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Old 11-30-2008, 05:15 PM   #64 (permalink)
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Originally Posted by instarx View Post
Oops, indeed.

Read the title: "Inviscoid, Incompressible Fluid". An incompressible fluid means water or some other liquid, not a compressible fluid like air. You picked an inappropriate diagram to support your position. Seems you and NASA are talking about different things.
Yes the calculations for this were done using an incompressible fluid model. They did this to simplify the problem so that it was easier to teach. Sort of like how you are applying the combined gas law which is a generalized application of the ideal gas law which assumes your working with an ideal gas ( Ideal Gas Law ). Note, air is not an ideal gas.

Bernoulli's principal applies to compressible and incompressible fluids alike. The full text of the Nasa example is here chapt3 . If you wish to expand upon it and calculate compressible flow effects I suggest you read this .

So Nasa and I are talking about the same thing.

The equation is the Combined Gas Law, and it applies to ALL systems - closed, open and in between. No exceptions. The Gas Laws: They're the Law, not Suggestions.
It does???

"The combined gas law states that for a closed system ......"

Combined Gas Equation-cs


As for ignoring the volume component: There are not two container volumes in this system, only one. It just happens to be a long, thin container where hot air is cooled as it passes through the container until it exits the other end. The values for V1 and V2 are therefore ignored.
The volume of the container isn't relevant. The volume of a unit of gas is.

"There is one more technique you need to know to apply the gas laws to things like the atmosphere.

In all the scenarios considered in section 1, the volume V was just the volume of the container. So the question is, what is the relevant V when we are talking about the atmosphere, or any other system where there is no relevant container?

The answer is that we talk about a parcel of gas. That is, take some region of space, and imagine marking all the gas that is initially within that region. We then follow the parcel of marked gas as time goes on. The parcel will move. The parcel will change shape, but that doesn’t matter, except insofar as the volume changes (compressing or expanding the parcel). The boundaries of the parcel might become a bit blurry, as molecules diffuse across the boundary to/from adjacent parcels, but this won’t greatly affect the properties of our parcel, especially if neighboring parcels have approximately the same properties anyway."

Gas Laws


Yes I do. It's attached to my car. What does that have to do with anything.
I asked because I thought you might like to make measurements.

With this one I am through. The IC cools the air. Cool air molecules have less energy therefore they exhibit less pressure and can be packed closer together. Cold air is more dense than hot air.
This contradicts your statement in post #55 "V1 and v2, the volume of the contained air, can be ignored because they remain essentially constant". Density of the air could not change and still have your calculations in post #55 be valid. In order to pack lower pressure air molecules together and make the air more dense you must raise the pressure
This is basic, and is entirely consistent with the gas laws. This gets back to my very first statement that the pressure drop measured across the IC is due to the drop in temperature. I have shown that the drop is predicted by the Gas Laws, and that the Gas Law even predicts the amount of temperature drop correctly. The equation is simple enough for anyone to understand. I gave a reference. I gave a link to a calculator for anyone to use. Yet you still want to argue that the temperature drop isn't responsible for the pressure drop. Fine.

If for some reason you refuse to accept the proven fact that lowering the temperature also lowers the pressure, it is not my responsibility. Believe what you will. If you want to argue that the Gas Laws are wrong, go dig up Robert Boyle or Jacques Charles and argue with them.
Your narrow view on how the combined gas law can be applied totally neglects the Charles' Law form.

Charles's law - Wikipedia, the free encyclopedia

In fact the way you describe things working it could be construed as a denial it exists.

If you had an ideal intercooler working on an ideal gas this would be use to describe its behavior because it results in the the maximum reduction of volume of the gas. This is the behavior engineers try to approximate when designing an intercooler. Unfortunately they cannot design one without flow restriction and the pressure drop across the intercooler varies in proportion to the square of the amount of air flowing through it.
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