- Jul 15, 2020
Great post. Thanks.
- As fuel evaporates from the surface, the fuel near the surface becomes both cooler and more resistant to evaporation. Yeah, there is some mixing, but it is not high order in most gasoline tanks, and so the fuel remaining becomes more resistant to evaporation, and the fuel at the surface even more so;
- Evaporation is a powerful cooling effect on mixed liquids - wide ranges of molecular weight - and so will cool fuel quite strongly with even modest evaporation.
Its important to note that the fuel is actually boiling (not really evaporating) however the effect is the same as you say, ie the fuel cools by giving up its enthalpy of vaporization and stays liquid. And also, yes, the lightest molecular weights (eg isopentane) boil off first. Its important to note the change in Octane of the fuel when the lighter fractions are lost.
As we climb, to altitude the lighter fractions boil/evaporate and the fuel loses its octane the higher you go. The irony, of course, is that engines rely on higher octane to avoid detonation, vapor lock etc. but as we climb, the fuel is losing the very octane needed to fly high. I guess this fact hurt the B29's trying to get to Japan hence all the fuel research at that time. The 'fix' was to fly lower which I always thought was to improve bombing accuracy, but maybe it was the fuel as well.
Its also worth noting how much fuel can be lost through boiling/evaporation. Up to 20% in the charts up to 50,000 examined in the old NACA E5H27 report.
I guess some of this discussion is finding its mark as Veloce 600 Specs now lists 25,000 as the cruise ceiling. A lot less to worry about at 5.45 PSIA.