> On a left top entry right bottom exit radiator.
> Downflow would have faster travel over the top and down the right side
> while the left bottom would not see much flow. The hotter water would
> flow faster through the downflow resulting in less cooling time.
> Cross flow would be much more even with the water spending more time in
> the radiator resulting in better cooling.
Run away now if math bothers you in the least. It isn't that simple.
True, less "hot" flow would provide a greater temperature difference
between inlet and outlet, but from a system standpoint, you'd be in
trouble. Remember, the coolant is the "hot" flow in the radiator, but it
is also the "cold" flow, or sink, in the engine. You want as great a
flowrate of coolant as you can get . The heat transfer is directly
proportional to the mass flow of the working fluid (at a given temperature
difference) - more flow, more heat transfer. The heat transferred to the
coolant (or Ethylene Glycol Water mix, EGW), Q, is equal to the (EGW
massflow) * (specific heat of EGW) * (temp difference of EGW), with the CP
of EGW (in btu/lb-degF units) in a 50-50 mix is .0005T + .7498, with T in
deg F. Less EGW flow = less heat removal from your engine. Think about it
- if your water pump now supplied 1/10th it's normal flow, is that a good
thing?
For the radiator, what thermal control systems guys try to design for is a
radiator sized for an effectiveness near 1. The hot side effectiveness of
your radiator is defined as:
(T_hotin - T_hotout)/(T_hotin - T_coldin). {cold side effectiveness is
T_coldin - T_coldout in the numerator}.
Therefore the best you can get is an effectiveness of 1 when T_hotout =
T_coldin. This will occur when you've got lots of cold flow (air, in the
radiator's case) and only a little hot flow. However, you've just cooked
your engine. Regarding heat exchanger design, for a single-pass unit,
cross flow vs. downflow is a push. The heat transfer will be driven by the
fin design and density as well as the tube design. This is more nasty
stuff that is well documented in Kays and London's "Compact Heat Exchanger
Design" (it is the Vizard book of heat exchanger design), and is dependent
upon fin material, pitch, density, etc. Not worth getting into.
Everyone asleep yet? Sorry.
Tcold cold DT COLD/DT MAX
|