Convection

Learning goal

On this page, you will learn thermal energy transfers through fluid motion by a mechanism called convection. In air, convection transfers heat much faster than conduction. There are two types of convection: natural convection and forced convection.

❶ Natural convection between a warm plate and a cold plate

You may have seen a science experiment that shows convection with ink. The experiment uses cups of hot water to warm up a small tank of room temperature water. An ink drop is added to the bottom of the tank. The warm water rises and carries the ink, thus allowing the process to be seen. (See this YouTube video.)

The simulation on the right below shows this process on the computer. Unlike the real experiment, we do not have to use ink and we can show the convective flow everywhere in the liquid—not just the portion dyed by ink. Follow these steps:

1. Click "Run" to start it. Observe what happens. Select "Graph On" to bring up the temperature graph. Monitor how the temperature recorded by the upper thermometer rises.

2. Click "Reset", and then click "Off" to turn off convection (something you can't really do in the real experiment). Repeat the steps in 1).

3. Click "On" to activate the convection mechanism again. Select "Low" to reduce the bottom temperature, and repeat the steps in 1).

4. Click "Reset". Move the heating plate (the red slab) to the middle of the container (you can do that by dragging it or clicking on it and then using the Up/Down arrow key to nudge it). Compare the heating in the upper part and the lower part. Can you explain what you observe?

5. Click "Reset". Change the surface area of the heating plate that is exposed to the fluid using the "Surface area" options. Observe the heat transfer to the upper plate.

The convection in this experiment is called natural convection, because no mechanical force is used to cause the flow of the fluid.

Question: (Click the link below to answer)

1) Can you tell if heat transfers faster when convection is allowed?
2) Does a higher temperature at the bottom increase the heat transfer rate?
3) Describe and explain what you observe when the heating plate is moved to the middle.
4) Describe what you observe when the surface area of the heating plate is reduced.



Graph: On Off       Convection: On Off
Bottom temperature: High Low 0°C
Surface area: Full Medium Small

❷ Accelerating heat transfer through forced convection

In the following simulation, there are two compartments that are separate from each other. Two identical objects with the same initial temperature are placed in each compartment. In the upper compartment, there is a fan that is blowing to the right. In the lower one, there is no fan. Start the simulation and compare the speed that the objects are cooled down. Click the "On" button to bring up the temperature graph for a better comparison.
If the coffee is too hot, you know that blowing it will cool it down. By blowing the air towards the hot coffee, you cause the air to flow faster and, therefore, carry away heat more quickly. This is called forced convection.



Question: (Click the link below to answer)

Explain the wind chill effect based on what you learned from this simulation.


Wind speed: Low Medium High
Graph: On Off       Velocity: On Off

Summary

The amount of thermal energy ΔQ transferred from a surface area A in a time period Δt through natural convection of fluid is approximately determined by the following equation:

ΔQ/Δt=-hA(T-T)

where T is the temperature of the surface, T is the temperature of the environment, and h is the convective heat transfer coefficient of the fluid. This equation is sometimes called Newton's Law of Cooling. The heat transfer through forced convection is more complicated as it depends also on the fluid speed as a result of the external force.

Question: (Click the link below to answer)

Explain Newton's Law of Cooling based on the above simulations.

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