Correlation Between Pressure Recovery of Highly Loaded Annular Diffusers and Integral Stage Design Parameters

2017 ◽  
Author(s):  
Dajan Mimic ◽  
Bastian Drechsel ◽  
Florian Herbst
Keyword(s):  
Boundary Layer ◽  
Dynamic Pressure ◽  
Boundary Layer Separation ◽  
Flow Coefficient ◽  
Tip Vortex ◽  
Pressure Recovery ◽  
Design Parameters ◽  
Steam Turbines ◽  
Pressure Losses ◽  
Induced Velocity

Exhaust diffusers significantly enhance the available power output and efficiency of gas and steam turbines by allowing for lower turbine exit pressures. The residual dynamic pressure of the turbine outflow is converted into static pressure, which is referred to as pressure recovery. Since total pressure losses as well as construction costs increase drastically with diffuser length, it is more than favourable to design shorter diffusers with rather steep opening angles. However, those designs are more susceptible to boundary layer separation. In this paper, the stabilising properties of tip leakage vortices generated in the last rotor row and their effect on the boundary layer characteristics are examined. Based on analytical considerations, for the first time a correlation between the pressure recovery of the diffuser and integral rotor parameters of the last stage, namely the loading coefficient, flow coefficient and reduced frequency, is established. Both, experimental data and scale resolving simulations, carried out with the SST-SAS method, show excellent agreement with the correlation. Blade tip vortex strength predominantly depends on the amount of work performed in the rotor, which in turn is described by the non-dimensional loading coefficient. The flow coefficient influences mainly the orientation of the vortex, which affects the interaction between vortex and boundary layer. The induced velocity field accelerates the boundary layer, essentially reducing the thickness of the separated layer or even locally preventing separation.

scholarly journals Correlation Between Pressure Recovery of Highly Loaded Annular Diffusers and Integral Stage Design Parameters1

2018 ◽  
Vol 140 (7) ◽  
Author(s):  
Dajan Mimic ◽  
Bastian Drechsel ◽  
Florian Herbst
Keyword(s):  
Boundary Layer ◽  
Dynamic Pressure ◽  
Boundary Layer Separation ◽  
Flow Coefficient ◽  
Tip Vortex ◽  
Pressure Recovery ◽  
Steam Turbines ◽  
Construction Costs ◽  
Pressure Losses ◽  
Induced Velocity

Exhaust diffusers significantly enhance the available power output and efficiency of gas and steam turbines by allowing lower turbine exit pressures. The residual dynamic pressure of the turbine outflow is converted into static pressure, which is referred to as pressure recovery. Since total pressure losses and construction costs increase drastically with diffuser length, it is strongly preferred to design shorter diffusers with steeper opening angles. However, these designs are more susceptible to boundary layer separation. In this paper, the stabilizing properties of tip leakage vortices generated in the last rotor row and their effect on the boundary layer characteristics are examined. Based on analytical considerations, for the first time, a correlation between the pressure recovery of the diffuser and the integral rotor parameters of the last stage, namely, the loading coefficient, flow coefficient, and reduced frequency, is established. Experimental data and scale-resolving simulations, carried out with the shear stress transport scale-adaptive simulation (SST-SAS) method, both show excellent agreement with the correlation. Blade tip vortex strength predominantly depends on the amount of work exchanged between fluid and rotor, which in turn is described by the nondimensional loading coefficient. The flow coefficient influences mainly the orientation of the vortex, which affects the interaction between vortex and boundary layer. The induced velocity field accelerates the boundary layer, essentially reducing the thickness of the separated layer or even preventing separation locally.

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

2018 ◽  
Vol 2 ◽  
pp. I9AB30 ◽  
Author(s):  
Dajan Mimic ◽  
Christoph Jätz ◽  
Florian Herbst
Keyword(s):  
Boundary Layer ◽  
Gas Turbines ◽  
Pressure Loss ◽  
Total Pressure ◽  
Total Pressure Loss ◽  
Pressure Recovery ◽  
Design Parameters ◽  
Pressure Losses ◽  
Tip Leakage ◽  
Highly Loaded

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.

