VAW Mitteilung 66: Air Slots for Flow Aeration

70 Pages·2012·6.85 MB·English

Nr. 66 Mitteilungen
der Versuchsanstalt fOr Wasserbau,
Hydrologie und Glaziologie
an der Eidgenossischen Technischen Hochschule Zurich
Herausgegeben von Prof. Dr. D. Vischer
Air Slots for Flow Aeration
Determination of shape, size and spacing
of air slots for the San Roque Dam Spillway
Peter'Volkart
Zurich, 1983
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PREFACE
Over the past few years there has been a trend worldwide for the con
struction of larger and more daring spillways from artificial reservoirs.
With a high spillway chute, large flow velocities resulting from high
specific discharges can cause extensive damage due to cavitation erosion
if no special protective measures are taken. However, recently the in
troduction of air to the flow at the bottom of the chute has given good
results in terms of preventing cavitation erosion. Special devices
facilitate the introduction of this air. Providing answers to the ques
tions as to the best way to introduce the air into the water and the
subsequent behaviour of the entrained air were the main aims behind a
model investigation performed at the Laboratory on behalf of Electro
consult {Milan). ~ese problems were studied specially for the spillway
of the planned San Roque project in the Philippines.
Due to the willingness of Electroconsult it is now possible for the
final report on this project {May, 1981) to be published as a communi
cation of the Laboratory. ~e kind agreement of Electroconsult to this
endeavour is acknowledged with thanks.
Oipl.-Ing. A. Chervet
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ABSTRACT
The Laboratory of Hydraulics, Hydrology and Glaciology
of the Swiss Federal Institute of Technology, Zurich, was
commissioned by Electroconsult, Milan, Italy, to carry out
model tests for the San Roque Dam Project in the Philippi
nes. With a 1:25 scale hydraulic model , air-slots for flow
aeration were investigated with the aim to prevent cavita
tion erosion of the dam spillway.
I
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GROOVE
Different shapes of air-slots were tested: deflectors (A),
offsets (C), deflectors with offsets (B) an.d a groove with
offsets (D). After comparison of general behavior and air de
mand of the slots for specific discharges between 20 m3/s·m
and 120 m3/s·m, a 0.75m high offset with a 0.50m high de
flector was selected.
With the selected air-slot, air concentration measurements
were made which resulted in an air-slot spacing diagram. The
results are compared with those of projects already in opera
tion.
Finally, model-prototype conformity is considered and
substantiated with trials to show the effect of scale on air
demand.
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List of Contents
page
List of Symbols 4
List of Figures 6
References 7
l . INTRODUCTION 8
l. l Commission and Project Description 8
l .2 Cavitation and Methods of Controlling it 9
1.3 Scope of Investigations 9
2. HYDRAULIC MODEL 10
2. l Model Construction 10
2.2 Measuring Instruments 10
2.2. l Measurement of Flow 10
2.2.2 Measurement of Water Levels 13
2.2.3 ~leasurement of Pressure Heads 14
2.2.4 Measurement of Velocities 14
2.2.5 Measurement of Air Demand 14
2.2.6 Measurement of Local Air Concentration 14
2.3 Similarity Problem 17
2. 3.1 Froude-Analogy 17
2.3.2 Influence of Air 18
2.3.3 Influence of Surface Roughness 19
2.3.4 Influence of Channel Width 21
2.3.5 Influence of Air Supply System 23
2.3.6 Scale Effect on Air Demand 24
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page
3. DETERMINATION OF SHAPE AND SIZE OF THE AIR-SLOT 29
3.1 Background 29
3. 1.1 Outline of Possible Shapes 29
3.1. 2 Genera 1 Outline of Investigations 30
3.2 Deflectors 31
3.2.1 General Behavior 31
3.2.2 Main Parameters 33
3.3 Groove 37
3. 3.1 Initial Design 37
3.3.2 Drainage of the Groove 39
3.3.3 Side Deflectors 40
3.4 Offsets 43
3.5 Selection of Air-Slot 47
4. SPACING OF THE AIR-SLOTS 49
4.1 Air Concentration Measurements 49
4.1 .1 Conditions Upstream of the Air-Slot 49
4.1 .2 Selected Cross Sections 49
4.1 .3 Concentration Profiles along the Channel Bottom 54
4.2 Criterion against Cavitation 56
4.3 Determination of Air-Slot Spacing 58
4.3.1 Applying Anti-Cavitation Criterion to the Measure- 58
ments
4.3.2 Additional Considerations 59
4.3.3 Results from other Spillways 61
5. SUMMARY AND CONCLUSIONS 63
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List of Symbols
Symbol Dimension Defintion
--------------------------------------
A [m2] area
a=f(e:) [ - ] friction parameter
b [mm] ~1i dth of the model chute
Oa
c [%l air concentration: C=-
Oa + o~~
= _£__
C* [%l air content: c* = .9E_ l + C*
Ow
D [m] distance between air slots
[ -] correction factors for air demand
Fr [ - ] Froude-number: Fr = __v_ _
/gL
9 acceleration due to gravity: g 9.81 m;s2
h [m] flowdepth
[m] deflector height
[m] offset height
k [r:1113;s] Strickler coefficient
K [ -] constant
L [m] characteristic length
p [m water] pressure as static ~1ater head: lm = 9.8l·l03Pa(N/m2)
q [m3/s·m] specific discharge
[m3/s·m) water flov1 in the drainage system, per meter channel
width
Q [m3;s,R./s] discharge
Re [ -] Reynold's number: Re = _v·_L_
\)
[ -] Reynold's number: Rex
2g·V-Sln Cl
u [m/s] velocity outside of boundary layer
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Symbol Dimension Definition
V [m/s] velocity
[m/s] absorption velocity
lm/s I characteristic velocity = velocity 25 cm above bottom
p·L
We [-) Weber-number: We= v (-)
g·o
x,y,z [m] coordinates: x =a 1o ng the axis
y =transverse
z =vertical
[degree] slope, angle to the horizontal
[m] boundary layer thickness
[m] displacement thickness
[ lllll} equivalent roughness height
E
[-] friction factor (Darcy-Weisbach)
[ -J geometric model scale
\l
[m2/s] kinematic viscosity, v = 1 .3·10-6 m2;s
\)
p [kg/m2J density: water 103kg/m3, air = 1.293 kg/m3
0 [N/m] surface tension: oair-water = 74.2·10-3 N/m
Indices a air
m mean
mix mixture
M model
p prototype
tot total
w water

