OK this has been bothering me off and on for years and I never did anything to
try to calculate the Cubic Feet per Minute (CFM) of a stock AH100 radiator fan.
In light of the recent discussion of air flow in the engine compartment
(Some text borrowed from the net)
According to the general fan law for axial fans, you will need to know the fan
blade diameter and pitch (blade tilt or attack angle) along with the rpm to
calculate CFM. An Austin Healey 100 radiator fan has a four blades with a 15
inch (1.25 foot) diameter and an 4-inch effective pitch (one inch pitch times
four blades). This means that each revolution of the running fan blows the 1.25
foot diameter column of air coming passed the fan 4 inches. There are
efficiency losses but we are talking generalities here . For the two ends of
the spectrum the engine is idling at 700 rpm or powering along the road at 3000
rpm. The pulley setup on the AH100 is nearly the same diameter on the water
pump and crank, so the fan rpm is approximately the same as engine rpm.
Calculate the linear velocity of the air through the running fan. If each
revolution moves the air 4 inches, then 700-revolutions per minute multiplied
by 4 means the air is being moved at 2800 inches per minute, or 233 feet in one
minute. For 3000 rpm engine speed, the answer is 12000 inches or 1000 feet.
Calculate the CFM (volumetric flow of air) at 700 rpm. The volume of the column
of air described above is pi (3.1416) x fan radius squared (7.5 inches or 0.625
feet squared) times the column length in feet. This would be 3.1416 x 0.390525
square feet x 233 feet = 285 cubic feet per minute at 700 rpm. At 3000 rpm the
CFM is 1226.
I knew that the stock fans do not move much air but my calculation says it
really is horrendous. Suspect my math is off somewhere. Math teachers and
mechanical engineers and anybody else for that matter, Comments please.
Perry
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