Vapor Pressure

1
Vapor Pressure

• Vapor Pressure, pv f (T)
• Boiling is initiated when the absolute pressure
  in the fluid reaches the vapor pressure
• B.P decreases with elevation
• Cavitation occurs when vapor bubbles are formed
  in a flowing fluid they are swept along into
  region of higher pressure where they suddenly
  collapse

2
Example 1

• In a water distribution system, the temperature
  of water is observed to be as high as 30 0C.
  Determine the minimum pressure allowed in the
  system to avoid cavitation

3
Surface Tension

• Drop of blood forms a hump on a horizontal glass
• Water droplets from rain or dew hang from
  branches or leaves
• Hg drops on plate

4
Surface Tension

• Forces develop in interfacial surface
• of two immiscible fluids
• Cause the surface to behave as if it
• were a skin or membrane stretched
• over the fluid mass.
• The intensity of the molecular
• attraction per unit length along any line in
  the surface is called the surface
• tension.
• Units are lb/ft and N/m.

Figure 1.7 (p. 25)
Forces acting on one-half of a liquid drop.
5
Figure 1.8 (p. 25)

• Effect of capillary action in small tubes. (a)
  Rise of column
• for a liquid that wets the tube. (b) Free-body
  diagram for
• calculating column height. (c) Depression of
  column for a
• nonwetting liquid.

6
Example 2

• What diameter of clean glass tubing is required
  so that the
• rise of water at 20C in a tube due to capillary
  action is less
• than h1.0 mm?

Figure E1.8 (p. 26)
7
Fluid StaticsPressure

• A vacuum gage connected to a chamber reads 5.8
  psi at a
• location where the atmospheric pressure is 14.5
  psi. What
• is the absolute pressure?

Is pressure a vector quantity?
8
Fluid Statics

• Pressure at a point normal force per unit area

Figure 2.1 (p. 39)
9
Basic Equation for Pressure Field

• Surface force due to the pressure Body force due
  to the weight

10
Pressure variation in a Fluid at Rest

• For a fluid at rest, a0

For incompressible fluid
11
Figure 2.3 (p. 43)

• Notation for pressure variation in a fluid at
  rest with a free
• surface.

Pressure difference or pressure head
Pressure at a distance h from the free surface ,
12

• Pressure is the same at all points along the line
  AB
• irrespective of height

Figure 2.4 (p. 44)
Fluid equilibrium in a container of arbitrary
shape
13
Example 3

• Because of a leak in an underground storage tank
  water
• has seeped in to the depth as shown. If the
  specific gravity
• of the gasoline is SG 0.68, determine the
  pressure at the
• gasoline-water interface and at the bottom of the
  tank.

14
Figure 2.5 (p. 45)

• Transmission of fluid pressure.

Hydraulic device hydraulic jacks, lifts and
presses
15
Using a hydraulic jack, a 1000 kg-car can be
lifted by applying a force of 100 kgf 908 N
16
Measurement of Pressure
Figure 2.7 (p. 48)
Graphical representation of gage and absolute
pressure.
17
Barometer to measure atmospheric pressure

• h for Hg column is 29.9 in for water, it will be
  34 ft

Invented by Evangelista Torricelli
1atm 760 torr 1 torr 133.3 Pa
Figure 2.8 (p. 50)
Mercury barometer.
18
Effect of shape of the Barometer tube
19
Effect of altitude on atmospheric pressure

• At high altitudes, a car
• engine generates less
• power and a person gets
• less oxygen because of
• the lower density of air

20
Examples 4 5

• Consider two identical fans, one at sea level
  and the
• other on top of a high mountain, running at
  identical
• speeds. How would you compare (a) the volume
  flow
• rates and (b) the mass flow rates of these two
  fans?

Determine the atmospheric pressure at a
locationwhere the barometric reading is 750
mmHg. Take the density of mercury to be
13,600 kg/m3.