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 h=1.0 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, a=0 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. Example3 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.