radiator inlet/outlet ratio

[azmios]

Hey guys,

I have been wondering about the old and new formula 1 cars. I noticed that the older cars tend to have a large opening behind the radiator whereas the new ones have much smaller outlet opening. I checked around with the aerothermal engineers and they told me that the ratio is around 1:1.1-1.25. The bigger opening is to anticipate the expanded hot gas after the radiator. What bothers me is that once the air passes the radiator, the flow velocity is lower and I wonder if the large opening is still needed. Furthermore, the latest formula 1 has small openings. Anyone can comment on this?

 

[Lou Scannon]

By what analysis did you determine that the air velocity decreases as it moves through the radiator?

"Schiefgehen will, was schiefgehen kann" - das Murphygesetz

 

[patprimmer]

If the mass of air flow passing through the system is the same at all points, which it must be, the air density controls the velocity. Hot air is less dense unless it's under considerably higher pressure. If the outlet is at higher pressure, airflow is reduced.

Regards
Pat
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[azmios]

hemi, your claim is interesting. Can you state the reason on why the velocity increases after radiator?

 

[patprimmer]

azmios

That is nothing like what he said

Regards
Pat
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[azmios]

Pat, you're right. I was reading it in a hurry.

 

[Compositepro]

Heating expands air and air flows from high to low pressure. So air velocity must be higher after the radiator. A diffuser can then bring this velocity back down.

 

[140Airpower]

azmios, the mass of air going into the radiator is the same as the mass exiting since no air is generated or disappearing. But the air coming out is a lot hotter and less dense so has a greater volume. So the velocity coming out is greater than going in. This means that it is possible to generate thrust at the exit and suction at the intake. This would be most effective at high speeds.
Road cars show an increase in CD of about 0.04-0.06 (15%-20%?) representing the aero drag cost of cooling. F1 is very clever at shaping and using the flow energy of radiator exhaust (and the air off the headers, etc) to reduce or reverse aerodynamic drag due to cooling. For that purpose the design of ducting of hot air out of the chassis and the location and shaping of the exits are carefully configured. You may also be able to use the thrust out of radiator exhaust to do a little wake filling and induce slightly higher airflow over the lip of the diffuser and the underside of the rear wing (engine exhaust is even more effective here).
You may be aware that the WWII P51 was famous for getting significant thrust from its radiator outflow. It was claimed that the thrust was equivalent to a 10% reduction in overall drag. It is less well known that the FW190 also got net thrust from its cooling.

 

[azmios]

Air,

In the place where I work, the old school F1 aerothermal engineers believe in opening the outlet as big as possible. The younger ones who came across Adrian Newey's "chimney" outlet believe otherwise. Both have their own reasoning. The recent high performance production cars also have the outlet much smaller than the inlet opening.

I am trying to get neutral opinions from outsiders. I am a combustion engineer and though this area is not my area, I am curious to know the scientific reasons.