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National Advisory Committee for Aeronautics, Technical Notes - Comparison of Several Methods of Predicting the Pressure Loss at Altitude Across a Baffled Aircraft Engine Cylinder

Several methods of predicting the compressible-flow pressure
loss across a baffled aircraft-engine cylinder were analytically
related and were experimentally investigated on a typical airecooled
aircraft—engine cylinder. Tests with and without heat transfer
covered a wide range of cooling-air flows and simulated altitudes
from sea level to 40, 000 feet ” M‘—

Both the analysis and the test results showed that the method
based on the density determined by the static pressure and the stag-
nation temperature at the baffle exit gave results comparable with
those obtained from methods derived by one-dimensional flow theory.
The method based on a characteristic Mach number, although related
analytically to one-dimensional flow theory, was found impractical
in the present tests because of the difficulty encountered in
defining the proper characteristic state of the cooling air.

Although the cylinder-baffle resistance coefficient determined
by the density method was consistent for a wide range of heat-transfer
values, a distinct difference was observed between the values with
and without heat transfer that could not be explained by one-
dimensional flow theory. Accurate predictions_ of altitude pressure
loss can apparently be made by these methods provided that they are
based on the results of seaelevel tests with heat transfer.

The high operating altitudes of both military and commercial
aircraft have greatly increased the severity of the engine air-cooling
problem” The decrease of the density of the air with increased alti—'
tude necessitates the handling of a greater volume of_air at higher
velocities and, as a result, the flow of cooling~air withinwthe fin
passages attains high Mach numbers_and a large decrease in the cooling-
air density occurs across the engine. The pressure loss increases 1°
with Mach number and consequently a greater pressure drop is needed
to force a given weight of cooling air across the engine at high '
altitudes than would be required for the same weight flow of air at
lower altitudes.