# Fluid Dynamics of helium gas

Hello

here is a schematic of the problem: http://imgur.com/CgcybVT

A stream of helium gas will be used to carry aerosolized particles.

I am assuming conservation of mass (and mass flow)

The mission is to find the exit stream velocity of the gas
. For the time being, i am assuming the weight of particle is negligible and doesn't reduce helium velocity.

I am using Bernoulli's equation... would it be correct to only focus on the inlet and exit ?

I feel the changes in area matter because as area increases, velocity decreases to maintain constant mass flow.

Thanks!

Best,

EDIT: I forgot to mention the system is under vacuum, and he outlet pressure of 0.065 psi is that of the vacuum chamber

here is a better diagram: http://imgur.com/64b7WLT

Last edited:

Gold Member
1) You can't use only the inlet and outlet since there are a lot of things in between there that are "lossy" in terms of pressure. In other words, not all of that pressure is going to be converted to velocity.

2) It appears that what you are reporting here is gauge pressure. If so, that strongly implies to me that this flow is likely compressible and you can't use Bernoulli's equation at all.

Chestermiller
Mentor
1) You can't use only the inlet and outlet since there are a lot of things in between there that are "lossy" in terms of pressure. In other words, not all of that pressure is going to be converted to velocity.

2) It appears that what you are reporting here is gauge pressure. If so, that strongly implies to me that this flow is likely compressible and you can't use Bernoulli's equation at all.
There's a compressible form of Bernoulli equation, right?

Gold Member
There's a compressible form of Bernoulli equation, right?

More of a "compressible generalization", but yes. Unfortunately, it is not very simple to use.

1) You can't use only the inlet and outlet since there are a lot of things in between there that are "lossy" in terms of pressure. In other words, not all of that pressure is going to be converted to velocity.

2) It appears that what you are reporting here is gauge pressure. If so, that strongly implies to me that this flow is likely compressible and you can't use Bernoulli's equation at all.
Hi:
1) You can't use only the inlet and outlet since there are a lot of things in between there that are "lossy" in terms of pressure. In other words, not all of that pressure is going to be converted to velocity.

2) It appears that what you are reporting here is gauge pressure. If so, that strongly implies to me that this flow is likely compressible and you can't use Bernoulli's equation at all.

Hi there,

it appears I forgot to mention the system is under vacuum... and the outlet pressure of 0.065 psi is that of the vacuum chamber

here is a better diagram: http://imgur.com/64b7WLT

We can assume vacuum extends upto the inlet Helium stream. This means density of Helium will change once it leaves the gas tank and that its compressible. As such, it makes calculate the density (rho = P*MW/(RT)) at vacuum pressure, right ?

With regards to your comment in 1): since i am assuming constant mass flow, couldn't i use rho*V*A between sections and work my way up to calculate the outlet velocity?