Bill Freeman wrote:
Thermal conductivity is of minor importance for a heat exchange liquid
which is flowing in a turbulent flow. The primary heat transfer
resistances
from the engine to the air are the convective heat transfer coefficients
from the metal to the liquid and vice versa. This is what Jeff was
pointing
our in his nucleat boiling comments. The point is that you can't transfer
enough heat from the rotor housing to the coolant without local boiling.
The bubble of coolant vapor forms (absorbing a HUGE amount of
energy in phase change) and then as it is swirled away from the heat
source it immediately recondenses in the cooler fluid stream (dumping a
HUGE amount of energy by phase change). This process gives MUCH
higher heat transfer than normal turbulent flow convection can support.
Conduction would only be of first order importance if you were in
a static situation.
Bill
-----------------
Bill,
What you say is true for the area around the hot spot inside the engine
block, where nucleate boiling is needed for the high flux, however I will
maintain that high thermal conductivity is important in the overall cooling
SYSTEM, which includes the heat rejection (radiator) as well as the heat
absorption.
In standard heat transfer equations (for turbulent flow), there are three
dimensionless groups that allow one to see the effects of fluid properties,
Nusselt number = h*D/k where h is the inside heat transfer coefficient, D is
the characteristic diameter of the tube, and k is the thermal conductivity of
the fluid. HIGH Nusselt number is desired, to get a high film coefficient.
The Nusselt number has been shown to correlate to two other dimensionless
numbers, the Reynolds number, and the Prandtl number. (Again I am addressing
the convective heat transfer from the fluid to the radiator tube wall to get
the heat OUT of the system.)
Reynolds number is D*G/mu, where D is the characteristic diameter of the
tube, G is the mass flow rate, and mu is the viscosity of the fluid. We want
Large Reynolds number for high Nusselt number, therefore want low viscosity.
Prandtl number is Cp*mu/k where Cp is the heat capacity or specific heat.
These can be worked around for liquids in turbulent flow (inside the radiator
tubes), and the film coefficient can be expressed as:
h = 0.023*((G^0.8*k^0.66*Cp^.33)/(D^0.2*mu^0.47))
Ignoring the coefficients in the equation above, what this tells us is that
for good heat transfer from the liquid to the tubes in the radiator, we want:
High flow rate through the tubes (G)
High thermal conductivity (k)
High heat capacity (Cp)
Small tubes (D)
Small viscosity (mu)
If one looks at the table that I sent to Paul, the best fluid is water,
followed by a 50/50 Ethylene glycol water mix. All the other heat transfer
oils will not perform as well in the radiator.
Again, in the engine around the sparkplugs where the flux is high, you want
the nucleate boiling and the above information is secondary. But the overall
SYSTEM needs to be balanced, and getting the heat out of the fluid into the
air is equally important.
Bill Schertz
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