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Topfun99 08-02-2014 11:51 AM

Basics of Radiator Cooling
 
I had some time on my hands so I thought I would post some basic information related to the car cooling systems. Basically, as we all know, water moves through the block & heads, absorbs heat, then moves through the radiator where it releases heat. This process is known as convective heat transfer. The study of heat transfer is known as Thermodynamics.

Part 1 - Let's start with the formula for convective heat transfer.

Qrate = h x A x delta T

Qrate = rate of heat transfer
h = convective heat transfer coefficient
A = heat transfer area
delta T = Temperature difference between the water and the component that heat is being absorbed/released to

We have 2 heat transfer processes occurring. One is internal to the engine where for a specific car, the heat transfer area "A" (engine internal surfaces) will always be the same. As the engine makes more power, the delta T will rise as the engine/head surfaces heat up so this will increase the heat transfer. The other factor, convective heat transfer coefficient, is different for various fluids but all increase with higher FLUID VELOCITY. Generally, the higher the fluid velocity the higher the coefficient. The way to understand this is to imagine fluid flowing along a surface. It tends to "stick" to the surface so the velocity at the wall is lower than the average velocity. As the fluid near the wall heats up, the rate of transferred heat goes down. When the fluid velocity is higher, it creates more turbulent flow and this helps to stir or mix the fluid so the heat transfer coefficient goes up and you transfer more heat into the fluid (water).

The same convective heat transfer process is occurring in the radiator but of course it is releasing heat to the radiator core which subsequently is being cooled by air passing past the external fins.

Simply put, with a fixed area "A" to transfer heat, the faster the water passes through the engine (and radiator), the more heat it will absorb. That means for best heat transfer, we do not want to slow the water down.

Same factors apply for radiators. But we can increase the size of the radiator to increase the size of the heat transfer area "A". Obviously, that's why big radiators with large surface areas and big fans are best for controlling water temperatures.

Note: Water is a GREAT fluid for heat transfer because it has a high specific heat capacity. That means it will absorb a lot of heat relative to it's mass.

Next is Part II - Water pumps.

chevyIIheavy 08-02-2014 12:29 PM

Re: Basics of Radiator Cooling
 
excellent info , thank you for sharing ...I'll be looking forward to the next segment on water pumps !!

Topfun99 08-02-2014 01:26 PM

Re: Basics of Radiator Cooling
 
Part II - Water Pumps

As we all know, it takes a water pump to return the heated water from the engine to the radiator and then pump it back through the engine. A typical car water pump is a centrifugal pump. Centrifugal pumps have operating characteristics that are governed by what's known as the Affinity Laws. These are as follows:

Effects on centrifugal pumps with limited change of speed (RPM) or impeller diameter.

Capacity (Q) varies directly as the speed (N) or impeller diameter (D).

What this means is if we double the speed or the size of the impellor, the flow will increase by a factor of 2x.

Head (H) varies as the square of the speed or impeller diameter.

This means that if we double the speed or impeller diameter, the head(output pressure) will increase by a factor of 4x.

BHP varies as the cube of the speed or impeller diameter.

Double the speed or impeller diameter means the power required to pump the water at 2x the flow, 4x the head(output pressure) will increase by a factor of 8x.

This is theoretical and doesn't apply to all ranges of our engine operations because the engine pumps are designed to flow adequate water at engine speeds from idle up to say 2000-3000 rpms and it is a closed system with restrictions to flow. At higher rpms we are not really getting the proportional large increases in flow and head nor are we using that much more power.

In a particular application, the pumps will have a set impeller diameter. But we can vary the speed by changing the diameters of the drive pulleys.

Many guys will change their stock pulleys to undersize pulleys to lower parasitic HP loss to drive the alternator and water pump. This will slow the water flow and if a mechanical fan is used, also reduce the amount of air flow even thought the mechanical fan is not a centrifugal fan. This may work in drag racing applications, but for street cars that are struggling to maintain water temperatures, this is not a good idea unless the radiator and fan are upgraded to offset the loss cooling capacity due to lower water flow.

ELECTRIC PUMPS - This is based on the research I did when I was considering putting an electric water pump (Meziere) on my car. If someone wants to correct me and/or add additional information, please feel free to do so.

Most aftermarket electric pumps are also centrifugal pumps. However, the are designed to operate at a set speed and pump a fixed amount of water. The speed and volume will vary slightly if system voltage drops such as at low idle but not significantly enough to be a factor.

It appears that they usually work very well for drag cars. But, from what I have read, their flow capabilities are not always adequate to keep up with the cooling demands or street/strip cars that cruise at higher rpms (2500+) for extended periods. Where a mechanical pump will pump more fluid as the engine rpms go up, the electric pump flow is fixed so it simply may not be able to pump adequate water. However, in a drag racing application, this is not a factor since the engine is only running at the high power level for a short time (12 seconds or less). During this short period the water temperature will not rise dramatically, but on a street car cruising down the highway on a hot day, the temperature will continue to rise and possibly get too high.

