Smilin_Ed

Will:
I believe we are in agreement and I think any heating needs to be upstream in the wing to help prevent icing in the pipes as was experienced in testing.

Chris, yes it is the fuel but I think it more practical (cheaper) to modify the planes so that they can better tolerate water than to change the specs. Trying to keep water, and other undesirable materials, out of the fuel would be much more expensive and more frustrating than a strategically located heater in the airplane.

barit1

Liquid-fuel rocket engines use regenerative cooling, wherein the cryogenic fuel and/or oxidizer is used to cool the case and nozzle of the engine, and in so doing vaporize the liquid.
Kill two birds with one stone.

In a gas turbine engine, there is a strong thermodynamic case to be made for mid-stage cooling of the compressor airflow. It would not be a pretty picture, with additional air ducts or fuel lines running hither and thither, but it would be a useful way to heat fuel in the tank and simultaneously optimize the engine cycle.

I'm sure it's been considered and rejected because of cost and complexity, but who knows? If fuel approaches $10/gallon, should it be reconsidered?

Mr Optimistic

Have read through the thread but can't quite see if there is positive evidence that the blockage/impingement is at the FOHE or if it's by logical analysis/plausability argument only. Could some patient soul enlighten me ? Thanks.

WhyIsThereAir

Don't know about the thread (I've only scanned it) but the last version of the report I read said that yes, ice* blockage in the heat exchanger can cause the symptoms seen. However, it went on to say that the timing of things observed at the engine would tend to put the blockage some 20 feet farther up the fuel line, and they are at a complete loss currently as to how that can happen - they can't find a point 20 feet back that can feasibly be blocked by ice.

I think the summary (of the report, not necessarily this thread) is: did ice* blockage cause the problem? Yes. Was it in the heat exchanger? Don't know. Was it farther back? Maybe, but don't see how that could happen. More research is required, and more research is happening.

* The report goes to some lengths to note that the "ice" is NOT 100% water. It is a frozen slush of water and fuel, and the water makes up a fairly small percentage of the slush.

Swedish Steve

Ice blockage of the FOHE inlet has been demonstrated in lab testing. It was not expected and operational procedures have been introduced to alleviate it. RR has designed a new FOHE which is now subject of an AD.
There is a picture in this thread, and also in Flight Global.

Smilin_Ed

barit1:
Please don't even think about ducting bleed air out of the engine. I had to investigate an accident where three of my colleagues died because of a bleed air leak. Yes, it was decades ago, and yes materials have improved, but bleed air is highly dangerous stuff. Introducing bleed air into a fuel tank frightens me and I'm fearless.

Will Fraser

barit1

In theory, if what you mean is transferring some of the heat of compression to the Fuel, it makes sense. Cooler air can be compressed further, and adds efficiency. The Thermodynamics are beyond me, I'm more a mechanical sort, and tend to agree that running Fuel through or even around the second hottest part of the engine, seems problematic.
Intercooler?

Smilin Ed

727, LAX ??

Douglas rules the waves. Boeing, not so much

Smilin_Ed

No. Navy A-3 in about 1972.

Dairyground

Back in post #2473, Pinkman says:

Using actual engines and fuel systems is probably neither the least expensive or most efficient way to go about investigating fuel system icing. Small-scale lab bench and workshop rigs, with a bit of mathematical modelling, will be a much more effective way of identifying the relevant parameters and their interaction.

On a lab scale, it should be relatively easy to identify the dependency of ice accreation rates on pipe external temperature, pipe size, internal surface characteristics, fuel temperature, fuel flow, fuel composition, including water content, effect of flow rate on the stability of the ice, and so on. Only when these elements are reasonably well understood will it be possible to predict what will happen in a full-size system, and reasonable to verify those predictions.

Perhaps such investigations are already under way in the industry. If not, it seems likely that they are a potential source of PhD theses in accademia.

On a slightly different tack, the AAIB report suggests that it was difficult or impossible in many cases to observe ice build-up inside their test rigs. I should have thought that techniques such as ultrasound would have been effective, and that it would not have been too difficult to send a miniature camera through the system.

