Exhaust velocity of a fire extinguisher

In summary, the conversation is about using a fire extinguisher on a kart to simulate a rocket and calculate the thrust force. The NASA site provides two ways to calculate the mass flow rate, but an exhaust velocity is needed for both. Two guys on a kerbal forum calculate the exhaust velocity using internal energy and thrust, but the person in the conversation is stuck. They are unsure of how much internal energy is transformed into kinetic energy to propel the gas. The specs of the extinguisher are 5kg of CO2 fuel, pressurized to 70 bar, weight 12.3 kg excluding fuel, and dimension of 670 mm x 170 mm. There is also a discussion about the feasibility of using a fire extinguisher
  • #1
Thijs982
< 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

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  • #2
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?
 
  • #3
I have but we saw on videos online that it is possible to accelerate a human on a chair using an extinguisher
 
  • #4
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:
 
  • #5
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
 
  • #6
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 .
 
  • #7
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.
 
  • #8
Some of those vids can be used to estimate the flow rate using the mass and duration.
 
  • #9
Thnx guys I'll try those ideas
 

1. What is the exhaust velocity of a fire extinguisher?

The exhaust velocity of a fire extinguisher refers to the speed at which the fire suppressant agent is expelled from the extinguisher during use. This velocity can vary depending on the type of extinguisher and the amount of pressure it is expelled with.

2. How is the exhaust velocity of a fire extinguisher measured?

The exhaust velocity of a fire extinguisher is typically measured using a tool called a pitot tube, which measures the pressure of the expelled fire suppressant agent and calculates the corresponding velocity.

3. Why is the exhaust velocity of a fire extinguisher important?

The exhaust velocity of a fire extinguisher is important because it determines how quickly and effectively the fire suppressant agent can reach the fire and extinguish it. A higher exhaust velocity means the agent can reach the fire faster, potentially reducing the amount of damage caused by the fire.

4. What factors affect the exhaust velocity of a fire extinguisher?

The exhaust velocity of a fire extinguisher can be affected by several factors, including the type of extinguisher, the pressure at which it is expelled, and the size and shape of the nozzle used to expel the agent.

5. How can the exhaust velocity of a fire extinguisher be increased?

The exhaust velocity of a fire extinguisher can be increased by using a larger or more powerful extinguisher, increasing the pressure at which it is expelled, or using a different type of nozzle that is designed for higher velocity output.

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