David, I didn't want to come back on this either, although I wrote a reply this morning that I haven't published. But as I'm not alone in this, well, I can't help myself
And sorry Lucas to high-jack your build thread.
Quote:
Originally Posted by davidimurray
If you put something directly in front of the radiator, you will stop the air flowing to the core in that area, conversely if you put something directly behind the radiator air cannot flow out of that area so no flow. Think of blowing down a straw – doesn’t matter if you put a finger over the top or the bottom end when you blow down it either way you still get no flow! Even if the fan is slightly spaced off the back the air needs time to turn to get around the fan.
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That is where I think you are mistaken. Anything mounted in front of the radiator will stop 100% of the natural air flow. But anything mounted behind a radiator will not stop it 100%, event if the fan is against the core. I will slow the airflow down for sure, obstruct part of it it with no doubts, but some air will still go through the core there : Why? Because hopefully, the blade's hub (which is +/- the same diameter as the motor), even though it is quite close to the core, is not touching the core! And that is the big difference.
If I take your example of the straw : put your finger in front of the straw, not touching it, but very close (as the blade's hub is not touching the core when you mount the fan against it), and blow: air will not go into the straw. Now, put your finger at exactly the same distance of the straw, but behind, and blow: air will go through it.
The only obstruction you can consider equal in the front and behind the radiator is the outer ring which actually touches the core. but it is so thin that is not a problem.
As I wrote this morning but haven't published, I' might not be an engineer, or the best mechanic in the world, far from it, and I recognize it. I'm certainly the one on this forum who is asking the most obvious and stupid question to you guys who were born with a wrench in your hands. I'm totally aware of this, and I thank all of you for helping me when my knowledge is not good enough for doing what I have to do on the car. That is what a forum is done for: sharing knowledge, isn't it?
So I don't know a lot about welding, exhausts, material resistance and so on... But I have been working for 15 years, (and still am I), for a company which is a specialist of fluid conduction in motor sports vehicles. As you already know, brake lines are part of it. But this company also deals with the other kind of fluids: fuel, oil, water, and air. And perhaps I can share my knowledge about that.
During all these years, I have had contacts with the best companies designing custom cooling systems for motor sports. As most of you are English, I am sure you know them, as most of these companies are English too. I don't want to mane any of them here, because what I say here involves me and only me, but if you know the good ones, they are the ones I am talking about.
These guys are engineers, and they know what they are talking about. For our customers, my company and theirs have studied and designed together radiator + fan assemblies for car racing Le Mans or the Paris-Dakar rally raid to name just a few. And not only one or two. When they were racing in the Sahara, they knew what a hot day is...
But as I said, I'm not an engineer, so I stupidly repeat what I have heard, learnt from them, and read from their literature during the last 15 years. Because my job is to write and edit the French literatures, catalogues (I'm currently working on the 2014 edition), technical files and support for our customers.
So believe me or not, but there is a proven fact that a rear mounted fan against the core is more efficient than a front mounted one. These guys have wind tunnels where they can prove it any time. Which does not mean that a front mounted or a ducted fan does not work. It does work, just not as good as a rear mounted one against the core for the reason I wrote at the beginning of this post.
I can't imagine the engineers of these companies advising a rear mounted ducted fan or a front mounted fan if there is a possibility to put a rear mounted fan against the core. I' have never seen that anyway.
A fan is just an assistance when the car is not in motion or when its speed is not sufficient to allow the natural airflow to extract calories from the water through the tubes and fins of the core. OK, on some applications where there is no or too few natural airflow from the speed of the car, it can work much longer, more often, or even constantly. But it is not our problem with the Haynes Roadster which has a big front opening.
As you say, there are 2 situations
1) Car stationary and fan running
2) Car moving
Ducting the fan on the radiator is considering only the number one. I'm sorry, but yes, the duct will obstruct the air flow more than if there were no duct and the fan mounted against the core. Again, I don't agree with you that it will be better, and that the supposed venturi effect will be even beneficial because an acceleration of the airflow. A venturi is a very specific aerodynamic phenomenon, and I'm not sure a radiator duct has this effect at all. The airflow going through all the outer fins will be slowed down by the duct, to me there is no doubt about that. The consequence of ducting the fan, in the "car moving" situation, is what is called a "charge drop" (I'm not sure this is the proper English term, sorry if it is not, but I think you know what I mean) which is the enemy of any kind of fluid conduction. Ducting a fan on the radiator is considering that the natural airflow will never be enough, and the fan will do most of the job. Of course, that way, the cooling in the “fan moments” will be very good, certainly better than a non-ducted equivalent set-up. But what share of the time does that concern in our situation?
So ducting is very good if there is no, or too little natural airflow from the speed of the car, because most of the cooling will be done thanks to the fan. But if in the normal conditions, the natural airflow is sufficient, which should be on the Roadster, in my opinion, it is another obstruction, and bigger one than the fan mounted against the core. The duct sacrifices the radiator area in front of it in normal use condition (when the fan is not needed), not 100% of it of course, but it does.
I'm not angry or anything, no problem for me. I just wanted to make my point, because I just hope I can help sometimes as well.
Thanks for the attention if you have read this post to the end!