scholarly journals Investigation of Separation Phenomena in a Radial Pump at Reduced Flow Rate by Large-Eddy Simulation

2016 ◽  
Vol 138 (12) ◽  
Author(s):  
Antonio Posa ◽  
Antonio Lippolis ◽  
Elias Balaras
Keyword(s):  
Reverse Flow ◽  
Leading Edge ◽  
Suction Side ◽  
Secondary Flows ◽  
Vaned Diffuser ◽  
Flow Rates ◽  
Turbulent Statistics ◽  
Large Eddy ◽  
Radial Pump ◽  
Reduced Flow

Turbopumps operating at reduced flow rates experience significant separation and backflow phenomena. Although Reynolds-Averaged Navier–Stokes (RANS) approaches proved to be usually able to capture the main flow features at design working conditions, previous numerical studies in the literature verified that eddy-resolving techniques are required in order to simulate the strong secondary flows generated at reduced loads. Here, highly resolved large-eddy simulations (LES) of a radial pump with a vaned diffuser are reported. The results are compared to particle image velocimetry (PIV) experiments in the literature. The main focus of the present work is to investigate the separation and backflow phenomena occurring at reduced flow rates. Our results indicate that the effect of these phenomena extends up to the impeller inflow: they involve the outer radii of the impeller vanes, influencing significantly the turbulent statistics of the flow. Also in the diffuser vanes, a strong spanwise evolution of the flow has been observed at the reduced load, with reverse flow, located mainly on the shroud side and on the suction side (SS) of the stationary channels, especially near the leading edge of the diffuser blades.

Jet, Wake and Intrinsic Motion in Impellers of Centrifugal Compressors

1996 ◽  
Author(s):  
Y. N. Chen ◽  
U. Seidel ◽  
U. Haupt ◽  
M. Rautenberg
Keyword(s):  
Reverse Flow ◽  
Vortex Pair ◽  
Leading Edge ◽  
Rotating Stall ◽  
Secondary Flows ◽  
Low Flow ◽  
Experimental Result ◽  
Vaned Diffuser ◽  
Measured Velocity ◽  
Intrinsic Motion

It was shown in a previous paper of the authors (1991) that jet and wake in the flow of the impeller of the centrifugal compressor are developed from the Dean’s type vortex pair formed in the curvature of the blade channel. The jet rotating against the sense of the impeller is weakened, and the wake rotating in the sense of the impeller is enhanced during travelling with the flow toward the outlet. This property is attributed to the conservation of the potential vorticity of the vortex. The experimental result obtained by Krain (1984) has confirmed this theory. The secondary flows found by Farge and Johnson (1990) enable the determination of the vorticity of the wake at the outlet of the impeller. It amounts to 6.9 Ω and 5.8 Ω for the radial-blading and the 60°-backswept blading impeller, respectively. The intensity of the vortex jet is weakened to undetectable value for both the impellers. The patterns of these secondary flow fields are also quite different between these two kinds of impellers. Whilst that of the former is controlled by the intrinsic motion, that of the latter is governed by the relative velocity along the blades. Furthermore, the experimental result obtained by the injection of colored dye at the impeller outlet and the measured velocity field around the impeller reveal an intense reverse flow in the radial blading impeller, travelling from the outlet toward the inlet, along the shroud. It can be shown that this reverse flow is caused by the intrinsic motion occuring in this impeller and impinging on the leading edge of the diffuser vane. As the rotating stall is introduced by the reverse flow, the low-solidity vaned diffuser, and still better the vaneless diffuser can therefore shift the stall line to a very low flow rate.