The good news on the electric pump in a drag racing application is that while idling and driving around at low rpms (1500 or less) before/after the race the electric water pump will probably pump more water than a mechanical pump and help to cool the engine down. And having the capability to pump water with the engine off to quickly cool off the engine in a short time is definitely an advantage.

Part III - Radiator fans.

bottlefed1 08-02-2014 01:38 PM

Re: Basics of Radiator Cooling
 
Great thread,

Always seen lots of threads where guys say that you need to slow down coolant to allow the cooling system to work effectively, often saying things like you need to give the heat enough time in the radiator to transfer to the air etc.. I have never agreed with this, unless it were in a non pressurized or very poorly designed system where the speed of the coolant would cause turbulence and lack of contact or foaming. My thoughts were that even though your coolant might not loose all the heat in a given pass the fact that it would be making more passes through the radiator per minute would more than make up for this and if anything a more even coolant temp would be seen.

It would appear you also do not subscribe to the slow coolant speed method or am I misreading your post.

PS just to make clear I am not talking about increasing pump speed, where you could run into cavitation issues. I am talking about placing a restrictor in a system to slow coolant.

Topfun99 08-02-2014 02:48 PM

Re: Basics of Radiator Cooling
 
Part III - Radiator Fans

There's not many secrets here - typically the bigger the better especially if it is an electric fan that can be cycled on/off.

Mechanical fans - Not typically used by car manufacturers like in the past. They were designed to be good at moving air at low/moderate engine speeds. In the 60's and 70's, it was typical to see them bolted directly to the water pump shafts and then the Engineers started using clutches to couple the fans that would slip more when cold then tighten up at higher temperatures.

The obvious bad things about mechanical fans is they are pushing air and using power when you don't need them during warm up and can't be turned on/off at highway speeds when you don't need all of the air flow. Also a lot of engines today rev much higher than 20+ years ago and a mechanical fan becomes a liability to blow up when they spin fast.

The good points are a good mechanical fan will move a lot of air and is a simple device - no electrical components to fail. This is a good thing for a lot of high performance street and street/strip applications especially if the car originally had a mechanical fan. Mechanical fans can be good for some racers because it is simple and doesn't draw on the electrical system if a car doesn't run an alternator. It just has to be slowed down or a good design that will pump enough air and survive the high rpms.

Electric fans offers big advantages to the car manufacturers over the mechanical fans. They can be turned off when not needed saving wasted energy and some even have multiple speeds to let the fan(s) run at a slower use less power when higher fan air flow rates are not needed. The other more subtle advantage is while the engine is heating up, they are not running so as the water starts to circulate in the radiator, it losses less heat with no air flowing through it. Eventually this allows the engine to reach it's designed operating temperature that much faster - important to for OEM applications for fuel mileage and emissions.

For high performance cars (street/strip and strip only cars), electrical fans are more expensive and complicated but if you have an electrical system it may be the best choice if one is willing to spend the money and take the time/effort to get the right one and install it. They can then be turned off when needed either automatically, or by using a manual switch (or automatically along with a back-up mechanical "override" switch.

One need for all fans is to have a good shroud design. The more surface of the radiator face it covers, the better it is. It helps to make sure it seals well around the radiator so it only draws air through the radiator, not from leaks around the edges of the shroud. If a mechanical fan is used, make sure it's spaced correctly relative to the shroud opening to maximize the amount of air it pulls out of the shroud.

The good thing about electric fans is there are a lot of options out there. Some good information on OEM electric fans available for use in custom applications can be found in this thread.

https://www.yellowbullet.com/forum/sh...d.php?t=966394

OEM electric fans usually work well because they've been well engineered to move a lot of air in a small fan/shroud package. The OEM fans have to move enough air to cool a radiator even when the engine compartment is crowded (blocking free air flow) and they have to pull air through an A/C condenser which not only blocks air but heats it up making it necessary to move even more air to cool the radiator.

Next up:

Part IV - Thermostats

Topfun99 08-02-2014 03:49 PM

Re: Basics of Radiator Cooling
 
Part IV - Thermostats

Lots of controversy and opinions here. Basically the OEM Engineers design a car's cooling system to be able to operate at a set temperature primarily for fuel efficiency, emissions and to allow the car's heater to function effectively on cold days. They design the cooling system to operate adequately on a hot day with the thermostat fully open at idle, stop and go driving sustained high speed driving at high temperatures, with the A/C on even at higher elevations where the air is less dense (thin air doesn't cool the radiator as well). In modern cars, once the thermostat is open it usually stays fully open and the coolant temperature is controlled by cycling the electric fan(s). Driving down the highway on a cooler day without the A/C on, the electric fans may not be needed. But if you are doing stop/go driving on the same day, the electric fans may cycle a lot. The exception to that is on a very cold day when you are running the car's heater full blast, the thermostat is probably closing partially at low car speeds when the engine isn't making much power. The car's heater acts like a radiator.

