Radiator Air Intake?

[Steve_D]

Hi All.
I’m modifying a rear engine, air cooled, car to water cooled so need to install a rad up front.

I will create a ducted ‘mouth’ in the lower part of the nose as the air intake. I hope to use the rad that comes with the donor engine (Subaru flat 4) if I can make it fit.

Now the question for the scientists amongst us.
If the rad has a frontal area X do I just make my intake ‘Mouth’ the same area or is there a secret ‘fluid dynamics’ formula?
Thanks for your thoughts?
Steve

[garagewidow]

there probably is.
but don't forget you are transferring hot coolant from rear to front of the vehicle which will dissipate quite a lot of heat in the process due to the extra surface area of the associated pipework(dependant on material of course)so you could probably use the standard rad with a smaller intake,plus it will probably handle a power upgrade if you wanted virtue the above.

Edited by garagewidow on Sunday 10th November 01:29

[Pumaracing]

In an ideal world the inlet to any airflow channel will be radiused to maximise the flow efficiency. However the radius at normal air speeds doesn't need to be very large to bump up the flow coefficient from "sharp edged" to "streamlined". On something as small as the ram pipe to a carburetor this radius only needs to be about 1/4" but that's a large percentage of the flow channel size if the carb inlet is only 1.5" in diameter. However the same radius on a 2 foot cooling radiator inlet flange is negligible.

So no it's not a great concern on something that size. Look at the radiused inlets to passenger jet aircraft engines. They aren't very large compared to the diameter of the turbine blade. Even on something that size (8 feet or more in diameter) a radius of a few inches does the job.

http://www.google.co.uk/imgres?imgurl=http://uploa...

So aim for a flared entry perhaps an inch bigger all round than the rad itself. Or just don't fret about it because it'll make sod all difference.

[PaulKemp]

F1 teams angle radiators to present a larger area to tunnel size
Yes they build rads with funny shapes and twisted core but you can angle the rad in your application to suit
In my 7 replica the rad is angled at 30 degrees anduch larger than the grill hole
Getting the hot air out is as important as getting cool air through and you should plan for this

[one eyed mick]

As paul says above getting the hot air out is just as important!!

[Steve_D]

You're all saying what I wanted to hear on the inlet size so that's good.
I read somewhere that the hot air exit needs to be 3 times the inlet size...any thoughts.

Also, yes, I plan to run copper pipes from front to back and they will be under the car in the airstream so cooling on the move should be fine. Sitting in traffic could still be an issue if I don't get it right.

Steve

[Evoluzione]

This sums it up pretty well:



There is a rough rule of thumb, but I don't have it with me right now. It's difficult to be an exact science as there are many variables.

Do a search on Autospeed.com, the chap on there did a lot of easy to follow real world testing on this kind of thing - use an undertray to form a low pressure zone to pull the air out of the back.

Edited by Evoluzione on Sunday 10th November 11:56

[DrDeAtH]

Aluminium coolant pipes might be cheaper than copper.

[Steve_D]

DrDeAtH said:

Aluminium coolant pipes might be cheaper than copper.

True but trying to work out just where the bends should be and what angles will be time consuming compared to some lengths of tube and a box of solder fittings. Time is money in a business.
It also means I can install the pipes from back to front long before I resolve how they will finally terminate to the engine and radiator.

Steve

[Nick1point9]

PaulKemp said:

F1 teams angle radiators to present a larger area to tunnel size

True but that's partly because you need to slow down the 200+ mph air to give you a lower mean surface speed. Do that in a road car and you could end up with a very low heat transfer coefficient because the air isn't travelling fast enough.

[PaulKemp]

Nick, you only comment on one aspect of F1 solution
My post was more about surface area much less about speed
I run a 30degree rad in a 7 to good effect
On the subject of hot air out any extra venting will help as engine bay heat will be lower pressure and speed
Cooling is usually multiple solutions
Rad size, position, air flow in and out
Oil coolers thermostat opening temp etc.