Scott Gettings wrote:

This should give a higher-than-expected temperature rise.
Factors making the temp lower than in real operation include: no
incoming heat from the fuel rail return, and no heat from the backup
pump running during takeoff and landing.

I'm not certain I agree with the above statement. I believe you are
ignoring the heat gain the fuel receives simply by passing through the
engine compartment. Most engine compartments are darned toasty,
especially so during the Summer.  I would not be surprised if the fuel
rail temp were 150 degF.  If you take 2.5 gal of 90 degF fuel and
recirculate it through the 150 degF fuel rail, in a few passes your fuel
will be darned close to 150 degF.

So, I don't mean to poo-poo your test; the test is good as it quantifies
the pump's contribution to the fuel heating issue. But, I think we need
some more data before anyone can claim to have data proving that fuel
heating isn't a problem with your proposed setup.

What's the worst case temperature for the fuel loop? I'd guess that it
would be extended climb-out in AZ in August at 15:00 (local) after the
aircraft has sat on the ramp all day. The whole engine compartment has
been heat soaked, the airflow is low, the engine is making best power
(and throwing gobs of waste heat), and this is about the worst time for
the engine to quit.  Or maybe touch-n-goes in the same environment,
since the fuel flow is quickly reduced when you level off, but the
system is still hot as blazes.

We need to get a handle on how much heat the fuel can acquire.  I'm
going to assume that all of the fuel lines (both flexible and rigid) in
the engine compartment are wrapped in fire sleeve, which is a good
thermal barrier.  So most of the heat gain from the engine compartment
is going to happen while the fuel is in the fuel rail and the pressure
regulator (assuming it's on the end of the fuel rail).  We need to
measure the fuel rail's surface temperature to get an idea of how much
heat could be transferred. Anyone got a plane we can use to measure
this?  The measurement could be as simple as using the temperature
indicating paints or crayons, or maybe one of those IR-based temperature
sensing guns.

Russell Kent



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