Heat transfer is a compromise of factors
tailored to specific operation conditions.
The entire ideal is to transfer heat from
engine to air. Holding the coolant longer in the block (via a flow
restrictor) will indeed increase the temperature and therefore the heat content
of the coolant. However, my view is the ideal is NOT to hold the heat in
the block but to promote its rapid exit from the block to the radiator.
At times with extremely high rpm engines,
restrictors have been used (and perhaps necessarily) to increase coolant
pressure on the pump side to reduce/prevent cavitation of the water pump.
This use of restrictors has often (wrongly) been used to come up with the
rational that slower moving coolant/water cools better. When if fact, it
has been the reduction of cavitation that has improved cooling – not the
slower flow.
But, my view is we are not (normally)
seeing those kinds of rpm in our application. A Mazda Rx-7 racing
engine may benefit for those reasons – but I don’t see any benefit
in our application. Also fuel efficiency is generally enhanced by running
an engine just short of melt-down – racers generally want to run as close
to this point as possible (without going over it) – so a hotter engine is
not necessarily bad from their viewpoint – unless it gets too hot of
course.
Anything that impedes the flow of coolant
from engine to radiator is going to impede the transfer of heat from block to
air no matter what other benefit it may bring.
The basic heat transfer equation Q (btu) =
mf*Cw*DT show there are two significant ways of increasing heat transfer –
one way is to increase the DT by increasing the
temperature of the coolant – but this is also increases the internal
temperature of the block (not necessarily what we want) – or by
increasing the mass flow “mf”. Increasing coolant temp in the block
also has a point of diminishing returns as the coolant temp goes higher there
is less transfer from block to coolant. The benefit comes when the hotter
coolant reaches the radiator and the greater DT promotes
more heat transfer from coolant to air. But, this benefit can be
diminished or eliminated if the flow is too slow in removing heat from the
engine.
The second method of increasing heat
transfer is by Increasing mass flow perhaps by increasing pump speed up
to the point that flow losses become prohibitive is probably the best way of
increasing heat transfer from engine to radiator. However, normally we
can only effect pump rpm by using different size pulleys – and there are
not a lot of ready made sizes to choose from – so we generally stick with
our stock water pump and pulley set up.
The bottom line as we know is that coolant
is one big compromise of opposing factors and the system needs to be tailored
to your application not some other application where the environment, operating
parameters, or incentives may be different.
My $0.02
Ed
From: Rotary motors in aircraft
[mailto:flyrotary@lancaironline.net] On
Behalf Of Al Gietzen
Sent: Tuesday, August 18, 2009
2:01 AM
To: Rotary
motors in aircraft
Subject: [FlyRotary] Re: Swirl
pots
I respectfully disagree with the idea of
the putting a flow restriction at the exit of the engine. True, it does
increase the coolant pressure in the block, and that’s good. But it
reduces flow, and that’s bad. Reduced flow increases the delta T
around the loop and reduces the cooling capacity of the radiator, and in an
aircraft application we don’t like to add additional radiator size.
As far as increasing the pressure in the block, you can get about the same
affect by plumbing the filler/pressure cap to the inlet side of the pump rather
than the outlet, so the pressure drop around the loop is not seen by the cap.
A swirl pot works, but is not
necessary. You need an air bleed from the high point of the engine
(usually the outlet of the pump, and from other places that the air may get trapped,
like the rad tank. A small bleed line (3/16” o.d. aluminum is good)
from the point the air gets trapped back to the filler neck/bottle. Since
the filler neck is plumbed to the low pressure side of the loop, any air gets
drawn back there.
I connected the turbo coolant port on
the rear iron of my 20B back to the port on the pump outlet to bleed air from
the rear iron, and a manual bleed plug at the pump outlet to vent air during
fillup. I have small bleed lines from each of two custom radiator tanks
back to the filler neck (which is connected to the line just before the pump
inlet). After fillup, one run cycle purges all the air back to the filler
neck.
BTW; the cold to hot expansion for my
20B system is about 2/3 of a quart.
Works for me, but of course each
installation is different; but the concept is the same.
Al G
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