B Knudsen Flow: High School Student Q&A

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Knudsen flow occurs when the mean free path of gas molecules is comparable to or smaller than the size of the flow channel, leading to a situation where gas particles collide more with the container walls than with each other. This flow is characterized by low pressure and cannot be modeled as viscous flow due to the lack of significant molecular collisions. Creating Knudsen flow can involve using small openings or channels, but simply digging a hole may not suffice to achieve the desired conditions. The thickness or radius of pores in membranes can influence the sieving of different molecules, as they relate to the channel size affecting flow behavior. Understanding the relationship between channel size and air pressure is crucial for generating ultra-vacuum conditions.
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I am a high school student trying to carry out an experiment about fluid. Thus I am studying Knudsen flow and come up with following questions.

1. How can a Knudsen flow occurs?
2. Can I simply dig a small hole on a board and make Knudsen flow?
3. What the difference between viscous flow, Knudsen flow and moduleur flow?
4. How can such flow sieve different molecules?

( I’m not good at English. Sorry for the poor structure of this thread.😥
 
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jedishrfu said:
What have you found so far on Knudsen flow?
It results from the situation that the mean free path becomes equal or smaller than the channel’s size. And if Knudsen number(mean free path divided by diameter of flow channel) becomes higher, it will turn to molecular flow and enter ultra vacuum.

The cause and effect confuse me.I couldn’t get the causal relationship between channel’s size and air pressure. Does the Knudsen flow means that pipes size can generate ultra vacuum condition?

And I see some applications using pores on membrane to sieve different molecules, but I’m not sure whether the thickness or radius of the pore refers to the channel’s size.
 
In Knudsen flow, the fluid behavior can no longer be modeled as a viscous continuum, since the laws of viscous flow are based on significant numbers of molecular collisions. Knudsen flow occurs when the pressure of the gas is made very low.
 
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