Paul Lamar wrote:
Steve,
Going back to my notes I got an intake pipe velocity of 5 cubic
feet per second or 8640 cubic inches per second divided by the
area of the two pipes or 6.28 gives 1375 inches per second.
Dividing by 12 gives 114 feet per second.
Absolute zero is -460F. Absolute intake temperature is therefore
360 for the intake when it is 100F.
No, absolute temperature of +100 deg F is 311 deg K, about 558 deg F
above absolute zero. (I think you subtracted when you should have
added.)
At 1600F -460 or 1140 absolute for the exhaust. Does this look right?
Well, the answer (1140 deg K) is correct (well... close to 1144 deg K),
but the formula ("1600F-460") is wrong. It should be one of the
following:
A: (1600 degF - 32 degF) * 0.55555 degK/degF + 273 degK
B: (1600 degF + 460 degF) * 0.55555 degK/degF
V is normally volume but in this case we treat it as volume per
unit time.
V2 is the unknown.
V1 is 114 feet per second. Do we need this in cubic metric units
of some sort?
I don't think so. The formula is a variation of the Ideal Gas Law which
is stated in MKS units so people usually work it that way. Consider,
however, that the English-to-Metric factor would be on both sides of the
equation, so it has no effect. But keep your units in mind. This does
not hold for the Fahrenheit-to-Celsius conversion, however, because it
is not a pure multiplication.
P1 = P2 = atmospheric.
At the macro scale, true. I'm not certain that it holds at the micro
scale, however.
(P1 x V1)/T1 = (P2 x V2)/T2 P1 = P2 so V2 = V1/T1 x T2 = 361 FPS.
With units: V2 = (114 F/s / 311 degK) * 1144 degK
The "degK" cancels, leaving: (114 F/s / 311) * 1144 = 419 F/s
You got 420 feet per second. Where did I go wrong?
With your conversion of "ambient" +100 degF air to 360 degK, which is
wrong.
Russell Kent
Thanks Russell. I am still dumb founded on why we are getting 130,000
RPM with a exhaust velocity of only 420 feet per second.
Paul Lamar
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