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Aug 17, 2023 at 14:14 comment added David Bailey Thanks @SmallPieceOfBread. Yes, the top of the water will cool, but whether it freezes depends on how low a vacuum is dynamically generated above the falling water and whether it has time to freeze before reaching bottom. To vacuum freeze water, I believe the pressure above the water needs to be <0.083 psi (600 Pa), but in final equilibrium, the pressure is ~0.5 psi. In any event, I don't think that a small amount of ice forming at the top would greatly affect the 2km column of water falling with a terminal velocity of at least 10m/s (according to my crude estimate for 15cm diameter pipe).
Aug 16, 2023 at 20:12 comment added SmallPieceOfBread You forgot to mention that due to evaporation the water would quickly cool, possibly to the point of freezing. Also, I wouldn't be surprised if the water would fall significantly below 10m as the evaporating water would provide some extra pressure, I'm not sure how much though
Aug 16, 2023 at 19:33 history answered David Bailey CC BY-SA 4.0