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  #1  
Old 01-01-2005, 11:33 PM
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Default Mass air flow for cooling

OK, stupid points, but correct me if i am wrong, and i'm sure you will, we have a MAF sensor on our planes, if the IAS stays constant as we increase altitude then the same MAF should be there for cooling. Does the velocity of the air substantially reduce cooling effect?
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Old 01-02-2005, 12:11 AM
DustinD DustinD is offline
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I do not think air velocity effects cooling unless the air is going so slowly that it gets saturated with heat. I think the only thing that matters is the number of air molecules and how much heat each molecule takes with it (air temp rise). I think that at sub sonic speeds faster will always cool better, but if the air is not able to heat up very much do to not having much time in contact with the heat exchanger you will not see a very large bonus do to the higher air velocity. For example, with wind chill, after 40 mph your skin does not add much more heat to the air that is in contact with your skin. Thus you can not feel a difference, and higher wind speeds are not figured into the charts.

Inside your airplane's cowling at altitude you will have a lot fewer air molecules. At a lower pressure the same cubic foot of air will have less heat capacity, but I think it will still heat up at the same speed, assuming the temps are the same.

I do not think dynamic pressure counts for anything other than the small air density increase in contact with the exchanger.

Keep in mind that I am not an expert, and could be wrong about anything I just said.
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Old 01-02-2005, 01:12 AM
Nathan Gifford Nathan Gifford is offline
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I was thinking about this too today Dust. If the MAF is truly measuring mass, then as you increase in altitude the mass of air should decrease also. Since the intercooler depends on transferring the heat to the air medium, less air means less media to transfer the heat to.

As you have observed, the air is considerably colder at higher altitudes. All things being equal, the higher deltaT across the intercooler would mean it would be more efficient.

However, the part that is not equal is the air density itself. Someone with more knowledge than I will have to determine just how great an impact of lower air density will be on heat exchange. I do know that at 100Kft cooling of electronics is mostly by conduction or radiation.
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Old 01-02-2005, 01:22 AM
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ok i get all that yup i do....but what is maf?









and can u show me it in this pic ?
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Old 01-02-2005, 01:23 AM
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Mass Air Flow
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Old 01-02-2005, 01:36 AM
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ty i was going down the turbo path and got stuck on Manifold opening ????
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  #7  
Old 01-02-2005, 01:25 PM
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MAF is the same as indicated air speed, right? They both are an indication of the number of air molecules flowing through an opening.

Don't confuse the two when you refer to MAF as the amount of air flowing into an engine. The engine can be off (no air flow), idle is little air flow and full throttle is alot. All of these can happen while the plane is in a dive or on the runway.
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Old 01-02-2005, 09:23 PM
Nathan Gifford Nathan Gifford is offline
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Quote:
Originally Posted by mplafleur
...
Don't confuse the two when you refer to MAF as the amount of air flowing into an engine...
I know you are an expert on these things, is the MAF sensor on the engine good for any altitude or only down low?
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Old 01-02-2005, 10:57 PM
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Quote:
Originally Posted by Nathan Gifford
If the MAF is truly measuring mass, then as you increase in altitude the mass of air should decrease also. Since the intercooler depends on transferring the heat to the air medium, less air means less media to transfer the heat to.
well, if i see it right, if your IAS (indicated air speed) stays constant with an increase in altitude, then the amount of air for cooling stays the same, although it is traveling faster.

if you put a MAF (mass air flow) sensor at the naca duct, it would read the same
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Old 01-02-2005, 11:31 PM
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Default Mass airflow/ intercooler

Mass airflow available for you rad or intercooler at altitude is a function of TAS, temperature and density. At 18,000 feet or so you will have a little more than half your SL mass flow available. TAS increases a bit with higher altitude and temperature also decreases with altitude so these help offset this tendency a bit. Climbing on a hot day and high altitude will test your heat exchanger effectiveness the most of all conditions.
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Old 01-02-2005, 11:44 PM
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Quote:
Originally Posted by Nathan Gifford
I know you are an expert on these things, is the MAF sensor on the engine good for any altitude or only down low?
Oh, it's good everywhere. It is used to tell the engine strategy the MASS of the air coming into the engine. The strategy is trying to put enough fuel to get to a certain air/fuel ratio. The A/F ratio is based on mass, and the mass of fuel is known (adjusted for temperature), so it's an easy calculation to figure out how much fuel you need to squirt in to get the right mixture.

The MAF sensor return a voltage that is proportional to the airflow. For example, the range for a typical sensor is 0-750 Kg/hr for a 0-5V output. Keep in mind that the operating temperature range may be about -40C to 120C.
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Old 01-02-2005, 11:47 PM
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Quote:
Originally Posted by rv6ejguy
Mass airflow available for you rad or intercooler at altitude is a function of TAS, temperature and density.
Understand, but if i climb at 120 IAS and hold 120 IAS to 25000 feet, then i have the same mass flowing through the intercooler at all altitudes, correct?, except for the effect of humidity, minor, and temperature, major.
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  #13  
Old 01-03-2005, 12:13 AM
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Quote:
Originally Posted by Dust
Understand, but if i climb at 120 IAS and hold 120 IAS to 25000 feet, then i have the same mass flowing through the intercooler at all altitudes, correct?, except for the effect of humidity, minor, and temperature, major.
Yes..
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  #14  
Old 01-03-2005, 12:51 PM
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Default Mass flow vs. altitude

IAS is the difference between dynamic and static pressure. Mass flow is not the same. TAS is roughly 180 at 120 IAS at 25,000 feet but air density is only about 11 inches Ab. Mass flow is therefore about 55% that of sea level excluding temperature. Your climb rate at 120 IAS at 25,000 would be very low.
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Old 01-03-2005, 02:55 PM
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OK, you did the computation for IAS to TAS, the same goes for the air volume, 11 AB + 50% 5.5 = 16.5 which is ambient sea level pressure. puts you back to the same Volume of air going through the intercooler over the same amount of time, but the velocity has changed.

does the velocity decrease cooling efficiency?

We know the decrease in ambient temp at altitude increases efficiency
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