Effects of Rotating Blade Wakes on Separation and Pressure Recovery in Turbine Exhaust Diffusers

2008 ◽  
Author(s):  
Olaf Sieker ◽  
Joerg R. Seume
Keyword(s):  
Boundary Layer ◽  
High Energy ◽  
Flow Coefficient ◽  
Pressure Recovery ◽  
High Mass ◽  
Scale Model ◽  
Swirl Number ◽  
Annular Diffuser ◽  
Bladed Rotor ◽  
Turbine Exhaust

Highly efficient turbine exhaust diffusers can only be designed by taking into account the unsteady interactions with the last rotating row of the turbine. Therefore, a scale model of a typical gas turbine exhaust diffuser consisting of an annular and a conical diffuser is investigated experimentally. To investigate the influence of rotating wakes, a variable-speed rotating spoke wheel with cylindrical spokes as well as with NACA bladed spokes generates high-energy turbulent wakes simulating turbine rotor wakes. For the rotor with the NACA blades, the drive of the wheel is run in motor as well as in generator mode. Additional measurements in a reference configuration without a spoke wheel allow the detailed analysis of changes in the flow pattern. 3-hole pneumatic probes, static pressure taps, as well as a 2D-Laser-Doppler-Velocimeter (LDV) are used to investigate velocity profiles and turbulent characteristics. Without the wakes generated by a spoke wheel, the annular diffuser (with a 20° half cone opening angle) separates at the shroud for all swirl configurations. Increasing the swirl results in increasing pressure recovery at the shroud whereas the hub boundary is destabilized. For a non-rotating spoke rotor and low swirl numbers, the 20° annular diffuser separates at the shroud. Increasing the swirl number, a strong deceleration of the axial velocity at the shroud is generated without separation and a higher pressure recovery is achieved. The boundary layer at the shroud of the 20° annular diffuser separates for all operating points with the bladed rotor. A partly stabilized 20° annular diffuser can only be achieved for much higher values of the flow coefficient than that for the design point. At this high mass flow, the NACA-bladed rotor operates as a turbine, resulting in the generator mode of the electric drive. Contrary to the numerical design calculations, the flow at the shroud of a 15° annular diffuser does not separate for all swirl configurations in the experiment. Pressure recovery of the 15° annular diffuser can be increased by increasing the inlet swirl whereas the hub boundary layer is destabilized. For the NACA bladed rotor, the flow in the 15° annular diffuser as well as the pressure recovery strongly depend on the flow coefficient. For flow coefficients lower than the design value, the flow partly separates at the shroud whereas large flow coefficients result in increased pressure recovery. The pressure recovery also depends on the direction of swirl and thus the swirl number.

Numerical Analysis of Pressure Losses in Diffuser and Tube Steam Partition Valves

2013 ◽  
Author(s):  
C. Bianchini ◽  
M. Micio ◽  
L. Tarchi ◽  
C. Cortese ◽  
E. Imparato ◽  
...  
Keyword(s):  
Working Conditions ◽  
Dynamic Pressure ◽  
Characteristic Curve ◽  
Valve Lift ◽  
Design Stage ◽  
Steam Turbines ◽  
Simple Method ◽  
Entire Study ◽  
Pressure Losses ◽  
Wide Range

