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Date: Wed, 23 Mar 2005 22:52:48 -0500
From: Ernest Christley <echristley@nc.rr.com>
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To: Rotary motors in aircraft <flyrotary@lancaironline.net>
Subject: Re: [FlyRotary] Re: EWP: How much is enough
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William wrote:

> I know that calculations are frowned upon by some, but the 
> relationship between flow rate and delta-T is not rocket science to 
> calculate very accurately.
>
> Q = m*Cp*deltaT
>
> Q is the amount of heat needed to be transferred,
> m is the mass flow rate
> Cp is the heat capacity at the temperature of operation of the fluid
> deltaT is the temperature rise/drop across the engine/radiator
>

Naw! We don't mind the math.  It's the assumption that there are 
accurate figures to plug into the equations, added to the assumption 
that 3 factors cover the whole story that bothers people.  The 
difference between theory and practice is that in theory they are the 
same thing.  8*)  Fer instance, in this case I would say that there can 
be significant variances in Q.  We have some rule of thumb numbers, and 
some numbers published by Mazda, but the first are suspect (as are all 
rules of thumb) and the second more so (Mazda's numbers are for a 
completely different setup).  As the temperature of the engine rises 
from 170F to 200F (just making up numbers), will more heat be rejected 
in the exhaust or even through the oil.  That is, will the percentages 
of heat rejection move around a bit.

> The greater the deltaT across the engine/radiator, the *lower* the 
> average temperature of the water in the radiator. This is important 
> because in our application, the most difficult part is getting the 
> heat out of the radiator into the air stream. We would like the 
> greatest temperature difference between the water and the air -- if 
> the water going into the radiator is 200F, and exits the radiator at 
> 170F, we have an approximate driving force of 185F - (temp of air).  
> On the other hand, if we go in at 200F and have a 60F drop (1/2 the 
> flow rate) then the approximate driving forces is only 170F - (temp of 
> air).
>

I'm going to add "with the average coolant temperature being constant", 
'cause I know you meant to.  I had to read it several times for it to 
make sense, until I added that caveat.  And with that I will agree, 
except to say that all this stuff is non-linear, so the real numbers 
won't be quite so simple.  Which is sort of a tautology, else this issue 
would've petered out a LONG time ago 8*)

-- 
This is by far the hardest lesson about freedom. It goes against
instinct, and morality, to just sit back and watch people make
mistakes. We want to help them, which means control them and their
decisions, but in doing so we actually hurt them (and ourselves)."