Hi Gilles,
A very interesting website, http://contrails.free.fr/
Your radiator duct experiments are fascinating. Very nice recommended
book section too.
B.t.w. I'm missing the old tutorial bits on parallel/series voltage
regulators by you and Jrme Delamarre.
I don't quite agree with your fuel pump discussion though.
As I see it there are exactly 2 ways of connecting 2 pumps.
Parallel:
To remove a pump you replace it with an open circuit. A series check
valve for each pump makes that possible (no backflow).
Series:
To remove a pump you replace it with a short circuit. A parallel check
valve for each pump makes that possible (no blockage).
There is no 3rd way.
A pump has a flow rate versus pressure characteristic. How steep is the
curve?
At one extreme is a pump that supplies a set pressure regardless of the
flow required to achieve that.
Two identical such pumps in parallel would supply the same pressure and
flow into a certain fixed orifice as one.
Two identical such pumps in series would supply double the pressure and
flow into a certain fixed orifice as one.
At the other extreme is a pump that supplies a set flow rate regardless
of the pressure required to achieve that.
Two identical such pumps in parallel would supply double the pressure
and flow into a certain fixed orifice as one.
Two identical such pumps in series would supply the same pressure and
flow into a certain fixed orifice as one.
Where in the range between one extreme and the other the Piermont pump
falls I do not know. I suspect that Rotax found experimentally that it
is somewhat closer to being a " flow source" (the second extreme) than
to being a "pressure source" (the first extreme). Hence the series setup
recommendation. But the difference is apparently not large.
Practical questions:
Is there a likelyhood that a check valve fails stuck open? That would
weaken the case for the parallel setup.
Is there a likelyhood that a check valve fails stuck closed? That would
weaken the case for the series setup.
In both cases the pump with the failed check valve must remain on.
But maybe the backflow in the parallel case with the Piermont pump
turned-off is not too bad?
Or, maybe the blockage in the series case with the Piermont pump
turned-off is not too bad?
On your website you also mention the possibility that a failed pump may
shed debris.
That probably weakens the case for the series setup indeed.
B.t.w. The "pressure source" (first extreme case above) has the battery
as its electrical analogue: one voltage (approximately), any current
(almost). The "flow source" (second extreme case above) has no readily
available electrical analogue, although current sources are quite common
constructs in electronics. But pretending that a pump is just like a
battery is not useful in a parallel versus series discussion.
Hoping I didn't offend.
Jan de Jong
461, second wing
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