Basil

Some time ago I'd attempted, unsuccessfully, to find the relationship of TAT to wing boundary layer temperature with a view to predicting fuel temperature trends.
Suddenly the latest, September 2008 Interim Report on the G-YMMM accident comes up with:

Expensive piece of research

Re-entry

Basil. This also interests me. Sadly I have no answer for you.

I have searched NASA and come up with nothing.

However, in my experience I have only ever seen fuel temps trend towards the TAT (and then remain at TAT). When moving to warmer air again, it trends back up to the new TAT, so it could be below TAT for the time it takes to warm up again.

I suspect this is what they are referring to re 'up to 3 deg below'.

But I stand ready to be corrected by an aerodynamicist.

kijangnim

Greetings,
Fuel coolings depends upon fuel initial temperature, the warmer the initial fuel temperature is, the faster and the further the fuel will cool down. So operating from Europe will give you a slow cooling, taking off from the Middle/Far East will give you a faster one

777guy

Greetings!

It has been my observation that the fuel on my company 777-200ER a/c will cool to a temperature equivalent to TAT or 1-2 degrees cooler. I attribute this difference to the location of the TAT observation, which is just below the Captain side window, and the actual temperature of the boundary layer air. The increased local air velocity obviously results in a temperature 1-2 degrees less than TAT.

TAT is a great predictor of future fuel temp. Given enough time exposure, the fuel will cool to TAT or 1-2 degrees below. Having said this, flight during daylight hours will retard the cooling rate, and night flight will facilitate the cooling rate.

Attempts to warm the fuel by increasing Mach are inefficient, and you must first overcome the thermal inertia of cooling before fuel warming can occur, in the meantime your increased fuel burn exhausts contingency fuel. Once the fuel begins to warm, the rate will only be about 1/2 degree per hour, so it is best to avoid cold fuel in the first place.

In the USA we are fueled with Jet A, which has a Fuel Freeze spec of -40c. Elsewhere in the world Jet A-1 is standard which has a Fuel Freeze spec of -47c. Obviously Jet A-1 is preferable if flying in a very cold airmass region.

Dispatchers should be careful to closely analyze forecast airmass temperatures, and when -65c SAT or below is forecast, great care should be exercised as to routes and altitudes for flight planning.

Crews are well advised to track fuel temp, perhaps on your Airep form, thus permitting a high level of fuel temp awareness. Employ efforts to avoid cold fuel EARLY, for the reasons described above.

Cheers,
777 Guy (KDFW) Texas, USA

Re-entry

Hey Basil. I found this:-

As the stream velocity U becomes larger, its kinetic energy, U2/2, becomes comparable to stream enthalpy, cpT, where cp is the specific heat at constant pressure and T is the absolute temperature. Changes in temperature and density begin to be important, and the flow can no longer be considered incompressible. Liquids flow at very small Mach numbers, and compressible flows are primarily gas flows. See also Gas; Mach number.

In a flow with supersonic stream velocity, the no-slip condition is still valid, and much of the boundary-layer flow near the wall is at low speed or subsonic. The fluid enters the boundary layer and loses much of its kinetic energy, of which a small part is conducted away although most is converted into thermal energy. Thus the near-wall region of a highly compressible boundary layer is very hot, even if the wall is cold and is drawing heat away. The basic difference between low and high speed is the conversion of kinetic energy into higher temperatures across the entire boundary layer.

In a low-speed (incompressible) boundary layer, a cold wall simply means that the wall temperature is less than the free-stream temperature. The heat flow is from high toward lower temperature, that is, into the wall. For a low-speed insulated wall, the boundary-layer temperature is uniform. For a high-speed flow, however, an insulated wall has a high surface temperature because of the viscous dissipation energy exchange in the layer.

Here is the link

chksix

A great atmospheric calculator here:
Aerospaceweb.org | Atmospheric Properties Calculator

Enter altitude and speed at the top and get the results below.

I entered 40000 ft, M0.79 and no temp increment (standard day)

TAT will be -29 °C while the laminar temp is -33 °C if I understand things correctly. -56 °C is the "ambient temp".

HarryMann

That's a good find!

I think you interpret correctly, laminar b.l. being worst case by 1 degreeC, and likely first half of wing chord nominally laminar in cruise, so TAT - 4C

Agree, OAT/SAT == Kinetic Temperature in first results table = -56.5 C

========================

Boundary layer Cf and thicknesses are interesting...

An old rule of thumb was that the b.l. thickness on a Spitfire wing at 400mph was 1"

This gives that for a 6 ft chord ref length at 20,000 ft...

BelArgUSA

We have a fuel temperature gage reading tank temperature from nº1 main tank.
That tank is located on the wind above nº1 engine.
We can read the fuel temperature received by each separate engine as well.
xxx
We are to keep the fuel 3ºC above fuel freezing temperature.
Jet A-1 freezes at -40ºC - That makes the minimum temperature to be 37ºC.
Our "book" says to use TAT in case the fuel temperature probe is INOP.
xxx
Generally, the TAT is generally around -30ºC, give or take 5ºC.
Generally takes a few hours, for fuel to go down to that temperature.
We have fuel heat (upstream from FCU) which can be selected using bleed air.
Another way is to increase Mach number = increase rise = increase TAT.
Or... descent...
xxx
Old PanAm trick of the trade - Keep 5,000 kg (10,000 lbs) of fuel in CWT until later.
Fuel in CWT (the belly near air packs) is much warmer.
If the fuel went very cold (often on 747SP) we could use warmer fuel from the CWT.
xxx
Obviously I do not know how goes with a 777, but likely to be about same.

Happy contrails