Environmental Engineering Reference
In-Depth Information
Table 2.4 Thermal Properties of Water at Various
Temperatures
....
Table 2.5 Viscosity of Water at 101.3 kPa
Absolute (Dynamic)
Temperature (
◦
C)
10
−
3
s/m
2
)
Specific Heat
Thermal
Viscosity,
ν(
×
·
N
Capacity,
Conductivity,
Temperature (
◦
C)
C
w
(kJ/kg K)
a
λ
w
(W/m K)
b
5
1.519
10
1.310
5
4.204
20
1.009
10
4.193
0.582
30
0.800
15
4.186
40
0.654
20
4.183
0.560
50
0.548
25
4.181
0.608
30
4.179
0.615
Source
: Modified from Tuma, 1976.
35
4.178
40
4.179
0.629
tensions) that are much more negative (Young, 1989).
It has also been possible to produce conditions in the
laboratory where water can be shown to sustain tensions
of more than 10 atm (Ridley and Burland, 1993; Guan and
Fredlund, 1997a). The water used in the laboratory must
first be conditioned through a pressurization process where
the cavitation nuclei in the water have been collapsed.
Water can sustain high tensile stresses and this phe-
nomenon would at first appear to be in conflict with our
fundamental understanding of the behavior of water. The
topic of water cavitation warrants closer examination. It is
important to understand the conditions under which water
can sustain a high tensile stress. The topic of cavitation is
of particular interest in geotechnical engineering since it has
been possible to produce suction sensors that can provide
a direct measurement of relatively high soil suctions (i.e.,
500 - 1500 kPa). Direct, high-suction-measuring devices
take advantage of the high tensile strength associated with
“conditioned” or pressurized water (Ridley and Burland,
1993; Guan and Fredlund, 1997b).
Cavitation is the creation of a new cavity or the expansion
of a preexisting cavity in a liquid (Young, 1989). The cavity
may commence in the form of a bubble suspended in the
liquid or it may be trapped in tiny cracks along the boundary
between a liquid and a solid. The bubbles may contain gases
or water vapor. If the bubble contains water vapor, then
reducing the pressure at a constant temperature results in
an “explosive” vaporization called cavitation. If the bubble
expands as a result of an increase in temperature the process
is known as boiling.
Young (1989) described four ways by which bubble
growth can be induced: (1)
gaseous cavitation
can occur as
a result of pressure reduction or an increase in temperature,
(2)
vaporous cavitation
can occur when a vapor-filled
bubble is subjected to pressure reduction, (3)
degassing
can occur when gas comes out of a bubble through the
process of diffusion, and (4)
boiling
can occur when there
is sufficient temperature rise.
Boiling and cavitation may be initiated either from a
macroscopic bubble or a microscopic bubble. If the gas or
50
4.182
0.640
60
4.185
0.651
70
4.191
0.659
80
4.198
0.667
90
4.208
0.673
100
4.219
0.677
a
The Engineering ToolBox, www.EngineeringToolBox.com
Water-Thermal Properties.
b
Data from Sengers and Watson, 1986.
2.3.2.4 Viscosity of Water
All fluids resist a change of form or the action of shearing,
and this property is called viscosity. The absolute (dynamic)
viscosity,
ν
, of a fluid is defined as the resistance of the
fluid to shearing force applied by sliding one plate over
another with the fluid placed in between. Absolute viscosity
depends on pressure and temperature. However, the influ-
ence of pressure is negligible for the range of pressures
commonly encountered in civil engineering applications.
The viscosity of water under atmospheric pressure condi-
tions (i.e., 101.3 kPa) and at various temperatures is given
in Table 2.5. The viscosity of liquids is shown to decrease
with an increase in temperature.
2.3.2.5 Cavitation of Water
The cavitation point of a liquid is generally assumed to be
equal to the absolute vapor pressure of the liquid under
consideration. If the vapor pressure of water at a particu-
lar temperature is 4.0 kPa and the absolute air pressure is
101.3 kPa, then cavitation should occur at
−
101
.
3
+
4
.
0
or
97
.
3 kPa on the gauge pressure scale. This is the vac-
uum pressure under which the water should start to vaporize
(Pearsall, 1972).
The above example suggests that water will cavitate and
commence boiling once the tension in the water approaches
the absolute vapor pressure. However, water in a soil is
known to be able to sustain pore-water pressures (i.e.,
−
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