Cooling package ductwork

Hi All,

I just watched the recent Racecraft Video on YouTube on ducting, I was interested particularly in the ratios for size of the opening aperture and the length of the duct for the entry.

When it came to the exit from the coolers this wasn't mentioned in the same detail, do the same rules apply, in that ideally the exit would be the same length as the coolers are high and that the exit aperture would be 1/4-1/3 the area of the cooler?

Cheers :)

Mike

Hi Mike, I hope you found the ducting video helpful! Here is the video link for anyone that wants to watch the video.

As a baseline starting point, the rules of thumb for sizing the inlets and minimum duct length also apply to the outlets. One thing to consider on the outlet is the change in energy within the fluid after it has passed through the cooler(s) of which there are two competing effects.

1. The fluid has had some thermal energy added to it because it has had its temperature increased (because of the heat exchanger interaction)

2. The fluid has lost some kinetic energy because of viscous and turbulence losses from passing through the coolers and ducting

Both of these competing effects have opposing effects on the ideal duct outlet sizing.

If you want to get really carried away, to properly size the outlet, you are trying to get the outlet flow velocity to approximately equal the free stream velocity in the vicinity of the outlet (which obviously isn't necessarily the same as the average free stream velocity of the vehicle). This requires a lot of CFD or wind tunnel time, which restricts this analysis to teams that have access to these resources. In this case, they will spend time tuning the inlet and outlet sizes for different drag and vehicle configurations.

So, in summary. For the rest of us in the real world, if you make the inlet and outlet areas similar, make the duct lengths as long as possible (ideally at least equal to the high of the heat exchangers), make all radiuses as large as practical and all transitions smooth, you will get good results!

Hope that helps! 💪

Hi Tim,

Thanks for the detailed and quick response! Cheers :)

Mike

Hi Tim,

Have you also included a fan in this setup? How would this affect the design strategy?

Background is (small) rally cars: Extended high load-low speed sections, followed by a few mid-speed stretches plus occasional traffic and idling times make the use of a fan a must.

The benefits of ducting are clear, but in the 30~100km/h zone, is there any benefit to be had from extensive duct work?

From my experience, a larger rad, a bigger fan and an increased cross section thermostat will do most of the job and basic ducting to guide the air through the rad and potentially shrouding the fan, are the next steps.

Usually, bonnet mods for exhausting are not allowed, so evacuating the hot air is also tricky.

Regarding fans, i lean towards front mounted ones, for extra space in the bay and that extra bit of impact protection in front of the rad. You have a hope to nurse the car back to service, without a fan, but not with a pierced rad.

No problem Mike!

Armaki - Yes we have a pair of fans mounted within the outlet duct. It doesn't have much affect on the design strategy apart from placing another small restriction to disturb the flow.

The speed range for where the ducting becomes beneficial - this would be different for every application. But I would think that at any forward speed where the cooling system isn't relying on the fan to help the flow the ducting would be helping the situation.

Packaging and rules always have a big impact on the cooling system ducting. Even if you don't have the space to build a fully ducted system, then at least sealing the radiator to the inlet and fan to maximise the flow is a good starting point. Mounting the fan to the front for a bit more impact resistance makes sense in your application too! 😅

Thanks for the input Tim,

Assuming no other type of ducting, would a fan shroud on a front mounted fan have any benefit? I believe not.

I would integrate the fan to the outlet ducting, as a kind of shroud, but at the front.

Regards,

Nondas

Without seeing a picture or diagram of the layout you propose it's hard to say if that would help. However, my first thought is that I agree with you about a shroud for a front-mounted fan. It would likely improve the cooling in the situation where the fan is providing all the flow (car stationery or slow) but would hurt the cooling in other situations.

Any recommendations for material to use for mocking up parts and for straight pieces to “block” flow? Was thinking of something like black corrugated plastic, but not sure it would be stiff enough. Would eventually transition to fiberglass or carbon fiber

Hi Aaron,

Depending on the span and stiffness required, there are lots of affordable options. Perspex or Polycarbonate sheet is cheap and easy to work with. Another favorite of mine is aluminium/plastic sandwich board material, I've put an image below for reference. This is what you'll often see signs and trailer bodies made out of. Being a bonded composite it's very stiff and as a bonus quite easy to work with! A bit of Googling around your local area is bound to find a few different suppliers.

Regarding fan shrouds, some comments.

A great number of people will not only block off significant areas of the radiator around the fan(s), but compound this restriction by placing the shroud very close to the matrix, which further restricts airflow around the perimeter. Further, they are generally flat rather than 'tapering' out to the fan.

A good compromise is to fabricate 'flapper' valves using thin rubber sheeting (inner tyre tube works well) over openings in the shroud. During normal driving the rubber flap allows some air through the matrix, but when the fan is operating it will draw/push the flaps over the openings, sealing them, and so allow the fan to operate optimally.

Something like this (if the link works) https://portfolioabout.me/diy-radiator-fan-shroud/

I've been looking to aviation to better understand duct work and aero. Consider the Merideth Effect - the P51 actually gained (minimal) thrust from its cooling system. Granted, our operational speed ranges are much, much slower, but I still think it's a valuable example to consider.

https://history.nasa.gov/SP-445/ch5-5.htm

Hoerner's "Fluid-Dynamic Drag" is a standard text on the subject, and Chapter IX deals with drag due to internal-flow systems (i.e. cooling systems). For ducted radiators and air-cooled engine installations, the best configurations come close to zero drag coefficient, but none are negative (creating thrust). Aircraft are often able to incorporate good duct configurations, and the P51 is a pretty good example. Even a formula car would have trouble incorporating an ideal duct, but cooling drag can definitely be reduced greatly with some modest effort at diffusing the inlet flow, sealing the gaps, and exhausting the hot air in at least the right general direction.

One really enterprising guy attempted to put the question of whether a cooling system could generate thrust to the test in his home-bulit aircraft. It makes for interesting reading, and maybe food for thought when it comes to automotive applications.

https://www.kitplanes.com/the-meredith-effect-fact-or-fiction/

Paul, first reference to that I know of was the Spitfire, which preceeded the P-51.

James, I agree, I didn't get it but apparently it was due to the heating of the air passing through the matrix and the resulting pressure/volume increase. Perhaps similar in practice to a basic ram-jet?

Ah, just finished reading that article, James, it would seem there is potential but not in WW2 aircraft. If you look through the comments, there's a link - https://www.researchgate.net/publication/253649721_The_bonus_of_aircraft_piston_engines_an_update_of_the_Meredith_effect - which looks like it would be an interesting read for you, and others, as not only was there mention of the Meredith Effect, but the use of diesel engines in some military aircraft. As it requires registration, I didn't download it to check it out fully.

I didnt realize they did that with the Spitfire, cool. If I remember right they also tried to make use of exhaust thrust on the Spitfire, but the experiment was short lived.