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> then your air velocity through your cooler will be
> > approx 0.1 - 0.3
> > of your airspeed. So for a 200 MPH speed you might have 20 -60 mph air
> > velocity through your cooler fins not 200. Just though I would
> > mention it.
>
> Hi Ed;
> Do you really think that it will be this fast? Even say through the Evap
> cores? The reason I ask is that I've just installed a fan on the back side
> of one of my evap cores to be used in conjunction with one of my EWP's to
> reject cowl/engine heat after shutdown. I assumed/guessed that the airflow
> exiting through the evap core will be slowed down to <5mph. I don't have
any
> data to support this, it just seemed reasonable.
> I didn't add a shroud around the fan as I didn't want to hurt in-flight
> cooling by forcing all of the air through the fan blades, even though this
> will make the fan less efficient. But I think if it does see up to 60 mph
> airflow then it's lifespan will indeed be short.
>
> S. Todd Bartrim (completing panel modification/upgrade)
> Turbo 13B RV-9Endurance
> C-FSTB
> http://www3.telus.net/haywire/RV-9/C-FSTB.htm
>
>
Todd, I seriously doubt the air velocity would be slowed down to 5 mph even
if you could, in theory, do this. If you hold you hand behind your core
with prop running at idle, you can tell you already have 2 -3 mph of
airflow. Besides consider this:
If you airflow were 5 MPH = 5280*5/60 = 440 ft/min = 7.33 ft/sec velocity
of the air
With a core area of approx. 95 sq. inch (0.65972 sq. ft) that would give
you 7.33*0.65972 = 4.838 cubic feet of airflow/sec through the core. A
cubic foot of air has approx. 0.076 lbm/ft^3. So 4.838*.076 = 0.367
lbm/sec of air mass flow through your core.
A mass flow of 0.367 lbm/sec would dissipate approx. 95 BTU/Minute
(according to my calculations) whereas to cool at say 120 HP, your rotary
oil cooler would need to get rid of approx. 1850 BTU/Minute. So clearly
you would need considerably more cooling than 5 mph air mass flow through
the oil cooler would give you. In fact, working it out with no diffuser,
you would need close to 100 mph of air flow to provide that cooling.
However, by using a diffuser to increase the pressure in front of the core,
you can increase your core/air heat transfer coefficient which in turn will
provide more heat transfer from the core to a cubic inch of air which in
turn would permit less air mass flow which means you really don't need (nor
want) 100MPH through your core.
My understanding of how the trade off of air velocity for increase pressure
works is through two factors
1. Slower air flow produces less core cooling drag (that's good)
2. Higher pressure (assuming constant air density in this case) means the
air molecules are on the average traveling at a higher velocity/speed. In
fact pressure is the manifestation of average air molecule velocity. In
fact without increasing air density, higher pressure only comes with an
increase in average molecular velocity. This increase can be induced by
temperature increase or in this case by the kinetic energy (1/2pV^2) of the
air stream into the diffuser turning into increased pressure.
The higher pressure (higher average molecular speed) means that the air
molecule makes more contact with other air molecules (and the hot metal
walls of the core) per unit time. This increase frequency of contact per
unit time means that the air molecule absorbs/transfers more heat per unit
time. That means the same cubic inch of air can have more heat transferred
to in per unit time than air at a lesser pressure where the molecules move
slower.
Since its the air molecule that carries away the heat, ultimately, its how
many air molecules make contact with the core walls (and each other) per
unit time. Increased airflow can provide this (but that also brings with it
increased cooling drag) or increased air pressure (resulting from lowering
the velocity of the intake air through the diffuser) which not only
increases heat transfer but lowers cooling drag due to the lesser velocity
of the air through the core.
So generally increasing pressure and lowering air velocity through a core is
better for cooling and drag, but its a compromise. Lower the air velocity
too much and you simply do not have enough air mass (at any pressure
obtainable with a diffuser) to carry away sufficient heat.
There! more than anyone wanted to know
Ed Anderson
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