Pressure or velocity for better cooling?

[tomislavp4]

Guys, I was under the impression that greater velocity leads to better cooling but yesterday I found an article stating the opposite! You can find the article here: http://www.3autos.com/Paris-Motor-Show- ... -cutaways/

it states "Additional ductwork behind the fascia creates a gently diverging shape that will slow down the airstream while increasing its pressure, thereby maximizing its ability to absorb energy from the heat exchangers."

Was I wrong all the time or is the author wrong? Thanks

[tomislavp4]

Ok, found it, sorry. I was wrong, you need pressure to force the air thorugh th radiator not speed.....

[xpensive]

Right, the way I learned things, forced convection between a solid surface and a gas or fluid is achieved by relative speed, where I have little memory that the media's static pressure should had any significance.

[Jersey Tom]

If you try to accelerate the air going at the radiator it will start to "bounce off" and you can get flow reversal back OUT of the sidepod.

Slowing the air down helps ensure it actually goes through the radiator.

Ideally you'd want maximum velocity for the highest average heat transfer rate, but there are limitations.

[rjsa]

The maths say pressure times velocity, so you need both.

[xpensive]

What an intriguing statement rjsa, would you care to elaborate on that one?

[Scuderia_Russ]

Our new car uses the side pod to create a pressure build up in front of the rad forcing the air through. We also managed to cool the coolant temp by an amazing amount by also adding a small winglet along the side of the rad helping to drag the exiting hot air out. Just looked for a piccy but I haven't got one.

[tomislavp4]

Well, offcourse you need both rjsa But the thing is,
a)do you need more pressure and less speed in front of the radiator than at the inlet or
b)less pressure and more speed in front of the radiator than at the inlet...

Thanks for the input Scuderia_Russ, confirming that pressure helps from some practical experience...
I have read an article about the p51 mustang aircraft some time ago, and it uses pressure build up in front of the radiator too, I can´t find the article though.

[riff_raff]

Actually, for cooling you care about mass flow.

With compressible flow like in a radiator duct, you can trade velocity for pressure. That's why the duct usually diverges as it approaches the core, in order to slow the airflow and increase the pressure ahead of the core. In order to get airflow across the core, you must have higher pressure ahead of it than behind it, otherwise the flow will reverse. The airflow also becomes more energetic as it absorbs heat passing thru the core.

[alexbarwell]

Isn't there also the consideration of exposing surface area to the cooling airflow? Not to mention as air (or any other gas really) is compressed the temperature increases (adiabatic??), reducing the pressure reduces the temperature. High pressure might well 'force' the cooling, but creating the high pressure in the first place requires energy so can be quite power-sapping. Guess there is always a trade-off, ultimately I suppose it comes down to how the numbers fall.

[xpensive]

As the pressure increase from the "ram-effect" of the air-speed is very small, a couple of percent of the atmosphere, I think the temperature increase from that is almost negligeable.
Regarding the lately so popular "chimneys", I have always thought they were there to create a lower pressue behind the radiators to help air-flow, can somebody shed a light on that?

[rjsa]

What an intriguing statement rjsa, would you care to elaborate on that one?

As riff_raff said, mass flow is an important factor, and it is the product of speed, cross section and pressure and temperature (P.V = nRT)

My thermodynamics is 20 years rusty but have a look at this example where you can play with values:

http://www.efunda.com/formulae/heat_tra ... e.cfm#calc

EDIT: deleteing the cookie for the web site above will free the 2 or 3 trials limit on the calculator.

[xpensive]

Most interesting lazy-dog sheet rjsa, but I cannot find pressure anywhere in the inputs.
However, density is important and to a cetain extent dependent of static pressure of the fluid, but as been said before, the difference in pressure made by speed is only a couple of percent of the atmosphere.

[tomislavp4]

Thanks everyone, very educational... About about the chimneys, yes they were there to do just that, create low pressure to suck hot air out

[Belatti]

Thanks everyone, very educational... About about the chimneys, yes they were there to do just that, create low pressure to suck hot air out

Let me add: its the same principle as the one used to make a liquid go up inside a tube by blowing air parpendicularly to one end. A joke many of us once did when where kids -and probably even does