Re: Venturi question


"RP" <no_mail_no_spam@xxxxxxxxx> wrote in message
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| Sorcerer wrote:
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| > |
| > | Sorcerer wrote:
| > | > "RP" <no_mail_no_spam@xxxxxxxxx> wrote in message
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| > | > |
| > | > | Sorcerer wrote:
| > | > | > "RP" <no_mail_no_spam@xxxxxxxxx> wrote in message
| > | > | > news:1158122514.441410.277650@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
| > | > | > | 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
| > | > | >
| > | > | > "Heat" is the mean kinetic energy of the molecules of the gas.
| > | > | > "Temperature" is the mean kinetic energy of the molecules of the
gas
| > | > | > per unit volume.
| > | > | > Thus a glowing cigarette is red "hot", but only locally, it is
not
| > going
| > | > | > to significantly raise the temperature of a room.
| > | > | >
| > | > | > In simple terms, "hot" gas pushes the piston simply by bumping
into
| > it,
| > | > | > that increases the volume, the piston absorbs the mean kinetic
| > energy
| > | > | > of the gas molecules which it transfers to the wheels, the
molecules
| > | > have
| > | > | > less kinetic energy and are "cooled".
| > | > | > When a diesel engine piston compresses air, it squeezes the gas
| > | > | > into a smaller volume and raises the temperature. Fuel is then
| > | > | > injected and the temperature is high enough to ignite the fuel.
| > | > | > As the piston passes TDC the burning process forces the piston
| > | > | > down to turn the flywheel and give the car kinetic energy.
| > | > | > The exhaust valve opens and the kinetic energy of the gas
| > | > | > is wasted as "heat", that heat is then distributed into a
greater
| > | > | > volume and the temperate of the car's interior is raised to a
| > | > | > level of human comfort. When too high, the driver opens the
| > | > | > windows and thus increases the volume, lowering the quantity
| > | > | > of heat per unit volume.
| > | > | > Androcles
| > | > |
| > | > | Temperature is the KE per molecule.
| > | >
| > | > How awkward, my living room has rather a lot of molecules.
| > | > The temperature is somewhere between 0 Kelvin and 3000 Kelvin.
| > | > Adjust the thermostat for me, please.
| > | > Androcles
| > |
| > | :) You're much too predictable.
| >
| > I would hope so, I like to be consistent. Better that than stupidly
| > claiming temperature is the KE per molecule.
| > Androcles.
|
| Then you should explain that you have redefined temperature to mean
| energy per unit volume before launching a discourse on thermodynamic
| processes, otherwise those who go by the standard definition of
| temperature might unjustly find error in your arguments. I'm reminded
| of the words of Lewis Carroll....oh right, you already know that story
| :)


Humpty Roberts in Wonderland:-
| Tom Roberts tjroberts@xxxxxxxxxx

Newsgroups: sci.physics.relativity
From: Tom Roberts <tjrobe...@xxxxxxxxxx> - Find messages by this
author
Date: Sat, 17 Sep 2005 17:57:18 GMT
Local: Sat, Sep 17 2005 6:57 pm
Subject: Re: Does the 'Curvature of Spacetime' cause gravity?


"Yes, tests of strong fields are few and far between, but there are
some:
the binary pulsars, and observations of accretion disks near black
holes

`I don't know what you mean by "observations",' Alice said.

Humpty Roberts smiled contemptuously. `Of course you don't -- till I tell
you.
I meant "there's a nice knock-down argument for you!"' <shrug>

`But "observations" doesn't mean "a nice knock-down argument",' Alice
objected.

`When I use a word,' Humpty Roberts said, in rather a scornful tone,
<shrug>,
`it means just what I choose it to mean -- neither more nor less.' <shrug>

`The question is,' said Alice, `whether you can make words mean so many
different things.'

`The question is,' said Humpty Roberts, `which is to be master -- that's
all.' <shrug>

Alice was too much puzzled to say anything; so after a minute Humpty Roberts
began again. `They've a temper, some of them -- particularly verbs: they're
the proudest -- adjectives you can do anything with, but not verbs --
however,
I can manage the whole lot of them! Impenetrability! That's what I say!'
<shrug>

"And you never responded to how a 2-d surface in a flat 4-d spacetime can
have nonzero curvature, and why that shows that the curvature of such
2-d surfaces is useless in "describing" the geometry of the 4-d
manifold...." he
droned on.

"If you say that the curvature of 2-d surfaces is useless in
"describing" the geometry of the 4-d manifold....I am willing to agree
with you. But I just wanted you people to help me visualize the
intrinsic curvature of 3-d Schw. space. I was told that the Gaussian
curvature of certain 2-d surfaces will represent the intrinsic
curvature of 3-d Schw. space. When I wanted these 2-d surfaces to be
identified, Jan PB had given some interesting suggestions. But now you
say it is *useless*....." said Alice.

"_SOME_ 2-d surfaces can be useful in describing the geometry of 4-d
spacetime, in particular those spanned by a 2-d vector space of
geodesics. But you were discussing 2-d surfaces defined by coordinates,
and _those_ are useless because coordinates are completely arbitrary,
and introducing that arbitrariness destroys their usefulness" replied Humpty
Roberts.

"That means the notion of intrinsic curvature of space is either too
complex that it cannot be visualized or it is just invalid." exclaimed
Alice.

"No. But in many cases using a ball of dust particles is a better
visualization tool than 2-d surfaces.", said Humpty Roberts, teetering
on his wall.

"Mathematically it is good enough to state that in Riemannian geometry
the Riemann tensor is non-zero. Where is the necessity of associating
it with a cooked up fictitious term 'curvature of space'? " asked Alice,
thinking of the cooked up egg she had for breakfast.

"Mathematicians and physicists are human. We share the common desire to
communicate with each other easily, accurately, and concisely -- that's
why technical vocabularies were invented." said Humpty Roberts scornfully
and pretending he is human by saying "we".

Alice pondered this for moment, then asked "Was it required to fool and
mislead the 'layman'?"

"Your problem, not mine", said Humpty Roberts, then realizing his
Freudian slip, he was pretending to be human, added "(ours).
But this technical vocabulary is not secret or unfathomable, it just
takes _STUDY_. <shrug>"

Alice then went back to say "The term *curvature* basically applies to
the bending of curves and 2-d surfaces."

Ho ho, thought Humpty Roberts, "Not in differential geometry or GR.
The term "curvature" was borrowed by analogy with 2-d surfaces, and
has come to mean the Riemann curvature tensor. That is, a manifold of
_any_ dimension with nonzero Riemann tensor is said to be curved."
and he shrugged like this :- "<shrug>"

Alice asked "Why *said* to be curved when it is actually not curved?"

Humpty Roberts let out a great sigh.
" <sigh>", he said.
"The nuances of English. I was discussing the usage of words and
not the concepts they represent."
-- Tom Humpty Roberts tjroberts@xxxxxxxxxx
news:ZDmYf.51582$2O6.5573@xxxxxxxxxxxxxxxxxxxxxxxxxx
The end.
With thanks to Lewis Carroll.

The reader should take careful note here.
Humpty Roberts is not discussing the concepts words represent, he is
discussing the meaning of words. The rest of us use a dictionary.

Professor Androcles.








.

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