This post is an attempt at create a reference page that is as reader friendly as possible. If you think any of the examples are too dense, please feel free to comment, it means I need to work on getting better at communicating.
This blog, and a lot of mechanical engineering work has a really big question to answer, when you do any kind of work, moving a car, use electricity to power a chip, or launch a rocket into space, you have heat, more often than not that heat is not something you want in your project. To get rid of heat engineers generally have 3 major ways to cool things off, conduction, convection, and radiation (not the nuclear kind).
First general rule of heat transfer, heat moves from where it is warmest to where it is coldest. For that heat to get from the warm spot to the cool spot, any combination of conduction, convection, or radiation can be used.
Cooling through conduction. Pick up a glass of ice water and your hand starts to feel cold. This is the conduction of heat from your warm body to the cold glass. Now instead of picking up a glass of ice water, you pick up a styrofoam cup filled with ice water, your hand doesn't feel nearly as cold. The water in the cup is no cooler than the glass, but your hand feels colder, this is because of something called thermal resistance. The greater the thermal resistance the harder it is for something that is hot to lose heat to something that is cold.
Convection is a bit more complicated in practice (but who cares, this is an intro). Imagine using a fan on a hot day, even though the air the fan is blowing on you isn't any colder you feel more comfortable. The reason you feel cooler in front of a blowing fan is because of something called "forced convection". The faster you move air molecules away from your body the cooler you will feel, this is because your body is naturally conducting heat to the air around you, but the air does not necessarily want to move that heat away from your body, by blowing air, the warm atoms are more likely to move away from your body.*
Radiative cooling. Instead of transferring heat by means of physical contact, radiative cooling lets heat escape through electro-magnetic radiation. This is what allows the Predator to hunt Arnald Schwarzenegger in the first Predator movie. The warmer an object is the more thermal radiation it will emit. Where the concept of radiative cooling gets complicated, is that everything is emitting some amount of thermal radiation. What does this mean for you or me, most of the time nothing that you need to worry about? For blog posts like how the Death Star deals with its Heat, or more recently Window Cooling (better title suggestions are always appreciated) this concern for radiative cooling is really important, if the cooling system isn't pointed at something really cold, they won't work that well. The best direction to point a radiative cooler is at the cold emptiness of space.
Radiative cooling is often called black body cooling. The reason for this term is that the closer an object is to being as black as humanly possible the better it is at cooling. If you want your spaceship to avoid losing heat, you make it as shiny and reflective as possible, to help with cooling, you want it to be as black as possible, this is one of the reasons scientists are obsessed with making really black materials.
*there are cases where you can use convection to warm something, but the hope here is to keep the intro as straightforward as possible.
This blog, and a lot of mechanical engineering work has a really big question to answer, when you do any kind of work, moving a car, use electricity to power a chip, or launch a rocket into space, you have heat, more often than not that heat is not something you want in your project. To get rid of heat engineers generally have 3 major ways to cool things off, conduction, convection, and radiation (not the nuclear kind).
First general rule of heat transfer, heat moves from where it is warmest to where it is coldest. For that heat to get from the warm spot to the cool spot, any combination of conduction, convection, or radiation can be used.
Cooling through conduction. Pick up a glass of ice water and your hand starts to feel cold. This is the conduction of heat from your warm body to the cold glass. Now instead of picking up a glass of ice water, you pick up a styrofoam cup filled with ice water, your hand doesn't feel nearly as cold. The water in the cup is no cooler than the glass, but your hand feels colder, this is because of something called thermal resistance. The greater the thermal resistance the harder it is for something that is hot to lose heat to something that is cold.
Convection is a bit more complicated in practice (but who cares, this is an intro). Imagine using a fan on a hot day, even though the air the fan is blowing on you isn't any colder you feel more comfortable. The reason you feel cooler in front of a blowing fan is because of something called "forced convection". The faster you move air molecules away from your body the cooler you will feel, this is because your body is naturally conducting heat to the air around you, but the air does not necessarily want to move that heat away from your body, by blowing air, the warm atoms are more likely to move away from your body.*
Radiative cooling. Instead of transferring heat by means of physical contact, radiative cooling lets heat escape through electro-magnetic radiation. This is what allows the Predator to hunt Arnald Schwarzenegger in the first Predator movie. The warmer an object is the more thermal radiation it will emit. Where the concept of radiative cooling gets complicated, is that everything is emitting some amount of thermal radiation. What does this mean for you or me, most of the time nothing that you need to worry about? For blog posts like how the Death Star deals with its Heat, or more recently Window Cooling (better title suggestions are always appreciated) this concern for radiative cooling is really important, if the cooling system isn't pointed at something really cold, they won't work that well. The best direction to point a radiative cooler is at the cold emptiness of space.
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| The blackest material made potentially helping NASA keep spaceships cold source extremetech.com |
Radiative cooling is often called black body cooling. The reason for this term is that the closer an object is to being as black as humanly possible the better it is at cooling. If you want your spaceship to avoid losing heat, you make it as shiny and reflective as possible, to help with cooling, you want it to be as black as possible, this is one of the reasons scientists are obsessed with making really black materials.
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| How NASA keeps satellites from getting too hot (or cold) source NASA |
*there are cases where you can use convection to warm something, but the hope here is to keep the intro as straightforward as possible.
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| How hot things can get (added 10/19/2016) |
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