Coolant flows too fast - myth or truth?

[shimniok]

When talking about engine cooling systems some folks will often say that the engine coolant will not do its job if it goes 'too fast' through the system.

Is this myth or is there truth to this?

I am creating a new thread so we can take this discussion out of another thread that is a troubleshooting thread. Didn't want to dilute the troubleshooting with back and forth theory discussion.

Thanks,
Michael

[shimniok]

To me this is one of those complex situations like exhaust system flow (and "backpressure") where we can't tell myth from fact because we don't really have good, widespread, scientific knowledge to support any conclusive analysis.

This topic is all about thermodynamics, as far as I can tell. So unless an argument is backed up with thermodynamic theory it really can't hold a lot of weight.

The following example offers NO support for the argument. I'd want to know WHY the flow of coolant has anything to do with the amount of heat that is exchanged between engine and radiator.

http://www.wefixittogether.com/?cat=11

There is an old automotive myth that removing the thermostat will allow the engine to run cooler. This is absolutely wrong. In fact the opposite is true. Without the thermostat to regulate flow the coolant will travel through the system too fast to pickup enough heat from the engine and too fast to release this heat to the radiator.Also, without the thermostat holding coolant in the engine it would take a very long time for the engine to heat up. This delay will cause the engine to use more fuel and have higher emissions than it should. In addition, on cold days, the heater inside the car would not get hot because is not heating up to the proper temperature.

[shimniok]

Another unfounded argument from Novak

http://www.novak-adapt.com/knowledge/cooling.htm

One of the myths being perpetuated in the aftermarket is that of high-flow water pumps - either conventional or electric - being able to cool all engines better. Faster is seldom better in terms of coolant flow and heat transfer. The coolant needs enough time in the engine to pull the heat out of the block and heads. Likewise, coolant needs enough time in the radiator to shed its accumulated heat before returning to the engine. As such, fooling with coolant flow speeds without a very solid basis for doing so can be counterproductive.

Again, no scientific/analytical/technical support for why coolant flow rate and heat exchange are related.

Just saying "it works this way" or "it works that way" proves nothing.

Michael

[cajun_lad]

To ask for thermodynamic theory is really biting off more than one can chew. It's been a while since I've had any formal training in the subject, but I assure you I have a good understanding. Let's start with the first most simplest aspect. There are 3 types of heat transfer: conduction, convection, and radiation. Let's visit the one that pertains to us for automotive cooling:
[/quote]
Convection is a combination of conduction and the transfer of thermal energy by fluid circulation or movement of the hot particles in bulk to cooler areas in a material medium. Unlike the case of pure conduction, now currents in fluids are additionally involved in convection. This movement occurs into a fluid or within a fluid, and cannot happen in solids. In solids, molecules keep their relative position to such an extent that bulk movement or flow is prohibited, and therefore convection does not occur.
Convection occurs in two forms: natural and forced convection.
In natural convection, fluid surrounding a heat source receives heat, becomes less dense and rises. The surrounding, cooler fluid then moves to replace it. This cooler fluid is then heated and the process continues, forming a convection current. The driving force for natural convection is buoyancy, a result of differences in fluid density when gravity or any type of acceleration is present in the system.
Forced convection, by contrast, occurs when pumps, fans or other means are used to propel the fluid and create an artificially induced convection current. Forced heat convection is sometimes referred to as heat advection, or sometimes simply advection for short. But advection is a more general process, and in heat advection, the substance being "advected" in the fluid field is simply heat (rather than mass, which is the other natural component in such situations, as mass transfer and heat transfer share generally the same equations).
In some heat transfer systems, both natural and forced convection contribute significantly to the rate of heat transfer.[/quote]

[cajun_lad]

Next, everyone should study up on Heat Exchangers, because that is ultimately what a radiator is.
[/quote]
A Heat exchanger is a device built for efficient heat transfer from one fluid to another, whether the fluids are separated by a solid wall so that they never mix, or the fluids are directly contacted. Heat exchangers are widely used in refrigeration, air conditioning, space heating, power production, and chemical processing. One common example of a heat exchanger is the radiator in a car, in which the hot radiator fluid is cooled by the flow of air over the radiator surface.
Common types of heat exchanger flows include parallel flow, counter flow, and cross flow. In parallel flow, both fluids move in the same direction while transferring heat; in counter flow, the fluids move in opposite directions and in cross flow the fluids move at right angles to each other. The common constructions for heat exchanger include shell and tube, double pipe, extruded finned pipe, spiral fin pipe, u-tube, and stacked plate. More information on heat exchanger flows and arrangements can be found in the heat exchanger article.
When engineers calculate the theoretical heat transfer in a heat exchanger, they must contend with the fact that the driving temperature difference between the two fluids varies with position. To account for this in simple systems, the log mean temperature difference (LMTD) is often used as an 'average' temperature. In more complex systems, direct knowledge of the LMTD is not available and the number of transfer units (NTU) method can be used instead.[/quote]

