Hot gases flow better than cold gases...why?

5.0stang

I have read that hot gases flow better than cold gases.

Why is this so? Less dense? Any good readings on it.

I am thinking of incoming intake charge (cool) in an engine, and the outgoing (hot) gases.

Thanks.

Danger

I don't know, but I'm thinking that hot gasses flow better because there is more distance, and thus less interaction, between the molecules.
As for a cold intake flow, the main reason that one wants that is because you get a denser charge (ie: more air and fuel per unit of volume). That means more released energy when you light it.

Astronuc

Hot gases are less dense and have higher viscosity, however density decreases faster than viscosity increases.

This is an interesting demonstration of the effect of increased viscosity.
http://www.physics.montana.edu/demon...mperature.html

Danger

Thanks, Astro. Now here's a chance for my ignorance to really shine.
I'm starting to question my understanding of viscosity. Isn't it the tendency of molecules to cling to each other? If so, how does heat increase that? Motor oil flows better hot, so why would a gas be contrary?

Astronuc

Well, in a liquid, the warmer the liquid the lower the intermolecular/interatomic bond, so that reduces the adhesion if you will. In gas, there is already low intermolecular interaction or force, but with an increase in temperature, each molecule has a greater momentum, and thus more momentum can be lost/transferred in a collision (e.g. molecular collision with the containment).

BTW, Danger - congrats on 5000 posts.

Danger

Quote by Astronuc

congrats on 5000 posts.

Y'know, I never even noticed that. Thanks.
Also thanks for the very clear and concise explanation of gas viscosity. That makes perfect sense to me.

mikelepore

Gas molecules are like billiard balls in that they move in little straight lines and collide with the walls and sometimes with each other. It may seem like we can "push" as gas through the length of a a tube, but really we have to wait until the molecules get to their new location by bouncing around. A higher temperature means that their average speed is greater.

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5.0stang	Hot gases flow better than cold gases...why? Tweet I have read that hot gases flow better than cold gases. Why is this so? Less dense? Any good readings on it. I am thinking of incoming intake charge (cool) in an engine, and the outgoing (hot) gases. Thanks.

Danger Recognitions: Gold Member	I don't know, but I'm thinking that hot gasses flow better because there is more distance, and thus less interaction, between the molecules. As for a cold intake flow, the main reason that one wants that is because you get a denser charge (ie: more air and fuel per unit of volume). That means more released energy when you light it.

Astronuc Admin	Hot gases are less dense and have higher viscosity, however density decreases faster than viscosity increases. This is an interesting demonstration of the effect of increased viscosity. http://www.physics.montana.edu/demon...mperature.html

mikelepore	Gas molecules are like billiard balls in that they move in little straight lines and collide with the walls and sometimes with each other. It may seem like we can "push" as gas through the length of a a tube, but really we have to wait until the molecules get to their new location by bouncing around. A higher temperature means that their average speed is greater.