naca-tn-2456
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National Advisory Committee for Aeronautics, Technical Notes - Analytical Method for Determining Performance of Turbojet Engine Tail Pipe Heat Exchangers
The performance of parallel—flow—type tail-pipe heat exchangers is
analytically investigated. An equation is developed relating the output
of unfinned heat exchangers to the inlet— and outlet-air temperatures,
engine- temperature ratio, corrected engine gas flow per unit of tail—
pipe area, and heat-exchanger dinwnsions.
The calculated performance of heat exchangers, based on assumed
characteristics of a hypothetical engine (rated compressor pressure
ratio, 4 and rated engine—temperature ratio, 3. 4), are presented in the
form of generalized working charts for heat-exchanger outlet air tem—
peratures of 7000 to 11000 R and flight conditions representative of
climb and cruise operation. The use of these working charts to predict
the performance of heat exchangers installed on engines having higher
compressor—pressure ratios will give results that are optimistic. The
method of calculating heat-exchanger performance as set forth in this
analysis, however, is still applicable.
The use of the generalized working charts to predict the performs
ance of unfinned heat exchangers is explained and the consideration of
such factors as pressure drop through the air side of the heat
exchanger and effect on engine performance is discussed. It is shown
that, in general, the extraction of heat from the tail-pipe gas by
means of a ramroperated unfinned tail-pipe heat exchanger has only a
slight effect on engine performance. The addition of longitudinal fins
to the air side of the heat exchanger in order to increase the heat out-
put per unit of heat exchanger length is also considered.
A comparison cf the performances of parallelaflow-type finned and
unfinned tail-pipe heat exchangers on the basis of equal pressure drop
through the air side at high heat outputs indicates that the unfinned
heat exchanger because of its lighter weight and simpler construction
is probably preferable to the longitudinally finned heat exchanger.
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