RE:Water Cooling

[Randy McCall <randym@schiller.com>]

Steven Buchholz wrote:

It seems like we are talking about two situations, external use of water to
help cool the intake charge and water injection into the intake charge
itself.  For the first situation I doubt that there is a real need to 
create
a fine mist of water as what you want to do is take advantage of the 
cooling
effects of the water evaporating away from the intercooler.  AAMOF, it 
seems
to me that if you had something that already created a vapor it would be
relatively useless as the liquid water had already absorbed the heat of
vaporization from somewhere else before it even got to the intercooler.  I
am less clear on the water injection into the intake charge, but it sure
seems to me that even there you want a fine spray of liquid water in the
intake so that it can vaporize in the cylinder ...



Wetting the intercooler is an indirect means of cooling the air stream. 
 Indirect evap coolers for HVAC can usually get around 15-20 (F) degrees 
depression depending on rate of evaporation off of the heat exchanger.  The 
effectiveness for a turbo IC is dependent on the mass flow rate through the 
IC, rate of vaporization for the water on the outside of the IC (that's 
where most of the cooling comes from, as a phase change from liquid water 
to vapor), heat transfer film coefficients for inside and outside of the IC 
(conduction of heat through the boundary layers to the metal), emmisivity 
of the metal, plus a bunch of other stuff. Theory for this stuff can be 
tough, and is usually more of an empirical "try it and adjust accordingly", 
or engineering manuals where someone else has done the experiements.

The other approach Steven mentioned is water injection which has been done 
many times.  Similar physics apply, except you want to control the amount 
of water (liquid - and atomized very finely) injected into an air stream so 
that all of it is evaporated into the airstream (after the compressor heat 
is added to intake air).  Depending on the beginning (ambient conditions) 
humidity ratios and dry bulb temperature, direct injection can get you from 
a little (high relative humidity, high temps) to a lot (low relative 
humidity) of intake air cooling through phase change absorption of heat.  I 
seem to remember hearing anecdotal stories of cleaner engines and valve 
train, less detonation, and of course additional air packed into a turbo 
motor (hence more gas can be burned for a given outside air temp)- all 
provided it is done correctly!  The catch is in keeping liquid water out of 
the combustion chambers.  Introduction of small amounts of liquid water 
(large amounts will become uncompressible and blow the head off, bend rods, 
etc) will absorb heat first, make steam second (with a big expansion 
coefficient) and probably erode some of that expensive aluminum in the head 
and pistons.

I personally haven't done any analysis for temperature depression from 
direct injection, but the method's temperature effects can be predicted 
using psychrometric charts provided you know the dry bulb temperatures at 
various points in the system, ambient conditions for humidity and dry bulb 
temperature, and mass air flow rates through the turbine.  It would be an 
interesting experiment for somone.....I can see someone mapping all of the 
various state points for a customized injection system that responds 
correctly to climate conditions!

Don't know if this is useful, but it's a "cool" thought .....

-Randy
'90 200Q w/ no direct water injection or indirect IC cooling, but thinking 
.....(of fixing all of the other problems first!)