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Chapter 6: Problem 2

What is forced convection? How does it differ from natural convection? Is convection caused by winds forced or natural convection?

Short Answer

Expert verified
Answer: Forced convection refers to heat transfer where a fluid is forced to flow over a surface by an external source (e.g. pump or fan), rather than by the fluid's own buoyant motion. Convection caused by winds can be considered as an example of natural convection, as it results from the uneven heating of the Earth's surface and the buoyancy forces created by temperature differences.

Step by step solution

01

Defining Forced Convection

Forced convection is a mode of heat transfer where a fluid is forced to flow over a surface by an external source, such as a pump or fan, rather than by the buoyant motion of the fluid itself. The heat is transferred from the surface to the fluid by conduction and then transported away by the fluid flow.
02

Defining Natural Convection

Natural convection is a mode of heat transfer where the movement of fluid is primarily driven by buoyancy forces that arise from differences in temperature. In this case, no external forces, like fans or pumps, are needed to drive the fluid flow. When a fluid gets warmer, it becomes less dense and rises, while cooler, denser fluid sinks, creating a fluid flow and transferring heat.
03

Comparing Forced and Natural Convection

The main difference between forced and natural convection lies in the driving force behind the fluid flow. In forced convection, an external agent, such as a pump or fan, is responsible for moving the fluid, while in natural convection, buoyancy forces arising from temperature differences within the fluid drive the flow. Forced convection typically has higher heat transfer rates than natural convection, as the fluid velocities are generally higher in forced convection.
04

Convection caused by Winds: Forced or Natural Convection?

Convection caused by winds can be considered as an example of natural convection. This is because winds are not generated by external agents like pumps or fans, but rather by the uneven heating of the Earth's surface and the resulting buoyancy forces. The warmer air near the Earth's surface rises while the cooler air from higher altitudes sinks, creating wind patterns and contributing to the natural convection process.

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Most popular questions from this chapter

Friction coefficient of air flowing over a flat plate is given as \(C_{f}=0.664(V x / \nu)^{-0.5}\), where \(x\) is the location along the plate. Using EES (or other) software, determine the effect of the air velocity \((V)\) on the wall shear stress \(\left(\tau_{w}\right)\) at the plate locations of \(x=0.5 \mathrm{~m}\) and \(1 \mathrm{~m}\). By varying the air velocity from \(0.5\) to \(6 \mathrm{~m} / \mathrm{s}\) with increments of \(0.5 \mathrm{~m} / \mathrm{s}\), plot the wall shear stress as a function of air velocity at \(x=0.5 \mathrm{~m}\) and \(1 \mathrm{~m}\). Evaluate the air properties at \(20^{\circ} \mathrm{C}\) and \(1 \mathrm{~atm}\).

For the same initial conditions, one can expect the laminar thermal and momentum boundary layers on a flat plate to have the same thickness when the Prandtl number of the flowing fluid is (a) Close to zero (b) Small (c) Approximately one (d) Large (e) Very large

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

A \(5-\mathrm{m} \times 5-\mathrm{m}\) flat plate maintained at a constant (?) temperature of \(80^{\circ} \mathrm{C}\) is subjected to parallel flow of air at \(1 \mathrm{~atm}, 20^{\circ} \mathrm{C}\), and \(10 \mathrm{~m} / \mathrm{s}\). The total drag force acting on the upper surface of the plate is measured to be \(2.4 \mathrm{~N}\). Using momentum-heat transfer analogy, determine the average convection heat transfer coefficient, and the rate of heat transfer between the upper surface of the plate and the air.

Metal plates are being cooled with air blowing in parallel over each plate. The average friction coefficient over each plate is given as \(C_{f}=1.33\left(\operatorname{Re}_{L}{ }^{-0.5}\right.\) for \(\operatorname{Re}_{L}<5 \times 10^{5}\). Each metal plate length parallel to the air flow is \(1 \mathrm{~m}\). Determine the average convection heat transfer coefficient for the plate, if the air velocity is \(5 \mathrm{~m} / \mathrm{s}\). Evaluate the air properties at \(20^{\circ} \mathrm{C}\) and \(1 \mathrm{~atm}\).
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