joe_padavano

The popular double pass and triple pass radiators you see today always struck me as gimmicks. The physics just didn't add up - for the same size core, you have the same surface area for heat transfer to the air but half the flow area for coolant and a much longer coolant residence time in the core. The rate of heat transfer (from the coolant to the tubes to the air) is a function of delta T - the greater the difference in temp, the faster heat is transferred. Increasing residence time means that the coolant in at least half the radiator has a lower temperature and thus heat transfer is reduced, making that part of the radiator less efficient. Here's the math.

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.

Here's additional info from Stewart Components, a manufacturer of high performance cooling components primarily for race engines.

Double pass radiators require 16x more pressure to flow the same volume of coolant through them, as compared to a single pass radiator. Triple pass radiators require 64x more pressure to maintain the same volume. Automotive water pumps are a centrifugal design, not positive displacement, so with a double pass radiator, the pressure is doubled and flow is reduced by approximately 33%. Modern radiator designs, using wide/thin cross sections tubes, seldom benefit from multiple pass configurations. The decrease in flow caused by multiple passes offsets any benefits of a high-flow water pump.

AJFink

So why is it that human nature wants to always fix what has already been engineered and proven.

joe_padavano

Because manufacturers can charge more for the gimmick with nearly no increase in production costs. Kaaa-CHING!

83hurstguy

Great info, I’m sure someone will come along to tell everyone to slow down the coolant to improve heat transfer...

There are cooling system improvements over stock, including better radiators, but most people don’t want to spend the money on them.

VC455

As hurstguy implied, another cooling system myth is that water can flow too fast through the radiator to cool properly. From a heat transfer standpoint that statement is nonsense.

I do think something is behind the myth.

I believe it is that faster water pump rotation results in impeller cavitation, which lowers flow. And lower flow causes the observed overheating.

Higher water temperature also increases risk of cavitation and consequent low flow rate.

joe_padavano

You are correct, and in fact those two links I provided also talk about the cavitation issue.

olds 307 and 403

I assume the closed impeller on the heavy duty pump helps lower cavitation while still increasing flow due to a faster ratio pulley?

VC455

You're right, a closed impeller reduces cavitation and can be turned faster to increase flow.

And other factors that can lower cavitation are the shape of the impeller blades, the clearance of the blade tips to the volute (inside of casting), and the clearances of the front and back of the closed impeller to the casting.

Generally, the factory did not design much efficiency (and, therefore, freedom from cavitation) into their pumps.

The reason being that, when the car was born, its cooling system had adequate capacity. The air-conditioned cars and heavy-duty cooling cars had the better pumps.

Now our cars, for various reasons, need even better cooling and we search for the best pumps and radiators.

olds 307 and 403

I am also a firm believer that a large 2 core will out perform a 4 core rad. Less restriction for coolant and air flow.

Koda

There's a couple things going on with the slow down to cool more misinformation.

You're going to have more coolant in the radiator than in the jackets, so slowing the coolant down would cool the individual mass of water more, but there is an issue, which is that we are not concerned with the change in temp per mass, we are concerned with the change in temp per time.

dT/dm * dm/dt = dT/dt. (change in temp per unit mass time change in mass per unit time cancels out to be change in temp per unit time)

Example: I am going to give you a quarter every 3 seconds. Or, I am going to give you a dime every second. Which one gives you more money? The dimes are worth less, but you are getting them faster enough that you end up with more money. Same theory with fast coolant flow, provided, like said above, you avoid cavitation and turbulent flow.

FN723

It's because an engineer gets paid to change things. It doesn't have to be an improvement, it just has to have the perception of an improvement.

joe_padavano

Sorry, but as an engineer I have to call BS on that statement. Most of the "different must be better" parts you see were developed by marketing people, not engineers. Exhibit A is the tubular manifolds marketed by a certain repro parts vendor. No engineer went anywhere near these.

VC455

i worked as an engineer for a marketing-driven company for 25 years and can verify what Joe said.

Marketers are evaluated on their ability to come up with new products; they constantly tinker with otherwise good products in an effort to stand out from their brethren.

Engineers are evaluated on their ability to make things that work well; they have a quality mindset.

AJFink

I being a Civil Engineer can't shake this image, there is so much wrong here .........Where do you come up with this stuff Joe?

Koda

Joe is professionally disgruntled at bad engineering, as am I.

Another thing engineers do is NOT CHANGE something that works. I'm a production engineer, I loathe doing anything more than periodic maintenance on equipment because then you have to confirm, and, even if you do, you may get a midnight call that the line is down.
Thus, no engineer would make anything new in the area of Olds exhaust. You have manifolds to look correct, and headers for performance. Use one.

Engineers, like doctors, get profound disrespect from the ignorant who think they they know just as much because they don't know what they don't know. This is usually seen in machinists, who think they know everything . A good engineer will know that each profession
has their own knowledge, and their own ignorance, on the other hand. Personally, I love my machinists. They like my original designs, and the donuts I bribe them with.

That exhaust manifold was thought up by Kryta looking up at his HVAC ducting in his building.

joe_padavano

Truth be told, there IS one type of engineer who changes things just for the sake of change. That would be software engineers, particularly those who work for Microsoft.

VC455

Joe, the MS engineers do have a reason for change.

They poll people who do not use their products to learn the type of interface they favor.

They make that change to attract those non-users.

Fun71

That made me remember this Dilbert cartoon from the 90s: