. Glob. Power Propuls. Soc.

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“…Turbine outflows are inherently unsteady and incorporate swirl and inhomogeneous flow conditions due to tip leakage flows and wakes. These conditions were also found to affect the diffuser flow considerably [3][4][5][6][7]. Sieker and Seume [5] showed that highly turbulent, unsteady inflow can stabilize the flow field even if the diffuser was predicted to separate according to the diffuser charts.…”

Section: Introductionmentioning

confidence: 99%

“…Sieker and Seume [5] showed that highly turbulent, unsteady inflow can stabilize the flow field even if the diffuser was predicted to separate according to the diffuser charts. Kuschel and Seume [7] showed the impact of unsteady coherent vortices at the blade tips using unsteady hot-wire probes. Furthermore, they came to the conclusion that the Reynolds stresses of the rotor outflow at the blade tip are anisotropic.…”

Section: Introductionmentioning

confidence: 99%

On the Numerical Prediction of the Influence of Tip Flow on Diffuser Stability

Drechsel

Joerg

Herbst

2016

JGPP

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In previous studies, the pressure recovery of highly-loaded annular diffusers was identified to correlate with the Reynolds shear stresses at rotor outlet in the blade tip region. The origin and propagation of the Reynolds shear stresses, however, have not been experimentally clarified yet due to measurement probe constraints. Hence in the present work, the origin of these stresses, as well as the transport throughout the flow channel is analyzed by simulating the rotor with the scale adaptive turbulence model SAS-SST is used. Using the SAS approach, the Reynolds shear stress characteristics of the simulation are validated by the experimental results, whereas common RANS approaches are shown not to be appropriate. The tip leakage vortex is found to be the source of the Reynolds shear stress production. The interaction between vortex and mean flow leads to turbulent momentum transport. The Reynolds shear stresses propagate into the rotor far-field connected to the blade tip vortices which mix about four chord lengths downstream of the rotor trailing edge.

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“…The Kluss study demonstrated through comparison with experiment that URANS CFD calculations simulating an upstream rotor, coupled with an anisotropic turbulence model are necessary to provide a reasonable prediction of turbine exhaust diffuser performance. Further efforts [79] on the same test rig confirmed the need for URANS calculations and anisotropic turbulence modelling, and revealed that turbine rotor tip clearance flows are a key feature in diffuser performance. Additional momentum close to the shroud surface, afforded by the tip gap, supports the boundary layer against the adverse pressure gradient within the diffuser and enables efficient operation of a more compact diffuser.…”

Section: Introductionmentioning

confidence: 80%

“…Contrasting the inlet delivery system, interactions between components are significant in determining performance and optimum geometry of the diffuser. Recent studies [78,79,80,81,82] indicate that rotor -diffuser interactions are significant, and that fully coupled, URANS stage simulations are necessary to resolve diffuser component performance. The use of such a modelling framework leads to more compact, higher performing diffuser geometries than would otherwise be designed had the interactions not been accounted for.…”

Section: Modelling Segregationmentioning

confidence: 99%

On the design of small to medium scale radial inflow turbines for supercritical CO2 power cycles

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In rural Australia, concentrating solar power at sub 10 MWe scale is a candidate technology to displace current fossil fuel based technologies [1]. For concentrating solar power to be competitive for this application, coupling with an advanced power cycle is essential. A candidate is the supercritical CO 2 Brayton cycle, which is suitable for higher turbine inlet temperature operation, and has the potential to exceed the efficiency of the steam Rankine cycle. Furthermore, due to lower volumetric flow variation over the cycle, simpler turbomachinery may be used, which enables the cycle to be downscaled while maintaining turbomachinery and cycle efficiency. Of the constituent components in the cycle, turbine efficiency has the greatest impact on cycle efficiency [2]. Radial turbomachinery is a key technology for energy conversion in the supercritical CO 2 Brayton cycle at small scales (i.e. below 30 MW shaft power). While the design of efficient turbines for the supercritical CO 2 Brayton cycle is critical to realising efficient concentrating solar power plants, only a limited number of prototypes have been tested, and none at representative inlet conditions. Radial inflow supercritical CO 2 turbines are characterised by small dimensional scale and high shaft speeds. Prototype turbine designs are characterised by low expansion ratio and medium specific speed to accommodate mechanical restrictions on shaft speed. Medium specific speed designs are selected for prototypes designs due to their implied optimal efficiency. Demonstration test facilities have utilised multiple stages, or lower expansion ratio cycles in order to accommodate designs of this specific speed without exceeding mechanical limits. Increasing inlet conditions to representative cycle conditions for concentrating solar power applications will require higher shaft speeds, or additional stages if specific speed of the stage is to be maintained. Alternatively, the use of single stage low specific speed designs may pose a feasible alternative, however these designs are generally disregarded owing to the implied efficiency penalties shown on gas turbine derived architecture selection charts. Better understanding of loss characteristics of low specific speed supercritical CO 2 turbomachinery is required in order to make an assessment on the feasibility of single stage expansions. Further to sizing of the stator and rotor, stage design also includes components upstream and downstream of the rotor. The broadest challenge for supercritical CO 2 turbomachinery is designing components within a system for high pressure, high density, high temperature, and high shaft speed operation. These constraints may have significant impact on the geometry of these hot gas path features. Details of these features are not clearly detailed in published prototype designs. Considering the knowledge gap for supercritical CO 2 turbines, one key aim of this thesis is to quantify the performance of low specific speed radial inflow turbines using numerical methods. A further aim is t...

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Influence of Unsteady Turbine Flow on the Performance of an Exhaust Diffuser

Cited by 9 publications

References 0 publications

Correlation between total pressure losses of highly loaded annular diffusers and integral stage design parameters

Correlation between total pressure losses of highly loaded annular diffusers and integral stage design parameters

On the Numerical Prediction of the Influence of Tip Flow on Diffuser Stability

On the design of small to medium scale radial inflow turbines for supercritical CO2 power cycles

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