This Minute Physics video explores some of the thermodynamics behind that process. Inside the sealed can, this chemical sits in an equilibrium of part-liquid, part-vapor. But when pressure is released by opening the nozzle, the liquid boils, generating extra vapor and cooling whatever remains in the reservoir. That first equation you see at in the video for isentropic adiabatic expansion is key to what happens in a nozzle with supersonic flow.
As the flow accelerates to supersonic speeds, its temperature drops dramatically. When I was in graduate school, we actually had to preheat our hypersonic wind tunnel in pretty much the same way you would preheat your oven at home before we ran at Mach 6 because otherwise the temperature inside the test section would drop so low that the oxygen would liquefy out of the air!
When the high-pressure gas expands, or relaxes, considerable cooling happens and eventually the gas becomes a very cold liquid.
In fact, most gases turn to a liquid when they cool down. In the lab, Leachman filled a balloon with air and dropped it in the liquid nitrogen.
At first, I thought it would pop. But the balloon actually shriveled up, as the air inside turned to liquid oxygen and nitrogen. When the air inside the balloon got really cold, the particles started to slow down and take up less space.
When he took the balloon out the process reversed. The balloon went back to its original shape. When the liquid boils in room temperature, the molecules in it move faster. And Turned the Ignition. By Loukia Papadopoulos. Follow Us on. Sponsored Stories. If you continue to use this site, you consent to our use of cookies.
Stay on top of the latest engineering news. The evaporation of the liquid inside the can results in a drop in its internal heat energy, and it absorbs a large amount of heat from the surrounding air and environment -- in this case, the metal can.
When the liquid inside absorbs the heat from the can's metal body, the can cools down rapidly. As the expanding gas leaves the can, it also absorbs heat energy from the nozzle and straw, and anything else the gas comes into contact with. For example, if you spray a keyboard, you'll see a thin white layer of frost form on the keys briefly. With prolonged use of the canned air, you might notice that the force of the air stream weakens over time, and the can becomes too cold to comfortably hold in your hand.
The heat energy from the can has all gone into evaporating the liquid inside; when the can itself becomes cold, not enough heat remains to vaporize more liquid. To remedy the "out of breath" condition, set the can down and let it warm for a few minutes. This restores the strength of the air bursts. The can carries a warning label telling you to avoid spraying onto your skin; the rapid absorption of heat can easily cause frostbite.
0コメント