I was thinking about the Bernulli effect, where the pressure of a fluid in motion decreases as the velocity increases. I can understand that some characteristic has to decrease in order to not have an overall increase in energy: but why is it not the temperature of the fluid?
M Walter
Dear Mark Walter:
Bernoulli does not describe entropy. Temperature is a measure of entropy.
Bernoulli describes the interchange between kinetic energy (velocity) and potential energy (pressure). Temperature is along the lines of frictional losses... energy that is no longer available as either kinetic or potential.
David A. Smith
Gas cools going through a nozzle and warms going through a diffuser in isentropic flow. The temperature can calculated using the ideal gas law and the heat capacity ratio Cp/Cv of the gas.
Bret Cahill
Dear Bret Cahill:
"vortex coolers"
But Bernoulli is inviscid, so yields NO temperature change.
David A. Smith
Not so, look at the compressible form of bernoulli's eqn. There is no absolute requirement for inviscid flow.
-- Ed Ruf ( snipped-for-privacy@EdwardG.Ruf.com)
For example:
The mechanical energy balance also holds for supersonic gas flow. Here's a fun applet:
Bret Cahill
Engineers: Why does a vacuum only hold water down to 28 feet?
Galileo: "Maybe nature abhors a vacuum down to 28 feet and after that nature no longer abhors a vacuum."
Dear Ed Ruf:
"Ed Ruf (REPLY to E-MAIL IN SIG!)" wrote in message news: snipped-for-privacy@4ax.com...
There are caveats to what you say:
Along with this is the general form where "potential" equates to "kinetic" along streamlines. No frictional loss term for increasing temperature.
David A. Smith
True. Bernoulli is for ideal flow -- no viscosity, etc.
The mechanical energy balance is the first thing you use to determine a situation. If that suggests that there is significant shear or turbulence then you must go to the equation of motion and friction factor charts to account momentum transfer.
Bret Cahill
Everyone is I think missing what my question is. We all agree that there is an increase in energy with the increased velocity. Since the total energy can't change something has to decrease. Bernoulli's equation (which I have no doubts about it's validity) says that the pressre decreases. Why doesn't the temperature of the fluid decrease to make up for the decrease in the velocity (instead of pressure)?
M Walter
*kinetic*
It *can* change. The change is usually small.
It does, but *very* slightly for Mach numbers less than 0.3.
Think "Second Law of Thermodynamics"... and what temperature represents. In fluids, it represents a measure of entropy. In a closed system (such as along Bernoulli's streamlines) entropy is required to stay the same (fixed temperature) or increase (increasing temperature). So if the streamline is closed, entropy cannot leave the system...
David A. Smith
You're wondering why the temperature of a liquid doesn't drop to give you a velocity head?
You're wondering why a nozzle cannot suck heat out of sea water to run an impulse turbine?
EVERYBODY wants to do THAT but we can't.
The reason is the Second Law of Thermo.
It's kind of like the Catch - 22 of engineering.
Bret Cahill
"There was only one catch and that was Catch - 22."
-- Heller
Thanks all. I knew there was some obvious reason (wasn't to me but..)
M Walter
Bret Cahill wrote:
Most people call Bernoulli the mechanical energy balance. Mechanical energy is the good stuff.
You can have a good time with mechanical energy.
Unless you are freezing heat is the bad stuff, even though it is measured in the same units as mechanical energy.
Just staying warm isn't my idea of a good time.
Just as you can go from democracy to despotism but not the other way, you can go from mechanical energy to heat energy but not the other way.
Whole forests have been leveled trying to keep mechanical energy from turning into heat energy.
What am I saying? The future of human life on this planet is 100% dependent on engineers preventing mechanical energy from becoming heat energy.
Even a cure for cancer cannot make that claim.
Bret Cahill
PolyTech Forum website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.