Page 83 - Industrial Ventilation Design Guidebook

48 CHAPTER 4 PHYSICAL FUNDAMENTALS

If the viscosity is a function of dv/dy, the fluid is classed as a non-Newto-
nian fluid. Fluids of this type are outside the scope of this chapter.
A special case of a Newtonian fluid is that of an ideal fluid, in which the
viscosity /x = 0. Ideal fluids do not exist; however, in many noncritical appli-
cations, the friction can be ignored to simplify calculations.
Thus viscosity is not a function of dv/dy, and it is independent of pres-
sure. However, it is a function of the temperature.
The viscosity of noncompressible fluids depends on the temperature as

where
/A is the viscosity at any temperature T,
/A O is the viscosity at any temperature T 0,
B and C are constants, depending on the nature of the fluid.
The viscosity of a gas depends on the temperature according to

where S is a constant specific for the gas. A simplified version sometimes

The kinematic viscosity v is the ratio of the dynamic viscosity /x and density p:

The kinematic viscosity of a gas is a function of the pressure, and its dimen-
1
2
sion is the square of length divided by the time, its unit being m s" ,
In old literature the cgs system is found, in which the dynamic viscosity is
2
measured in centipoise =0.1 poise = 0,001 dyne s cm~ :

The non-Si unit of kinematic viscosity is the centistoke:

4.1.2 Constants for Water
For water the values of the constants discussed in the previous section are
given in Table 4.1. The value of the elasticity modulus increases as the pres-
sure and temperature increase. At a pressure of 10 MPa and temperature
9
373 K, the elasticity modulus E is 2.7 x 10 Pa.
The equations do not give exact results, but the error is small and in many
cases can be ignored.