One trick some of us already know is if your car starts to overheat on a hot day, you can sometimes get the temperatures to stabilize by turning off the A/C (if you have it and it's on) AND turning on the car's heater (if you have one of course) with the fan on high. Of course you better roll down your car's windows also though this will be evident very quickly. I once nursed a Chevy conversion van that had a messed up, undersized radiator all of the way from Florida to New Mexico during one summer by running the heater full blast in heat of the day. 95-100 ambient air temperature outside and hotter inside the van.....;)

Typically, the thermostat starts to open just before it's design temperature and is full open at the design temperature +5deg or so. Does the thermostat restrict the flow? Though I don't have any real data I would say it depends. At idle and slow engine speeds when the coolant flow is minimum I would say no. The opening is large enough to not impede flow. As engine speed increases past say 2000rpm, I would say it probably restricts flow but remember, the pump is capable of pumping more water anyway. I know this: if you have a cooling system that is adequate, a thermostat will not cause overheating.

As far as using a thermostat or not, I would say if your system has borderline cooling capacity and/or if you feel you don't need to get your engine up to a minimum "operating temp" quickly then don't use one at all or only use one of the aftermarket restrictor washers. The car will probably run cooler without a thermostat. Only problem I can think of without a thermostat is the lower hose on some applications can collapse with no thermostat especially at higher rpms because the water pump will be trying to pump enough water to cause a pressure drop at the inlet that will suck the inlet hose closed. This can be solved by putting a wound coil wire in the lower hose. I've seen this wound coil wire use on various OEM hoses.

bottlefed1 08-02-2014 04:00 PM

Re: Basics of Radiator Cooling
 
On some cars only one fan runs based on engine temp, however both run when the AC is turned on. On these cars if they are overheating you are better off leaving the AC on and pulling the wire from the compressor clutch, with the coil de-energized to prevent flyback from the clutch windings from blowing the AC clutch circuit components.


Great thread!

rat infested 08-03-2014 01:29 PM

Re: Basics of Radiator Cooling
 
I assume the next part will be air supply through radiator?

Above 40 MPH it is not uncommon to see over 4000+cfm of air available to the front side of the rad.
If you have 1 or 2 electric fans sealed to the back side of the rad with a full coverage shroud you have to keep them spinning or they
will become an obstruction to air flow.

I mount electric fans directly against the rad for maximum air flow/draw through the rad/fan at that point, and allow free air to move from the front through the rad without obstruction everywhere the fans are not. Air flaps that seal shrouds under vacuum and allow air through when there is pressure present also work well as seen in a lot of modular dual fan set ups.

Oh yeah, in high HP cars and hot rods that don't see cold climates, just say no to thermostats.

Topfun99 08-03-2014 01:42 PM

Re: Basics of Radiator Cooling
 
Quote:

Originally Posted by rat infested (Post 22897545)
I assume the next part will be air supply through radiator?

Above 40 MPH it is not uncommon to see over 4000+cfm of air available to the front side of the rad.
If you have 1 or 2 electric fans sealed to the back side of the rad with a full coverage shroud you have to keep them spinning or they
will become an obstruction to air flow.

I mount electric fans directly against the rad for maximum air flow/draw through the rad/fan at that point, and allow free air to move from the front through the rad without obstruction everywhere the fans are not. Air flaps that seal shrouds under vacuum and allow air through when there is pressure present also work well as seen in a lot of modular dual fan set ups.

Oh yeah, in high HP cars and hot rods that don't see cold climates, just say no to thermostats.

I recommend using electric fans with shrouds that cover most of the radiator so they are more effective. Yes, when not on, the electric fans do create a restriction but if the air doesn't get through fast enough with them off, they will cycle on/off to cool the water down.

stiverson 08-03-2014 02:46 PM

Re: Basics of Radiator Cooling
 
ok what way should the fan go pull or push?

Topfun99 08-03-2014 02:56 PM

Re: Basics of Radiator Cooling
 
Quote:

Originally Posted by stiverson (Post 22899913)
ok what way should the fan go pull or push?

Some are designed for pushing, some for pulling, some you can flip the blade and make it either a pusher puller.

Typically a pump (which a fan is) likes to pump with as little resistance on the draw or suction side. So that would mean mounting it as a pusher would allow the fan to move as much air as possible.

The problem with trying to push air on the front of a radiator is without a shroud on the front, all of the air doesn't end up going through the radiator, a portion of it deflects around it and recirculates itself back in the fan suction.

So that means it's best to use the available shrouds with a puller electric fan on the back of the radiator.

I have a Black Magic Puller on the engine side of my radiator. I also have a large pusher fan on the front side that I turn on manually along with the black magic fan to drive the temps down as quickly as possible when I'm hot lapping the car during the 3 back-to-back passes in True Street. Otherwise, the black magic typically keeps the temps at 185 or less on a hot summer day in central Florida.

stiverson 08-03-2014 03:21 PM

Re: Basics of Radiator Cooling
 
i have a shucks 17 inch and it on the motor side that pulls. when driving it at 180 ish all day long

mindinfection 08-04-2014 03:03 AM

Re: Basics of Radiator Cooling
 
subscribed

mindinfection 08-04-2014 03:04 AM

Re: Basics of Radiator Cooling
 
great thread !!!!!!!!!!!!

mopwr2004a 08-04-2014 12:03 PM

Re: Basics of Radiator Cooling
 
Some good info


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