I've often thought fully shutting off the main grill and having small secondary radiator with highly optimized airflow ducting would be a neat trick - one that's not new. But then I saw this thread.
Obvious flaws usually have obvious solutions.
Inadequate suface area -> Obviously don't try to cool the engines entire thermal load with this, just highway cruising loads.
Hot surfaces -> Limit the temps from coolant.
Bellypan hitting something -> Seperate cooling loop. Lower pressure and flow rate, lower total liquid volume = less mess.
So I think you can't do away with your regular rad with electric fans.
So:
Keep the existing rad and coolant setup and only insert a small water-water exchanger to a secondary cooling loop that uses bellypan and/or hood, roof or even the rear lid/quarter panels.
If engine coolant is ~90 C the secondary coolant perhaps reaches ~80 C, if you flow control the fluid then you can control the surface temperature of the car down to safe levels. For a belly pan, frankly I wouldn't care if it got to 80C
A temperature vs flow controlled secondary cooling loop runs through a belly pan rad, where it cools to around 60C or below degrees, then a Aluminum hood or hood sections where it cools perhaps to +20 above ambient. This then returns to pick up heat from the exchanger in the main loop.
Why 60C? This would lead to a hood surface temperature of roughly 55-60 C. This feels hot enough to give you a start and make you stop leaning on that guy's weird looking hood, but takes about five to ten seconds to cause burns. Long enough for you to get your hand of it.
This system would run full time full flow once engine is at operating temp. It would do its work in a temp range below a thermostat that triggers opening of a movable grill block, which in turn would be below the temperature the thermo-fans kick in.
Given this much lower target of dissipating 10-20kw of heat it's much easier to make it work. I would bet that this could dissipate adequate heat for highway cruising that you could substantially shut the front grills except for a trickle of airflow in to the engine bay.
Someone want to take a guess at what a few square meters of aluminum at 55-80 C with highway speed airflow might achieve?
(Some cars have substantial amounts of aluminum body work and most of the chassis ...)
Interesting.
But the most interesting thing in this thread was the Meredith effect.
The Meredith Effect - AviationBanter
Probably negligible benefit below Formula 1 speeds and needless added weight. Sure but this tidbit intrigued me:
Quoting:
"the jet effect would be greatly enhanced if the exhaust system could
be piped to discharge within the exhaust ductwork that carried the
heated air from the radiator. This is because any additional heat
would expand the air, increasing the velocity of the discharge and
therefore the thrust attained."
"This does two things, 1. It accelerates the air through the ductwork.
2. It creates a negative pressure behind the radiator which sounds
like the same thing as 1, but really isn't. 3. It can produce
positive flow through the cooling ductwork even sitting on the ground
with tail to the wind. Ok, that's three things."
Crazy idea 1: Secondary cooling loop feeds an aluminum belly pad radiator that does double duty smoothing airflow. Probably not worth the extra weight and complexity?
Crazy idea 2: Radiator ductwork that uses an exhaust exit to move air through it. Probably a good use of otherwise wasted energy?