Date:         Mon, 1 Jul 2002 00:46:21 +0100
Reply-To:     Clive Smith <clive.harman-smith@NTLWORLD.COM>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         Clive Smith <clive.harman-smith@NTLWORLD.COM>
Subject:      Re: Humid Air - More  Comments and a bit of theory
Content-Type: text/plain; charset="iso-8859-1"

Yup, I'm aware that we'd have to settle for a single typical auto radiator,
but that'd presumably still give the hunidity trend, which I think we're all
beginning to realise is probably small, or confused by so many other
factors - of course, apart from density (almost negligible), humidity will
change the effective Reynolds No, and should this be at or near a critical
transitional value, air>surface heat transfer could change a fair bit.. Many
radiators use a square cross section water tube in the matrix, principally
to raise effective Re., thus improving performance.

The biggest changes that can be made to radiator performance, if it's deemed
necessary, are:

1) keep it clean and free of dirt/grease and macro blockages (flies);
2) try to achieve a low pressure area behind the matrix, as you say, reduce
the 'back-pressure' (but I hate that expression), ensuring that the radiator
exhaust can flow freely away. An air dam might be a good way to achieve
this, but as with most aerod. mods, a bit of experimentation and
instrumentation (a cheap thermocouple probe behind the rad) is usually
required -even on aircraft, everything is tested to ensure a nett gain is
actually being achieved, whatever the theory - and frequently they still
find cases where things aren't quite working as expected.
3) prevent intake air going around the rad, avoiding the matrix!
4) site the intake at the stagnation point (point of max. static pressure -
the nose, but also often just below the windshield on a hooded car, where
the cold air inlets often are).
5) create a convergent duct into the matrix cross section - but only if
you've got (2) and (3) well sorted, particularly (2) which will dominate all
other aerodynamic design features.


ASHRAE ? Automobile society? Here the RAeS data sheets might contain such
data, but they're v. expensive and not open source.

Regards,

Clive Smith
'89 T25 Syncro Transporter Van

----- Original Message -----
From: "Nancy Hughes" <keithahughes@QWEST.NET>
To: <vanagon@GERRY.VANAGON.COM>
Sent: Sunday, June 30, 2002 5:23 PM
Subject: Re: Humid Air - More Comments and a bit of theory


> Well...
>
> > I thought that enthalpy would come into it somewhere - honest I did...
>
> Kinda the basic underpinning, IME.
>
> > ... but what we need are some graphs of the total heat transfer capacity
of
> > a radiator matrix, at a given International Standard Atmosphere, for a
range
> > of humidities to establish whether the curves increase or decrease.
Let's
> > assome a dry surface too for now, with no water droplets around, nor
thin
> > films. Any takers?
>
> Unfortunately, this is impossible to do *unless* you calculate the
> result for a *specific* heat exchanger configuration. You would
> then have calculated what's known as the "Effectiveness" (similar
> to what most folks think of as "efficiency") of that particular
> exchanger, under the conditions of the test.
>
> The effectiveness will be dependent on numerous factors, such as
> both static and velocity pressure of the air stream, and airstream
> turbulence, that make each installation of any specific heat
> exchanger different. Take for example, our Vanagon configuration.
> There's a great deal of backpressure on the radiator matrix due to
> the exit flow path restrictions. This will increase static pressure
> within the matrix, decrease velocity pressure, and *probably*
> increase turbulence although I think it would be hard to
> generalize.
>
> There may, indeed, be some 'standard' heat exchanger calculations
> for varying humidity profiles hidden amidst the ASHRAE stuff, but
> I've not run across them. They have a great deal of esoteric stuff,
> such as a "standard test day" (including total insolation [INcoming
> SOLar radATION], with incident angle profile, standard system draw
> rates, temp/wind/humidity specs., and sky temp specs) for testing
> solar/thermal devices (yes, I used to do this back when the world
> was young:-), so I suppose a standard fin/tube heat exchanger
> profile isn't unlikely.
>
> Keith Hughes
> '86 Westy Tiico "Marvin"
>