[cajun_lad]

To sum it up in simple terms, the coolant maintains the temp in the engine by circulating via the use of a water pump. The coolant then must cool in the radiator by the flow of air across the radiator (speed of car, type of fan used, and outside temp of air are all factors). If the water pump is pushing the water throughout the system at a HIGHER RATED speed than the RADIATOR can keep up with, you WILL have OVERHEATING problems. Ambient temperature of the air "cooling" the radiator, the type of fan(electric or mechanical) used and condition of fan clutch (if one is used), the condition of the radiator cooling fins are ALL factors that MUST be considered. NOT just the water pump. Theoretically it is true that a higher rated water pump could cause overheating issues if you are using a stock 20+yr old radiator and fan. It is also true that a higher output water pump could resolve overheating issues if you have a radiator that can keep up with it.

[Don S]

..
Michael;

I put my finger on the radiator and calculated it is a myth four decades ago.

A high out put water pump, assuming it is pumping a higher volume than really needed, is going create more heat in the engine because it will require/consume more power/fuel. Think... over kill!
This is the way we were taught at the South Hampton Institute of Technology...

Have a good one.. Don S..
Certain ethnic groups in the Middle East have many less retards than the USA. They blow them up! Here in the US we allow them to be Senators?
Now the whole world is going to hate America for burning food in our trucks. Can you spell Corn Ethanol?

[Lindel]

In a nutshell, the longer the fluid stays in contact with the cooling area of the radiator, the more heat will be pulled out of the fluid, to a point. Airflow plays a key role here, and without airflow, the radiator becomes nothing more than an inadequate heatsink.

Older style radiators are considerably less efficient than more modern examples, primarily due to fluid and air flow rates, amount of cooling surfaces and the material it's made out of. Copper conducts heat a little better than aluminum, but there are other trade offs to consider (aside from the price of copper at the moment). Weight and durability are probably the primary concerns.

[GWChris]

It certainly is a heat exchanger - heat is being exchanged from hot metal in the engine to the coolant, then transported to the radiator, transferred to the metal of the radiator, and from there into the air.

It's interesting that no one suggests that there might be too much AIR flow to cool the radiator, but somehow that the liquid moves too fast. I've never heard, "hey, you got too much air going over that radiator, slow it down some".

The flow rate of the coolant has no effect on a molecular level - the liquid is essentially an infinite reservoir, as there are always more molecules available. They will continue to absorb energy until the coolant in that region boils. The molecules are not shooting over the surface too fast to pick up heat due to a pump that is too big. However, lack of circulation could lead to stratification and local boiling.

What flow rate does effect is the temperature of the coolant in any given region of the system. Clearly too low a flow will allow a larger temperature gradient to exist from inside the heads to the radiator. You want that coolant circulating as fast as possible so that it moves the heat from the engine to the radiator as fast as possible. This will result in the greatest possible temperature difference between the coolant and the engine, and between the coolant and the radiator surfaces, which will give maximum efficiency of heat transfer.

The problem in implementation is that trying to circulate the coolant too fast results in other problems like cavitation and local pressure drops, so it doesn't work.

The reason why high flow pumps are not always helpful is not because the coolant is moving too fast to transfer heat, but because you can't just go jamming on a bigger pump and expect a system to get proportionately better – that's what engineering is for.

[radjeeper]

At one time on my '78 Waggy AMC 401 (w/ Edelbrock Streetmaster cam, manifold, and carb) I was running a little whimpy plastic flex fan (someone recommended it) and the old stock three core radiator. On hot days stuck in trafic it would get hot. I changed the thermostate to a 160 highflow Robert Shaw and it ran about 20 degrees cooler. In fact, in the winter time it was too cool and I put in a 195. I recommended one to my mechanic with AMC 360 coolling problems and it dropped his temps as well. My point being is that the Robert Shaw stat increases coolant flow through the system and thus drops the temp significantly. So, I'm thinking that perhaps the Flowcooler pump's problem is not speed as much as it is the casting design?

With all the new mods made to the 401 (headers, small chamber heads, holley carb) when we fired it up the heat was building past 220. Perhaps the old Robert Shaw 195 stat was sticking? Took out the 195 and put in a 160 (my mechanic found that the cooler he could keep his carburated 360 the better it ran) which solved the problem. We'll see how it does in the winter.

Cooling system also consists of a BJ's alum. radiator (the inlet and outlet had to be modified to come straight out vs angled), a Flowcooler water pump, and a seven blade steel fan with thermal fan clutch . A fan shroud is also in the plans. It is too soon to tell how it will all work. Last trip out it was a cool Moab day. I'll keep you posted though.

My guess is that the cooling system is more than speed of coolant, etc., etc. It is based on a system of parts and how well they work together to exchange heat from the engine to the air. I would bet that you have to have everything working in "symphany" to get it right. Not too fast, not too slow, etc.

