Ever wondered what goes on inside a radiator? Yeah, me either!!

earlbrown

runs with scissors
If you want to cool the steel and you're in a hurry, do you use a trickle or a firehose? The specific heat of water is 1BTU per F per pound. A cubic foot of water takes 62.3 BTUs to cool it just 1*F. Since we're worried about time, there's two ways to do it in a hurry.... You can hit it with something VERY VERY VERY cold (not an option with our setups) -or- you can hit it with a BUNCH of things that are slightly colder.
Would you go slow and get $15 an hour from now or a penny every second? :)



A radiator is a heat exchanger (technically it's a 'conductor/convector' as that's how it does most of it's cooling). The goal when keeping engine temps under control is to conduct heat from the engine into the coolant. Then a pump, pumps the coolant with the stored energy into the radiator. Coolant weighs over 62#'s per cubic foot and air weighs .08#'s per cubic foot. All things being equal (and 100% efficient), it takes a bunch of air just to average the temps of the two.


When it comes to heat rejection, the main factor is 'Delta T' which is a fancy way of saying 'difference in temperature' which is a fancy way of saying 'your cooling system works better in Antarctica'.

If the ambient air outside (or the back of the A/C condenser) is 100F and you desire 160F you only have a '60 degree Delta T' to work with. If it's 150F due to a front mount and running the A/C in a parking lot you only have a 10F Delta T. That's not a flow issue, that's a physics thing.


Now when it comes to cooling going so fast it can't lose heat, that's just wrong. It takes some abstract thought to wrap your mind around what's going on. If you want to lose the most amount of BTU's per unit of time the way to increase that is to make the coolant entering the radiator hotter!


When it exits the engine, it's as hot as it's going to get (assuming it's cooler than 180F outside :) ). The hot coolant enters the radiator on the DS and that's where Delta T is the greatest. That first inch leaving the tank will have the most amount of heat transfer as from it's point of view the air 'feels the coolest' going across it. As the coolant works it's way across the flues, it's getting cooled and getting closer to ambient. As the coolant cools down, the Delta T gets smaller and smaller so less heat gets moved into the airstream.


Let's say you slow the flow level down so that the coolant can crawl through the radiator so it can't 'go too fast to lose heat'. Lets say in this example when it's halfway across the fins, the coolant temp is the same as the outside air... You just wasted 1/2 the radiator.
At the same time in that example the #1 and #2 combustion chambers are boiling the crap out of the head and detonating like crazy because there's not replacement coolant flushing the heat away. Since the coolant is going so slow, it enters the engine 'cold as hell' and had the front cylinders ice cold.... as it absorbed buttloads of heat from the three cylinder walls, then each of the other 2 combustion chambers before getting to the front two cylinders. You have a severe mismatch in temps across the engine, localized boiling, and all kinds of interesting (very bad things) that water does when it's pressures and phases are dancing all over the place. To make things worse, the steam will act as an insulator for the iron and act as a physical buffer to keep the coolant from even touching the melting engine. (that's how firewalkers walk across coals. The steam layer keeps the heat source from setting fire to their feets)


Now lets at what happens when the coolant 'goes too fast to cool'... When it's comes out of the engine at full temp and enters the radiator, just like in the other example it has the same Delta T relative to ambient conditions. But since it's moving along at a good pace, the radiator gets to use the entire flue length to reject heat energy (the equivalent of doubling you radiator width/ doubling useable airflow). Plus with the added velocity and pressure there will be increased tumble and 'scrubbing' going on between the coolant and flues increasing efficiency.
The faster the coolant is going, the move even the temp is inside the engine. When the gauge reads '180F' for example, it's only that temp at the location of the sending unit. The temp is progressively cooler as you work your way backwards through the head and block towards the radiator outlet.

Now with the pump, velocity matters here too. For a pump to move mass you have two sides to consider... The high pressure side and the low pressure side. For a pump to be working right it needs a pressure head. When that fails you have cavitation and other nasties going on. (If you've ever seen a pump cavitate on the dyno, you'd wonder how the radiator hoses didn't disintegrate. They can go berzerk!)

For a pump to be loaded with the head it needs to do it's job it had to have a pressure differential. On a sealed cooling system like in a car it pulls from the bottom of a radiator through a rubber hose. (the hose needs to be meaty and/or have a spiral wound metal coil in it to keep it from collapsing or being a flow restriction). It then pumps the water into the block, to the back of the block, into the heads (series cooling in this example, not parallel cooling), through the heads and into the thermostat housing. Normally the thermostat itself is the orifice/restriction so that the block in under pressure to load the pump, raise the boiling point of the coolant, and minimize localized boiling and phase changes. With a normal GN/TTA if you remove the thermostat the radiator flues becomes the choke point, which is fine....


