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I wouldn't worry about the theory behind
mdot*Cp*deltaT too much. This has been tested thoroughly. As long as your Cp
number is correct the calculation will be nuts on. The place you get into
trouble is measuring the constants for Cp or for a heat transfer coefficient.
Your analysis looks good to me Ed. The point Ernest made is a valid one. Cp is
per unit mass. A more dense fluid will transfer more heat per volume flow than a
less dense fluid. Bill S. also makes some good points.
The main thing I have to add is: it would be nice
to know what the convection coefficient is for NPG. That is what gives the heat
transfer between the hot metal and the coolant. A more viscous fluid would tend
to have a thicker boundary layer and less turbulence. That could cause problems.
The turbulence and mixing of the boundary layer help to transfer heat.
I also would be cautious about the vapor pressure.
Boiling is not a bad thing. It is a good thing. The heat transfer coefficient
for a phase change (liquid to gas) is infinite. This helps to cool a hot spot.
We are talking about sub cooled boiling here where the bulk liquid is cooler
than the boiling point. Locally the liquid boils and transfers all the heat the
metal can move. The limiting factor is actually the metal conduction for this
case. The bubbles of vapor are cooled by the surrounding coolant and
collapse. Put a pot on the stove and watch as you transfer from sub-cooled
to nucleate and finally bulk boiling. You can see the process
happen.
Both bulk and nucleate boiling are
to be avoided. Sub cooled boiling, a thin boundary layer and turbulence are
all good things. NPG strikes out on all three. In addition it requires more
power to pump and the pressure drop through the evaporator core type coolers at
low temps is suspect.
In short:
The 13b was developed to use water/glycol as a
coolant.
To properly validate NPG you need a
dyno and a lot of thermocouples, plus a way to measure the mass flow of the
coolant, pressure drops, pump power, and the heat transfer coefficient.
Anybody got that laying around in their
hangar?
Do you want to be a guinea pig?
I would not use NPG.
Monty
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