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MORE: downflow vs. cross flow, fan position

To: spridgets@autox.team.net
Subject: MORE: downflow vs. cross flow, fan position
From: Ron Soave <soavero@yahoo.com>
Date: Wed, 4 Aug 1999 19:57:25 -0700 (PDT)
Reply-to: Ron Soave <soavero@yahoo.com>
Sender: owner-spridgets@autox.team.net
Editing error - when I said there is "no difference",
I meant for horizontal vs. vertical flow.  If you
stayed with the whole post. there is obviously a
difference for fan position.

--- Ron Soave <soavero@yahoo.com> wrote:
> 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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