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Normally, we think of combustion as occurring only in the gas phase. I think I understand the main reason for this - in the gas phase it's very hard for a bimolecular reaction to produce a single product, which makes it more likely for radical chain reactions to propagate rather than being terminated - but I'm curious about whether it's possible for combustion to occur in the liquid state at all.

For example: suppose I mixed liquid $\ce{CH4}$ with liquid $\ce{O2}$ under high pressure and reasonably low temperature. Would they react immediately? If not, could I 'ignite' the mixture in the same way as I could in the gas phase? (Note that the combustion of $\ce{CH4}$ into $\ce{H2O}$ and $\ce{CO2}$ doesn't change the mole number, so the volume change should be relatively small.) Would the reactions that occur be similar to the reactions that occur during combustion of $\ce{CH4}$ in the gas phase, or would they be completely different?

If this reaction wouldn't resemble combustion, is there some other example of a self-propagating oxidation reaction in liquid or aqueous phase with similar properties to combustion?

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Your question is addressed by Explosive Systems Containing Liquid Oxygen: Liquid Oxygen-Liquid Methane Mixtures. J. Chem. Eng. Data, 1959, vol. 4, pp. 127–131.

If liquid methane and liquid oxgygen at 90K are mixed, they are miscible in all proportions. Mixtures over a wide range of proportions detonated with use of a blasting cap. 100 grams of mixture punched a 10cm hole in a 3/16 inch steel plate.

For comparision, the TNT control only dented the metal plate!

Further tests on the stoichimetric liquid methane/oxgygen mixture found that it was detonated by a .22 caliber copper bullet at 40 foot range, flame and spark.

So nothing happens if the liquids are simply mixed, but a powerful explosion occurs if a stimulus is applied.

Liquid oxygen and liquid methane have also been used in rocket engines. See Liquid Methane/Liquid Oxygen Injectors for Potential Future Mars Ascent Engines for example.

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  • $\begingroup$ Nice, thank you. I suppose the explosion is due to the very sudden generation of lots of heat. I was interested in the theoretical case of constant volume (so that the products would also be liquids), but I guess that would be rather hard to achieve in practice! $\endgroup$ – Nathaniel Mar 9 '15 at 13:54
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Perhaps your question is rather what would it take to have a fluidic version of a controlled "burn" rather than an explosion. Interesting! The necessary conditions would be introduction of one liquid ("fuel") into another ("oxidizer") via a nozzle, with a reaction occurring at the interface at a rate sustained by continuous introduction of more of the "fuel" component. Methane and oxygen react too violently - maybe you could go to fluoromethane or difluoromethane and tone it down. Or go to hydrogen peroxide as the oxidizer.

A lot of work has been done on developing extremely compact, lightweight energy sources that can power an engine in extreme environments. For example, liquid lithium and sulfur have been used to power a reciprocating engine.

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