Radiator Flows

[Evodelta]

Hello all,

I'm currently working on a chargecooler kit (water to air intercooler for all you guys in the US) for my road going integrale and I have a question in relation to the way the water should flow in the rad, I guess this is relevant to vertically mounted oil coolers too. (I have one of these aswell)

Instead of having the water or oil flow going from side to side, it's going up and down, but which way should I pump it, upwards or downwards, does it make any difference?

On the oil rad I have made it go upwards as I believe it should go this way to keep up the pressure and also it may slow the oil own fractionally to give it more time to cool.

My chargecooler rad (or pre-rad as they are known) favours down flow, this is only because of the postioning of the pipes, It will be more awkward to get it to flow upwards, but possible, does it matter?

A fairly minor point really, but I would like to get it right.

Martin.

[Guy Croft]

The fluid you're cooling should enter so that the hottest fluid enters physically higher than the exit. If it was a vertical radiator, hot water enters at the top, cools and comes out the bottom.
The cooling medium and fluid to be cooled should contra-flwo ideally, one goes one way, the other opposite direction.

GC

[Evodelta]

The fluid you're cooling should enter so that the hottest fluid enters physically higher than the exit. If it was a vertical radiator, hot water enters at the top, cools and comes out the bottom.

Thanks very much for the reply, this works even better in my situation than I would have hoped! I have to ask though, why is this? Something to do with heat rising?

The cooling medium and fluid to be cooled should contra-flwo ideally, one goes one way, the other opposite direction.

GC

This I can understand completely, I've also heard it called 'Countercurrent'

Martin.

[Guy Croft]

Yes, heat rises is a good way to remember which way round to put the connections. Hottest part of a heat exchanger is always the 'top'.

GC

[cheitane2003]

Guy:
Wich physical reason is behind this rule to put the hotter the higher?Or is it only for positionals reasons?For example,when you want to get a room cooler,the air conditioning is always at the top because the hot air goes to the roof always...And in the oposite,all the warmers should be on the floor...
Best Regards,
Many thanks!

[Uros Piperski]

a/c is on the top of the wall because the hot air is up there.
In a radiator you feed the hot water to the top, and as it cools it flows down. In the oposite case, the hot water entering the lower section of the radiator would tend to stay low instead of flowing up to the exit pipe.

[Evodelta]

In a radiator you feed the hot water to the top, and as it cools it flows down. In the opposite case, the hot water entering the lower section of the radiator would tend to stay low instead of flowing up to the exit pipe.

But it's being constantly pumped, so how can it stay where it wants to? Surely it has no option but to go whichever way it is sent.

[Testament]

In a radiator you feed the hot water to the top, and as it cools it flows down. In the opposite case, the hot water entering the lower section of the radiator would tend to stay low instead of flowing up to the exit pipe.

But it's being constantly pumped, so how can it stay where it wants to? Surely it has no option but to go whichever way it is sent.

If it's being pumped it doesn't really matter, as you say it as to move that way. It's not the heat that rises its the fluid/vapour with a lower density rises, this cannot happen in a system where the flow is forced rather than moving by natural convection. This said it makes reasonable sense to plumb it up that way if there is no preference due to space constraints.

[Uros Piperski]

But it's being constantly pumped, so how can it stay where it wants to? Surely it has no option but to go whichever way it is sent.

Yes, it will go the way it's pumped, but the fluid itself will work against the forced flow.
The correct way is the way Guy described, but as Testament said, "plumb it up that way if there is no preference due to space constraints."

[Evodelta]

It's gonna be going down then, it suits the pipe runs and the theory!

Thanks all for your help.

[Guy Croft]

Remember it's not being constantly pumped - the rad is still working when the hot engine is turned off. That cooling phase - heat soak is critical. That's why electric fans run on. The hottest point is the top of the engine, so quite naturally one puts the entry at the highest point of the radiator.

Pumping and heat transfer: The radiator operates under a forced convection regime, the heat transfer is dependent on the Reynolds number - Re - (among other things, Prandtl, Nusselt numbers) and high velocity - of which Re is a function - improves convective heat transfer.
You can ignore completely the effect of pumping velocity on transfer of energy molecule to molecule and their conductive heat transfer from fluid to pipe wall. that's going to be unaffected. Pumping it is a good thing.
Moreover turbulence - and Re is used to work this out (also used in good oil cooler and intercooler design) - is also a good thing because turbulence scrubs off the static boundary layer at the pipe wall. Same principles are used with engine body cooling, if you don't scrub that boundary layer (and additives like antifreeze are vital here) you get hugely impaired heat transfer and nucleate boiling of the coolant can occur - zero heat transfer in some regions.


GC

[SteveNZ]

Same principles are used with engine body cooling, if you don't scrub that boundary layer (and additives like antifreeze are vital here) you get hugely impaired heat transfer

GC

So glycol is advantageous in this situation? Ive read plain water has a better heat transfer co-efficient than glycol coolant.

[Guy Croft]

Yes and no.

Glycol is an important 'antifreeze' agent and also it raises the boiling point of the water, bit like adding salt. The coolant mustn't boil. It also inhibits corrosion, or at least I think it does (or maybe something else is added to to it to make up commercial 'antifreeze' as we know it).
However yes it does have a lower specific heat capacity than water, plain water is better from a heat removal point of view but in engines you must add a wetting agent to the water to reduce the surface tension or you get that nucleate boiling thing I mentioned above.
Silkolene among others produce a wetting agent, does that without reducing the shc of the coolant. Don't know of an antifreeze that does everything though, maybe someone does.

GC

[Marc]

Guy ,
I use Red Line Oils"Wetter Water" product which does just that.
Without Antifreeze it can reduce temps up to 30 degrees according to the MFG.With antifreeze 10 degrees.I tried it and it works as claimed under both conditions, without and with antifreeze.Great product.

[sumplug]

Yes, seen this "wetter water" used in a couple of Turbo engines, and its amazing how it brings running temps down!

Andy.