Last week I introduced the three heat transfer mechanisms: Conduction, Convection, and Radiation and performed a deep dive into Conduction heat transfer. This week I want to continue that discussion by going deeper into Convection.
First, let’s define convection heat transfer. Convection is heat transfer through a moving fluid. Note: Liquids and gases are considered fluids in science and engineering. The moving part is critical; if the fluid was not moving at all then heat transfer would be through conduction that we discussed last week. Convection is important because the fluid motion enhances heat transfer between hot and cold areas. There are two types of convection, natural and forced convection.
In forced convection, the fluid is “forced” to move by some external means, such as a pump, fan, or the wind. Natural convection on the other hand is buoyancy driven by differences in fluid density in the presence of gravity.
Natural Convection
You’ve probably heard the phrase “heat rises”. That’s true when we are talking about natural convection. Heat does not rise when we are discussing any other type of heat transfer. When a fluid such as air is in contact with a warm surface, heat flows from the warm surface to the air via conduction. As the air warms up, the molecules in the air move further apart reducing the density of the air. In the presence of Earth’s gravity, the less dense air becomes buoyant and begins to rise. The warmer, less dense air then displaces the cooler, more dense air above it. The cooler, more dense air first moves away from the rising air and then falls towards the floor. The cooler air is now in contact with the warm surface, it now picks up heat and begins to rise, repeating the cycle. We’ve now created a circulation pattern, which you can sometimes feel as a draft. Please see illustration below.
Here are a couple of YouTube videos that show this process clearly. Hot vs Cold Water Experiment and Best Convection Currents!
Natural convection is also why heat tends to collect up near the ceiling of a room, especially in rooms with high ceilings. If you could levitate your easy chair up to the ceiling, it will be nice and cozy warm up there. A ceiling fan can force that warm air back down to the floor, but that’s a topic for a future blog.
Forced Convection
Turning now to forced convection. In our daily lives, we’ve all experienced the cooling effect of forced convection. We’ve all blown on a spoonful of hot soup to make it cool faster so we can eat it. During a hot summer day, we’ve all experienced how a nice breeze can make you feel much more comfortable. On the other hand, during the winter, that same breeze can make the day feel much colder than the thermometer reads. We’ve all heard the weather person talking about the wind chill factor during the winter. It’s a very real phenomenon, caused by forced convection. The subject of forced convention is quite difficult and well beyond the scope of his article. But, the thing to really know about forced convection is, the faster the wind speed, the greater the rate of heat transfer.
In last week’s blog, we discussed how heat moves through walls, ceiling, and floor by conduction. But conduction never occurs in isolation, convection is also working to remove heat from your home. This is especially true if the house is leaky. On a blustery day, the heat lost through convection can exceed the heat lost through conduction alone. On a windy day, your furnace will have to work much harder to keep your home warm than on a calm day at the same temperature.
Stay tuned for next week’s blog on the effects of radiation heat transfer.