Benoulli.

Anything flowbench related

Benoulli.

Postby Chad Speier » Fri May 27, 2011 8:01 am

For some reason on here this man's concepts aren't very common yet they impact what we do greatly..... so THESE ARE THE LAWS OF PHYSICS WHICH ARE NOT QUESTIONABLE... smart people figured these out centuries ago:

First... air is a COMPRESSABLE fluid, oil and water are INCOMPRESSABLE. If we don't start with that statement people get lost down the line

Second... the principle of conservation of energy requires that the sum of kinetic energy and potential energy remain constant. Energy can't be created just transformed.

Third.... fluids only move from a area of high pressure to a area of low pressure. Period. The best example we can relate to is a piston, as it moves down a bore it is creating a lower pressure than that of what is in the port above it so the air fills the lower pressure void until it's pressure is higher than that of what is filling it. FWIW the pressure increases in the cylinder because the mass of air increases and the volume to store that mass stops increasing in size. Basically it's filled up at that pressure level.

Fourth.... a orifice can only flow so much volume thru it at a set pressure drop. Think of a orifice plate used to calibrate a flow bench. It is set at a certain diameter so that at a given depression 28" of H2O it will move a set CFM level, then you adjust the bench to read what it should.

Pressure and Temp...

Higher pressure = Higher temp...... Lower pressure = Lower temp

Think about it this way, why do we need an intercooler on a supercharged engine? Because the air is being compressed increasing the pressure which is putting energy INTO the air (since it's a compressable fluid) which causes the air molecules together and they make HEAT. So we need to cool that higher pressure air down before we put it in the motor with a intercooler.

Same applies to your compressed air tank, when you put the air gun on the line and use it to dry or clean off parts... it's colder NOT BECAUSE of evaporation (you can actually measure this with a temp gun) but because of the expansion of the air. Conversely when you compress the air into the air tank the tank gets warmer again something you can check with a temp gun.

Velocity and Area...

Area effects the speed of the air, the larger the area the slower that same volume of air passing thru it will be. I know there are some guys who don't get this because I have seen the crap they grind on, but when you need the velocity in a cylinder head to decrease you increase the area at that point. Which leads into....

Pressure and Velocity...

The faster the velocity the lower the pressure. The hood scoop or carb venturi on a race car is a perfect example of this...

A carb only draws fuel into the air stream because as the area decreases (going into the venturi), the speed increases and the pressure drops. Then it expands again due to the larger area which increases the pressure and slows the velocity.

In terms of the hood scoop the set area only allows a set volume into the scoop. (4th rule above) The air exits the scoop orifice and slows down into the larger area of the scoop the pressure INCREASES due to the conservation of energy. You are turning the kinetic energy of the velocity into the potential energy of PRESSURE. As started above in the 3rd rule... the more pressure you have above the valve the more you can put in the cylinder... hence why a good hood scoop will increase the speed/HP of the car going down the track.

So.....

Higher Pressure = Bigger Area = Lower Velocity = Lower Temp
Lower Pressure = Smaller Area = Higher Velocity = Higher Temp



THANKS BRET!
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Postby SStrokerAce » Tue May 31, 2011 9:53 pm

LOL, you like that one? I have heard enough about what people don't get... that should be plain enough so that some guys can understand.

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Postby blaktopr » Sat Jun 18, 2011 4:19 pm

I don't get it :D
Wow that thread got a little hairy with what is and what isn't.
Good Sticky.
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Re: Benoulli.

Postby 358T » Sun Jul 31, 2011 9:39 am

Chad Speier wrote:
Fourth.... a orifice can only flow so much volume thru it at a set pressure drop. Think of a orifice plate used to calibrate a flow bench. It is set at a certain diameter so that at a given depression 28" of H2O it will move a set CFM level, then you adjust the bench to read what it should.



I think the statement above is a little misleading. Flow through an oriface is also effected by the shape of the entrance, shape of the exit, general shape/texture and length. These variables set up the flow coefficient. An oriface with a higher flow coefficient will flow more at the same pressure.
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Re: Benoulli.

Postby Chad Speier » Sun Jul 31, 2011 10:16 am

358T wrote:
Chad Speier wrote:
Fourth.... a orifice can only flow so much volume thru it at a set pressure drop. Think of a orifice plate used to calibrate a flow bench. It is set at a certain diameter so that at a given depression 28" of H2O it will move a set CFM level, then you adjust the bench to read what it should.



