Technical Fan shroud science. A commonly held thought is that fan blades should recess 50% into the shroud for

• A commonly held thought is that fan blades should recess 50% into the shroud for optimum cooling. To get your fan in the correct position, use fan spacers or shims

So we all have heard that before.
But what happens if the shroud is deeper and the fan is fully inside the shroud?

Let’s say it’s flush-
Let’s say it’s a few inches deep-

What should be expected from fan/shroud relationship like that?

 

I don't know about your other scenarios, but if you look at older rear wheel drive cars, the fans are half in and half out of the shroud. They never did overheat, so my guess is the engineers knew what they were doing, and keeping the engine temperature down is helped by the fan position in the shroud

 

Right, pretty much copy what OEM engineering and R&D did and you’ll likely get the same satisfaction and results.

I’m working on a project that’s got a tight budget, a deadline, and fan that’s pretty deep in the shroud. I’m Working with supplied parts and I honestly don’t know if it’s going to be a major issue or not.

Finding or making a shorter shroud is gonna blow both the budget and deadline.

 

I'd try it as is. As long as the fan is not so deep into the shroud that air can pass around it, should work.
Maybe not optimum, but.....
Got any pics?

 

Personally I don’t like it, I’d prefer to do it more along the conventional thinking and proven ideas.

However it’s not my problem to fix.

I honestly don’t know what’s gonna happen and not sure if I should squeeze for more time and dough or let it ride.

 

There is more wrong in the second photo than blade placement. The shroud doesn't even cover the entire radiator.

You say it's not your problem? I say it's not your worry.

 

I thought the general rule was fan 1/3 of the way into the shroud.
I would think it has a lot to do with the fan blade and shroud design
and that there is not one hard and fast rule.

 

Seeing how I'm in the middle of building a fan/shroud set-up, I gave that some thought...

I personally think it has more to do with serviceability than anything else. If you look at some electric fan set-ups, having the fan fully inside the shroud isn't uncommon. Maybe not as far as what you have, but as long as the fan blades tips are within 1" or less of the shroud opening walls (which what you have appears to be) it should work the same. I'll agree with the above poster that the bigger issue is not covering the entire radiator surface.

Going deep into the shroud will make installing the fan or replacing a belt more difficult... a serviceability issue.

If you look at non-automotive applications, having the fan fully inside the ductwork is common, if not preferred outright.

 

I'd leave the shroud off and see how it does. If it needs a shroud, get or make one that actually fits properly.

 

That shroud swallows up the whole fan. 5" isn't just a little bit in, it's all the way in. I'd say the shroud isn't going to be doing any good at all (but I'm no aeronautical engineer).

 

It's not the same thing, but on these appliances I repair for a living, the manufacturers also want the fans to stick out of the shrouds a little. I suspect it has something to do with the centrifugal force of the air being slung off the blade.

What if you cut a bunch of holes or even slits around where the shroud ought to terminate?

Sent from my SM-G965U using The H.A.M.B. mobile app

 

Agreed after seeing the pics.....that shroud isn't doing much good, if any.

 

When I build my shrouds , I add a 3/4 to 1" band around the circumference of the opening . In my goofy mind it gives the fan something to suck against. Bill

 

The shroud is intended to focus and maximize the air density around the fan blades, to increase the volume of air the fan is able to move in a given revolution. Having the leading edges of the fan blades "in" the shroud allows them to efficiently access that dense air, and having the trailing edges "out" of the shroud allows them to efficiently release that air into the relatively lower pressure engine compartment.

It may be true that having the shroud at exactly the middle of the fan blades is optimal. I would just make sure you have some of the fan in, and some out, of the shroud and you'll get most of the benefit of having it. Don't forget to make sure the heated air has a way out of the engine compartment, too.

 

Hey 48, don't you think the flange pointing forward into the "box" of the shroud would cause turbulence inside there? I think the air should be able to flow backwards as fast as possible and that little wall all the way around the opening will force the air from the far corners of the radiator back into the path of more air.

