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I have a question about the behaviour of air with a small mass fraction of water vapour at extremely low pressures and temperatures.

I am currently running experiments in a large vacuum facility that is designed to produce low-density, high-speed flows. The lab that I work in has a central compressed air supply that I have been using to inject into the facility, producing a plume of Mach ~6. The readings I get when injecting 0.5 g/s into the chamber are:

  • Static pressure: ~2 Pa
  • Stagnation pressure: ~3000 Pa
  • Stagnation temperature: 385 K
  • Static pressure (far from the plume): 293K

When taking total temperature readings in the plume, I've been recording temperatures which are actually greater than the stagnation temperature upstream of the nozzle (~410 K). The only explanations I can come up with for this are:

  1. Sensor error (I use K type thermocouples and I'm not sure how well they operate in highly rarefied flows).
  2. Condensation of the water vapour in the flow, releasing latent heat.

My question is: would air with a small mass fraction of water vapour (well below the ambient relative humidity) condense after an extreme expansion into a vacuum, and hence account for the high temperature readings?

For reference, I've done some simple isentropic expansion calculations, and they indicate that the flow at near the nozzle exit will get down to ~30 K. Pitot readings indicate that the stagnation pressure in the same spot is ~150 Pa.

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