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Module 2: Characteristics of Gases - Flow Characteristics

 

Features
Laminar and Turbulent Flow
Practice Problems

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Objective

  1. Determine if the gas stream is laminar, transitional, or turbulent by calculating the Reynolds number for the gas stream.

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Laminar and Turbulent Flow

When fluids such as gases are moving slowly, the bulk material moves as distinct layers in parallel paths as illustrated in Figure 1 (Laminar Flow). The only movement across these layers is the molecular motion, which creates viscosity. This is termed laminar flow.

Figure 1

As the velocity of the gas stream increases, the bulk movement of the gas changes. Eddy currents develop which cause mixing across the flow stream. This is called turbulent flow, and it is essentially the only flow characteristic that occurs in air pollution control equipment and emission testing-related situations. Turbulent flow is illustrated in Figure 2.

Figure 2

A dimensionless parameter called the Reynolds number is used to characterize fluid flow. It is the ratio of the inertial force which is causing gas movement to the viscous force which is restricting movement. The Reynolds number is calculated using Equation 1. Consistent units must be used to ensure that the Reynolds number is dimensionless.

equation 1

Table 1

Reynolds numbers less than 2,000 are associated with laminar flow conditions. Due to the relatively low velocities associated with this type of flow, they are rarely encountered in air pollution field situations.

Reynolds numbers above 10,000 are associated with turbulent flow. In many field situations, the Reynolds numbers exceed 100,000.

Between Reynolds numbers of 2,000 and 10,000, turbulent flow conditions have not fully developed. This is called the transitional flow range. Flow in this transitional range has characteristics of both laminar and turbulent flow.

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Example Problem 1.
Calculating the Reynolds Number, Circular Duct

What is the Reynolds number for a 28,000 ACFM gas stream moving through a duct having a diameter of 3.2 ft?

example equation 1

Solution:
Calculate the Reynolds number using the equation:

example equation 1(a)

  1. Calculate the gas velocity, v.

    example equation 1(1)

  2. Calculate the viscosity, µ, using the following equation:

    example equation 1(2)a

    Convert temperature from 300°F to °K.

    example equation 1(2)b

  3. Convert the viscosity units of measure to pounds mass per foot second.

    example equation 1(3)

  4. Calculate the Reynolds number.

    example equation 1(4)

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Example Problem 2.
Calculating the Reynolds Number, Rectangular Duct

What is the Reynolds number for a 28,000 ACFM gas stream moving through a rectangular duct having a width of 4 ft and a height of 2 ft? Assume the gas temperature is 300°F and the moisture and oxygen concentrations are identical to Example Problem 1.

Solution:
This problem is identical to the one above except for the rectangular duct. The equivalent hydraulic diameter for the rectangular duct must be calculated and used as the characteristic length in the Reynolds number equation. The equation for the hydraulic diameter is shown below.

equation 2

As in the previous example, calculate the Reynolds number using the following equation:

example equation 2(a)

  1. Calculate the value for the linear dimensions, L. In this case, L = Dh

    example equation 2(1)

  2. Calculate the gas velocity, v.

    example equation 2(2)

  3. Calculate the Reynolds number.

    example equation 2(3)

Essentially all gas flow situations are turbulent in air pollution control systems, emission test and monitoring equipment, and dispersion modeling studies. However, this does not mean that the gas stream is entirely well mixed as indicated in Figure 3 [Well-Mixed Gas Stream Flow (Turbulent Conditions)].

Figure 3

In reality, the side-to-side mixing (and even mixing in the direction of flow) can be limited as indicated in Figure 4 [Stratified Gas Stream Flow (Turbulent Conditions)]. For this reason, it is possible to have different concentrations of pollutants at different positions in the duct. This is called pollutant stratification. It can be caused by a variety of factors: combining of two separate gas streams into a single duct, temperature differences in the gas stream, and in-leakage of cold ambient air into the duct.

Figure 4

Stratification does not exist in most industrial gas handling systems. However, it is important to check for this condition prior to installation of continuous emission monitors or other instruments, which are located at a single sampling or measurement point in the gas stream. These measurements can be in error if stratification is severe.

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Practice Problems
Gas Flow Characteristics

Instructions:
Complete the Practice Problems before proceeding to the next lesson. Click on the button below.
Practice Problems

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