Re: Venturi question


<matt271829-news@xxxxxxxxxxx> wrote in message
news:1158097941.649861.101980@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
| Sorcerer wrote:
| > <matt271829-news@xxxxxxxxxxx> wrote in message
| > news:1158004973.423968.104990@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
| > | Hi
| > |
| > | It seems to be a well-established fact that the temperature drops
| > | inside a venturi tube (e.g. causing icing in carburettors). The only
| > | explanation I've been able to find is that "the expansion of fluid as
| > | it passes the throat causes a temperature decrease". I'm not sure what
| > | "passes" means here. The pressure inside the throat is lower than the
| > | pressure either side, right?
| >
| > No, not right. A venturi is narrower in the middle.
| > The gas is squeezed down, that RAISES its pressure and temperature,
| > it cools by losing heat to the tube, but then it cools again as it
leaves
| > the exit upon expansion.
|
| The standard explanations (e.g.
| http://en.wikipedia.org/wiki/Venturi_tube) say that pressure is lower
| in the narrow part of the tube.

"Wikipedia does not have an article with this exact name. Please search for
Venturi tube) in Wikipedia to check for alternative titles or spellings."

Wackypedia is a disaster, any idiot can write it, and they do.

Let's put it this way... As a piston falls in the engine, the lowest
pressure is in the manifold, the highest is atmospheric in the air filter.

Venturi tubes are also used to measure water flow and the introduction
of chlorine gas to kill bacteria; the lowest pressure is in the reservoir,
the highest at the faucet. Likewise air pressure is lowest at the top of the
atmosphere.

Thus in the case of a carburettor the flow is from high pressure to low,
but in the water example it is from low pressure to high. If you say
the pressure is lower in the narrow part of the tube, lower than what,
inlet or exhaust?

| If that really means that the density
| of the gas (in terms of molecules per litre, say) is lower in the
| narrow part, then the gas would be expanding as it *entered* the narrow
| part, and compressing as it *exited* the narrow part. This is what I
| can't get my head round. Is it really true that the gas is being
| compressed as it *exits* the narrow part??

The gas is decompressed as it leaves. If you let the air out of a truck
tyre the valve will frost up. The pressure in the valve stem is
slightly lower than the pressure in the tyre (100 PSI), but is much higher
than atmospheric (15 PSI).
So if you have a restriction the gas or water squeezed down at
the start of the tube and then drops pressure as it leaves, but
in the water example the exit pressure is greater than the entry
pressure.
In numbers, 100 ft head of water at the faucet, 0 feet head at the
reservoir which is on a hill.
Faucet closed :
At 50 feet, there is 50 feet head of water on each side the venturi.
Faucet open:
Water flows, there is then 50 feet head on the inlet side of the
venturi, 49 feet head on the other, and 99 at the faucet.

In other words the faucet is supporting the weight of 99%
of the water, the venturi 1%.

Look up Boyle's Law and Charles's Law.
Androcles





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