Designing a combustion chamber (solid mechanics problem)

  • Thread starter Hobold
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Here's my problem: I want to design a cylindrical combustion chamber for a hybrid propelent rocket. Basically, I will burn paraffin and use oxygen as oxidant. As I will be using stainless steel, I thought I could approximate the maximum stress that must be resisted by the material through a simple pressure vessel, so it would be t = σ/(p*r), where σ is the maximum stress resisted by stainless steel with a secutiry coefficient, p is the internal pressure and r is the internal radius and then I would have the needed wall thickness t.

There is one problem, though: the combustion temperature is about 1000°C and I have no idea if it will affect greatly the resistence of the material. I believe, as paraffin is not a great heat conductor, there won't be much heat transference to steel and, as the combustion is quick (~10s), there won't be a great increase of temperature.

Am I doing right?
 

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Yes, the allowable stress of your stainless steel will be significantly reduced at 1000 degF.

You can find these allowable stresses in ASME B31.1, B31.3, or BPV VIII, Div. 1. You can use the hoop stress formula you listed or the ones in these codes.

You may need to switch to 316H to make this work.
 
  • #3
AlephZero
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There is one problem, though: the combustion temperature is about 1000°C ... I believe, as paraffin is not a great heat conductor, there won't be much heat transference to steel and, as the combustion is quick (~10s), there won't be a great increase of temperature.
The heat transfer will not be limited because paraffin is a poor conductor. It will mostly be convection and radiation from the hot gas.

If your design doesn't include a cooling system, I would assume the combustor temperature could rise to the maximum flame temperature, which will higher than the average exit temperature of the gas.
 

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