B Knudsen Flow: High School Student Q&A

[Chain Shawn]

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.）

 

[jedishrfu]

What have you found so far on Knudsen flow?

 

[Chain Shawn]

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.

 

[jedishrfu]

There's some description in the wikipedia article:

https://en.wikipedia.org/wiki/Knudsen_diffusion

Basically the gas particles in a knudsen flow are more likely to collide with the walls of the container than with each other.

 

[Chestermiller]

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.