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Module 4: Liquid Characteristics - Viscosity
Introduction
Absolute Viscosity
Kinematic Viscosity
Surface Tension
Practice Problems
Objectives
- Explain how the viscosity of liquids can affect some important processes
in air pollution control systems.
- Explain how temperature affects liquid viscosity.
- Explain the relationship between surface tension and viscosity.
The viscosity of a liquid is a measure of that fluid's resistance to flow when acted upon by an external force such as a pressure differential or gravity. Viscosity is a general property of all fluids, which includes both liquids and gases. While the basic concept of viscosity is the same for liquids and gases, changes in temperature affect the viscosity of liquids and gases differently. For more on gas viscosity, see Module 2, lesson on Viscosity.
In most cases, lower viscosity is beneficial to air pollution control applications. The following five phenomena, commonly encountered in air pollution control, are directly affected by liquid viscosity:
- Pump performance
- Size distribution of liquid droplets
- Settling rate of droplets
- Absorption of gaseous and particulate pollutants by liquids
- Gravitational settling of solids in liquid
Viscosity relates directly to pump performance and therefore, to the system connected to the pump. Prior to designing a pump, it is important to determine the viscosity of the fluid at the expected operating conditions. An increase in liquid viscosity generally increases the required net inlet pressure and the required pump input power. Furthermore, an increase in the viscosity generally corresponds to a decrease in the maximum allowable pump speed.
The viscosity affects the size of liquid particles. Liquid droplets can be formed by vapor condensation in stack gas or by spraying liquid into the gas stream (i.e. air pollution control equipment). Increasing the viscosity tends to increase the size of the droplets, which in turn increases their gravitational settling rates.
For a given mass of liquid, smaller sized droplets (lower viscosity) yield greater total surface area than do larger droplets. Greater surface area provides increased contact between the gas stream and the liquid and is generally beneficial for cooling a gas stream and collecting pollutants. Absorption (discussed later in this Module) is an important technique used by many types of air pollution control equipment to collect pollutants. The rate of absorption is partly dependent on both the size distribution and settling rate of droplets entering the gas stream, both of which are affected by viscosity.
Suspended solids in a liquid of a relatively quiescent state will settle by gravity if the solids have a greater density than the surrounding liquid, which is generally the case in air pollution work. Suspended solids settle more quickly in liquids with lower viscosities than with higher viscosities. You know from experience that a marble, dropped into a bucket of water, reaches the bottom more quickly than if that same marble were dropped into a bucket of honey.
There are two basic methods for expressing viscosity: absolute
(dynamic) viscosity 
and kinematic viscosity 
.
Absolute viscosity is the more commonly used form of viscosity and is defined by Equation 1.
- Where:
In Figure 1, two parallel surfaces of a liquid are a certain distance apart (d) and each has the same area (A). A force, F, is applied to the upper surface, resulting in a movement of this plane defined by velocity, v.
-
#1
- Assuming the same force is applied, how does increasing the liquid viscosity (in Figure 1) affect the velocity of the upper plate?
In an industrial context, pumps must apply more force, or torque, to a high-viscosity liquid than to a low-viscosity liquid to attain the same liquid flow rate. As the torque required to move a liquid increases, the power requirements of the pump increase and the life of the pump decreases.
The units of absolute viscosity are as follows:
The common units used for absolute viscosity are given in Table 1.
Another common unit of measure for absolute viscosity, which is used
throughout the world, is the centipoise (cp). A poise (p) unit is equivalent
to a dyne 
sec/cm2 unit and 1 poise is equivalent to 100 cp. The absolute
viscosity of water at room temperature is approximately 1.0 cp. Table
2 shows the relationship between the absolute viscosity of pure water
and temperature.
-
#2
- Using Table 1, what effect does temperature have on the absolute viscosity of a liquid? Explain.
Note: The temperature dependence of liquid viscosity is the opposite of the temperature dependence of gases. In the case of gases, the viscosity increases with temperature.
The kinematic viscosity is simply the absolute viscosity divided by the density of the liquid as shown in Equation 2.
- Where:
The units of kinematic viscosity are as follows:
The common units for displaying kinematic viscosity are shown in Table 3.
Since the density of a liquid is not altered significantly by temperature, the effect of temperature on kinematic viscosity is the same as on absolute viscosity. Limited data concerning the viscosity of aqueous mixtures, slurries, and ionic solutions are available in standard reference texts.
The same intermolecular forces that determine viscosity create surface tension. Surface tension is a measure of the internal forces generated by molecules due to their position in the surface of a liquid or the interface between two liquids. Surface tension forms what appears to be a membrane on the free surface of a liquid that allows insects to rest and pine needles to float on the water surface.
Liquid droplets tend to form a spherical shape since this shape produces the least amount of surface area for a given volume.
Since viscosity and surface tension are closely related, surface tension determines the size of liquid droplets in the same manner as viscosity. As surface tension increases, so does droplet size. For example, mercury at room temperature tends to form larger droplets than water and oil do because mercury has a higher surface tension.
Surface tension corresponds to the efficiency of inertial impaction for particulate matter. (Inertial impaction was discussed in the lesson on Collection Mechanisms in Module 3.) Particles can more easily penetrate liquid droplets with lower surface tensions than ones with higher surface tensions. Therefore impaction is inversely proportional to surface tension.
Practice Problems
Viscosity
- Instructions:
- Complete the Practice Problems before proceeding to the next lesson. Click on the button below.