Re: Venturi question

Sorcerer wrote:
<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


I think the question is "where did the heat go?" So far we've
outlined three distinct cooling processes between us. Actually four,
because you and tadchem mentioned the vapor compression cycle
(refrigerator). This cooling cycle is not however due, or even remotely
related to the venturi effect. This would fall under the category of
plain old evaporative cooling instead. On the other hand evaporative
cooling can occur at the venturi outlet providing that the mixture
exiting the outlet contains liquid in some form. But this cooling
effect wouldn't be related to the venturi effect, it would only be
incidental.

The process that you outlined isn't adiabatic. It requires a heat
sink. This was dubbed the "gas cycle" when it was first invented. The
restricted portion of the venturi would have to be maintained at a
temperature lower than the gas within it. If no heat sink is present,
then the cooling effect will only occur temporarily, i.e. until the
venturi and gas temps equalize.

The adiabatic process that tadchem outlined causes cooling of the
entire volume of compressed gas, not just the gas at the outlet. This
is evident in his example of the aerosol can. It is the entire can that
cools rather than just the nozzle. The throttling effect (Joule-Thomson
process) that I mentioned causes a temperature drop that is
proportional to the drop in pressure, or in other words a cooling of
the gas exiting any orifice, be it a venturi or otherwise. Though there
would be some cooling within the connecting tubing and in the venturi,
it would be a much smaller effect than that occurring at the outlet
where the pressure drop is greatest. Evaporative cooling would occur
in similar fashion, since the throttling effect is essentially just a
further evaporation of the gas, so to speak. In the throttling effect,
even though the gas is already in vapor form, it still has internal
"cohesive" PE, and depending upon conditions, that PE may be either net
positive or net negative. The gas may cool down, or it may heat up,
depending upon its initial density and temperature, and of course it's
composition. This is one of the reasons that real gases aren't ideal.

So take your pick, or mix and match.

Matt, can you describe in more detail the actual system that you have
in mind? I'm no expert on this subject, but I might be able to describe
the process in terms other than this or that "effect". Basically, any
time a gas cools, it's because it has lost heat energy. That's a
bit of a tautology, but at least it tells us that the heat had to go
somewhere. I *think* your question is two-fold: How did the heat
transfer occur, and where did the heat go. Based upon your questions
so far, I'm really not sure what you're looking for other than a
better understanding.

But now that you've brought the subject up, I'd like to know a little
more about it too. Maybe someone else can help us all out.

Richard Perry

.

Relevant Pages

  • Re: Venturi question
    ... |> | it passes the throat causes a temperature decrease". ... A venturi is narrower in the middle. ... |> The gas is squeezed down, that RAISES its pressure and temperature, ... of chlorine gas to kill bacteria; the lowest pressure is in the reservoir, ...
    (sci.physics)
  • Re: Venturi question
    ... |> | it passes the throat causes a temperature decrease". ... A venturi is narrower in the middle. ... |> The gas is squeezed down, that RAISES its pressure and temperature, ... of chlorine gas to kill bacteria; the lowest pressure is in the reservoir, ...
    (sci.physics)
  • Re: Venturi question
    ... |> | it passes the throat causes a temperature decrease". ... A venturi is narrower in the middle. ... |> The gas is squeezed down, that RAISES its pressure and temperature, ... of chlorine gas to kill bacteria; the lowest pressure is in the reservoir, ...
    (sci.physics)
  • Re: Venturi question
    ... |>> but in the water example it is from low pressure to high. ... This cooling cycle is not however due, ... |> related to the venturi effect. ... This was dubbed the "gas cycle" when it was first invented. ...
    (sci.physics)
  • Re: Venturi question
    ... |> pressure is in the manifold, the highest is atmospheric in the air ... Is it really true that the gas is being ... This cooling cycle is not however due, ... It requires a heat ...
    (sci.physics)