I think the statement above is a little misleading. Flow through an oriface is also effected by the shape of the entrance, shape of the exit, general shape/texture and length. These variables set up the flow coefficient. An oriface with a higher flow coefficient will flow more at the same pressure.



While all of this is true, there is still a certain amount of "given" when it comes to flow bench measuring/calibrating/using..

A "standard" calibration plate is around .125 thick and a machined sharp edge. The "standard" coefficient for such hole is .62.

The "standard" formula to figure cfm through said plate is: (13.29 X hole D²) x (SQRT test pressure)

In other words what I'm saying is if we all used the same type plates, things would be more even.
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Re: Benoulli.

Postby SStrokerAce » Sun Jul 31, 2011 11:59 am

358T wrote:
Chad Speier wrote:
Fourth.... a orifice can only flow so much volume thru it at a set pressure drop. Think of a orifice plate used to calibrate a flow bench. It is set at a certain diameter so that at a given depression 28" of H2O it will move a set CFM level, then you adjust the bench to read what it should.



I think the statement above is a little misleading. Flow through an oriface is also effected by the shape of the entrance, shape of the exit, general shape/texture and length. These variables set up the flow coefficient. An oriface with a higher flow coefficient will flow more at the same pressure.


Correct... and Chad is correct as well, there is a coef for each type of orifice... but relating to cylinder heads we use the MAXIMUM 146 cf/sq in standard at 28"... but my point is for this topic that the cylinder head dynamically will flow a certain amount depending on both the lift and the depression. If the demand for the head at a certain lift is less than what it flows at 28", it will flow at a lower depression... and if it has more demand than what it flows at that lift, it will flow at a higher depression. Pumping losses and valve jobs are keys too that...

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Re: Benoulli.

Postby 358T » Mon Aug 01, 2011 12:37 pm

Thanks for the explanation guys.
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Re: Benoulli.

Postby Blue Oval Ralph » Mon Aug 06, 2012 7:13 am

What happens if the air goes sonic through the restictor will it flow more air? Have been told by some good engineers both ways it will and it won't
Thanks


Chad Speier wrote:
358T wrote:
Chad Speier wrote:
Fourth.... a orifice can only flow so much volume thru it at a set pressure drop. Think of a orifice plate used to calibrate a flow bench. It is set at a certain diameter so that at a given depression 28" of H2O it will move a set CFM level, then you adjust the bench to read what it should.



I think the statement above is a little misleading. Flow through an oriface is also effected by the shape of the entrance, shape of the exit, general shape/texture and length. These variables set up the flow coefficient. An oriface with a higher flow coefficient will flow more at the same pressure.



While all of this is true, there is still a certain amount of "given" when it comes to flow bench measuring/calibrating/using..

A "standard" calibration plate is around .125 thick and a machined sharp edge. The "standard" coefficient for such hole is .62.

The "standard" formula to figure cfm through said plate is: (13.29 X hole D²) x (SQRT test pressure)

In other words what I'm saying is if we all used the same type plates, things would be more even.
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Re: Benoulli.

Postby Chad Speier » Mon Aug 06, 2012 7:34 am

Blue Oval Ralph wrote:What happens if the air goes sonic through the restictor will it flow more air? Have been told by some good engineers both ways it will and it won't
Thanks




This is what I think happens. At the .55 mach the pumping losses reach a point where the energy used is more than the energy gained from the inertia ram effect. In other words, the engines converting pressure drop into velocity and keeps doing so in an increasing manner until 690ft/sec. At that point the conversion of energy into air speed and inertia is more than the return one gets from the inertia. This does not mean the flow stops. It means the power increases stop and both TQ and HP drop like a rock. All your doing past 690ft/sec is stretching air molecules out and getting nothing in return (inertia ram effect) as far as cylinder fill.
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Re: Benoulli.

Postby jmarkaudio » Mon Aug 06, 2012 7:47 am

It also depends on whether you are drawing air in naturally or compressing they air, like using a blower. The shape of the port is still a factor on forced induction but I believe will limit it less than a N/A engine.
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Re: Benoulli.

