Thesis (PhD)--Stellenbosch University, 2012. / ENGLISH ABSTRACT: The aims of this thesis are to identify all possible modes of operaton for a multi-stage axial flow
compressor; then to characterise the performance, attempt to numerically model operation, and
determine the main flow field features for each mode.
Four quadrant axial flow compressor operation occurs when the direction of flow through
the compressor or the sign of the pressure difference across the compressor reverses, or any
combination of these. Depending on the direction of rotation of the compressor, six modes
of operation are possible in the four quadrants of the performance map. The rotor rotates in
the design direction for three modes, and in the opposite direction for the other three. The
stationary-rotor pressure characteristic is S-shaped and passes through the second and fourth
quadrants.
A three-stage axial flow compressor operating in the incompressible flow regime was used
for the experimental investigation. Flow through the compressor was reversed or augmented by
means of an auxiliary axial flow fan. Compressor performance was measured by means of static
pressure tappings, a turbine anemometer calibrated to measure forward and reversed volumetric
flow and a load cell for torque measurement. The inter-blade row flow fields were measured with
pneumatic probes and 50 μm cylindrical hot film probes.
Three dimensional single blade-passage Navier-Stokes simulations were performed using the
Numeca FineTurbo package. Steady state simulations used a mixing plane approach. A nonlinear
harmonic approximation was used for time-unsteady simulations.
Unstalled first quadrant operation was unremarkable, and good agreement was obtained between
experimental and numerical data. A single stall cell was detected experimentally during
stalled operation, which was not modelled numerically.
In the fourth quadrant for positive rotation, (windmilling), the compressor acts as an inefficient
turbine. Flow separates from the pressure surface of the blade, rendering the steady-state
mixing plane approach unsuitable.
The performance characteristic curves for second quadrant for positive rotation, are discontinuous
with those of first quadrant operation. The temperature rise in the working fluid is
significantly higher than at design point. Periodic flow structures occurring across two blade
passages were detected at all flow coefficients investigated, invalidating numerical modelling assumptions.
Better agreement was obtained between experimental and numerical data from a case
found in literature.
If the compressor operates as a compressor in reverse (third quadrant operation), significant
separation occurs on the pressure surface of all blades, and flow conditions resemble severe first
quadrant stall. Separation becomes less severe at larger flow rates, allowing numerical simulation,
though this is sensitive to the initial flow field.
In the the part of the second quadrant, where the compressor rotates in reverse, it operates
as a turbine. The blade angles and the direction of curvature match the flow angles and turning
well, leading to high turbine efficiencies. Numerical simulations yielded good agreement with
measured results, but were again sensitive to the initial flow field.
Fourth quadrant operation with negative rotation occurs when flow is forced through the
compressor in the design direction. Large separation bubbles are attached to the pressure surfaces of rotor and stator blades, so virtually all throughflow occurs near the hub and casing / AFRIKAANSE OPSOMMING: Die doelwitte van hierdie tesis is om al die moontlike werkmodusse vir ’n bestaande multi-stadium
aksiaalvloei kompressor uit te ken; om dan die gedrag te gekarakteriseer, ’n poging aan te wend
om die werking numeries te modelleer, en die belangrikste vloeiveldkenmerke vir elke modus te
bepaal.
Vier-kwadrant aksiaalvloei kompressor werking vind plaas as die rigting van die vloei deur
die kompressor, of die teken van die drukverskil oor die kompressor omkeer, of enige kombinasie
daarvan. Afhangende van die rigting van rotasie van die kompressor is ses operasionele modusse
moontlik in die vier kwadrante van die kompressorkaart. Die rotor draai in die ontwerprigting
vir drie van die modes, en in die teenoorgestelde rigting vir die ander drie. Die stilstaande-rotor
drukkarakteristiek is S-vormig gaan deur die tweede en vierde kwadrante.
’n Drie-stadium onsamedrukbare vloei aksiaalvloei kompressor is vir die eksperimentele ondersoek
gebruik. Vloei deur die kompressor is omgekeer of aangehelp deur middel van ’n aksiaalvloei
hulpwaaier. Kompressor werking is gemeet deur middel van statiese druk meetpunte
in die omhulsel, ’n turbine anemometer wat gekalibreer is om vorentoe en omgekeerde volumetriese
vloei te meet, en ’n lassel vir wringmoment meting. Interlemryvloeivelde is opgemeet met
pneumatiese sensors en 50-μm silindriese warm film sensors.
Drie-dimensionele Navier-Stokes simulasies is uitgevoer vir ’n enkele lem van elke lemry,
met behulp van die Numeca FineTurbo sagtewarepakket. ’n Mengvlakbenadering is gebruik
vir bestendige toestand simulasies, terwyl ’n nie-linere harmoniese benadering gebruik is vir die
tyd-afhanklike simulasies.
Ongestaakte eerste kwadrant werking was alledaags, en goeie ooreenkoms is gevind tussen die
eksperimentele en numeriese data. ’n Enkele staak-sel is eksperimenteel ontdek tydens gestaakte
werking. Gestaakte werking is nie numeries gemodelleer nie.
In die vierde kwadrant vir positiewe rotasie, (”windmeulwerking”), werk die kompressor as ’n
ondoeltreffende turbine. Vloei-wegbrekinging op die lem drukoppervlaktes maak die bestendige
toestand mengvlakbenadering ongeskik.
In die kenlyne vir tweede kwadrant positiewe rotasie, is daar ’n diskontinu¨ıteit in die prestasie
karakteristiekkrommes vir die eerste en tweede kwadrant werking. Die temperatuurstyging in die
werk- vloeistof is beduidend ho¨er as as by die ontwerppunt. Periodiese vloeistrukture wat oor twee
lemme plaasvind is gevind by alle vloei ko¨effisi¨ente wat ondersoek is, en dit maak die numeriese
modellering aannames ongeldig. Beter ooreenkoms tussen die eksperimentele en numeriese data
iss verkry met ’n geval wat uit die literatuur gevind is.
Indien die kompressor werk as ’n kompressor in omgekeerde (derde kwadrant weking), vind
beduidende wegbreking op die drukoppervlak van al die lemme plaas, wat lyk soos ernstige
gestaakte eerste kwadrant werking. Die vloeiskeiding raak minder ernstig by ’n groter vloeitempo,
wat numeriese nabootsing toelaat, maar die nabootsings is sensitief vir die aanvanklike vloeiveld.
In die tweede kwadrant, by omgekeerde rotasie, werk die kompressor as ’n turbine. Die
lemhoeke en die rigting van lemkromming stem ooreen met die vloeihoeke en verwringing, wat lei
tot ho¨er turbine doeltreffendheid. Numeriese nabootsings stem goed ooreen met gemete resultate,
maar is weereens sensitief vir die keuse van die aanvanklike vloeiveld.
Vierde kwadrant werking met negatiewe rotasie vind plaas wanneer die lug gedwing word om deur die kompressor in die ontwerprigting te vloei. Groot skeidingborrels sit vas aan die
drukoppervlaktes van alle lemme, sodat meeste deurvloei naby die naaf en die omhulsel plaas
vind.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/20075 |
| Date | 03 1900 |
| Creators | Gill, Andrew |
| Contributors | Von Backstrom, T. W., Harms, T. M., Stellenbosch University. Faculty of Engineering. Dept. of Mechanical and Mechatronics Engineering. |
| Publisher | Stellenbosch : Stellenbosch University |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | Unknown |
| Type | Thesis |
| Format | 194 p. : ill. |
| Rights | Stellenbosch University |
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