Any heavy hitters here who work with heat exchangers much? I want to get a good idea on pressure loss or flow resistance through a radiator core at different speeds. I'm trying to figure out if I should keep my fans on at higher speeds on track or whether they are actually impeding airflow. I can do basic calcs to figure out air velocity through the fan and if the OE fan is pushing ~1200CFM then the airflow is ~18MPH. Does that mean that the fan is impeding airflow through the radiator if I'm traveling at speeds above 18MPH?

I'm particularly concerned about the pusher fan. I roadrace this car and need best possible cooling. I want to know if it is better to have the fans on all the time or off over X speed (considering I frequently travel over 100mph)

I'm an ME so I understand the concepts, but I don't remember the crazy equations and I don't have a computational fluid dynamics program anymore.

 

Noone? Come on, I know there are some engineers here.

 

Come on, someone's bored at work and wants to help me out here.

 

wow... okay.

 

I'm not an engineer but have worked quite a few years in racing and would say that running the fans is a waste.
Think about it like this, you used 1200cfm as an example, calculate out your frontal area of the radiator. Mulitiply that number with your speed, average on a lap if you roadrace, in feet per minute.
You can't find ANY fan in the world that moves that kind of volume!!!

Don't think it will restrict the overall flow, but it is unnecessary.

 

If you are worried about cooling, you can probably find better ways to achieve you goals then worry about the fan being turned on. For sure you will not achieve the same amount of air flow as a running car, but the bigger question is the design of the radiator and the channeling of air to and from it. Since the air flow won't be laminar, but rather turbulent try to find a radiator that will take best advantage of that.

 

That's a good point but the real question is this:

If you're going 60MPH, and the fans are off is air going through the radiator at 60MPH? I'm sure it's not. The pressure drop across the radiator plays a big role in how fast the air is able to get through it. Now what I'm trying to get at is this: At what speed does the fan become a restriction rather than help, considering that the fan will push air through at about 20MPH.

Once I know that I can see if I'm over that speed for an appreciable amount of time while on the track that would validate me taking the pusher fan off.

 

I would say anything over 20mph or so, if the fans could only flow 18mph of airflow. Esp if the fan was mounted fore of the radiator.
However, if you're not experiencing any cooling issues, you're probably going to have to run a cooler thermostat.
Exe: If you have a 165*F thermostat, no matter how cool your airflow (or the amount of) over the Radiator core is, your car is still going to run at 165*F.

If it's a track car only, ditch the fans. Use a squirrel cage fan if you have to run it at idle for long periods of time.

Remember, if the system is working properly, you car will never run cooler than whatever degree thermostat you're running.

 

I understand the "ditch the fans if it's a race car" thing.. but you must understand... I have an engineer's mind. I won't do it unless it can be proven.

You see, just because the fans only flow 18mph through the radiator, what makes you think that if I took them off and went 60MPH the air would be going 60MPH through the radiator? I assure you it will not. I need some way to estimate the resistance that the radiator creates to airflow. Is it 10%? Is it 50%?

Do this:
-Take a fan and stick it to a radiator (like it comes stock) and measure the airflow on the other side of the radiator. It will be high. Almost no loss because the fan is right on the radiator creating a very large pressure difference.
-Take the fan off the radiator and put it 2 feet away. Now measure the airflow on the other side of the radiator. I guarantee it is much much lower because the fan isn't forcing the air through.

Now realize that theory in real life... at what travel speed will we see >18MPH through the radiator? Let's guess the "velocity loss" through the radiator will be something like 50% as a total guess. That would mean my fans would be useless if my mean speed is always greater than 40MPH. Now that's easy... my mean speed is definitely over 40MPH, but what if the loss is more like 70%?

 

Would it not make more sence to have a fan that is trying to pull more air into the radiator then trying to push it. You know that air hitting the front of the radiator at 100 mph isnt coming out of the back side of the radiator at 100 mph, so why dont you put it on the back side? Have it pulling air so it works with you and not against you

 

...bro. You're not reading. If you are travelling 100MPH air is DEFINITELY not going through the radiator at 100MPH. The question here is: "Then what speed is the air going through the radiator at?"

...and if you can keep a mean speed of 100mph on any of the tracks I drive on my hat is off to you.

Also this isn't a debate of puller or pusher fan. I just want to figure out how effective a fan is and up to what point speed it is effective. Someone has to have some sort of race testing or fluid dynamics experience to ehlp.