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National Advisory Committee for Aeronautics, Technical Notes - The Effect of Air Passage Length on the Optimum Fin Spacing for Maximum Cooling

The effect on cooling of baffle length with optimum
cylinder finning is discussed. Results from tests of
several streamlined cylinders are given. It 1s shown that
by employing several baffles the cooling can be increased
several times.

The problem of cooling a finned engine_ cylind§r_may
be reduced to a determination of the cylinder shape, the
fin depth, the fin spacing, the fin thickness, and the
baffle arrangement that will dissipate the most heat from
the cylinder for a given available pressure drop Ap. A
study of the available Ap has been made (reference 1)
and the effect of the various elements of the baffle has
been studied (reference 2). The effects of fin spacing
and fin thickness have been given for one baffle length
(reference 3), and the effect of fin width has been re—
ported (reference 4). There remain, then, three unex—
ploréd possibilities for the improvement of cylinder cool—
ing: (1) the shape of the cylinder, (22 the length of-
the baffle, and (3) the lowering of the heat transfer from
the hot gases inside the cylinder to the cylinder wall by
suitable changes in the cylinder design, The_ first two
possibilities will be considered in this note.

The power dissipated in cooling a cylinder is QAp,
whore Q is the volume of air passing through the fins
per unit time, and Ap is the pressure drop across the
cylinder—baffle arrangement. This power, with well—de—
signed cylinder finning and baffling, amounts to the or—
der of 1 percent of the horsepower of the engine. This
power for cooling obviously does not include the power
consumed in overcoming the profile drag of the cowled en—
gine, which power must be considered when any camparison
of various types of engine is made. The poWer that an
engine develops is often limited by the cooling provided.
When such a condition exists, the power required for cool—
ing is of very small consideration. fSince this power is
relatively small in any case where’Eood cylinder and baf—
fle design is used, the solution of the cooling problem
involves the use of more power.for cooling. The real
cooling power is the product of_the volume of air passing
between the fins and the actual pressure drop of the air
passing between the fine. The work reported in reference
B'was concerned with making this power as large as possi—
ble with a given over—all drop in pressure across the cyl-
inder—baffle arrangement by variations in the baffle ar—
rangement.