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National Advisory Committee for Aeronautics, Technical Notes - An Investigation of Aircraft Heaters - XXXIII - Experimental Determination of Thermal and Hydrodynamical Behavior of Air Flowing Along Finned Plates

An experimental investigation has been performed on the thermal
and hydrodynamical behavior of air flowing along a steam—heated plate
to which were attached cylindrical fins 5/16 inch in diameter
and 5/8 inch long. Local values of unit thermal conductance per row
of fins were obtained for air flowing in a "variable-width" rectangular
duct (variation of hydraulic diameter). In this duct Were placed for
the various tests a plate having metallic fins, one having nonmetallic
fins, and also a plate without fins. The range of Reynolds modulus was
from 67,800 to 418,000, based on the hydraulic diameter of the duct.
The results obtained with all three plates are compared and the effects
of "finning" and of the added turbulence along the unfinned portions
of the plate due to the presence of the fins are shown. The effect
of duct width on heat transfer is also given. The results of temperature
and pressure traverses across the duct section normal to the main-stream
flow are presented for one duct width and one weight rate. A method of
predicting the performance of a pin-finned plate is presented and
compared with experimental results.

Although the system presented here consisted of cylindrical pin
fins the method of analysis may be extended to systems with other types
of fins.

Very few data on cylindrical or pin fins are available for the _
design of heat exchangers. At present it is necessary for the designer
to postulate a composite system based on separate data for flow over
tube banks, or cylinders, and a flat plate (reference 1). This proce-
dure neglects one of the most important items, namely, the added heat
transfer due to the turbulence induced by the fins on the air adjacent
to the unfinned portion of the plate. Consequently, data concerning
the effect of pin fins upon the heat-transfer characteristics of a flat
plate were obtained. Because this effect is composed of two parts,

(1) the effect of an extended surface and (2) the turbulence-promoting
effect of the rows of fins, the tests were carried out with steel fins
which would produce both effects, with wooden fins which would produce
only the second effect, and with an unfinned "flat" plate which would
produce neither effect. By conmaring the performance of the steel-
pin plate with that of the unfinned flat plate, the over-all effect
of "firming" can be determined. Comparison of the performance of the
wooden—pin plate with that of the unfinned flat plate shows the effect,
on the unfinned portion of the plate, of the turbulence induced by fins
because very little heat (about 10 percent) is transferred by the wooden
fins. The effect of thermal conductivity in the fins is obtained by
comparison of the performance of the steel-pin plate with that of the
wooden—pin plate.