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?
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How is flow classified based on Reynolds number?
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Explain the nature of flow given the Reynolds number of less than 2000.
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What type of flow exists when Reynolds number is greater than 4000?
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Describe transient flow.
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What are the three types of flow mentioned in the content?
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What is the equation used to compute Reynolds number for flow in non-circular sections?
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The hydraulic radius is calculated as the ratio of ______ to wetted perimeter.
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What plays a role in determining friction loss in pipe flow?
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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.
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Description
This quiz covers the concepts of laminar, turbulent, and transition flow in closed circular conduits, including the Reynolds number and friction loss in pipe flow.