***historical data*** We've all heard the wives tales of how removing the thermostat will make your car overheat. This particular tale doesn't apply to us. On older SBC cars and cars with the radiator cap on the same tank at the top hose the extra velocity overwhelms the spring in the cap, slowly vents the coolant.... when the coolant mass isn't enough to keep up with heat rejection needs, overheating leads to phase change.... then it's all over.
Think putting your finger over the end of the garden hose. The house pressure is still the same but the extra velocity (force) will blast the bikini top off your houseguest.


On our cars, the cap is on the low pressure side of the radiator. It's basically just exposed to system pressure not pressure+velocity (PSIA .vs PSIT).




Moral of the story our hotrods make 'X' amount of waste heat (in the water jacket) which means with only have to get rid of exactly 'X' into the air stream. The faster the flow, the more even the temps across the combustion chambers ft to r.

Obsessing over certain flows or certain restrictions are usually just covering up a more basic issue that just isn't obvious.


Don't get me wrong, I love figuring out new and interesting ways to mess with stuff. You know, like figuring out how to survive without working, and talking nice young ladies into taking it up the exit pipe, and getting free hottubs off craigslist... but I've heat to figure out how to break the laws of physics.
 

turbo nasty

Turbo Dojo / MNTR
Wow hit a nerve huh.....lol

Earl have you tried it on your car?
Don't make me record it with a infared camera.lol
 

earlbrown

runs with scissors
^Didn't want to lose all that typing. I'm low on beer!!!

I have one of those IR guns too. Possibly the best 15 clams I ever spent! :)

And, yes, without the thermostat my car won't warm up with my super trick $80 Fbody radiator after replacing my clogged up orig radiator.
 

charlief1

RIP Charlie!
Yes, that's the subset of how you can walk barefoots on coals. I also mentioned being 100% efficient so boundary layer doesn't matter! :p

You can never count on 100% efficiency because there's no way to eliminate the boundary layer between the metal and the coolant Earl. Especially when the water is moving fast. Even with a brand new block and heads there will be some sort of steam layer between the metal and coolant.;)
 

longball

I fought the lawn...
[quote="earlbrown, but I've heat to figure out how to break the laws of physics.[/quote]

According to these same Laws of Physics you can not have two opposing views and have them both be right.
Either you are right or 'turbo nasty' is right but you both can't be right. Unless you are in different universe's...
How much beer DID you drink, Earl?

The way I understand the First Law of Thermandynamics I would suspect that you are right?

http://en.wikipedia.org/wiki/First_law_of_thermodynamics
 

turbo nasty

Turbo Dojo / MNTR
^Didn't want to lose all that typing. I'm low on beer!!!

I have one of those IR guns too. Possibly the best 15 clams I ever spent! :)

I used one of those as well...I'm talking a Flir thermography camera.

I figured my thread would start this.
I'll start another that titled " I used Rotella and my cam wiped its lobes" lol
 

bryesh

Member
I thought he had drunk too much again, but surprisingly it seems as if Earl's right. I was just now sitting in front of a fan set to low and I was cold. So I turned it to high and by gum, I was colder. Turned it off, and now I'm hot again.
 

earlbrown

runs with scissors
It's neet to see science work in real life!

Like the other day when I stuck my keys in the wall socket. Electricity really is blue!
 

mgmshar

Active Member
I find this thread even more humorous because the title of my Masters thesis was "Investigating the Transient Response of an Automotive Radiator Using Discretization Techniques". No kidding.

And now I design and sell ball bearings for a living. Obviously, I should have drank more. :confused:



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Nasty Wendy

Perverted Lurker
(that's how firewalkers walk across coals. The steam layer keeps the heat source from setting fire to their feets)


I'm sure it helps having all them feets as well.:D

Good write up. Been runnin' no stat for quite some time. Runnin' requires feets too.:D
 

Chuck Leeper

Toxic old bastard
Staff member
It's neet to see science work in real life!

Like the other day when I stuck my keys in the wall socket. Electricity really is blue!

Yep, it is! I just recently witnessed a guy doing a "color test" on a 480/3phase...It did have a tint of red in it...........Musta been the screw driver being vaporized.:eek:
 
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