Low Pressure Turbine Relaminarization Bubble Characterization using Massively-Parallel Large Eddy Simulations

2012 ◽  
Vol 134 (2) ◽  
Author(s):  
Shriram Jagannathan ◽  
Markus Schwänen ◽  
Andrew Duggleby
Keyword(s):  
Boundary Layer ◽  
Reverse Flow ◽  
Leading Edge ◽  
Suction Side ◽  
Low Pressure ◽  
Surface Boundary ◽  
Suction Surface ◽  
Hairpin Vortices ◽  
Low Pressure Turbine ◽  
Large Eddy

The separation and reattachment of suction surface boundary layer in a low pressure turbine is characterized using large-eddy simulation at Ress = 69000 based on inlet velocity and suction surface length. Favorable comparisons are drawn with experiments using a high pass filtered Smagorinsky model for sub-grid scales. The onset of time mean separation is at s/so = 0.61 and reattachment at s/so = 0.81, extending over 20% of the suction surface. The boundary layer is convectively unstable with a maximum reverse flow velocity of about 13% of freestream. The breakdown to turbulence occurs over a very short distance of suction surface and is followed by reattachment. Turbulence near the bubble is further characterized using anisotropy invariant mapping and time orthogonal decomposition diagnostics. Particularly the vortex shedding and shear layer flapping phenomena are addressed. On the suction side, dominant hairpin structures near the transitional and turbulent flow regime are observed. The hairpin vortices are carried by the freestream even downstream of the trailing edge of the blade with a possibility of reaching the next stage. Longitudinal streaks that evolve from the breakdown of hairpin vortices formed near the leading edge are observed on the pressure surface.

EFFECT OF AXIAL CASING GROOVE GEOMETRY ON ROTOR-GROOVE INTERACTIONS IN THE TIP REGION OF A COMPRESSOR

2021 ◽  
pp. 1-54
Author(s):  
Subhra Shankha Koley ◽  
Huang Chen ◽  
Ayush Saraswat ◽  
Joseph Katz
Keyword(s):  
Rotor Blade ◽  
Leading Edge ◽  
Secondary Flows ◽  
Low Flow ◽  
Flow Angle ◽  
Flow Rates ◽  
Piv Measurements ◽  
Tip Leakage ◽  
Large Vortex ◽  
Blade Leading Edge

Abstract This experimental study characterizes the interactions of axial casing grooves with the flow in the tip region of an axial turbomachine. The tests involve grooves with the same inlet overlapping with the rotor blade leading edge, but with different exit directions located upstream. Among them, U grooves, whose circumferential outflow opposes the blade motion, achieve a 60% reduction in stall flowrate, but degrade the efficiency around the best efficiency point (BEP) by 2%. The S grooves, whose outlets are parallel to the blade rotation, improve the stall flowrate by only 36%, but do not degrade the BEP performance. To elucidate the mechanisms involved, stereo-PIV measurements covering the tip region and interior of grooves are performed in a refractive index matched facility. At low flow rates, the inflow into both grooves, which peaks when they are aligned with the blade pressure side, rolls up into a large vortex that lingers within the groove. By design, the outflow from S grooves is circumferentially positive. For the U grooves, fast circumferentially negative outflow peaks at the base of each groove, causing substantial periodic variations in the flow angle near the blade leading edge. At BEP, interactions with both grooves become milder, and most of the tip leakage vortex remains in the passage. Interactions with the S grooves are limited hence they do not degrade the efficiency. In contrast, the inflow into and outflow from the U grooves reverses direction, causing entrainment of secondary flows, which likely contribute to the reduced BEP efficiency.

Numerical Predictions of Turbine Cascade Secondary Flows and Heat Transfer With Inflow Turbulence

2020 ◽  
Author(s):  
Yousef Kanani ◽  
Sumanta Acharya ◽  
Forrest Ames
Keyword(s):  
Heat Transfer ◽  
Boundary Layer ◽  
Leading Edge ◽  
Suction Side ◽  
Lateral Spreading ◽  
Secondary Flows ◽  
Pressure Side ◽  
Initial Flow ◽  
Numerical Predictions ◽  
Inflow Turbulence

