Technician1002 wrote:Remember the boiling pressure is higher in a high mix.
Higher, but actually, the enthalpy change of vaporisation of water is lower as pressure (and boiling temperature) go up..
Indeed, at more than water's ~650K critical temperature, it is zero.
You still have to heat said water, but you do not need to boil it.
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DYI is potentially right here. You'd ultimately need to work out whether the increase in SOS would be worth the lost pressure, and, as has been suggested, it would be inferior to say a fuel/oxygen/helium mix.
To use the 320x mix as an example, if the nitrogen in the air were replaced by a helium buffer gas, the speed of sound in the mixture goes from ~1000 m/s up to ~1930 m/s, peak pressure goes up from 3200 atm to 4060 atm - both of which owing some of that to lower heat capacity of the gasses increasing temperature from 2830K to 3590K.
Fiddle with the proportions a little (as, ultimately, the buffer gas does not affect the partial pressures of the fuel or oxygen), and >2000 m/s SOS values are possible, if at the expense of temperature (which is probably a good thing, as it will reduce heat loss).
DYI wrote:Perhaps the 320X butane/propane/air hybrid proposed by Ragnarok would be a candidate...
Actually, on that note, I've been thinking about whether it would be possible to achieve a hybrid where part of the fuel (be it propane, butane or whatever) is still liquid on the point of firing, using the heat of the combustion to vaporise the fuel.
Given that the enthalpy change of vaporisation for propane is 15.7 kJ per mole, but the enthalpy change of combustion is 2044 kJ per mole, provided you could still achieve ignition with the lean fuel/air mix (which would be the limiting factor, although spectacular ignition could help get you closer to the lower explosive limit), and solve the metering issues, then there would seem to be a considerable net gain.
Like that, I'd say 500x mixes of propane/butane/air could be possible.