Exhaust velocity of a fire extinguisher

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Homework Help Overview

The discussion revolves around calculating the exhaust velocity and mass flow rate of a fire extinguisher used in a project simulating a rocket. The original poster is exploring the relationship between internal energy and kinetic energy in the context of thrust generation.

Discussion Character

  • Exploratory, Assumption checking, Conceptual clarification

Approaches and Questions Raised

  • The original poster attempts to use internal energy to calculate exhaust velocity but is uncertain about how much internal energy converts to kinetic energy. Participants question the feasibility of using a fire extinguisher for propulsion and suggest alternative methods for measuring thrust.

Discussion Status

Participants are exploring various ideas and approaches, including the use of a spring balance and considering different sizes of fire extinguishers. Some guidance has been offered regarding the challenges of the calculations and the need for more information.

Contextual Notes

There are constraints regarding the available information for calculations, and participants are discussing the practicality of the project based on the thrust-to-weight ratio of fire extinguishers.

Thijs982
< Mentor Note -- thread moved to HH from the technical physics forums, so no HH Template is shown >[/color]

Hey,
For our project we have to think of an experiment that can kind of simulate a rocket. We decided to use a fire extinguisher on a kart, since its mass constantly changes (same with a rocket). To calculate the thrust force that the extinguisher provides we need the u ( exhaust velocity) and mdot (the mass flow rate).

The NASA site gives us two ways to calculate mdot (https://www.grc.nasa.gov/WWW/k-12/VirtualAero/BottleRocket/airplane/mflchk.html) But for both ways an exhaust velocity is needed.

On a kerbal forum (https://forum.kerbalspaceprogram.com/index.php?/topic/38334-how-much-dv-is-in-fire-extinguisher/) two guys calculate the exhaust velocity via two ways. One with the internal energy and the other one using Thrust. I am currently trying it with internal energy but i am kind of stuck.

I know that it is an adiabatic proces thus the change in U (internal energy) is equal to -3/2 * n * R * dT, but I don't know how much of the internal energy gets transformed in kinetic energy to propel the gas.

Can somebody please explain to me, or help me calculate mdot and the exhaust velocity of my fire extinguisher

Kind Regards,
Thijs

Specs of the extinguisher:
5kg of CO2 fuel
Pressurized to 70 bar
weight 12.3 kg excluding fuel
Dimension = 670 mm x 170 mm

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Thijs982 said:
< Mentor Note -- thread moved to HH from the technical physics forums, so no HH Template is shown >

Hey,
For our project we have to think of an experiment that can kind of simulate a rocket. We decided to use a fire extinguisher on a kart, since its mass constantly changes (same with a rocket). To calculate the thrust force that the extinguisher provides we need the u ( exhaust velocity) and mdot (the mass flow rate).

The NASA site gives us two ways to calculate mdot (https://www.grc.nasa.gov/WWW/k-12/VirtualAero/BottleRocket/airplane/mflchk.html) But for both ways an exhaust velocity is needed.

On a kerbal forum (https://forum.kerbalspaceprogram.com/index.php?/topic/38334-how-much-dv-is-in-fire-extinguisher/) two guys calculate the exhaust velocity via two ways. One with the internal energy and the other one using Thrust. I am currently trying it with internal energy but i am kind of stuck.

I know that it is an adiabatic proces thus the change in U (internal energy) is equal to -3/2 * n * R * dT, but I don't know how much of the internal energy gets transformed in kinetic energy to propel the gas.

Can somebody please explain to me, or help me calculate mdot and the exhaust velocity of my fire extinguisher

Kind Regards,
Thijs

Specs of the extinguisher:
5kg of CO2 fuel
Pressurized to 70 bar
weight 12.3 kg excluding fuel
Dimension = 670 mm x 170 mm

<Moderator's note: advertisement removed>
Welcome to the PF. :smile:

Most fire extinguishers have a very low thrust/weight ratio. Are you sure you want to use it for this project? I doubt you can even overcome axle friction on your cart with a typical fire extinguisher discharging... Have you looked into any other ideas?
 
I have but we saw on videos online that it is possible to accelerate a human on a chair using an extinguisher
 
Thijs982 said:
I have but we saw on videos online that it is possible to accelerate a human on a chair using an extinguisher
They were using a hidden string to pull the chair probably... :wink:
 
hahah maybe maybe, but we'll give it a shot. But can you help me because I don't quite understand how much of the internal energy of the gas can be used to propel the gas backwards
 
It's not a very easy calculation anyway but in this case it's made even more difficult because you don't really have enough information to work with .

Personally I would just use a spring balance .
 
I had a look at some of the vids online and I think it will work if you use the standard office size fire extinguisher rather than a small kitchen/residential or car extinguisher.
 
Some of those vids can be used to estimate the flow rate using the mass and duration.
 
Thnx guys I'll try those ideas
 

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