Just got back from a 2 day conference/class on
automotive heat exchangers given by the SAE, with a
fellow named Ram Shah as the instructor (if you're a
Mech E, you have probably used one of his 25 books in
school, and he works at Harrison, which does many many
of the radiators, condensers, evaps, oil coolers for
all cars). After 2 days of mind numbing equations, we
got down to some folksy questions, and the topic
ofvertical vs. horizontal flow came up,as did "sucking
vs. blowing" for cooling fans. All things being equal
(core size, and available airflow), there is no
difference. The reason a lot of manufacturers went to
horizontal was because of the reduction in vehicle
frontal height in modern cars, which require lower
profile radiators. Also, if the manufacturer wants to
increase the size of the core, a horizontal flow HX
allows for the same headers and collectors, the only
difference being longer tubes (or plates). In a
downflow radiator, it is unlikely that the height
could be increased, so more columns of tubes would be
required, increasing manufacturing cost. In the auto
industry, Shah said they start to worry if cost goes
up for something by more than 3 or 4 cents.
For fan position, it is better to have the fan behind
the radiator ("sucking"), with an inlet shroud
(important). If the fan is in front of the heat
exchanger, the flow pattern across the HX surface will
be less uniform, with the velocity pressure
concentration at the fan blade tip. Additionally, the
flow will be turbulent, (fan on or off) making the
pressure drop across the HX higher than for laminar
flow and reducing total flow (for those who care,
pressure drop increases with the square of velocity
for turbulent flow, and varies nearly linearly for
laminar flow). With the fan behind the HX, the flow
is more laminar into the HX, and the flow field is
more uniform, giving much better air distribution
across the face of the HX. With the fan behind the
HX, an inlet shroud is important, or the fan will draw
air in locally, and not from the front of the heat
exchanger.
Still awake?
Ron
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