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Thread: Diverging/Converging Radiator Ducting - Why?

  1. #1
    Okay, after 4 years of FSAE, I give up trying to understand FSAE radiator ducting.

    Convention is to have a diverging inlet (slow the air down), radiator, and then converging outlet (usually with a fan at the outlet). We've typically followed this convention

    Design judges support this, the Smith books support this, everyone seems to support this.
    However, everyone's support for this seems to stem from WWII heat exchanger research for aircraft flying at 400 mph.

    WWII aircraft Goals: Reduce Drag
    FSAE Goals: Don't melt the engine, screw drag

    What I mean by this is, in FSAE (at least for our team), our primary concern is making sure we don't overheat the engine when we are reaming on it at 25 mph for extended periods. Saving 4 lbs of drag at 29 mph by having a small entrance to the radiator duct means nothing.

    I guess it just frustrates me because every calculation I know how to do from fluids/heat transfer/common sense, tells me to make the entrance AT LEAST the same size as the radiator if you are concerned about mass flow rate and not drag.

    I just finished a CFD study of external flow over the body/ducting and it also shows highest mass flow rates at the radiator with a duct entrance area at least equal to radiator area.

    What are other's thoughts on this?

  2. #2
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    To be honest, I'd like to hear an answer on this, too. It's been a hot point among our team members this year. I also agree that "conventional" methods are probably for vehicles traveling much faster than our cars.

  3. #3
    Originally posted by Bus_Lengths:
    WWII aircraft Goals: Reduce Drag
    FSAE Goals: Don't melt the engine, screw drag

    I guess it just frustrates me because every calculation I know how to do from fluids/heat transfer/common sense, tells me to make the entrance AT LEAST the same size as the radiator if you are concerned about mass flow rate and not drag.
    If increasing mass flow was the only way to increase cooling, you may be right.

    Generally race-cars use a diverging duct to the cooler core to slow the air down as it passes through the cooler core. Slower air spends more time in contact with the core and gets hotter. So, it absorbs more heat energy per kg of air. An exit duct is less critical, once the air is out the back of the cooler core any downstream ducting is probably more for drag reduction, although don't dump your air to a high pressure region if you can help it.

    You ought to add 'keep it small and light with a low centre of gravity' to your FSAE goals. If you did that, then maybe a 'conventional' setup would have an advantage?

    I'm surprised more FSAE cars don't take inspiration from Formula Fords. If you look at a well known auction site item 300375806610 you will see a few photos of a Swift SC93 which has a very neat cooler core installation. The cores are very angled, creating the diverging inlet duct effect without adding any width to the car.

    Regards, Ian

  4. #4
    Originally posted by murpia:
    Slower air spends more time in contact with the core and gets hotter. So, it absorbs more heat energy per kg of air.
    Right but remember Newton's Law of Cooling. Your overall heat transfer will go down, because it takes longer to transfer the same amount of energy to hot air than it does cold air. As such, heat rejection is maximized from this perspective when mass flow approaches infinity.

    q = k*A*dT

    Other than modifying the heat transfer coefficients or the surface areas, increasing mass flow IS the only way to increase cooling.
    Wesley
    OU Sooner Racing Team Alum '09

    connecting-rods.blogspot.com

  5. #5
    I would have to agree with Wesley here, what you are seeking is not heat energy absorbed per kg of air more than heat energy absorbed, which will occur with a larger difference between the core and the air temperature. Therefore, the faster the flow in your radiator, the better should be the heat exchange, right?

  6. #6
    There is a paper that Porsche did covering their development of the cooling system for the 911. They were mostly trying to reduce drag while meeting cooling requirements, but the results show a pretty much linear inverse correlation between flowrate and coolant temperature. Also a direct correlation between both flowrate & duct outlet size and flowrate & aero drag.

    So if all you care about is coolant temp, get really big ducts and shove as much air through as possible. I imagine there is diminishing returns at some point, and I'm sure the judges will want some data to back up your design.
    Matt Brown

  7. #7
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    Originally posted by EPMAl:
    I would have to agree with Wesley here, what you are seeking is not heat energy absorbed per kg of air more than heat energy absorbed, which will occur with a larger difference between the core and the air temperature.
    I think this is the key.

    The reason "real" racecars have diverging ducts is because each kg of air comes with a real cost - that cost being drag. The fewer kg of air that has to pass through your cooling system, the less drag you have.

    So how do you maximize cooling with fewer kg? You slow down air flow, so that the air has the maximum amount of time to absorb heat: maximum cooling per kg of air.

    In our cars, drag isn't a huge disadvantage like it is in most racing series. Therefore, rather than try and maximize heat transfered per kg, just maximize the amount of airflow through the radiator. The efficiency in heat transfered per kg is lower, but the overall heat transfer is higher, at the cost of drag.

  8. #8
    We did some simple wind tunnel testing of a radiator with different size inlet ducts last year. Mass flow DOES increase with a smaller inlet vs radiator size (in our testing anyway and obviously only until separation begins). The reason we want to slow the air down as it enters the radiator is to increase the pressure difference across the radiator. Slower air = higher pressure. The only way air moves from one place to another is by creating a pressure difference.
    UoA 07'-?
    www.fsae.co.nz

  9. #9
    Originally posted by Jimmy01:
    We did some simple wind tunnel testing of a radiator with different size inlet ducts last year. Mass flow DOES increase with a smaller inlet vs radiator size (in our testing anyway and obviously only until separation begins). The reason we want to slow the air down as it enters the radiator is to increase the pressure difference across the radiator. Slower air = higher pressure. The only way air moves from one place to another is by creating a pressure difference.
    +1. We have a winner.

    If you try to "neck down" from the inlet to the radiator, the air will just blow right back out and around. High velocity but low pressure to force through the core. You don't want flow reversal.

    I'd been told an old UTA car had that issue, before they found out why in a wind tunnel. That could be just BS though..

  10. #10
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    I reading this wondering if someone was going the bring up the pressure difference across the radiator. +1 to Jimmy

    Also, I'm pretty sure this is covered in the radiator design topic on the forum. And RCVD also has something on it.
    ~~~~~~~~~~~
    Cheers

    plohl


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