Postby SStrokerAce » Wed Aug 22, 2012 11:28 pm

Yeap Chad....

Remember in a boosted application the air density is higher. So the problems of higher MACH play just as much, in fact more.
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Re: Benoulli.

Postby boneil » Tue Nov 27, 2012 5:46 pm

I would beg to differ, liquids are compressible. Though for all practible purposes it should be considered not compressible. Water molecules are tightly knitt, but not crammed together so that they cannot move, thus it still can be compressed, but the measurable amount would be negligible by comparsion to matter in a gaseous state. The pressure that would be needed (assuming the vessel you house the liquid in is strong enough) to compress water far exceeds any internal combustion engine. Thus hydraulic locking of an engine that has ingested water and bent a rod in the process.
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Re: Benoulli.

Postby Greenlight » Wed Nov 28, 2012 12:20 am

While most (if not all) flow benches have a predetermined discharge coefficient for each orifice, in reality the flow through the orifice is determined by the density of the fluid (moist air, which is likely different from one day to the next), the compressibility of the air in the orifice, and the Reynolds number (unitless dimension which is the ratio of the fluid's inertial force and the viscous force), the orifice area, and the pressure differential on each side of the orifice.

If you flowed a head on Darin's (Arlington, TX ~ 400 ft altitude) or Meaux's flow bench (Abbeville, LA ~ 20 ft altitude), then shipped the flow bench with the head still attached to Denver (~5000 ft altitude) and flowed the head again, you would get a considerable difference in CFM.

You stated something like, "for a given pressure differential an orifice will have a given flow". That's not exactly true.

Imagine if the air pressure on one side of the orifice is 2 psi and is 1 psi on the other side (~28" H2O). Now compress the air on the inlet side to 1000 psi with 999 psi on the other side (again 1 psi differential). The air is much more dense in this situation and the volume flow rate will be less (maybe 10 times less). Of course this is a grossly exaggerated example.

The discharge coefficient actually changes as the air speed through the orifice changes. That is the main reason that Super Flow wants you to perform test with the inclined manometer above 40% or so. The discharge coefficient line becomes "flater" at high velocities, but the compressibility increases which leads to a different measurement error.

Flow a port on "scale 5" at 20% on the inclined manometer, then flow it on "scale 3" (60%? 70% ??). The CFM difference will be much different. I'm sure you already know this.

To obtain a really accurate reading you should use the equation and graphs that I have shown here:
Image

and you should use this standard to calculate the moist air density.
http://iopscience.iop.org/0026-1394/45/2/004/

The formula you stated is only an approximation of the flow through an orifice.

You saw the difference in flow when the PTS guys did the "pass around plate" tests on about 20 different flow benches. The results were a real eye opener with as much as a 30 cfm difference on the exact same calibration plate.

Using the more precise method is time consuming and therefore the "Super Flow" method is somewhat of an acceptable compromise. It is an excellent tool for each individual head porter to compare flow rates within the confines of his/her own shop, not head porter "A" to head porter "B".
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Re: Benoulli.

Postby Chad Speier » Sat Dec 08, 2012 10:21 pm

If you flowed a head on Darin's (Arlington, TX ~ 400 ft altitude) or Meaux's flow bench (Abbeville, LA ~ 20 ft altitude), then shipped the flow bench with the head still attached to Denver (~5000 ft altitude) and flowed the head again, you would get a considerable difference in CFM.


Personally, I find this to be myth. I have done it several times and on a properly calibrated orifice bench, there was zero difference..

Flow a port on "scale 5" at 20% on the inclined manometer, then flow it on "scale 3" (60%? 70% ??). The CFM difference will be much different. I'm sure you already know this.


I find this not true either if it's calibrated properly. I fix the calibration of that orifice. This is why you need a set of plates that closely resemble the pressure drop across the hole you are calibrating. The SF bench sucks because of this.

You stated something like, "for a given pressure differential an orifice will have a given flow". That's not exactly true.


I think what was intended is a hole that is "X" in size can only flow so much air at a given pressure differential.
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Re: Benoulli.

Postby Greenlight » Mon Dec 10, 2012 1:09 am

Chad,

Here is an example of how the location (atmospheric conditions) effects an orifice style flow bench and how using the "standard" formula is not the best method for comparing flow numbers from head porters in different locations.


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