 

Shit can that shroud. It's worthless in your application. Get one made that actually covers the entire radiator. Than, after buying the correct one for your vehicle or having one custom fabricated, you need to ask yourself if you are using a long or short mechanical water pump. Then if need be, eliminate the mechanical fan and install a puller electric fan.

To further complicate things and make your head hurt more, take into the consideration the fan itself. How many blades does it have? What are the pitch of said blades? What is the length of each blade? You can't use a Pinto fan on a big block motor! Get my drift?

 

Sorry, I won't buy the idea that having the fan partway out of the shroud improves 'efficiency'. Air doesn't 'sling off' the fan blades, it's not water... If that were the case, propeller airplanes wouldn't fly, the thrust would go sideways, not in the direction of air flow.

I've seen lots of fan installations over the years (ventilation fans for hazardous gases, HVAC installs), and they use full ductwork to improve the efficiency. For an automotive application, look at most '50s-up heater fans; these are fully ducted for just that reason.

 

This thread reminds me of the "Old Wives Tale" of coolant not being in the radiator long enough to be cooled.

 

You may be correct but, The first one I added a band to was to my old Stude P/U about 25 years ago . The temp gage would start to climb at just the mention of a red light. After putting a shroud on it I had problems getting it up to a proper operating temperature. I applied the same to the one in my 48 Chevy P/U . 396 cu.in. I have sat at red lights in Pittsburgh through three cycles on a 90 degree and very humid day and warms to 190 degrees.In the eight years that I've been driving it , it has never once topped 190. I am using the same set up on my roadster that I'm building . So I to have contemplated the same questions as you. I'm not saying that it's right or wrong. I've done this way for 25 years and when my sphincter muscle is happy , I'm happy Bill

 

Oh yeah besides the budget and timeline,,,, there’s rules.
“The fan must be covered by a shroud.”

The rules are ambiguous by design, the car fits the rules. Leaving the shroud off isn’t possible.

I’m just not sure how this set up will work besides being away from conventional and what’s considered optimal. All it needs to do is cool good enough. It won’t be sitting idling in traffic but it will be either WOT or turning left.

Sooo I need the shroud science.

 

Heck, I'd just take a Sawsall and cut it off right behind the blade. 20 minutes and your good to go.

 

If the owner can live with the crappy access to the fan and for belt installs, I'd call it good...

 

Apparently in this application, the shroud is for safety, not cooling....why are you asking about how effectively it cools?

 

Nothing about this set up is "scientific", but it works. It never over heats! I take that back, it used to over heat with a 160* thermostat, so I put a 195* in it.

 

I want to offer an apology to 31 Vicky. I hadn't opened his thumbnail pictures until after I posted. 31 Vicky was hoping for a path of least resistance solution. What I offered I doubt would help. For that I apologize . Bill

 

Well I don’t want to see him leave with some weird vortex on the airflow that may actually cool less than no shroud.

 

cut the shroud back so it covers half the fan blade, and call it good?

 

Sorry, but you'd be wrong. More than 1/2 the airflow off an engine driven cooling fan is exiting off the blade tips outward.....air does indeed "sling off" the fan blades. This is the same reason you cannot infinitely spin a propeller faster and faster and get more speed in an airplane . Once the blade tips go supersonic, airflow is totally disrupted and thrust falls off badly.
Air reacts to propellers just like water does, same principles of fluid mechanics apply.

Thrust in a propeller plane is not from air pushed back by the prop, it is from the prop screwing itself forward into the air mass. Only a jet or rocket engine works off of mostly just air thrust backwards.

Fan pushed too far into a shroud just turns the shroud into a choke point.

 

On that note, would the shroud work even better if the fan was only 25% in? Or some other amount smaller than 50%?

 

X10.......right on!! The only thing I would add is that the blades (skew and pitch) need to match the environment (ducting, etc.) to work optimally.......including noise. Luckily, most hot rod applications will be OK with anything that "feels" right.....not great, but OK.