Control valves are one of the key steam turbine components both in terms of operational safety and flexibility. It is hence fundamental to correctly predict the valve characteristics at the various working conditions to accurately estimate machine performance and control logics. The aim of this work is to develop a simple method to predict pressure losses within the partition system to be used at preliminary design stage. Two types of partition valves typically employed in real industrial steam turbines of different power (from 1MW to 100MW) are analysed. The first type exploits a diffuser-like shape to maximize the dynamic pressure recovery before the discharge into the impulse stage. The second type, based on simple tube geometry, increases the allowable flow rate, for the same valve seat, at the cost of higher pressure losses. Geometrical dimensions have been varied to cover a wide range of configurations employed in industrial applications. An exception is made for the diffuser angle and the relative fillet radius which were fixed to guarantee product standardization among the various machine sizes. The flow is supposed axisymmetric and upstream reference condition for the entire study is 140 bar and 540 °C which are typical working conditions for such steam turbines. The influence of the shutter is also considered to properly characterize regulation of the steam flow on the basis of valve lift. Pressure losses are first modelled dividing the partition valve into singular homogeneous parts such as the intake, the straight pipe, the diffuser and the discharge, for which simple correlations are available in literature. The overall characteristic curve is validated using CFD computations conducted with the steady state RANS solver available in the commercial code CFX exploiting the SST turbulence model. The development of the correlation permitted to rapidly cover the selected range of geometries and conditions highlighting that dynamic pressure losses are the major sources of losses. Minimal passage area to discharge section ratio is hence a dimensionless value able to describe characteristic curves insensitively to any other geometrical parameter.

Influence of Turbulent Flow Characteristics and Coherent Vortices on the Pressure Recovery of Annular Diffusers: Part B — Scale-Resolving Simulations

2015 ◽  
Author(s):  
Bastian Drechsel ◽  
Christoph Müller ◽  
Florian Herbst ◽  
Joerg R. Seume
Keyword(s):  
Boundary Layer ◽  
Flow Characteristics ◽  
Boundary Layer Separation ◽  
Numerical Approach ◽  
Pressure Recovery ◽  
Experimental Investigations ◽  
Physical Parameters ◽  
Shear Stresses ◽  
Local Flow ◽  
Coherent Vortices

This paper examines the diffuser flow with consideration to turbine outflow conditions. The setup consists of a low-speed axial diffuser test rig, that represents a 1/10 scaled heavy-duty exhaust diffuser with an annular and a conical diffuser part. In part A of this paper it was shown through experimental investigation that the turbulent kinetic energy as well as the Reynolds shear stresses are the relevant physical parameters that correlate with diffuser pressure recovery. To complement the experimental investigations, unsteady scale-resolving CFD simulations are performed, applying the SST-SAS turbulence model. As a first step, the numerical approach is validated by means of the experimental data with regards to the diffuser’s integral parameters as well as the prediction of local flow characteristics. In a second step, the interaction of coherent vortices generated by the rotor and the diffuser’s boundary layer are analyzed by means of the validated SST-SAS results. These vortices are found to have a major impact on the boundary layer separation in the region immediately downstream of the rotor and at the diffuser inlet.

Sub boundary-layer vortex generators for the control of shock induced separation

2006 ◽  
Vol 110 (1106) ◽  
pp. 215-226 ◽  
Author(s):  
G. S. Cohen ◽  
F. Motallebi
Keyword(s):  
Boundary Layer ◽  
Total Pressure ◽  
Static Pressure ◽  
Pressure Rise ◽  
Boundary Layer Separation ◽  
Vortex Generators ◽  
Pressure Recovery ◽  
Flow Visualisation ◽  
Pressure Distributions ◽  
Pressure Profiles

Abstract The results of an investigation into the effects that sub-boundary layer vortex generators (SBVGs) have on reducing normal shock-induced turbulent boundary-layer separation are presented. The freestream Mach number and Reynolds number were M = 1·45 and 15·9 × 106/m, respectively. Total pressure profiles, static pressure distributions, surface total pressure distributions, oil flow visualisation and Schlieren photographs were used in the results analysis. The effects of SBVG height, lateral spacing and location upstream of the shock were investigated. A novel curved shape SBVG was also evaluated and comparisons against the conventional flat vane type were made. The results show that in all but two cases, separation was completely eliminated. As expected, the largest SBVGs with height, h = 55%δ, provided the greatest pressure recovery and maximum mixing. However, the shock pressure rise was highest for this case. The experiments showed that the mid height SBVG array with the largest spacing provided similar results to the SBVG array with the largest height. Reducing the distance to shock to 10δ upstream also showed some improvement over the SBVG position of 18δ upstream. It was suggested that total elimination of the separated region may not be required to achieve a balance of improved static pressure recovery whilst minimising the pressure rise through the shock. The effect of curving the SBVGs provided an improved near wall mixing with an improved static and surface total pressure recovery downstream of the separation line. The optimum SBVG for the current flow conditions was found to be the curved vanes of h = 40%δ, with the largest spacing, located at 18δ upstream of the shock. Overall, it was apparent from the results that in comparison to larger vortex generators with a height comparable to δ, for SBVGs the parameters involved become more important in order to obtain the highest degree of mixing from a given SBVG configuration.

