Radar System Engineering

422 R-F COMPONENTS [SEC. 1111
The increase in the coefficient is due mainly to creating a turbulent
layer where good mixing occurs close to the actual wall, and to reducing
the thickness of the poorly conducting layer. The cooling blast for a
magnetron approximates the necessary conditions, but maintenance of
turbulent flow over large areas takes considerable power.
The conclusion is that with moderate blowing on the inside and
natural convection outside, the loading can increase to 0.17 wat ts/inZ,
and with blowing on both sides can increase to 0.3 watt siin2, for an over-
, Blowers
T 1
I
I I I I
(a)
(b)
FIG.11 .26. —Methods of improving heat transfer through pressurized containers.
all rise of 30”C in air temperature. These figures are all approximate,
since the geometry is never simple and uniform conditions of air flow
over a whole vessel are never realized.
Radiation plays a minor role in heat transfer ~or the differentials here
involved. Painted rather than bright surfaces are worth while, however.
Fins are helpful in increasing effective area, especially with natural convection,
though not in proportion to actual surface. If the forced convection
is not along the fins they do not help at all, because the increased
area is compensated by impediment to air flow.
Two schemes to improve heat transfer are shown in Fig. 11.26. In
a, the internal and external blowers are directed at the same portion of
the wall, which has some fins at that point at least to increase effective
area. In the region of turbulent flow directly under the blower output
the transfer coefficient is high. In the second case, b, there is an outer
shell close to the wall of the pressure container. A powerful blower
maintains turbulent flow over the whole interspace and a high coefficient
results. This may be repeated on the inner wall of the pressure housing.