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FLUID MECHANICS - THEORY

   

The vaporization and condensation processes will be introduced in this section. The concept of vapor pressure is then presented. The conditions when boiling and cavitation occur will be discussed as well.

     
    Vaporization and Condensation


Vaporization (Evaporation)


Condensation: Contrails

 

When the molecules at the surface of a liquid or solid gain enough energy to overcome the cohesive force, the molecules will escape into the air. The substance undergoes a phase change and turns into vapor. This process is referred to as vaporization (e.g., evaporation and sublimation, see below). In general, the rate of vaporization increases with the temperature.

The process of phase change from liquid to vapor is called evaporation. For example, water will evaporate into vapor when the temperature reaches 100oC (for atmospheric pressure at sea level). A phase change directly from solid to vapor is called sublimation. An example of a sublimation process is dry ice at room temperature. The dry ice will become vapor. Also, ever wonder why moth balls will "disappear" over a period of time, but one can still smell the odor? This is another example of sublimation.

The process of phase change from a vapor to liquid is called condensation. The phenomenon of condensation can be observed in our daily lives. The contrail (condensation trail) left by an airplane is an example of the condensation process. As the hot humid exhaust air from the nozzle mixes with the surrounding air, a visible trail is formed due to condensation.

    Vapor Pressure


Vapor Pressure

 

Now consider a closed container partially filled with a liquid, as shown in the figure. As the liquid molecules at the surface gain sufficient energy to escape into the air (evaporation), some of the liquid molecules will collide with the wall or air molecules, bounce back and re-enter the liquid (condensation). Over a period of time, the system will reach a steady-state where the rate of evaporation is the same as the rate of condensation. At this instance, the pressure exerted on the liquid surface by the liquid vapor is called vapor pressure. Vapor pressure is a fluid property, and it is a function of the temperature. Generally, the vapor pressure increases with temperature. The value of vapor pressure for water and is summarized in the table below as a function of temperature.

     
20 C (68 F) pv
(kPa)
pv
(psi)
Carbon
Tetrachloride
1.3e4 1.9e0
Ethyl alcohol 5.9e3 8.5e-1
Gasoline 5.5e4 8.0e0
Glycerin 1.4e-2 2.0e-6
Kerosene 3.1e3 4.5e-1
Mercury 1.6e-1 2.3e-5
Various Liquids Vapor Pressure (absolute)
 
Temp
(oC)
pv
(kPa)
  Temp
(oF)
pv
(psi)
0 0.611   32 0.0885
5 0.872   40 0.1217
10 1.228   50 0.1781
15 1.666   60 0.2563
20 2.338   70 0.3631
30 4.243   80 0.5069
40 7.376   90 0.6980
50 12.33   100 0.9493
60 19.92   120 1.692
70 31.16   140 2.888
80 47.34   160 4.740
90 70.10   180 7.507
100 101.3   212 14.69
Water Vapor Pressure (absolute)
     
    Boiling and Cavitation


Boiling
  Boiling will occur when the absolute pressure of a liquid is less than or equal to its vapor pressure. One characteristic of the boiling process is the formation of vapor bubbles in the liquid. The formation and collapse of bubbles, primarily due to a reduction in pressure, in fluid flow is called cavitation (flow induced boiling). In engineering applications (e.g., pumps, turbines and hydraulic systems), it is a good practice to avoid cavitation because it can cause structural damage, produce noise, and reduce the overall efficiency of the system.
     
Elev
(km)
Pressure
(kPa)
  Elev
(ft)
Pressure
(psia)
0 101.33   0 14.70
2 79.50   5,000 12.24
4 60.12   10,000 10.11
6 47.22   15,000 8.30
8 35.65   20,000 6.76
10 26.5   25,000 5.46
Standard Atmosphere Pressure
at Various Elevations
  It is well known to backpackers that boiling of water is dependent on the current elevation above sea level. An increase in elevation reduces the atmospheric pressure. With a lower pressure, water (or any liquid) will boil at a lower temperature that matches the vapor pressure. For example, at an elevation of 4 km, the atmospheric pressure is about 60 kPa (see table at left). At that pressure, water will boil at about 86 C (see table above). This does not mean food will cook faster or slower, but boiling will just occur at a lower temperature.
     
   
 
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