scholarly journals Study of Sweep and Induced Dihedral Effects in Subsonic Axial Flow Compressor Passages—Part II: Detailed Study of the Effects on Tip Leakage Phenomena

2010 ◽  
Vol 2010 ◽  
pp. 1-13 ◽  
Author(s):  
P. V. Ramakrishna ◽  
M. Govardhan
Keyword(s):  
Axial Compressor ◽  
Axial Flow ◽  
High Sensitivity ◽  
Tip Clearance ◽  
Flow Coefficient ◽  
Tip Vortex ◽  
Pressure Losses ◽  
Tip Leakage ◽  
Compressor Rotor ◽  
Flow Compressor

This article presents the detailed study of rotor tip leakage related phenomena in a low speed axial compressor rotor passages for three sweep configurations [Unswept (UNS), Tip Chordline Swept (TCS) and Axially Swept (AXS)]. Fifteen domains are numerically studied with 5 sweep configurations (0°, 20°TCS, 30°TCS, 20°AXS, and 30°AXS) and for 3 tip clearances (0.0%, 0.7% and 2.7% of the blade chord). Results were well validated with experimental data. Observations near the tip reveal that UNS rotor shows high sensitivity than the swept rotors in the blade pressure distribution with change in tip clearance. AXS rotor has high loading capability and less tip clearance effect on blade loading at the near stall mass flow. Downstream shift of the vortex rollup along the chord is observed with increased flow coefficient and increment in the tip gap height. In particular, the effect of flow coefficient is more predominant on this effect. Tip vortex-related flow blockage is less with the swept rotors. Among the rotors, the AXS rotor is found to incur low total pressure losses attributable to tip leakage. Effect of incidence is observed on the flow leakage direction.

Flow Control in an Aggressive Interturbine Transition Duct Using Low Profile Vortex Generators

2014 ◽  
Vol 136 (11) ◽  
Author(s):  
Yanfeng Zhang ◽  
Shuzhen Hu ◽  
Xue-Feng Zhang ◽  
Michael Benner ◽  
Ali Mahallati ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Flow Control ◽  
Separated Flow ◽  
Adverse Pressure Gradient ◽  
Boundary Layer Separation ◽  
Vortex Generators ◽  
Pressure Losses ◽  
Layer Separation ◽  
Low Profile ◽  
Flow Mechanisms

This paper presents an experimental investigation of the flow mechanisms in an aggressive interturbine transition duct with and without low-profile vortex generators flow control. The interturbine duct had an area ratio of 1.53 and a mean rise angle of 35 deg. Measurements were made inside the annulus at a Reynolds number of 150,000. At the duct inlet, the background turbulence intensity was raised to 2.3% and a uniform swirl angle of 20 deg was established with a 48-airfoil vane ring. Results for the baseline case (no vortex generators) showed the flow structures within the duct were dominated by counter-rotating vortices and boundary layer separation in both the casing and hub regions. The combination of the adverse pressure gradient at the casing's first bend and upstream low momentum wakes caused the boundary layer to separate on the casing. The separated flow on the casing appears to reattach at the second bend. Counter-rotating and corotating vortex generators were installed on the casing. While both vortex generators significantly decreased the casing boundary layer separation with consequential reduction of overall pressure losses, the corotating configuration was found to be more effective.

The boundary layer separation from streamlined surfaces and new ways of its prevention in diffusers

2017 ◽  
Author(s):  
Arkady Zaryankin ◽  
Andrey Rogalev ◽  
Ivan Komarov ◽  
V. Kindra ◽  
S. Osipov
Keyword(s):  
Boundary Layer ◽  
Boundary Layer Separation ◽  
Layer Separation
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