Designing an optimum cooling system

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Midlana1
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Re: Designing an optimum cooling system

Post by Midlana1 » 29 Aug 2017, 07:38

But that still doesn't answer the questions.

On the freeway with the fan not running, there is no recirculating air current to be concerned about, so I still don't see how a shroud is a help.

Also, mechanical fans - with a clutch - are not a drag at freeway speed and may actually take less power to run that an electric. That said, the real advantage of an electric fan is that the ECU decides when to turn it on.

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Re: Designing an optimum cooling system

Post by rennkafer » 29 Aug 2017, 11:00

Midlana1 wrote:But that still doesn't answer the questions.

On the freeway with the fan not running, there is no recirculating air current to be concerned about, so I still don't see how a shroud is a help.

Also, mechanical fans - with a clutch - are not a drag at freeway speed and may actually take less power to run that an electric. That said, the real advantage of an electric fan is that the ECU decides when to turn it on.
It does... the shroud, by directing the airflow out of the radiator can help keep heated air from recirculating back in front of the radiator. Without one, it can and will recirc right back in front of the radiator depending on the design of the vehicle.

A clutched fan can help in that way but you're still spinning the mass of everything up to the clutch. My International, given its age, is pre-clutch so it spins the entire mass all the time, plus the resistance from airflow. I'm told that removing it and putting on an electric makes a noticeable difference (I haven't bothered... it's a 50yr old truck). I'd imagine putting on a clutched fan from a later IH would make a similar difference. The other thing is, IME, electric fans are more reliable than clutched mechanical fans. YMMV on that.
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Re: Designing an optimum cooling system

Post by Midlana1 » 29 Aug 2017, 11:30

rennkafer wrote:
Midlana1 wrote:But that still doesn't answer the questions.

On the freeway with the fan not running, there is no recirculating air current to be concerned about, so I still don't see how a shroud is a help.
It does... the shroud, by directing the airflow out of the radiator can help keep heated air from recirculating back in front of the radiator. Without one, it can and will recirc right back in front of the radiator depending on the design of the vehicle..
This sounds like the benefits of sealing off gaps around the radiator, and I agree with that. I just can't see how making the exit smaller can improve cooling throughput, but to be fair I won't know until I try it.

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Re: Designing an optimum cooling system

Post by John » 30 Aug 2017, 19:52

You could think of your cooling system like a ram jet. Your air intake ahead of the radiator (diffuser) slows the airflow velocity and converts some of the kinetic energy into pressure energy compressing the air slightly. The radiator then heats the air lowering its density a bit. Then by restricting the outlet area the right amount (in a nozzle/shroud) the pressure is converted back to kinetic energy (airflow velocity) before ejecting the air rather than just dumping the pressure energy overboard. In systems with a large enough compression and temperature rise (density drop) the kinetic energy recovered in the nozzle can exceed that consumed in the diffuser resulting in jet thrust. Spitfires used this principle during the 2nd world war to gain a net positive thrust (negative drag) from their cooling systems all be it at aircraft velocities rather than car velocities. I’m not saying you could have any hope of achieving negative drag in a car cooling system but its posible you could achieve improved air flow efficiency even at highway velocity.

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Re: Designing an optimum cooling system

Post by ShadowCat38 » 30 Aug 2017, 22:01

Midlana1 wrote: Also, mechanical fans - with a clutch - are not a drag at freeway speed and may actually take less power to run that an electric. That said, the real advantage of an electric fan is that the ECU decides when to turn it on.
There's a way around this as well. Our big engines use a mechanically driven fan off the front of the motor, but don't use a heat clutch. Instead, there's an air solenoid in the connection to the engine that engages the fan when air pressure is removed (safety design in the event of pressure loss). It is similar to the A/C clutch on those compressors, but with an air swivel core instead of an electric slip ring. The air supply line is toggled by a remotely located electric solenoid that is controlled by the ECU. The attachment to the front of the engine is not small, but neither is the engine. This thing is a 470 HP 13L Detroit Diesel engine, and when that fan kicks on, it robs 40 horses from the engine just to spin. It feels like you're standing in front of a jet turbine when it kicks on, there is so much airflow.
Midlana1 wrote:But that still doesn't answer the questions.

On the freeway with the fan not running, there is no recirculating air current to be concerned about, so I still don't see how a shroud is a help.
You're not wrong, here. At highway speeds, any assistance offered by a shroud is minimal, and may even be a bit detrimental if the design is sloppy and doesn't take that scenario into account. A ram-air effect will accomplish a lot more with greater ease without a shroud throttling the flow. But, you greatly limit yourself if you design for the best-case scenario, because then you can only operate reliably during the best-case scenario. By installing the shroud, you've improved the reliability and efficiency for those other conditions that the vehicle needs to operate at.

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Re: Designing an optimum cooling system

Post by John » 31 Aug 2017, 17:57

If you take the ram air effect into consideration it can guide the cross sections through the various stages of the system. Exit area should be a bit less than intake area because pressure energy recovery won't be 100% efficient. The proper place for pressure recovery is probably at the exit of the cowl rather than the shroud. So if the shroud area is larger than the intake area it shouldn't throttle the flow much. Minimising cooling drag is probably all a bit redundant on a 7 style car anyway. In a car around 10% of total aero drag comes from cooling air.

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Re: Designing an optimum cooling system

Post by John » 06 Sep 2017, 14:12

Check out the Meredith effect.

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