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Chapter 6: Problem 33

How does turbulent flow differ from laminar flow? For which flow is the heat transfer coefficient higher?

Short Answer

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Question: Explain the difference between laminar flow and turbulent flow and determine which type of flow has a higher heat transfer coefficient. Answer: Laminar flow is characterized by smooth, parallel layers of fluid with no mixing between adjacent layers, while turbulent flow is characterized by chaotic and disorderly fluid motion with random fluctuations in velocity and pressure. Turbulent flow has a higher heat transfer coefficient than laminar flow due to the increased mixing of fluid particles, resulting in a more uniform temperature distribution and enhanced heat transfer.

Step by step solution

01

Definition of Laminar Flow

Laminar flow is a type of fluid flow characterized by smooth, parallel layers of fluid moving along the flow direction. In laminar flow, there is no mixing between adjacent layers, and velocity gradients are steady and constant throughout the fluid.
02

Definition of Turbulent Flow

Turbulent flow, on the other hand, is characterized by chaotic and disorderly fluid motion. In this type of flow, there are random fluctuations in velocity and pressure, and fluid particles mix and move in an unpredictable manner.
03

Differences between Laminar and Turbulent Flow

The fundamental difference between laminar and turbulent flow lies in the fluid's motion pattern. Laminar flow has smooth and orderly layers of fluid, while turbulent flow is characterized by irregular and chaotic fluid motion. Turbulent flow has higher energy dissipation than laminar flow due to the more intense mixing of the fluid layers.
04

Heat Transfer Coefficient Comparison

The heat transfer coefficient is the proportionality constant between the convective heat transfer and the temperature difference driving the heat transfer. In general, heat transfer by convection is more effective in turbulent flow because of the increased mixing of fluid particles. This leads to a more uniform temperature distribution and enhances heat transfer. Therefore, the heat transfer coefficient is higher for turbulent flow compared to laminar flow.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Laminar Flow
Laminar flow is a smooth and orderly type of fluid movement, where the fluid flows in parallel layers, with minimal disruption between them. In this kind of flow, each layer, or streamline, flows at a consistent velocity, maintaining a steady and predictable path.
This predictable behavior minimizes friction and allows fluids to move quietly through pipes or channels. In laminar flow, there's little to no mixing across these layers, which keeps the flow stable and efficient.
  • Velocity of fluid is uniform across any cross-section.
  • Common in situations involving low flow rates and smaller tubes or flow channels.
  • The Reynolds number, a dimensionless indicator of flow type, generally falls below 2000 for laminar flow.
Turbulent Flow
In contrast, turbulent flow is characterized by chaotic changes in pressure and flow velocity. Unlike laminar flow, turbulent flow features eddies and swirls, making it far more unpredictable.
Turbulent flow is common in larger channels and at higher flow rates where the inertia forces are much greater than the viscous forces.
  • Fluid particles move in seemingly random paths, leading to increased mixing of the fluid.
  • Occurs at higher Reynolds numbers, typically above 4000.
  • It's this mixing that increases pressure drop and enhances the energy needed for pumping fluids through channels.
Heat Transfer Coefficient
The heat transfer coefficient is a crucial parameter in determining how effectively heat is transferred from one fluid to another or from a solid to a fluid. It reflects the ability of a fluid to conduct heat away from its surroundings.
A high heat transfer coefficient indicates more efficient heat exchange, which is especially important in engineering applications such as heat exchangers.
  • It depends on the nature of the fluid flow 鈥� being generally higher in turbulent flows due to enhanced mixing.
  • Measured typically in units of W/(m虏路K), signifying the heat transfer rate per unit area for a given temperature difference.
  • Critical for designing systems like radiators, HVAC systems, and chemical reactors to ensure optimal thermal regulation.
Convective Heat Transfer
Convective heat transfer is the process of heat transfer between a surface and a fluid flowing over or around the surface. This type of heat transfer can be enhanced by the nature of the flow 鈥� whether laminar or turbulent.
Convection involves both conduction and the movement of fluid which carries away energy from the surface.
  • Laminar flow results in lower convective heat transfer due to the lack of mixing and lower velocity gradients.
  • Turbulent flow, with its intense mixing and higher velocity differences, promotes greater convective heat transfer efficiency.
  • Key factor in determining the efficiency of systems where temperature regulation is critical, such as cooling systems or heat exchangers.

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Most popular questions from this chapter

What fluid property is responsible for the development of the velocity boundary layer? For what kind of fluids will there be no velocity boundary layer on a flat plate?

For laminar flow over a flat plate the local heat transfer coefficient varies as \(h_{x}=C x^{-0.5}\), where \(x\) is measured from the leading edge of the plate and \(C\) is a constant. Determine the ratio of the average convection heat transfer coefficient over the entire plate of length \(L\) to the local convection heat transfer coefficient at the end of the plate \((x=L)\).

For steady two-dimensional flow, what are the boundary layer approximations?

What is a similarity variable, and what is it used for? For what kinds of functions can we expect a similarity solution for a set of partial differential equations to exist?

A rectangular bar with a characteristic length of \(0.5 \mathrm{~m}\) is placed in a free stream flow where the convection heat transfer coefficients were found to be \(100 \mathrm{~W} / \mathrm{m}^{2} \cdot \mathrm{K}\) and \(50 \mathrm{~W} / \mathrm{m}^{2} \cdot \mathrm{K}\) when the free stream velocities were \(25 \mathrm{~m} / \mathrm{s}\) and \(5 \mathrm{~m} / \mathrm{s}\), respectively. If the Nusselt number can be expressed as \(\mathrm{Nu}=C \operatorname{Re}^{m} \operatorname{Pr}^{n}\), where \(C, m\), and \(n\) are constants, determine the convection heat transfer coefficients for similar bars with (a) \(L=1 \mathrm{~m}\) and \(V=5 \mathrm{~m} / \mathrm{s}\), and \((b) L=2 \mathrm{~m}\) and \(V=50 \mathrm{~m} / \mathrm{s}\).
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