I find it very gratifying to see so many informed and thoughtful people putting their heads together on the "outside the box" analysis underway. In the worst case, this discourse burns some time but stimulates flexible thinking about things that others might readily accept as "status quo". In the best case it may allow or inspire people in the official determination loop (and in the aircraft systems development stream) to work a bit harder at understanding the outer margins of probability for failure in the marvelous products they devise.
Comment on two recent posts:
at # 1375, PickyPerkins says:
The engines on the 767 performed just fine in suction all the way from sea level to 41,000 ft.
What they could NOT handle was a cessation of boost pressure during the climb.
The engines spooled down 14 seconds after the boost pumps were turned off.
.... gap ....
This seems a clear indication to me that the boost pumps were doing something which the HP pump could deal with so long as boost pressure continued.
But when boost pressure was discontinued, the engines spooled down after a delay of 14 seconds.
That something which boost pumps were doing could have been bringing air out of solution.
Which was OK with boost pressure on, but not OK when the boost ceased.
I would suggest that your concept is very promising, but the conclusion is incorrectly stated:
The effect of the submerged boost pumps running will be to
keep dissolved gas
IN solution, because the pressure gradient across the pump is relatively gradual (with fluid present at both inlet and outlet) and the pressure added by the pump is not so large an increment to the gravity feed pressure of the surrounding fuel.
If, however, the boost pump ceases operating, then a larger pressure gradient develops at its outlet side due to the suction of the HP pump and engine. This increased suction pressure gradient would likely be the cause of dissolved and otherwise entrained gasses increasing in volume and coming out of solution in a way that might disrupt the overall flow.
-------
Johngreen's detailed explanation of resonance is very informative as a baseline for discussion.
My understanding is that the resonant couple in a plumbing system like the 777 fuel supply path includes at least the two normal tuned systems, plus possibly other parasitic resonances. A first tuned/resonant path is the plumbing, pipes, etc, and the terminations formed by pumps and gates and valves. A second is the fluid, gas, or whatever may be rattling around inside the plumbing. Outside structures and forces may contribute to additional resonant loops.
A teasing difference of circumstance in the BA038 context is the history of very cold fuel. Someone has credibly noted that JPxx fuel viscosity is characterised only at -20c (iirc) in the standard fuel spec, meaning the viscosity might differ widely with varying fuels in the regions above and below -20c. Clearly, fuel viscosity will affect flow characteristics as well as the propagation velocity for shock waves (which controls resonance behavior). Could it be that the very cold fuel temps changed viscosity of a somewhat non-common fuel in such a manner as to put the resonance propagation velocity characteristics of the fuel outside the range of modeled behavior used in the 777 design calculations, and therefore outside the damping ability designed into the system?
If low temps, uncommon fuel, and resonance possibility were factually linked to the chain of causality, one might have a very plausible failure mode.