apexkw

this is what i was trying to say!!!!!!!!!! by touching the ice cube really fast you never cool down your finger and you never heat up the ice cube. i guess the way i worded it just came out wrong.

maxpesce

You all forget that a radiators capacity is measured in BTU/HR the hr part = TIM, so ALL the heat excange equations become TIME dependent -
the Greater the retention time in the Rad the more COOLING takes place (until the water temp = air temp) and the GREATER the Delta T between the ENGINE and the RADIATOR. the GREATER the the Delta the HIGHER the transfer efficency - you want the water in the block to get as HOT as possible w/o boiling and as COLD as possible in the Radiator. This is accomplished by carefully regulating the FLOW RATE through the system - the THERMOSTAT is the PRIMARY means of adjusting the flow rate in the RADIATOR.

paw140

Your logic is wrong. I'm not sure how to explain this any better... Jeff had a pretty good explanation for your icecube analogy. Like Jeff said, using your icecube example, you need to imagine a chain of icecubes. Let's say you drag this chain of icecubes over your hand very slowly. Your hand is ALWAYS in contact with the icecubes. Next, drag the chain of icecubes across you hand very rapidly. Again, your hand is always in contact with the ice cubes. In both examples, the transfer of heat from your hand to the icecubes will be roughly the same.

Another example. Take a large tub of water and heat it up to a boil. Place your arm in this boiling pot of water. While keeping your arm submerged in the water, move it around very rapidly. You will get burned, right? The heat transfer to your arm does not stop because you are moving it around faster. It will probably burn you worse if you move your arm around because your arm will actually cool down the water around it.

You are using time incorrectly. Also, the delta T you need to look at is between the radiator and the surrounding air, not between the engine and the radiator, since the energy is being transfered to the air.

This is getting complicated to explain, but the bottom line is that flowing faster generally accomplishes better cooling, and a more uniform coolant temperature. Residence time in the radiator is irrelevant.

Brad

Except for the fact paw140 was being sarcastic and saying that logic was bunk.

Brad

lol....guess I should read to the end of the thread before posting. DER!

maxpesce

You can only have a heat transfer if the RAD is COOLER than the Engine (rergardless of how the RAD is cooled) if the water leaves the outlet of the rad at the same temp as the water in the block the it WILL NOT cool the block (wnless the water gets hotter) as the heat transfer in the radaitor is NOT instantanious - (thermal inertia) the LONGER the water stays in the Rad the cooler it gets (untill it reaches the same temp as the heat sink ie the ambient air). But the point is MOOT since w/o a Thermostat the engine will run too COLD except at MAXIMUM output (ie WOT under LOAD) the THERMOSTAT is ther because the cooling system is designed to matain optimum temprature for ALL operating conditions - so RAD is SIZED to accomedate the heat transfer at MAX load and the the THEROMOSTAT maintains Optimum TEMP at anything LESS than MAX load by recirculating part (or MOST) of the coolant through the engine and BYPASSING the RAD (no Cooling).

Brad

I've got an idea. How bout we just squash it and say the official word is...

"IT IS NOT RECOMMENDED THAT THE THERMOSTAT IS REMOVED FOR ANY REASON AND DOING SO MAY CAUSE DAMAGE TO YOUR MOTOR"

paw140

This is incorrect and impossible. The radiator is always losing heat to the atmosphere, assuming it is above ambient temperature. And the radiator will always be cooler than the engine.

I agree that if, for some strange reason that defies the laws of thermodynamics, that if the inlet and outlet temps of the radiator are the same, then no cooling is taking place. But it is impossible for this to occur if there is a temp differential between ambient air and the coolant. By speeding up coolant flow, you are merely evening out the temperature distribution of the coolant throughout the system. For example, you can flow very slowly and have the radiator inlet temp at 250F and the outlet at 150F, or you can flow very rapidly and have the inlet at 201F and the outlet at 199F. In both cases approximately the same amount of thermal engery will be lost to the atmosphere, but you will be running much more consisent at the higher flow rate.

cover8

ok thanks...

I'll leave my thermostat in!

neit_jnf

iTrader: (17)

I'm bringing this back because I'm very surprised nobody mentioned the different heat transfer modes taking place in the cooling system.

If my memory serves me right...

Heat transfer due to a fluid (coolant or air) on a surface (engine, rad) is mostly due to convection, and the faster the flow the higher the heat transferred between the fluid and the surface. That's why higher flowing fans cool a radiator faster.

Now there's also a combination of conduction and convection there, maybe here lies the confusion many seem to have. The faster the fluid moves the better the convection but the lower the conduction of heat between the molecules as they don't have as long time together to transfer the heat. (Of course there's also radiation and natural convection involved but I won't go there) This is also true backwards, a still fluid in contact with a different temperature surface will conduct the heat better thru the molecules but will have minimal convection (some natural convection will still be there)...

So the question would be, which is the highest operating mode of heat transfer in the car's cooling system?

My perspective is something like this: (this is my opinion, I may be wrong)

Engine to coolant:
Mostly convective heat transfer; why? Because if it was conduction we would use a highly conductive solid connected to a radiator to shed the heat to ambient air and no fluid in the engine internals or rad except for the ambient air cooling it.

coolant to radiator: same as above

radiator to ambient:
Mostly convection as well but with a higher degree of radiation than the others, thus the name "radiator". The rad will radiate heat away even with no fans but at a far less rate not sufficient for proper operation of the car.

As far as heat transfer rates go I think (correct me if I'm wrong) that

convection > conduction > radiation

So after this half asleep dissertation going back to my college years I conclude that the better the flow the more the heat is transferred... as long as the coolant stays liquid and the pump doesn't cavitate.

Discuss?

I also wanted to add that I agree with this:

With the slower flow half the engine will be cooler than the other as well. With the faster flow the temperatures tend to even out, which is better for the engine.

KaiFD3S

iTrader: (37)

Just leave the stock one on, like I said on the other thread when I bought my car the previous owner had drilled 6 holes throught the t-stat which caused my "cooling problem" my car would never reach operating temps, it would always stay at 74c and sometimes drop down to 69c which really sucked, I could not go WOT since engine was still too cold.

I reaplaced it, now no more problem....

paw140

I love these discussions!

Quotes from my thermodynamics book:
"Conduction is the transfer of energy from the more energetic particles of a substance to the adjacent less energetic ones as a result of interactions between the particles. In gases and liquids, conduction is due to the collisions of the molecules during their random motion. In solids, it is due to the combination of vibrations of the molecules in a lattice and the energy transport by free electrons."

"Convection is the mode of energy transfer between a solid surface and the adjacent liquid or gas which is in motion, and it involves the combined effects of conduction and fluid motion. The faster the flulid motion, the greater the convection heat transfer. In the absence of any bulk fluid motion, heat transfer between a solid surface and the adjacent fluid is by pure conduction. The presence of bulk motion of the fluid enhances the heat transfer between the solid surface and fluid, but it also complicates the determination of heat tranfer rates."

(Cengel, Y. Introduction to Thermodynamics and Heat Transfer, McGraw-Hill, 1997)

So, yes, I would say you are right.