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Rather than me state my oft express viewpoint about slow
moving fluid and its cooling effectiveness, here are some folks with a world of
experience and knowledge and what they say about it.
Q = M*Cp*Delta T.
There is no doubt that if you move air slower through your
core, you are going to increase its temperature more than faster moving air
through the core (everything else being the same).
However, that does not necessarily mean you are removing
more heat from the total system. Let me try to show the relative effects
of M and Delta T of air flow through a 1 sq ft core
Velocity(ft/sec) Mass
Flow(lbm/min) Delta T(F)
Q(BTU/Min)
10
45.9
50
573.75 (Baseline)
Lets increase delta T to the maximum that I have heard
reported (100F)
10
45.9
100
1147.5
So doubling the delta T doubles the heat rejected - no
surprise there, but this is the largest delta T that I have heard
reported.
Now lets vary mass flow by varying velocity
20
91.8
50
1147.5
40 183.6
25
1147.5
So here, I can reduce the delta T by as much as 75
deg (100-75 = 25) but by modestly increasing the duct velocity from 10 ft/sec
( 6 mph) to 40 ft/sec (27 Mph) I can
reject the same amount of heat at only a 25F increase in air temps.
So my point is that we appear to hit the practical limit
on hear transfer increase by increasing delta T much sooner than we hit a
limit by increasing mass flow.
Now IF you have a very efficient core design and can get a
100 Delta T when the air velocity is 40 ft/sec then you are really cooking as
that would give you
40
183.6
100
4590 BTU/Min
So how do you get maximum heat transfer - I imagine
it is a combination of duct and core design optimized for that particular set of
parameter values - like smaller holes, more fins, wavy fins, thicker core,
etc. But alas those would only be optimum for one set of values and
because we want to cool from sitting on the ground to going 200 MPH,
we must accept a compromise that accommodates both of those regimes of
operation.
Cooling is sort of like life - a continuing series of
compromises {:>)
My 0.02
Ed
----- Original Message -----
Sent: Sunday, September 30, 2007 1:32
PM
Subject: [FlyRotary] Re: Another cooling
question
Snip
That's exactly what I HAD thought, until I was told
that the air could pass through too fast and not pick up as much heat.
Mark... this is what I was trying to communicate. It
could be totally wrong so let's get more opinions.
Ed??
With respect to exit area size only.
If the volume of air through the coolant radiator was
moving faster than optimum the air delta T would be lower
than it would be with slower air. Slower air at a higher delta T means less
air needs to exit the cowl. What is optimum? Every install is
unique so it needs to be viewed \ identified on each
installation. Ed uses a 30% duct air speed as a reference point. If I
understand this correctly the diffuser play a role in duct airspeed. If the
air is not being diffused optimally the airspeed could be much higher than 30%
through the part of the core. Dennis H. and I observed airspeeds through a
test core at 50% + without a diffuser.
Your inlets are 72sqin with 306sqin core face
for water and 24.75 sqin with 102 sqin for oil. Both are
competing for the same exit area. IF this is
an exit area problem only then enlarging it should improve both water and oil
proportionally. Water should realize 2/3 of the improvement and
oil 1/3. 306 sqin vs 102 sqin. Your improvements after opening the
exit seem to track this very close. So increasing your exit area further
should show more improvements in both oil and water to a point. But you do not
need more water cooling improvements right?
Core 306 sqin at an airspeed of
115mph.
40% or 46 mph = 8602 cfm 8602 cfm at a 50 deg
air delta T = 7741 btu's \ min
30% or 34.5 = 6451
cfm 6451 cfm at a 80 deg air delta T
= 7741 btu's \ min
In this example 2151 cfm less air needs to flow through
the water radiator to produce the same btu rejection. So what effect would
2151 cfm less air through the water radiator and exit area have on the oil
cooler's ability to flow more air? Would it improve the oil cooler air flow by
1/3 or 717 cfm? Not sure.
Can your radiator produce a 80 deg air delta T? It may
only produce 60, 70 or??
Am I in left field here?
Bobby
(flow testing new radiator ducts prototype
today)
ED wrote:
<snip>
Mark, if you really had excess air flowing through your radiators
the coolant would drop more than 4 Deg F. In fact, the more air flow the
more coolant Delta T you would drop through the radiator.
<snip>
That's exactly what I HAD thought, until I was told that the air could
pass through too fast and not pick up as much heat. This didn't make
sense to me. Maybe I wasn't listening closely and missed the point
altogether (wouldn't be the first time).
What I DO know is that the air is flowing faster through the water
radiator than the oil radiator. (I'm not sure I have the ASI's hooked up
correctly, but they're both hooked up the same). I have a pitot behind
each radiator hooked up to two separate ASI's. In slow cruise, say
125-130 kts, the water radiator ASI will read about 110knts and the oil ASI
will read about 90 kts. The way it was behaving before I opened up the
exit, it appeared that the air from the water radiator was trying to exit
backwards through the oil inlet. I say this because of how high the
oil temps were reading. I enlarged the cowl exit, and both
the water and oil temps dropped significantly.
The ASI's are referencing the static port for these readings; should they
be referencing cowl or cabin pressure instead? Airspeeds readings seem
awfully high to me.
Mark
(Going to the airport today to recalibrate temp sensors)
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