Posted: Mon Apr 23, 2012 12:19 pm
Sorry to intrude on this thread, but something said earlier last page REALLY needs to be corrected:
1. "... burn rate... incomparably bigger..."
Who cares? Butane burns fast enough. Lower burn rate is to be valued - it makes detonation easier to avoid and allows the burn to be maintained long after the disk ruptures, achieving the more level projectile base pressure profile which is desired by any gun designer. The more constant the projectile base pressure, the less stress is exerted on the gun for the same ballistic performance.
2. "energy that is lower than the powerful H2 combustion"
That "lower" energy? It happens to be 5315 kJ, or 354kJ/mol of gas in the chamber (oxidiser must be counted).
3. The H<sub>2</sub>O produced in the hydrogen/oxygen burn will be gaseous, not liquid. The actual energy release in the reaction given is 484kJ, not 572. This gives 161kJ / mol of gas in the chamber.
Starting at atmospheric pressure, the highest pressure oxygen/hydrogen mix produces about 9.6 bar on ignition, and that's with ideal adiabatic conditions. Butane/oxygen mixes under the same conditions are capable of over 24 bar. The energy considerations above are not the whole reason for that, but RJB's statement is just downright wrong. Hydrogen is, by mass, a very energetic fuel. Measuring fuels in terms of energy per unit mass is rather useless when we do all our work in the gas phase, where molar energy content is what matters.
[/rant]
Carry on, everyone.
There's just far too much wrong with that statement to let it go. I'll start from the beginning and work through.RJB wrote:The burn rate of H2 is incomparably bigger than the butane´s is.
2 C4H10(g) + 13 O2(g) → 8 CO2(g) + 10 H2O(g) + energy that is lower than the powerful H2 combustion:
2 H2(g) + O2(g) → 2 H2O(l) + 572 kJ (286 kJ/mol)
H2 is the future....
1. "... burn rate... incomparably bigger..."
Who cares? Butane burns fast enough. Lower burn rate is to be valued - it makes detonation easier to avoid and allows the burn to be maintained long after the disk ruptures, achieving the more level projectile base pressure profile which is desired by any gun designer. The more constant the projectile base pressure, the less stress is exerted on the gun for the same ballistic performance.
2. "energy that is lower than the powerful H2 combustion"
That "lower" energy? It happens to be 5315 kJ, or 354kJ/mol of gas in the chamber (oxidiser must be counted).
3. The H<sub>2</sub>O produced in the hydrogen/oxygen burn will be gaseous, not liquid. The actual energy release in the reaction given is 484kJ, not 572. This gives 161kJ / mol of gas in the chamber.
Starting at atmospheric pressure, the highest pressure oxygen/hydrogen mix produces about 9.6 bar on ignition, and that's with ideal adiabatic conditions. Butane/oxygen mixes under the same conditions are capable of over 24 bar. The energy considerations above are not the whole reason for that, but RJB's statement is just downright wrong. Hydrogen is, by mass, a very energetic fuel. Measuring fuels in terms of energy per unit mass is rather useless when we do all our work in the gas phase, where molar energy content is what matters.
[/rant]
Carry on, everyone.
