In liquids the cohesive forces predominates the

• 84

This preview shows page 40 - 51 out of 84 pages.

In liquids the cohesive forces predominates the molecular momentum transfer due to closely packed molecules and with the increase in temperature, the cohesive forces decreases with the result of decreasing viscosity. 40
Properties of Fluids But in the case of gases the cohesive force are small and molecular momentum transfer predominates. With the increase in temperature, molecular momentum transfer increases and hence viscosity increases. The relation between viscosity and temperature for liquids and gases are: 42 o 2 (ii) For a gas, 1 where for air o 0.000017, 0.000000056, 0.1189x 10 -9 , are constants for theliquid For water, μ o 1.79 x 10 poise, 0.03368and 0.000221 -3 (i) For liquids, o t t 2 where Viscosity of liquid at t o C, in poise o Viscosity of liquid at 0 C , in poise 10 m 2 1 poise 1 Ns o 1 t t
Types of Fluids 1. Ideal Fluid. A fluid, which is incompressible and is having no viscosity, is known as an ideal fluid. Ideal fluid is only an imaginary fluid as all the fluids, which exist, have some viscosity. 2. Real fluid . A fluid, which possesses viscosity, is known as real fluid. All the fluids: in actual practice, are real fluids. 3. Newtonian Fluid. A real fluid, in which the shear stress is directly, proportional to the rate of shear strain (or velocity gradient), is known as a Newtonian fluid. 4. Non-Newtonian fluid. A real fluid, in which shear stress is not proportional to the rate of shear strain (or velocity gradient), known as a Non-Newtonian fluid. 42
Types of Fluids 5. Ideal Plastic Fluid. A fluid, in which shear stress is more than the yield value and shear stress is proportional to the rate of shear strain (or velocity gradient), is known as ideal plastic fluid. 43
Example 3 If the velocity distribution over a plate is given by metre above the plate, determine the shear stress at y = 0 and y= 0.15 m. Take dynamic viscosity of fluid as 8.63 poises. 3 in which u is velocity in metre per second at a distance y 44 u 2 y y 2
45
Example 4 Calculate the dynamic viscosity of an oil, which is used for lubrication between a square plate of size 0.8 m x 0.8 m and an inclined plane with angle of inclination 30 o as shown in Fig. 1.4. The weight of the square plate is 300 N and it slides down the inclined plane with a uniform velocity of 0.3 m/s. The thickness of oil film is 1.5 mm. Fig.1.4 46
47
Example 5 The space between two square flat parallel plates is filled with oil. Each side of the plate is 60 cm. The thickness of the oil film is 12.5 mm. The upper plate, which moves at 2.5 metre per sec requires a force of 98.1 N to maintain the speed. Determine : · i. the dynamic viscosity of the oil, and ii. the kinematic viscosity of the oil if the specific gravity of the oil is 0.95. m
48
49
Thermodynamic Properties Fluids consist of liquids or gases. But gases are compressible fluids and hence thermodynamic properties play an important role.
• • • 