Mr Optimistic

Isn't it unsettling that even with an intact aircraft, all the systems ready to be examined in any detail you like, all the witnesses standing ready, fuel still available in the tank, all left in a convenient location and with time to think, there should still be any uncertainty ?

PETTIFOGGER

Mr Optimistic, Hi.
I am not sure what uncertainty you are referring to and why that would be unsettling, particularly after Swedish Steve's post just above. Can you say more?
rgds, pf

Mr Optimistic

Thanks for the reply. Have read through thread and seen discussions on the meaningfulness of specifications, anxiety about FAME (and about the possibility that there is some unknown factor which isn't even tested for), questions over twin engine ops for the airframe/engine combo, but there seems to be no clear smoking gun.

This gave me the impression that the ('interim' ?) findings are based on plausibility: cold long flight, fuel starvation, there is water in fuel - so ice is a main suspect, where might it most likely accumulate ? Yes we can mimic that failure mode in the lab, so lets go with that one then as NFF will have implications.

If it's so plausible now mother nature has pointed it out to us why didn't the original design anticipate it and why is it so rare an occurrence ?

All very well to cover office walls with fault trees, cut-sets and 10 to the power of minus 9, but this surely should be cut and dried forensic engineering.

And I haven't got that impression from reading this thread (no headline claiming 'CRACKED IT').

Doesn't this one need putting to bed more firmly ? (..the issue, not me !).

As an after thought, has the fuel system on the 777's with alternative engine supplier demonstrated differences in design to explain why it is only RR ?

4greens!

In oil refineries it is common practice to pump large amounts of heavy lube and fuel oil for distances of up to a mile.In order to keep the oil from cooling and becoming too viscous to pump "trace heating" is used. This consists of a small diameter pipe wrapped round the pipeline which is then heavily insulated. Low pressure steam is passed through the tracepipe.By substituting steam for electrical resistive heating tape and insulating the fuel pipe is it not possible to warm the fuel before it gets near to the engine? This would be a more elegant solution than piping bleed air around the wing. Just a thought.

Fargoo

This is exactly what is used in the cargo bays to stop water from freezing inside the service pipes (resistive heater tapes that is). I'd imagine it'd take a load of research and money thrown at it before they'd ever certify electrical heater tapes on a fuel system pipe.

cockney steve

Already well-proven in industrial apps. However, the current needed to pump that much heat into such an enormous flow-volume would require some seriously big alternators and cabling. HUGE power-loss!

mid-stage heat-exchange intercooling would appear to be the simplest solution,a portion of the surplus,returned, warmed fuel could recirculate through the wing/pylon-piping, whilst the rest returned to the tank.

although power-losses are involved, the thermodynamic gain would offset some or all of this, thus making a virtue from necessity.

Smilin_Ed

Steve, your analysis of electrical heating problems is right on. Not very practical. Also, I don't have much problem using air bled off the engine as long as it stays in the nacelle and can't leak out to burn things like wings. I'm really paranoid about that.

Markieboy

I still don't believe an ETOPS airliner can have BOTH engines display the same behaviour at the SAME time due to fuel freezing or boost pump contamination.

It can't happen. Sure, it can happen in one tank and after a while maybe even in the other but AT THE SAME TIME? Forget it.

The chances of fuel freezing in both tanks at the same time is mathematically irrelevant.

Just my .02.

barit1

Not knowing the details of the Trent/777 installation -

The HP fuel pump is typically sized to pump fuel for the SL takeoff condition, which means it is oversized in cruise, and VERY oversized in descent. It being a positive-displacement pump, the excess fuel must be routed somewhere, and depending on where the excess is dumped, it can either create an unwanted thermal runaway, or else solve an overcooled condition.

Or did I read that the RR HP pump is a variable-displacement type, in which case the above is null and void?

Swedish Steve

No. the HP fuel pump is a gear type positive displacement pump. Remember it is turned by the gearbox, so the speed is proportional to engine speed, and so therefore is the output.
Inside the FMU is a Pressure drop and spill valve which controls the downstream pressure by spilling fuel back to the HP pump intake.

barit1

That's what I thought.

My point is that pumping fuel up to burner pressure takes a lot of energy, and when there's an excess of high pressure fuel available, that's heat that can be put to good use.

Or not.