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I'm trying to estimate the temperature of fuel stored in wing tanks during the flight. In my situation, I'm considering two wing tanks of 2000 liters each, one on each wing, at 20°C before the flight

I've run some calculations (I can provide additional numbers if necessary), and I end up with a temperature of 17°C after more than two hours of cruise (Mach 0.5, FL250)

Does this number sound correct to you? I was expecting a much lower temperature. I took a constant mass of fuel, which is not the reality, but even 15°C for instance seems rather hot. What are the usual temperatures of fuel in airliners tanks?

Federico
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tintindu34
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  • I'd be curious to see exactly how you got to 17. – Notts90 Sep 08 '16 at 11:28
  • I used a mutliphysics simulation software (amesim), to model the convective exchanges. I assumed a tank outside surface of 5m² on each side of the wing – tintindu34 Sep 08 '16 at 11:33
  • This is related http://aviation.stackexchange.com/questions/2185/how-do-aircraft-keep-fuel-from-freezing-while-in-flight "For example, at an actual temperature of -56.5 C (SAT) and Mach 0.72 the TAT is -34 C, but at Mach 0.80 the TAT is -29 C." so I guess something is wrong with your calculations. – Mark Sep 08 '16 at 12:03
  • Yes I saw that, thank you. It confirms that fuel stays above -40 °C at least. But it does not tell if it is more 10, 0 or -20°C. For my calculations, I took the r – tintindu34 Sep 08 '16 at 12:07
  • For my calculations, I accounted for the ram effect: at 25'000 ft, the outside temperature is around -35°C, but at mach 0.5, the stagnation temperature is -22°C. Thus the cooling is not that strong – tintindu34 Sep 08 '16 at 12:17
  • Well explained in this Mayday episode about BA38 disaster in 2008. – mins Sep 08 '16 at 12:25

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Quite cold, in fact. External air temps at altitude reach -30 to -40 deg C. The fuel in the tanks wil cool down and reach those temps within a couple of hours exposure. Jet A has an icing inhibitor added to it to prevent freezing of the fuel in the tanks during extended periods in the stratosphere.

From a heat transfer standpoint, I would probably analyze this as a rectangular bar made from a composite sandwich of aluminum with a Jet-A center subject to convective cooling by air moving at a typical true airspeed, say 450KTAS at STP for FL250. For simplicity we would ignore compressibility effects. That may be where you erred.

Romeo_4808N
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    Jet A1 may have icing inhibitor added but it's not part of Jet A1 by default. Many airplanes have heaters or other anti-icing systems and do not require it. If it is added it is at the point of sale at extra cost. – GdD Sep 08 '16 at 12:27
  • Temperature can even reach -74°C over Ural as noted in this AAIB report. That's an extreme, but -54°C is common at the tropopause. – mins Sep 08 '16 at 12:33
  • Yes but in the situation of regional flights (like with ATRs), you don't spend a lot of time in cruise (<2hours) and you don't climb that high. In this case I'm not sure you get below 0°C – tintindu34 Sep 08 '16 at 12:41
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    @tintindu34: The lapse rate in the standard atmosphere in the troposphere is −6.5°C/km until reaching the tropopause (11 km). So 0°C is reached below 3 km AMSL (cruise altitude of an ATR-72 is about 5 or 6 km, so maybe -15°C, but well below -56C°, you're correct). – mins Sep 08 '16 at 13:49