Flow in Closed Circular Conduits

Questions and Answers

Define laminar flow.

Laminar flow is when the velocity of fluid is low, and fluid molecules move in an orderly fashion without mixing.

Explain turbulent flow.

Turbulent flow occurs when the velocity of fluid is relatively high, leading to the formation of eddy currents and mixing of fluid particles.

What is transition flow?

Transition flow, also known as transient flow, happens when the velocity is between laminar and turbulent flow, making it difficult to classify the flow.

What did Reynolds conclude regarding fluid flow through pipes?

Reynolds concluded that the type of fluid flow through pipes is dependent on factors like average velocity, pipe diameter, dynamic viscosity, and fluid density. Signup and view all the answers

How is flow classified based on Reynolds number?

Flow is classified as laminar if Re <= 2000, turbulent if Re >= 4000, and transient if 2000 < Re < 4000. Signup and view all the answers

Explain the nature of flow given the Reynolds number of less than 2000.

For Re < 2000, the flow is considered laminar. Signup and view all the answers

What type of flow exists when Reynolds number is greater than 4000?

Turbulent flow exists when Re > 4000. Signup and view all the answers

Describe transient flow.

Transient flow occurs when 2000 < Re < 4000, exhibiting characteristics of both laminar and turbulent flow. Signup and view all the answers

What are the three types of flow mentioned in the content?

laminar, transition, turbulent Signup and view all the answers

What is the equation used to compute Reynolds number for flow in non-circular sections?

Re = ρv(4R)/μ Signup and view all the answers

The hydraulic radius is calculated as the ratio of ______ to wetted perimeter.

area Signup and view all the answers

What plays a role in determining friction loss in pipe flow?

Friction factor Signup and view all the answers

Study Notes

Flow in Closed Conduits (Circular)

Laminar, Turbulent, and Transition Flow

Laminar Flow: occurs at low velocities, fluid molecules move in an orderly fashion, no mixing of fluid particles, and fluid velocity is maximum at the center of the pipe and zero at the pipe wall.
Turbulent Flow: occurs at relatively high velocities, eddy currents are formed, mixing of fluid particles occurs, and fluid particles have a random motion transverse to the main flow direction.
Transition Flow: occurs when velocity is between laminar and turbulent flow, a transition stage when the flow cannot be classified as laminar or turbulent flow.

Reynolds Number (Re)

Reynolds Number is a dimensionless parameter that determines the type of fluid flow through pipes.
It is dependent on four factors: average velocity of the fluid (v), pipe diameter (d), dynamic viscosity of the fluid (μ), and density of the fluid (ρ).
The Reynolds Number is calculated by: Re = ρvd/μ.
Laminar flow exists at low Reynolds Numbers (Re ≤ 2000), turbulent flow exists at high Reynolds Numbers (Re ≥ 4000), and transition flow exists when 2000 < Re < 4000.

Flow Classification

Reynolds Number classification:
- Re < 2000: Laminar Flow
- Re > 4000: Turbulent Flow
- 2000 < Re < 4000: Transient Flow

Flow in Closed Conduits (Non-Circular)

Hydraulic Radius and Reynolds Number for Non-Circular Conduits

Hydraulic radius (R) is the characteristic dimension for flow path in non-circular cross-sections.
R is defined as the ratio of the cross-sectional area (A) to the wetted perimeter (WP).
Wetted perimeter (WP) is the sum of the length of boundaries in contact with the fluid.
The unit for R is meters (SI unit system) or feet (U.S. Customary System).

Examples of Non-Circular Cross-Sections

Shell-and-tube heat exchanger
Air distribution ducts
Rainwater or sewage drains
Flow path inside a machine

Calculating Hydraulic Radius (R)

For a non-circular cross-section, R can be calculated using the formula: R = A / WP
A is the net cross-sectional area of the flow stream
WP is the wetted perimeter

Reynolds Number for Non-Circular Conduits

The Reynolds number (Re) for flow in non-circular sections is computed by replacing the diameter (d) with 4R, i.e., four times the hydraulic radius.
The formula for Re is: Re = ρv(4R) / μ
ρ is the fluid density, v is the fluid velocity, and μ is the fluid dynamic viscosity.

Note on Calculating Reynolds Number

The validity of this substitution can be demonstrated by calculating the hydraulic radius for a circular pipe.
This approach will give reasonable results if the cross-section has an aspect ratio (ratio of the width of the section to its height) not much different from that of the circular cross-section.