Rapid valve opening, Adiabatic compression and Oxygen

[dMemphys]

Hello everyone!

I'm currently on a project involving the oxygen compatibility of alloys. We are building a experimental setup which looks will recreate an quasi-adiabatic compression of oxygen onto a piece of metal (and see if it burns, or not).

The setup looks like this:

1000bar vessel ======|>Valve<|===1meter long tube===|Comb. Chamber|

The fast opening valve (fully opened in under 18ms) releases 1000bar pure oxygen into the (silver coated) combustion chamber (atmospheric pressure).

I'm looking for some way to predict velocities, pressures and temperatures inside the apparatus and have some difficulties selecting the models. For example: Should I consider the fast opening valve as a throttling valve? Or should I apply the Fluid Hammer models? How can I find the velocity in the tube leading to the combustion chamber?

Well, in fact, I think I just don't know where to begin with this problem. So any help/insight will be greatly appreciated.

Thank you for your time!

Denis Maloir

 

[monkeydog]

1000 bars for Oxygen seems too high.
Can you achieve that?

 

[dMemphys]

Hello!

Thanks for your reply!

All calculations have been made so that 1000bars could be achieved, yes. We have acquired a high pressure compressor (Howden - Burton-Corblin).

I'm thinking of some kind of water hammer model, but it's usually applied when a valve is suddenly closed. Can it be applied when a tank is suddenly opened on a dead end (closed combustion chamber)?

 

[iainuts]

If you're just looking for temperature of the gas as it's being compressed, we've typically modeled the long tube as a cylinder/piston arrangement. There isn't much mixing between the gas entering the long tube and the gas already in the long tube. Without this mixing of the incoming gas and the gas being compressed, the gas already in the tube when you pressurize it is being compressed as if it were compressed with a piston. So you can determine the temperature of the gas being compressed by assuming it follows a line along an isentropic compression. Of course, there's some heat exchange with the walls as well but that's more difficult to model with any accuracy. Once compressed however, the oxygen will obviously be extremely hot but in your situation, the initial pressure is very low so you don't have much mass and the heat capacity will be much lower than if you started with a higher pressure in your long tube. Starting with a higher pressure will result in a higher chance of ignition. Note that this is a fairly common situation in industry where oxygen being transfered to cylinders for example. In those cases, many times the Teflon hoses connecting the cylinders will have a length of brass or other non flammable material at the point where the gas is being compressed to prevent ignition from the high temperature.

So to answer your question, apply the first law of thermo to your tube and consider the gas already in the tube doesn't mix very quickly with the incoming gas.