Layout of spillway aerator air vents and its effect on air supply

27 Pages·2017·1.17 MB·English

DEGREE PROJECT IN VEHICLE ENGINEERING,
SECOND CYCLE, 30 CREDITS
STOCKHOLM, SWEDEN 2017
Layout of spillway aerator air vents
and its effect on air supply
GUSTAV DAGGENFELT
KTH ROYAL INSTITUTE OF TECHNOLOGY
SCHOOL OF ENGINEERING SCIENCES
Layout of spillway aerator air vents and its effect on
air supply
Gustav Daggenfelt
June 12, 2017
Abstract
Chute aerators are constructed to protect spillways from cavitation dam-
age. They function by launching the water flow as a jet and supplying air
underneath since having air entrained into the water is an effective way to
mitigate cavitation. This report looks at Bergeforsen, a dam in northern
Sweden, and its spillway as a basis for investigating how altering the aer-
ator’s outlet alters its performance. Five different designs are tested using
CFDwithANSYSFluent17. Thedesignsareevaluatedwithregardtototal
air flow, air flow distribution, duct pressure distribution, cavity length, and
air concentration in water jet.
Itwasfoundthatdesignswithmoreevenductpressuredistribution,that
transported air further toward the spillway center, had somewhat reduced
total air supply compared to designs that released more of its air in the
early part of the duct. Consequently there appear to be a trade off between
effectively distributing the air and providing more air flow. The current
design used by Bergeforsen strikes this balance quite well, better than the
other tested designs.
Luftinblandare byggs f¨or att skydda utskov fr˚an kavitationsskador. De
fungerar genom att l˚ata vattenstr¨ommen bli en str˚ale under vilken luft
tills¨attseftersomattblandainluftivattenstr¨ommen¨aretteffektivts¨attatt
motverkakavitation. Denh¨arrapportenunders¨okerBergeforsen, endammi
norraSverige,ochdessutskovf¨orattunders¨okahurluftinblandarensutsl¨app
p˚averkar dess prestanda. Fem olika designer testas med hj¨alp av CFD i
ANSYS Fluent 17. Designerna utv¨arderas med avsikt p˚atotalt luftfl¨ode,
f¨ordelning av luftfl¨ode, tryckf¨ordelning i luftg˚angen, l¨angd p˚alufth˚alrum,
samt luftkoncentration i vattenstr˚alen.
Det uppt¨acktes att designer med mer j¨amn tryckf¨ordelning i luftg˚angen,
som transporterade mer luft till utskovets mitt, hade n˚agot mindre total
lufttillf¨orsel j¨amf¨ort med designer som sl¨appte ut mer av luften tidigare ur
luftg˚angen. Det ser d¨arf¨or ut att kr¨avas en kompromiss mellan effektiv
f¨ordelning och totalt luft fl¨ode. Den design som anv¨ands i Bergeforsen idag
uppfyller den kompromissen ganska v¨al, b¨attre ¨an de andra som testades.
Acknowledgements
This work was done at the department for Hydraulic Engineering at Kung-
ligaTekniskaH¨ogskolanonbehalfofVattenfallABandsupervisedbyJames
Yang. Particular thanks to Penghua Teng for providing grids for CFD and
for plenty of help and advice along the way.
1
Contents
1 Background 3
2 Theory 5
2.1 Spillway . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.2 Cavitation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.3 Aeration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2.4 Bergeforsen . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
3 Method 9
3.1 Numerical model . . . . . . . . . . . . . . . . . . . . . . . . . 9
3.2 Air vent designs. . . . . . . . . . . . . . . . . . . . . . . . . . 10
4 Results 12
4.1 Duct pressure . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
4.2 Air Volume Flow . . . . . . . . . . . . . . . . . . . . . . . . . 13
4.3 Air Cavity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
5 Conclusion & Discussion 17
6 Appendix 18
6.1 Graphics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
2
Chapter 1
Background
Spillwaysareimportantstructures partsoftheflowadjacenttothewalls
servingasoutletsforexcesswaterfrom or bottom. A chute aerator is a de-
hydroelectric dams. In steeper and signthatusearampdeflectortolaunch
longer spillways, so more commonly thewaterintoajetunderneathwhich
in large dams, the streaming water there is an air duct outlet. The free
can reach high velocities that cause surface of the jet then has air en-
cavitation damage which erode the trained into it [2].
concrete along the spillway floor and Bergeforsen power station is lo-
walls. cated on Indals¨alven outside Timr˚a.
