I've been busy with Family vacation, dealing with the exceptional wet weather in
central Texas and my tennis playing but finally I have some more
thoughts on radiators and cooling. My cooling is marginal for Texas in
the summer. I want to climb at 120 kts and 26 + inches MP on a 100 deg
F day without exceeding 215 on water and oil.
I have the Griffin radiator (core 19 X 13 X 2.5 inches) and stock RX-7
'89 oil cooler as shown on pictures I have previously posted. The
radiators are mounted under the engine at about a 30 deg. angle. My
latest test flight with OAT of 92 deg F on the ground was encouraging.
I had temp. probes on the outlet side of the oil and water radiators
to measure the temp. of the heated air. The temp. probes had an upper
limit of 160 deg. F. The air exiting the water radiator exceeded the
160 Deg. limit soon after take-off. I estimate the air temperature
rise through the water radiator was at least 80-90 deg. Cooling water
temp. never exceeded 210 deg. F.
The air exiting the oil radiator was at 135 - 140 deg. F. (A delta T of
about 40 - 45 deg F.) Oil temperature rose to 213 deg. F. max and at
cruse 24 in. MP, 160 mph at 5500 feet the oil temp. decreased to 210 deg. F.
I'm close to ideal cooling but I've been surprised how little effect my air
flow modifications have have had on overall oil and water cooling. After
studying K&W Chapter 12 some more I've decided I mounted my cooling radiators
incorrectly!! As mentioned above, the radiators are below the engine at about
a 30 Deg angle (alpha = 60 deg.) to the incoming air stream. The tanks are
orientated fore and aft. This positions the fins across the air stream.
Ch. 12.2 of K & W Fig. 12.6 shows a radiator block at an oblique angle (alpha)
to the incoming air. The tubes are at the angle alpha to the flow. In the
K & W analysis the tubes are slightly aerodynamic in shape they turn the flow
as it enters the radiator fins. In the radiators I am using the tubes are
separated about 1/2 inch. My fins are separated by about 0.080 inch. Because
I mounted my radiator with the tanks fore and aft, the fins are at the angle
alpha to the flow and the fins turn the air. The fins are very sharp thin metal
and I believe air flow separation and turbulence is occurring at the leading
edge of each fin. Because the fins are very close together the flow is restricted
through the entire radiator surface. I believe the separated, turbulent flow at
the leading edge of the fins limits the amount of air flowing through the
radiator regardless of how "good" the diffusers are ahead of the radiators.
If I have to do it over, I will defiantly mount my radiators with the tanks on the left
and right side of the incoming air so that the tubes turn the air through alpha - not
the fins!!
Any comments - Am I out to lunch on this one?
PS. The end of the first paragraph in Ch. 12.2. states "We shall consider first the
simple case of parallel inflow at an angle alpha to the tubes, as shown in Fig. 12.6"
I have not found a consideration in Chapter 12 of the case of the fins being at
an angle alpha.
Dennis Haverlah
|