Hot gases flow better than cold gases why?

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Discussion Overview

The discussion revolves around the question of why hot gases flow better than cold gases, particularly in the context of engine intake and exhaust systems. Participants explore concepts related to gas density, viscosity, molecular interactions, and the implications for energy release in combustion processes.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • Some participants suggest that hot gases flow better due to their lower density compared to cold gases.
  • One participant proposes that the increased distance between molecules in hot gases leads to less interaction, facilitating better flow.
  • Another participant notes that while hot gases have higher viscosity, the decrease in density occurs at a faster rate than the increase in viscosity.
  • A participant questions the relationship between temperature and viscosity, expressing confusion about how heat increases viscosity in gases compared to liquids.
  • It is mentioned that in gases, higher temperatures result in greater molecular momentum, which affects collision dynamics and flow behavior.
  • One participant compares gas molecules to billiard balls, emphasizing their straight-line motion and collisions, and notes that higher temperatures lead to increased average speeds of the molecules.

Areas of Agreement / Disagreement

Participants express various viewpoints on the relationship between temperature, viscosity, and gas flow, indicating that there is no consensus on the mechanisms involved. Some concepts are clarified, but fundamental disagreements remain regarding the nature of viscosity and molecular interactions.

Contextual Notes

Participants acknowledge the complexity of the topic, with discussions touching on the definitions of viscosity and molecular behavior in different states of matter. There are unresolved questions about the interplay between temperature, density, and viscosity in gases.

Who May Find This Useful

This discussion may be of interest to individuals studying thermodynamics, fluid dynamics, or those involved in engineering applications related to combustion and gas flow.

5.0stang
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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.
 
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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.
 
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? :confused:
 
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. :approve: :cool:
 
Astronuc said:
congrats on 5000 posts. :approve: :cool:

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.
 
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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