Abstract Turbine passage secondary flows are studied for a large rounded leading edge airfoil geometry considered in the experimental investigation of Varty et al. (J. Turbomach. 140(2):021010) using high resolution Large Eddy Simulation (LES). The complex nature of secondary flow formation and evolution are affected by the approach boundary layer characteristics, components of pressure gradients tangent and normal to the passage flow, surface curvature, and inflow turbulence. This paper presents a detailed description of the secondary flows and heat transfer in a linear vane cascade at exit chord Reynolds number of 5 × 105 at low and high inflow turbulence. Initial flow turning at the leading edge of the inlet boundary layer leads to a pair of counter-rotating flow circulation in each half of the cross-plane that drive the evolution of the pressure-side and suction side of the near-wall vortices such as the horseshoe and leading edge corner vortex. The passage vortex for the current large leading-edge vane is formed by the amplification of the initially formed circulation closer to the pressure side (PPC) which strengthens and merges with other vortex systems while moving toward the suction side. The predicted suction surface heat transfer shows good agreement with the measurements and properly captures the augmented heat transfer due to the formation and lateral spreading of the secondary flows towards the vane midspan downstream of the vane passage. Effects of various components of the secondary flows on the endwall and vane heat transfer are discussed in detail.

Platform Film Cooling Investigation on an HP Nozzle Vane Cascade With Discrete Shaped Holes and Slot Film Cooling

2020 ◽  
Author(s):  
G. Barigozzi ◽  
A. Perdichizzi ◽  
L. Abba ◽  
L. Pestelli
Keyword(s):  
Heat Transfer ◽  
Film Cooling ◽  
Heat Transfer Performance ◽  
Leading Edge ◽  
Suction Side ◽  
Secondary Flows ◽  
Intensity Level ◽  
Transfer Performance ◽  
Film Cooling Effectiveness ◽  
Exit Mach Number

Abstract The present paper reports on an experimental investigation on the aerodynamic and heat transfer performance of different platform cooling schemes: two based on cylindrical and shaped holes and one featuring a slot located upstream of the leading edge plane simulating the combustor to stator interface gap. Tests were run on a 6-vane cascade operated at an isentropic cascade exit Mach number of 0.4 and a significant inlet turbulence intensity level of about 9%. The cooling schemes were first tested to quantify their impact on secondary flows and related losses for variable injection conditions. Heat transfer performance was then assessed through adiabatic film cooling effectiveness and heat transfer coefficient measurements. The Net Heat Flux Reduction parameter was then computed to critically assess the cooling schemes. When compared with the cylindrical hole scheme, shaped holes outperform for all tested injection rates, while the slot alone is able to thermally protect only the front of the passage. Discrete holes are required to cool the platform region along the whole pressure side and the suction side leading edge region.

Highly Resolved Large Eddy Simulation Study of Gap Size Effect on Low-Pressure Turbine Stage

2017 ◽  
Vol 140 (2) ◽  
Author(s):  
R. Pichler ◽  
V. Michelassi ◽  
R. Sandberg ◽  
J. Ong
Keyword(s):  
Kinetic Energy ◽  
Cross Sections ◽  
Control Volume ◽  
Leading Edge ◽  
Suction Side ◽  
Low Pressure ◽  
Small Time ◽  
Gap Size ◽  
Low Pressure Turbine ◽  
Large Eddy

Blade-to-blade interactions in a low-pressure turbine (LPT) were investigated using highly resolved compressible large eddy simulations (LESs). For a realistic setup, a stator and rotor configuration with profiles typical of LPTs was used. Simulations were conducted with an in-house solver varying the gap size between stator and rotor from 21.5% to 43% rotor chord. To investigate the effect of the gap size on the prevailing loss mechanisms, a loss breakdown was conducted. It was found that in the large gap (LG) size case, the turbulence kinetic energy (TKE) levels of the stator wake close to the rotor leading edge were only one third of those in the small gap (SG) case, due to the longer distance of constant area mixing. The small time-averaged suction side separation on the blade, found in the LG case, disappeared in the SG calculations, confirming how stronger wakes can keep the boundary layer attached. The higher intensity wake impinging on the blade, however, did not affect the time-averaged losses calculated using the control volume approach of Denton. On the other hand, losses computed by taking cross sections upstream and downstream of the blade revealed a greater distortion loss generated by the stator wakes in the SG case. Despite the suction side separation suppression, the SG case gave higher losses overall due to the incoming wake turbulent kinetic energy amplification along the blade passage.

