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Chapter: Mechanical - Fluid Mechanics And Machinery - Fluid Properties and Flow Characteristics

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Fluid Properties

1 Density or Mass density: 2. Specific weight or weight density: 3. Specific Volume: 4.Specific Gravity:

FLUID PROPERTIES

 

1 Density or Mass density:

 

Density or mass density of a fluid is defined as the ratio of the mass of a fluid to its volume. Thus mass per unit volume of a is called density.

 

2. Specific weight or weight density:

 

Specific weight or weight density of a fluid is the ratio between the weight of a fluid to its volume. The weight per unit volume of a fluid is called weight density.

 

3. Specific Volume:

 

Specific volume of a fluid is defined as the volume of a fluid occupied by a unit mass or volume per unit mass of a fluid

 

4.Specific Gravity:

 

Specific gravity is defined as the ratio of the weight density of a fluid to the weight density of a standard fluid.

 

 

 

VISCOSITY

 

Viscosity is defined as the property of a fluid which offers resistance to the

 

movement of one layer of fluid over adjacent layer of the fluid. When two layers of a fluid, a distance ‘dy’ apart, move one over the other at different velocities, say u

 

and u+du as shown in figure. The viscosity together with relative velocity causes a shear stress acting between the fluid layers

 

The top layer causes a shear stress on the adjacent lower layer while the lower layer causes a shear stress on the adjacent top layer. This shear stress is proportional to the rate of change of velocity with respect to y.

 

VAPOUR PRESSURE

 

The pressure at which a liquid will boil is called its vapor pressure. This pressure is a function 3 of temperature (vapor pressure increases with temperature). In this context we usually think about the temperature at which boiling occurs. For example, water boils at 100oC at sea-level atmospheric pressure (1 atm abs). However, in terms of vapor pressure, we can say that by increasing the temperature of water at sea level to 100 oC, we increase the vapor pressure to the point at which it is equal to the atmospheric pressure (1 atm abs), so that boiling occurs. It is easy to visualize that boiling can also occur in water at temperatures much below 100oC if the pressure in the water is reduced to its vapor pressure. For example, the vapor pressure of water at 10oC is 0.01 atm.

1.CAVITATION

 

Cavitation(flashing of the liquid into vapour) takes place when very low pressures are produced at certain locations of a flowing liquid. Cavitation results in the formation of vapour pockets or cavities which are carried away from the point of origin and collapse at the high pressure zone.

 

 

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