Environmental Engineering Reference
In-Depth Information
Using this method it was difficult to create a constant air flow
long enough to make reliable measurements, but la Cour now had
suitable equipment. In 1892, he invented the Kratostate, a device
which could smooth out the changing rotation of the mill to a
constant rotation and driving force on a dynamo. Now it could be
used to deliver a very constant air flow through the wind tunnel.
Figure 4.8
“Keglevindfang” on the right with twelve sails (left); The conical
wind catcher rotor as a roof mill with only six sails (right).
The first model Sørensen designed had twelve sails. La Cour
found almost immediately that performance improved when some
of the sails were removed. That was a surprise because the most
common opinion at that time—even among millwrights—was that
a mill should have as many sails as possible. This was supported
by the hitherto accepted scientific formula
L
= 0.0338
F
v
3
where
L
is power,
F
the sum of the frontal area of the sails, and
v
the velocity of the incoming wind.
The formula says that if the total front area of the sails is
increased, then the power output will increase proportionally.
La Cour proved that this was totally wrong. Only the term for the
cube of the wind velocity is still valid from this formula.
La Cour presented his findings in a paper “Experiments
with small mill models” at a meeting in the Danish Engineering
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