Off-Design Flow Measurements in a Centrifugal Compressor Vaneless Diffuser

1995 ◽  
Vol 117 (4) ◽  
pp. 602-608 ◽  
Author(s):  
A. Pinarbasi ◽  
M. W. Johnson
Keyword(s):  
Flow Rate ◽  
Centrifugal Compressor ◽  
Secondary Flows ◽  
Design Flow ◽  
Vaneless Diffuser ◽  
Flow Measurements ◽  
Vaned Diffuser ◽  
Flow Angle ◽  
Flow Rates ◽  
Study Results

Detailed measurements have been taken of the three-dimensional velocity field within the vaneless diffuser of a backswept low speed centrifugal compressor using hot-wire anemometry. A 16 percent below and an 11 percent above design flow rate were used in the present study. Results at both flow rates show how the blade wake mixes out more rapidly than the passage wake. Strong secondary flows inherited from the impeller at the higher flow rate delay the mixing out of the circumferential velocity variations, but at both flow rates these circumferential variations are negligible at the last measurement station. The measured tangential/radial flow angle is used to recommend optimum values for the vaneless space and vane angle for design of a vaned diffuser.

Numerical Predictions of Turbine Cascade Secondary Flows and Heat Transfer With Inflow Turbulence

2021 ◽  
pp. 1-34
Author(s):  
Yousef Kanani ◽  
Sumanta Acharya ◽  
Forrest Ames
Keyword(s):  
Heat Transfer ◽  
Boundary Layer ◽  
Leading Edge ◽  
Suction Side ◽  
Lateral Spreading ◽  
Secondary Flows ◽  
Pressure Side ◽  
Initial Flow ◽  
Numerical Predictions ◽  
Inflow Turbulence

Abstract Turbine passage secondary flows are studied for a large rounded leading edge airfoil geometry considered in the experimental investigation of Varty et al. (J. Turbomach. 140(2):021010) using high resolution Large Eddy Simulation. The complex nature of secondary flow formation and evolution are affected by the approach boundary layer characteristics, components of pressure gradients tangent and normal to the passage flow, surface curvature, and inflow turbulence. This paper presents a detailed description of the secondary flows and heat transfer in a linear vane cascade at exit chord Reynolds number of 500,000 at low and high inflow turbulence. Initial flow turning at the leading edge of the inlet boundary layer leads to a pair of counter-rotating flow circulation in each half of the cross-plane that drive the evolution of the pressure-side and suction side of the near-wall vortices such as the horseshoe and leading edge corner vortex. The passage vortex for the current large leading-edge vane is formed by the amplification of the initially formed circulation closer to the pressure side which strengthens and merges with other vortex systems while moving toward the suction side. The predicted suction surface heat transfer shows good agreement with the measurements and properly captures the augmented heat transfer due to the formation and lateral spreading of the secondary flows towards the vane midspan downstream of the vane passage. Effects of various components of the secondary flows on the endwall and vane heat transfer are discussed in detail.

Combustor Turbine Interface Studies: Part 1 — Endwall Effectiveness Measurements

2002 ◽  
Author(s):  
W. F. Colban ◽  
K. A. Thole ◽  
G. Zess
Keyword(s):  
Film Cooling ◽  
Total Pressure ◽  
Large Scale ◽  
Guide Vane ◽  
Leading Edge ◽  
Suction Side ◽  
Pressure Profile ◽  
Secondary Flows ◽  
Adiabatic Wall ◽  
Combustor Liner

