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Module 4: Liquid Characteristics - Viscosity - Practice Problems
- Instructions:
- Work these problems on a sheet of paper and check your answers against those provided below.
-
#1
- How does lowering the liquid temperature affect the (i) absolute
viscosity and (ii) kinematic viscosity of a liquid?
- Viscosity increases.
- Viscosity decreases.
- Viscosity is not affected by temperature.
- Viscosity increases.
- Answer:
- i. a. Viscosity increases.
- ii. a. Viscosity increases.
- As shown in Table 3, the absolute viscosity of a liquid increases as the temperature of the liquid is lowered. Since temperature does not significantly alter the density of a liquid, it affects the kinematic viscosity in the same way that it affects the absolute viscosity.
-
#2
- Liquid A has a viscosity of 1.75 cp and Liquid B has a viscosity
of 1.05 cp. Based solely on their viscosities, which liquid is the better
choice in the following situations? Assume the same operating conditions.
- Suppose you want to:
- Form droplets with greater surface tension.
- Use a liquid to absorb gaseous pollutants in a venturi
scrubber.
- Use a liquid in condensers and evaporative cooling towers to
cool hot gas streams.
- Lower pump power requirements for circulating a liquid through
a condenser.
- Increase the settling rate of suspended solids in a primary sedimentation tank.
- Form droplets with greater surface tension.
-
- Answer: i. Liquid A
- The surface tension of a liquid increases as its viscosity increases.
- Answer: ii. Liquid B
- Liquids of lower viscosity tend to form smaller droplets than liquids
of higher viscosity. Smaller droplets have more surface area than larger
droplets composed of the same total mass of liquid. Smaller droplets
absorb pollutants more readily than larger droplets due to their larger
surface area.
-
- Answer: iii. Liquid B
- A gas stream can transfer its heat more readily to a liquid having
greater surface area. Liquids of lower viscosity tend to form smaller
droplets than liquids of higher viscosity. As explained above, smaller
droplets have more surface area.
-
- Answer: iv. Liquid B
- Pumps must use more horsepower to move highly viscous fluids at the
same liquid stream flow rate as is required for less viscous fluids.
-
- Answer: v. Liquid B
- Suspended solids settle to the bottom of the settling chamber more quickly in less viscous liquids than in liquids of higher viscosity.