Re: Venturi question


Timo A. Nieminen wrote:
On Thu, 13 Sep 2006, matt271829-news@xxxxxxxxxxx wrote:

matt271829-news@xxxxxxxxxxx wrote:
As you say, various different potential cooling processes have been
mentioned. If expansion of the gas is in some way involved then I'm not
at all sure where this is supposed to occur - you say that the pressure
drop is greatest "at the outlet", but as I understand it the pressure
is lowest in the narrowest part of the tube, which seems to imply that
"expansion" would occur as the gas passes from the inlet into the
narrow part. Is that wrong?

Of course, I'm assuming that lower pressure = lower gas density =
expansion of gas, and higher pressure = higher gas density =
compression of gas. However, I'm starting to wonder about this. Is it
possible that the lower pressure in the narrow part of the venturi tube
is NOT actually associated with a lower gas density there? Don't know.

Richard Perry went through much of this already, but you might like to
consider the following anyway:

PV=nRT does suggest that you'll find a lower density (ie n/V) where P is
lower. However, it seems odd that as you squeeze the gas into the
constriction, the density would decrease rather than increase.

If you put an incompressible fluid (eg, water, approximately) through such
a constriction, the velocity must increase. An increase in velocity
requires an acceleration, the acceleration requires a force. The force is
due to a pressure gradient. Where the fluid accelerates, the pressure must
be decreasing. Where the fluid slows down, the pressure must be
increasing. Thus, the pressure must be lowest in the constriction.

For a gas, which is compressible, does the same thing happen? If the gas
is compressed in the constriction, the increase in velocity will be
smaller, and thus the drop in pressure will be smaller. So, the more
compression, the smaller the pressure drop. Thus, you do get higher
pressures associated with higher densities. So, try this: use Bernoulli's
equation to find the pressure in the constriction as a function of the
flow velocity. If the cross-sectional areas are A1 outside the
constriction and A2 in the constriction, what is the flow velocity in the
constriction as a function of the density in the constriction? Finally,
use the ideal gas law to find the temperature in the constriction as a
function of the density in the constriction.

There's a reason why a venturi tube is usually explained in terms of
Bernoulli's equation using an incompressible fluid: simplicity.
Bernoulli's equation is all about conservation of energy, and assumes (a)
no losses; the fluid is inviscid, and (b) all of the energy is kinetic
energy or gravitational potential energy. But it takes work to compress a
gas, and an expanding gas can do work, so it becomes quite a bit more
complicated. Does this work change the pressure difference above, which
just assumed Bernoulli?


Thank you for bringing to my attention a slight error in my argument.
Where I said (more or less) that "the velocity pressure must
continually decrease along the gradient in order to produce a flow",
this should have been "the velocity pressure times the cross sectional
area must continually decrease. This is of course equal to the force
gradient along the gas column, meaning that the net *force* must
continually decrease along the tube. Both the velocity pressure and
the static pressure can rise, fall, and rise again along the gradient,
but they must do so inversely to each other when the process is
adiabatic. In the throat of the venturi tube velocity pressure is at
its maximum while static pressure is at its minimum.

So like you were saying, when the speed reduces as the liquid enters a
region of greater cross sectional area, the velocity pressure here can
be lower in this wider section than it is at the outlet. Yet the force
is greater, due to the greater cross sectional area, so motion
continues along in the same direction, albeit at a reduced speed. This
is a hydraulic effect, much like a car lift in a garage, but in reverse
sequence. There are several effects coming into play in this system,
including all of the other cooling effects previously discussed, to at
least some degree no matter how slight. An ideal system is easier to
quantify, but in order to see what's going on in a real system a
broader perspective is required. Idealism can, and often does, lead to
inconsistencies and confusion.

I hope that I didn't confuse the OP even more with all of this. :)

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, ...
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  • 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
    ... 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, ... This cooling cycle is not however due, ...
    (sci.physics)
  • Re: Venturi question
    ... If expansion of the gas is in some way involved then I'm not ... at all sure where this is supposed to occur - you say that the pressure ... However, it seems odd that as you squeeze the gas into the constriction, the density would decrease rather than increase. ... If you put an incompressible fluid through such a constriction, the velocity must increase. ...
    (sci.physics)