Improved durability of gas turbine engines is an objective for both military and commercial aeroengines as well as for power generation engines. One region susceptible to degradation in an engine is the junction between the combustor and first vane given that the main gas path temperatures at this location are the highest. The platform at this junction is quite complex in that secondary flow effects, such as the leading edge vortex, are dominant. Past computational studies have shown that the total pressure profile exiting the combustor dictates the development of the secondary flows that are formed. This study examines the effect of varying the combustor liner film-cooling and junction slot flows on the adiabatic wall temperatures measured on the platform of the first vane. The experiments were performed using large-scale models of a combustor and nozzle guide vane in a wind tunnel facility. The results show that varying the coolant injection from the upstream combustor liner leads to differing total pressure profiles entering the turbine vane passage. Endwall adiabatic effectiveness measurements indicate that the coolant does not exit the upstream combustor slot uniformly but instead accumulates along the suction side of the vane and endwall. Increasing the liner cooling continued to reduce endwall temperatures, which was not found to be true with increasing the film-cooling from the liner.

On the Interactions of a Rotor Blade Tip Flow With Axial Casing Grooves in an Axial Compressor Near the Best Efficiency Point

2018 ◽  
Author(s):  
Huang Chen ◽  
Yuanchao Li ◽  
Joseph Katz
Keyword(s):  
Flow Rate ◽  
Rotor Blade ◽  
Leading Edge ◽  
Secondary Flows ◽  
Leakage Flow ◽  
Flow Rates ◽  
Tip Leakage ◽  
Flow Features ◽  
Blade Tip ◽  
Corner Vortex

Previous studies have shown that axial casing grooves (ACGs) are effective in delaying the onset of stall, but degrade the performance of axial turbomachines around the best efficiency point (BEP). Our recent experimental study [1] in the JHU refractive index-matched liquid facility have examined the effects of ACGs on delaying stall of a one and half stage compressor. The semicircular ACGs based on Müller et al. [2] reduce the stall flow rate by 40% with a slight decrease in pressure rise at higher flow rates. Stereo-PIV (SPIV) measurements at a flow rate corresponding to the pre-stall condition of the untreated machine have identified three flow features that contribute to the delay in stall. Efficiency measurements conducted as part of the present study show that the ACGs cause a 2.4% peak efficiency loss. They are followed by detailed characterizations of the impact of the ACGs on the flow structure and turbulence in the tip region at high flow rates away from stall. Comparisons with the flow structure without casing grooves and at low flow rate are aimed at exploring relevant flow features that might be associated with the reduced efficiency. The SPIV measurements in several meridional and radial planes show that the periodic inflow into the groove peaks when the rotor blade pressure side (PS) overlaps with the downstream end of the groove, but diminishes when this end faces the blade suction side (SS). The inflow velocity magnitude is substantially lower than that occurring at a flow rate corresponding to the pre-stall conditions of the untreated machine. Yet, entrainment of the PS boundary layer and its vorticity during the inflow phase generates counter-rotating radial vortices at the entrance to the groove, and a “discontinuity” in the appearance of the tip leakage vortex (TLV). While being exposed to the blade SS, the backward tip leakage flow causes flow separation and formation of a counter-rotating vortex at the downstream corner of the groove, which migrates towards the passage with increasing flow rate. Interactions of this corner vortex with the TLV cause fragmentation of the latter, creating a broad area with secondary flows and elevated turbulence level. Consequently, the vorticity shed from the blade tip remains scattered from the groove corner to the blade tip long after the blade clears this groove. The turbulence peaks around the corner vortex, the TLV, and the shear layer connecting it to the SS corner. During periods of inflow, there is a weak outflow from the upstream end of the groove. At other phases, most of the high secondary flows are confined to the downstream corner, leaving only weak internal circulation in the rest of the groove, but with a growing shear layer with elevated (but weak) turbulence originating from the upstream corner. Compared to a smooth endwall, the groove also increases the flow angle near the blade tip leading edge (LE) and varies it periodically. Accordingly, the magnitude of circulation shed from the blade tip and leakage flow increase near the leading edge. The insight from these observations might guide the development of ACGs that take advantage of the effective stall suppression by the ACGs but alleviate the adverse effects at high flowrates.

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