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Calculate the Velocity of Gas

04/03/2020 9:08 AM

What is the velocity of gas inside the tube with outer diameter (OD) 10mm flow rate of 100LPM?assume pressure at 1bar

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#1

Re: Calculate the Velocity of Gas

04/03/2020 11:34 AM

a. What's the volume of 100 liters?

b. What's the area of a 10 mm dia hole?

You should be able to work it from there...

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#7
In reply to #1

Re: Calculate the Velocity of Gas

04/04/2020 12:35 AM

Uh, I hate to be critical here, but the volume of 100L is 100L!

Or were you just trying to get OP to think a bit?

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#2

Re: Calculate the Velocity of Gas

04/03/2020 1:14 PM

First you would need the id measurement of the tube...If the "assume pressure at 1 bar" is the air line pressure, then you are at atmospheric pressure and no pressure differential exists, so no flow....

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#3

Re: Calculate the Velocity of Gas

04/03/2020 2:41 PM

Are you trying to get others to solve your Homework problem ?

You are soooooo clever !!!!

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#4

Re: Calculate the Velocity of Gas

04/03/2020 2:56 PM

It is impossible to solve with the information given. Without knowing the ID of the tube or at least the wall thickness so one could calculate it, there isn't even a starting point.

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#5

Re: Calculate the Velocity of Gas

04/03/2020 10:59 PM

Friend,

Others have given you hints regarding how to answer this question. You post that the pressure is 1bar, so I am interpreting this as gauge pressure (pressure above atmospheric) as opposed to absolute pressure (pressure above a vacuum). One of the answers you received pointed out this difference in meaning of "pressure at 1bar".

Next, when any fluid moves and is touching a wall, the velocity of the fluid will vary depending on how close to the wall one is. At the very intersection where the two touch the fluid (gas, in your case) is not moving, while towards the middle it will be moving at the fastest. If the speed is below a certain value (look up "reynolds number") the flow is linear, but if it is above the flow is turbulent with eddies and vortexes (vortices). I suspect that in your problem the answer is in terms of the average or mass flow without regard to any real-world variations in the flow depending on where in the tube you are.

Finally, as gas or any other fluid moves through a tube there is energy lost due to friction against the walls, so there has to be a difference in pressure from one end of the tube to another. In your problem, the length of the tube is not stated, so you can assume that it is short enough that any deviation in pressure from the stated 1bar can be ignored.

See all the fun you can get into when you ask a bunch of engineers and others a question like you have? We have at least three assumptions, one missing value (the tube ID (inside diameter), an uncertain definition, and need a formula along with numbers to be put into it. Are you ready to go forward?

--JMM

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#6

Re: Calculate the Velocity of Gas

04/03/2020 11:58 PM

Calculate Average Velocity, with Tube ID and Flow Rate

We know that, in a pipe, if you can determine the flow rate by multiplying the inside cross-sectional area of the pipe by the average velocity, you get the flow rate:

where:

VAve = the Average Velocity

Ax = the Cross-sectional Area of the Pipe ID

F = the Flow Rate

If you have two of these values, you can indeed find the remaining unknown value.

If you need to know the average velocity, rearrange the equation:

So, use your flow rate, and your inside diameter (to calculate the cross-sectional area) to get your answer.

But note this: the velocity number that you will get is the AVERAGE velocity. The velocity near the sides of the pipe are going to be lower that those in the center. This profile will vary depending on the character of the inside pipe surface, the magnitude of the flow rate, and characteristics of the fluid.

Make sure you keep your conversions consistent!

Good luck!

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#8
In reply to #6

Re: Calculate the Velocity of Gas

04/04/2020 4:33 AM

Since we are having fun with this,you neglected to mention the cooling of the gas as it exits the tube,the ambient or relative atmospheric pressure of receiving container pressure.

A better measurement would be a measurement of the mass exiting the tube rather than volume,and as stated above,this will still be an average velocity.

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#12
In reply to #8

Re: Calculate the Velocity of Gas

04/04/2020 2:35 PM

Nice distinction! I was not even thinking about how a mass flow rate would fit into this. I was thinking to keep it as simple as possible for OP, in light of the paucity of useable information!

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#9

Re: Calculate the Velocity of Gas

04/04/2020 4:43 AM

Hint:Once you calculate your area in mm,remember that 1cc=1milliliter.

Much easier now,right?

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#10

Re: Calculate the Velocity of Gas

04/04/2020 6:23 AM

Since most seem to be looking at average velocity, will there not be the occasional molecule/atom that is travelling at the speed of sound through the intermolecular spaces? I seem to remember that from some physics lecture back in the '70's.

This is why "smell" can move through a building faster than the air circulation rate.

I know that I could determine the speed of sound in gas at 1 barg. Maybe the velocity in the tube is greater than this.

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#11
In reply to #10

Re: Calculate the Velocity of Gas

04/04/2020 12:31 PM

Just an Engineer,

I strongly doubt your conjecture. Brownian motion can account for motion of a smell in a room. Any molecule that is moving will invariably encounter another, so a molecule we can smell (whether simple such as H2S or much larger) will take a rather significant random path.

The gas in the OP's tube is probably flowing at an average rate between 10% and 20% of the speed of sound in air (I am letting the OP do the calculation to get a more accurate number, and I am assuming a reasonable value for tube wall thickness). Even allowing for turbulence in the flow, which is very likely, no molecules will be traveling anywhere near the speed of sound.

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#13
In reply to #11

Re: Calculate the Velocity of Gas

04/04/2020 8:53 PM

Thanks for the feedback.

As you indicate, the calcs are relatively simple once you know the cross section. Most of my work recently has been with "water".

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#14

Re: Calculate the Velocity of Gas

04/18/2020 1:14 PM

Hint: π [pi] is involved somewhere...

Go for it.

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#15

Re: Calculate the Velocity of Gas

05/16/2022 5:47 AM

At what temperature?

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#16

Re: Calculate the Velocity of Gas

05/16/2022 5:48 AM

If the calculation shows >10m/s, then its diameter is too small.

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#17

Re: Calculate the Velocity of Gas

05/16/2022 6:56 AM

I presume he is asking for the Speed,not the Velocity of the gas.

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