Cavitation damage has been ob- The dam began its service in 1955
servedinspillwaysworldwidethrough-andiscurrentlyunderongoingmain-
out the 20th century. A dam fail- tenanceandmodernizationforfuture
ure at the Madden Dam in Panama use. Primarily a new spillway was
1935 first prompted research into the constructedin2014[8]. Thenewspill-
phenomenon [2]. Some of the most way has an aerator to reduce erosion
standoutcaseshavebeeninpipespill- caused by cavitation. The aerator
wayssuchasGlenCanyonDam1983 has 13 square outlets along a ramp
[1]whereinitialcavitiationdamageis spanning the width of the spillway.
lesslikelytobespottedearly. Inthat Potentially the aerator could be im-
case the erosion eventually led to vi- proved by changing the design of the
brations breaking apart large pieces outlet, and as a result improve the
ofconcreteandpullingoutmetalbars, protection against erosion.
opening up an 11 m deep hole. Due to the spillway’s unusually
Cavitationinspillwayscanbemit- large width the flow characteristics
igated in several ways; redesigning differ somewhat compared to other
the spillway for reduced flow veloci- smaller Swedish dams. The size also
ties, smoothing out the concrete sur- make experiments with small scale
face and removing edges or other ir- models less reliable. Therefor CFD-
regularities, andinstallinganaerator simulation will be the method used
upstream of the critical region. to investigate.
Aerators are devices that mix air
into the water, most importantly the
3
Figure 1.1: Bergeforsen’s new spillway, air vents from the aerator can been
seen along the bottom.
4
Chapter 2
Theory
2.1 Spillway process will cause small shock waves
and micro jets of water that can cre-
Aspillwayisanessentialpartofdam ate large pressure spikes on nearby
constructionasitistheoutletforex- surfaces such as the concrete floor or
cess water from the reservoir to the walls resulting in erosion and mate-
river beneath. Typically it can be rial fatigue.[1, chapter 1] In hydro-
separated into three sections; a gate, engineeringthisentirechainofevents
achannelorchute,andanenergydis- is usually included in the term cav-
sipator. Thegateregulatesthewater itation due to the temporary nature
flow from the reservoir into the chute of the vapor bubbles.
where it flows down into the dissipa- The collapse of a vapor bubble
torwherethewaterhasitskineticen- is cyclical; the bubble is first com-
ergy reduced before it’s released into pressed before it collapses into two
the river. smaller bubbles and releases a wa-
ter micro jet, the two then combine
2.2 Cavitation back into a single bubble and the cy-
cle begins again. After each cycle
Inhighvelocitywaterflowoveranir- the resulting bubble is smaller than
regular or rough surface there will be it was at the start.[1, page 8-10] If
pockets of flow separation resulting the collapse happens in the vicinity
in low pressure areas. If these pres- of a boundary the microjet tends to
sure drops are large enough the wa- be directed towards it, as shown in
terwillstarttotransitionintovapor, figure 2.1.
essentiallyaboilingphenomenondue When the two bubbles recombine
to decreased pressure rather than in- they generate shock waves and it is
creased temperature. The formation these and the micro jets that cause
of these vapor bubbles in the water cavitation damage. Which factor is
is called cavitation. moreimportantishardtodetermine,
Asthevaporreturnstothehigher partiallybecausetheshockwavesfrom
pressurelevelsofthesurroundingflow onebubblecollapsecantriggerachain
the bubbles will collapse back into reactioninothernearbybubbles. Such
water. As they do so the collapsing achainreactioncanresultinanultra
5
shock waves and the jets during bub-
ble collapse.[1, pages 11-12 & 34]
2.3 Aeration
Figure 2.1: Bubble collapse and re- As early as 1953 it was found that
bound cycle close to a boundary. [1, cavitation damage can be effectively
page 9] mitigated if there is between 6-8%
air entrainment in the water[5]. In
truth the air concentration required
jet whenmultiplemicrojetsfromdif-
is poorly understood as it is difficult
ferent bubbles combine into a larger
to measure air concentration at the
single jet with velocity in the order
area where damage occurs. It also
ofabouthalfthesonicvelocityofthe
depends on surface roughness, flow
liquid.[1, page 9-10]
velocity, depth and other unknown
Cavitationdamageisprogressive,
factors. The findings from several
which means that erosion of a sur-
studieshoweveragreethatonlysmall
face is going to increase the inten-
amounts of air are needed to signif-
sityofcavitationonthatsurface,and
icantly reduce cavitation damage[2].
