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fin
Air Cooling A
By DR. M. MARK
Fig. 1: Heat transfer vs.
coolant rate of flow and
pressure drop necessary
to move coolant
Fig. 2: Dummy magnetron
and typical cooling
fin assembly
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COOLING FIN
is,MAX-/
THE increase in output of the
magnetron and the continued
trend toward higher frequencies
have resulted in greater heat density
in the tube, emphasizing the
importance of cooling in its design.
Part temperatures consistent with
required performance and reliability
must be maintained; for airborne
equipment the objective is to
achieve this with a cooling system
imposing a minimum weight and
power penalty on the aircraft.
Free convection and radiation
can prevent unacceptable temperature
rises in magnetrons for very
moderate heat dissipations. However,
there must be sufficient space
surrounding the component surfaces
to allow for the development of the
fluid velocities associated with free
convection. Where reduced volume
or increased heat dissipation of the
magnetron results in excessive part
temperatures with only free con-
Fig. 3: Original fin
ANODE
BLOCK
X
OUTPUT
PIPE
IBN
vection for cooling, or for high altitude
operation where free convection
drops off rapidly, forced convection
becomes a necessary alternative.
In a forced convection system an
efficient set of cooling fins, used
with air as the coolant, can reduce
the amount of horsepower needed
for cooling. This article illustrates
the substantial gains made in reducing
cooling power for á particular
magnetron by improved fin design.
Forced Convection Cooling
The heat transfer process for
forced convection is a function of
the mass rate of flow of the coolant,
the flow regime, the coolant properties,
the geometry of the flow passage,
and the temperature difference
between the coolant and the
walls of the flow passage. Once the
coolant is picked, the only variable
becomes the flow passage dimensions,
which are a function of the
particular cooling system, the mass
rate of flow, and the temperature
difference mentioned.
Airborne installations require a
cooling system using minimum
power. Therefore, for a given coolant
density, the product of the coolant
mass rate of flow (W in lbs./
min.) and the pressure drop necessary
to move the coolant through
the cooling system (.p in psi)
should be a minimum. Where the
Ap available is fixed, minimizing
power means minimizing W; using
any less than the :gyp available is
inefficient, since the difference between
that required and that available
will be wasted in an orifice or
its equivalent.
The simplest means of employing
forced convection is by channeling
the fluid directly over the electronic
part, in which case the prime surface
area for heat transfer is fixed
by the given component. A more
complex and often more effective
cooling system involves a heat exchanger
to which the component is
I6 60
14 !, 70
a
12 i60
3
10 i 50
4 s 0
6 u 30
4 = 20
TEMP ERATURE RISE
PRESSURE DROP
2 10 _
10 20 30 40 50 60
AIR n061,CU.FT/MI11, DEMSITT0T226
6/CU. FT.
Fig. 4: Heat transfer and pressure drop
characteristics for original fin assembly
Fig. 5: II) Fin with deflector insert
and I 1 with streamlined exit
100
Tele -Tech & ELECTRONIC INDUSTRIES June 1956