Quote:
Originally Posted by stonebreaker
No, you like nearly everyone else, is forgetting half of Bernoulli's equation. The drop in pressure perpendicular to the flow, which is the part everyone remembers, is made up by an increase in dynamic pressure - everyone forgets this part. Go back to your wiki article and re-read it. Note that for a given streamline, the actual equation is dynamic pressure + static pressure = a constant. A CONSTANT. So if there is a drop in static pressure, it must be balanced by an increase in dynamic pressure. And it's the dynamic pressure that causes backpressure - if I need to force X amount of gas through a pipe in a given amount of time, I have to force it through using a given pressure (no pressure differential, no flow). If I want to increase the velocity of gas going through the pipe, the only way to do that is to increase the pressure driving it.
You made a mistake about 10 posts back and now you're sticking to it rather than re-examining your premise. This was fun for a while but now you're just being stubborn. I'll talk to you tomorrow - some of us have work.
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I just agreed with you about bernoulli.
What exactly is the "mistake" I made? If you're referring to bernoulli and the flow equation, read the first line again.
PS - I'm doing my work while chatting with you. It's a nicety for me.
Keep in mind, that you can increase the pressure driving a gas through a pipe by simply allowing it's expansion to drive the increase in pressure.
Like I said, there's more to it than simple pressure and velocity. Since the flow is compressible, and is not adiabatic (it loses heat and expands), keeping a lower pipe diameter will increase the flow's velocity due to expansion until the expansion is complete. If the pipe is longer than what is necessary to exhaust the flow after complete expansion, the flow will stack and pressure will build behind it. The larger the pipe diameter, the faster the exhaust gasses will expand to fill the pipe, and the lower it's velocity, since more expansion is perpendicular to flow, and less is parallel. Lower velocity has less potential energy, and thus, will create less pressure drop across the valve.
If you have some piece of information that refutes this, please post it for review.
To keep things clear, I have never suggested that a smaller pipe is universally the answer. I have been suggesting this whole time that pipe diameter should be based on mass of flow, and that larger is not universally better for a given application.
It appears that you're arguing from a performance standpoint primarily, which is likely why you're refusing to see my stance on the subject. When one isn't looking for more potential power than is actually necessary to do the work required, it changes many of the variables in determining the proper size of pipe required for maximum efficiency.
For the third time, there is
alot more to the model than force (pressure) and velocity.
So before you start thinking that I'm arguing with you simply to argue, It needs to be mentioned that I'm not saying that you're wrong at all. I just don't agree that the method you've used for determining what works best
in your situation necessarily applies to what is necessary for the best BSFC at lower RPM.
To take your suggestion to extremes, the best possible configuration is always the larger diameter pipe, which can quickly get out of hand. Ask anyone who's ever over-ported a head.