[Billygoat]

With the limitations of an engine mounted mechanical pump, I doubt you can get the coolant going to fast to allow proper heat to transfer to the air, then factor in the restrictions in the radiator and the engine, and I would bet it near impossible.

The problem in implementation is that trying to circulate the coolant too fast results in other problems like cavitation and local pressure drops, so it doesn't work.

That is the key, a pump with to high a flow rate can actually move less coolant.

My 360 water pump was getting a loose brg so I replaced it with a "high flow" model. What struck me was the impeller was exactly the same as the NAPA pump that was on it. So I guess the std NAPA pumps are "high flow" too.

[shimniok]

The quoted post still doesn't really support the statements with any kind of theory but it's something to add to the mix for now. It smells believable to me... at least in some/most situations...

From: http://www.physicsforums.com/archive.../t-198975.html

In the low flow limit the coolant leaves the engine at thermal equilibrium [ie, the same temp as the cylinder / water jackets] and additional heat can only be removed by increased coolant flow (assuming constant engine temperature). Similarly with the radiator, in the low flow limit the coolant leaves the radiator at thermal equilibrium [ie, same temp as the outside air] and additional heat can only be removed by increased coolant flow (assuming constant ambient temperature). In such a case the cooling can be described as flow-limited and increasing your flow rate will increase your cooling rate.

If you increase your coolant flow you will eventually get to the point where your coolant is no longer at thermal equilibrium with your engine. In other words, the coolant is leaving the engine at a lower temperature than above. At that point the system is no longer flow limited and further increases in coolant flow will serve to further reduce the temperature of the coolant leaving the engine.

If you still continue to increase your coolant flow you will eventually get to the point where your coolant is no longer at thermal equilibrium with your radiator. In other words, the coolant is leaving the radiator at a higher temperature than above. At that point the system is radiator limited and further increases in coolant flow will only serve to further increase the temperature of the coolant leaving the radiator. This would continue until the temperature of the coolant leaving the radiator was arbitrarily close to that of the coolant leaving the engine.

Must think and read more...

[CROCKETTBOONE]

The amount of cooling the radiator does may only be 5 degrees. So with that in mind, a variance of one or two degrees would make a big difference. Try running without a thermostat in the summer and see how overheated the engine becomes. Its all a balance of each system working properly to cool the engine.

Remember the air dams on the front of these beasts.......They're funtion is to not let air flow past the radiator too fast when driving highway speeds. Thus supporting the Truth side of this debate.

So......my vote goes for>>>>>>> Truth

[BRUTUS]

The amount of cooling the radiator does may only be 5 degrees. So with that in mind, a variance of one or two degrees would make a big difference. Try running without a thermostat in the summer and see how overheated the engine becomes. Its all a balance of each system working properly to cool the engine.

So......my vote goes for>>>>>>> Truth

Even though I agree with your conclusion, the above statement is all hearsay. You CANNOT quanitfy a radiator as it "cools 5 degrees". The ONLY way to quantify a radiator/heat exchanger is by its efficiency which in no way can be converted to a temperature change because there are too many other variables involved.

It has been a long time since I took thermodynamics (1998) but I do know this the variables in play are such:

E = Efficiency of the heat exhanger (constant, covers all physical dimensions and orientations of the radiator)
T1 = Water Inlet temperature
T2 = Water Outlet temperature
V1 = Water Flow velocity (water pump property)
T3 = Air Crossflow inlet temperature (ambient air temp)
T4 = Air Crossflow outlet temperature (rougly underhood temp)
V2 = Air Crossflow velocity (related to vehicle velocity and fan velocity)

If you assume that all factors are held constant and V1 is increased, T2 has to increase because the water has less time exposed to the heat exhanger.

My personal opinion is that high-flow water pumps are popular among racers because they increase V2 at the same time they increase V1 and the net effect of the two keeps the engine within operating temp range.

[tgreese]

I'm a physicist, and my physical intuition says "myth."

Think of it this way. If you slow down the flow of coolant, the result will be that the water in the radiator will get cooler before it moves out of the radiator and the water in the block will get hotter before it moves to the radiator. This isn't necessarily what you want to happen. The cooling efficiency is defined by the temperature differential between the coolant in the radiator and the air used for cooling.

What you would like is for the heat generated in the block to be moved immediately to the radiator, for radiation to the air. In most cases, the cooling system is too efficient if operated without a thermostat; the flow of the water pump is restricted by the thermostat, increasing the engine operating temperature. The opposite seems also to be true then: the more coolant flow you have in reserve, the more cooling capacity you have in reserve.

There will be a point where increased flow will not increase cooling capacity significantly - I expect this is where the radiator cannot radiate enough heat to the air, regardless of the flow. This could be because the air around the surface of the radiator is so hot that there is no heat flowing to the air, or when the surface area of the radiator in contact with the air is too small to radiate the amount of heat in the coolant.

I can't see how more flow can do anything but help cooling.