it’s therefore important to use pre-
In order to increase the air entrain-
ventive measures as damage can go
ment,particularlyinthecrucialparts
from being unnoticed to being dan-
of the flow close to the spillway floor,
gerous quickly.[3, page 95]
aeratorsarecommonlyinstalled. Aer-
Theriskofcavitationcanbeesti-
ators are airducts that suck in out-
mated using the cavitation index K
side air and distribute it from under-
h +h −h neath the water flow.
p a v
K = (2.1)
V2/2g
whereh isthebottompressurehead,
p
h is the atmospheric pressure head,
a
h is the vapor pressure head, V is
v
the water flow velocity and g is grav-
itational acceleration. A K-value at
0.2 or less indicates critical risk of
cavitation requiring aeration.[7]
Aerationreducescavitionbyhav-
ing air entrained in the flow. The
Figure2.2: Exampleofageneralaer-
main reason for this appears to be
ator design. Sideview crossection.
thefactthatanair-watermixturewith
more than 1% undissolved air, some-
Aerators typically use an offset
what counter-intuitively, can have a
andadeflectororrampinorderachieve
muchlowersonicvelocitythaneither
a water jet with an air cavity under-
fluidhasonitsown. Thisreducesthe
neath in which air can be supplied,
pressurespikesgeneratedbyboththe
6
air entrainment coefficient:
Q
A
β = (2.2)
Q
W
where Q and Q are the respective
A W
volume flows of air and water at the
aerator.
Another similar parameter is the
localaverageairconcentrationatthe
Figure 2.3: Water flow over an aera- aerator defined as
tor with P and L defined.
C = Q /(Q +Q ) (2.3)
a A A W
howeverbothoftheseparametershave
as shown in figure 2.2. The flow can
the weakness of not considering the
thereforbesaidtohavethreegeneral
bottomairconcentrationinthedown-
flowzones; thejetzone,thereattach-
stream region.[7]
ment and spray zone, and the far-
Whendoinginitialdesignofavent
fieldzone.[6]Thejetreattachmentpoint
aerator there are several important
is denoted P and its distance from
steps to take, the dimensions of the
the deflector and/or offset is the jet
ramp and offset are of primary im-
length L. The flow zones can then
portance but those will not be cov-
be defined as jet zone at 0 < x/L <
ered in this report. Instead focus is
1, reattachment and spray zone at
on the vent and duct design.
1 < x/L < 3, and far-field zone at
The air duct should be designed
x/L > 3. The reattachment point P
to avoid so called choked flow which
can be defined in various ways but
iswhereasonicflowregimeisreached.
in this report is defined by the point
A duct flow velocity limited to under
of peak pressure on the spillway bot-
100 m/s is therefor recommended as
tom. Figure 2.3 shows a possible ex-
well as a maximum pressure drop in
ample based on one of the CFD sim-
the duct of roughly 2-6% depending
ulations.
ontheamountoffrictionlossesinthe
Very little of the air entrained in
duct. The vent outlet area is recom-
thejetzoneremaininthelowerparts
mended to be less than the area of
of the flow after reattachment point
the duct intake.[1, page 70]
P,insteadmostoftheairisdetrained
bybeingtransportedupandreleased
fromthewatersurface. Aeratorscon- 2.4 Bergeforsen
sequently have only a small effect on
the bottom air concentration in the ThespillwayatBergeforsendam,shown
far-field zone [6]. Luckily not much infigure1.1, hasanopeningwidthof
air is required. 25mwhichmakesitthelargestspill-
A common ratio used to evaluate way in Sweden. It’s crest is at an el-
the effectiveness of an aerator is the evation 112.75 m and the aerator is
located 24.72 m downstream, more
measurements are given in table 2.1.
7