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1 . INTRODUCTION
1. 1 Commission and Project Description
The Laboratory of Hydraulics, Hydrology and Glaciology of the S~liss Fe
deral Institute of Technology in Zurich was commissioned by Electroconsult
Consulting Engineers, 11ilan, Italy (by letter dated ~1ay 2, 1980), on behalf
of the National P01•1er Corporation, 11anila, Philippines, to carry out model
tests for the spillway and the low level outlet of the San Roque Dam Project.
The project concerns a gravel fill dam on the Agno River. The dam's ma
ximum height \~ill be 210 m, its fill volume 43 million m3. It ~1ill create
a reservoir with a gross storage volume of 990 million m3 and active stora
ge of 670 million m3.
As a multipurpose project, the dam will be used for the production of
hydroelectric energy, irrigation and flood protection. The power instal
lations will consist of 3 vertical Francis turbines with a maximum head
of 190 m and a max. discharge of 306 m3;s.
The spillway is of the gated open chute type, designed for a flood of
12'800 m3;s. The width of the chute is about lOO m, its length 550 ~- ~ith
a slope of 1:4, maximum water velocities will be up to 40 m/s. At the down
stream end energy will be dissipated by a ski-jump into a stilling basin.
During the construction stage, the river will be diverted through two
rectangular tunnels, each ~1i th a capacity of 1950 m3;s, and one circular
tunnel with a capacity of 380 m3;s. This latter one will also be used as
the downstream section of the S-shaped low level outlet.
The hydraulic problems related to the low level outlet were investigated
by computer-simulation and hydraulic model tests at the Laboratory of Hyd
raulics, Hydrology and Glaciology, Zurich (reports Nr. 774 and 777). The
spillway was investigated with a 1 :lOO scale model at the National Hydrau
lic Research Center, Philippines. Additional studies concerning cavitation
problems of the spillway were made 1·1ith a l :25 scale model at the Zurich
laboratory and are described in this report (776).
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1.2 Cavitation and Methods of Controlling it
A surface irregularity in a hydraulic structure tends to deflect the
stream away from the surface and, consequently, to form a low pressure zone
just downstream from the irregularity. It the pressure falls to the vapour
pressure of water, vapour bubbles appear then suddenly collapse when they
reach a region of higher pressure. If they collapse against a solid boun
dary, serious cavitation erosion may occur. From damage to existing spill
ways and low-level-outlets, it is known that within a relatively short time,
erosion can reach a depth of l .5 to 2 m and remove tens of cubic meters of
high-strength concrete.
There are two traditional methods for preventing cavitation erosion,
firstly by finish-specifications for concrete surfaces (Ball, 1976) and se
condly by using high strength material, such as epoxy concretes, epoxy mor
tars, fibrous concretes or steel-lining. The first method defines objectio
nable irregularities. However, the standards are difficult to obtain, espe
cially with velocities higher than 20 or 25 m/s. Furthermore, the method is
relatively uncertain because of defects that can occur on the surface con
crete as a result of atmospheric, climatic or chemi·cal agents. The second
method is safer, however being expensive, is reserved for small areas such
as near outlet gates, or for repairing damaged surfaces.
More recently, a new method, which is proposed for the San Roque dam
spillway, has been developed to protect the surface by_aeration devices.
Devices on the spillway bottom (air-slots) produce a local subpressure by
which air is sucked into the flow. The compressibility of the air-water
mixture reduces the pressures of collapsing vapour bubbles and thus protects
the surface concrete from cavitation erosion.
l .3 Scope of Investigations
The following investigations were made with the. l :25 scale hydraulic model
by representing a 3 m wide section of the prototype with a single air slot:
Determination of shape and size for the air slot by comparing the over
all air demand of three different types of various sizes (deflectors,
grooves, offsets and combinations of these types)
- Studying of scale-effects by varying the scale between 1:18.75 and 1:30
Determination of the optimum spacing of the air-slots by measuring air
concentration along the bottom and at some selected cross-sections.