Interesting. Why, please?
Again, interesting. So you’re saying that the total volume of cooling passages, manifold water rail etc. in the engine is exactly equal to the volume of cooling passages in the radiator? Presumably the volume of the upper and lower radiator hoses, coolant reservoir tank etc. are not involved in the calculation?
Go to Stewart water pumps and read tech tip #3.
They make water pumps for NASCAR.
The facts and wives tales explained.
I would paste a link, but I am unfamiliar with this forums program.
That’s a pretty good article. Here’s the link:
It’s in line with my thinking that the higher the flow rate the higher the heat transfer because the temperature differential is higher. I can sort of believe choking the flow just downstream of the CC could cause a pressure build up there and maybe raise the boiling point, but I cannot understand why you would choke the flow downstream AND upstream of the CC.
But willing to be educated!
Rick $.02
A common misconception is that if coolant flows too quickly through the system, that it will not have time to cool properly. However the cooling system is a closed loop, so if you are keeping the coolant in the radiator longer to allow it to cool, you are also allowing it to stay in the engine longer, which increases coolant temperatures. Coolant in the engine will actually boil away from critical heat areas within the cooling system if not forced through the cooling system at a sufficiently high velocity. This situation is a common cause of so-called “hot spots”, which can lead to failures.
Above is the relevant part. Their reason for higher flow being better is completely incorrect BTW. But pertaining to your remark “faster flow prevents hot spots,” I interpreted your statement as arguing that hot spots decrease in number or severity, etc. as flow increases. What the “tip” says is that the coolant will boil if flow is inadequate. It doesn’t say that more flow beyond minimally adequate is better still. Hot spots depend on pressures, cavitation and other factors, as discussed earlier in this thread. IMHO. What about your “equal time” statement BTW?
What temperature differential do you mean?
BTW, as I mentioned they are explicitly wrong IMO in that they bring up the myth of keeping the coolant in the radiator to allow it to cool. You don’t want to cool the coolant, you want to cool the engine.
It surprises me how this silly theory of water going too fast to cool continures. No one wants to slow down the air through the radiator to cool better. No one closes the windows (with no A/C) to slow the air down through the car on a hot day to cool better. No one looks for the slowest room fan to sit in front of on a hot day. Everyone knows if you are cooling something hot you run more cold water over it, not less. But for some reason in a car engine, some want to slow the water down to cool better???
Tom
I was thinking back to a heat transfer course I took a million years ago (and would likely fail today:-). A theoretical case of a cold fluid flowing through a hot pipe. The delta T I was referring to is the pipe vs the fluid temperature.
What is the optimum flow rate for max heat transfer assuming a constant source of heat for the pipe?
If the flow was zero, the fluid and pipe would eventually be the same; zero delta T. If the flow was fast, would the heat transfer keep increasing or hit a “sweet spot” where any faster and the heat transfer is actually less. The faster the flow the higher the delta T since the fluid temp isn’t raising as much.
I can’t imagine there being a “sweet” spot. It seems to me the the higher the flow rate the higher the heat transfer. This of course ignores any effects raising the pressure with restrictions and raising the BP.
Or something like that:-)
Rick
I suspect this started with a cooling article in one of the hot rod magazines many many years ago. I remember reading it in my teens and thinking that it did not make much sense when thought about in the principles of high school physics. Probably circulated as an urban myth type thing for many years before that.
You’ve got it basically correct–you want the flow to be so fast that all parts of the hot pipe are in contact with new water that hasn’t had a chance to heat up at all. That would be at infinite flow rate. Lack of this theoretical ideal would be measurable as a temperature increase in the cold water exiting the pipe compared to its temperature as it enters. The less this out-in temp differential, the better the cooling.
But yes, there is a “sweet spot.” That’s because it takes work (pressure X flow rate) to move water through a pipe, and that work generates heat. So, disregarding the heat source for the pipe, the water will heat itself (and the pipe) due to fluid resistance. At low flow rates (Poiseuille flow) the work/heat is linearly proportional to flow rate. At higher flow rates (approaching or becoming turbulent–the actual situation) work/heat goes as flow rate squared or worse (not a precise analytic equation). So you need to keep the flow reasonable. IMHO.
This isn’t exactly what’s going on in a car radiator, though. The conduction of heat from the coolant to the walls of the radiator tubes is important, but the paradox related to coolant flow rate (more is better) applies to the temp gradient between atmosphere and radiator fins. To move more heat, you want the fins to be as hot as the engine, so that the gradient is max. If the coolant cools during transit through the radiator, then the last fins are cooler, and not as efficient in transferring heat to the ambient air. Also IMHO.
I have this, as far as I remember it matches my 4.2 long stud original, the larger holes should be on the exhaust side because that is where the water is pumped in and up out the head, before that I had composite gaskets that also have small holes: 6 and 10 mm.
I’m away from my files but the Australian coolant author was Norman something IIRC.
His mod involved adjusting flow path around the head.
On the assumption that water flow is biased towards the shortest path across a pressure gradient, Norman felt the flow was biased against reaching the distant rearmost cylinders. His hypothesis was that having entered the block low on the exhaust side, more of it would flow past front cylinders 4-6 towards the head and manifold exit passages, leaving the rear cylinders 1-3 to run hotter than the fronts. We had a similar pathway discussion with the late Pete Petersen and S2 downflow radiator inlet position on the top tank.
Norman’s solution was to restrict the front vertical slot between inlet ports #1 & 2 by about 60-odd percent, the slot between 3 & 4 by about 30 percent and the rear slot untouched. The 3.8s have no other passages and Norman reported successful cooling improvement in his hot Australian climate and repeated the experiment on other Es of 3.8,and 4.2L capacity.
He came to my house with his US friend who also posted here. I performed his mod on an engine but as I had no experience of that engine prior to the mod I couldn’t come to any conclusion.
Norman Lutz I think? He also had a modification for the V12 along the same lines.
The below articles would lead me to believe there is a “Goldilocks” flow rate; not too fast, not too slow, just right.
According to Motor Trend:
and a site named Engine Basics:
Read the first one. Slight error in their logic (see below). Laminar (Poiseuille’s Law) flow is the slowest– characterized by a low Reynold’s number. Turbulent flow is faster. Opposite of what they say. They are correct about the layers and why it’s not ideal for heat transfer. But that’s a reason for fast flow, not for slow flow. IMHO.
Moving fluid too quickly through an area can result in laminar flow, where the fluid forms layers. The layer closest to the surface moves slower than layers farther away from the surface. When this occurs, the layers act as insulators and the capacity to transfer heat is diminished.
Edit: Just read the second article. Seems pretty good IMHO.
Robert you are absolutely correct the the article has it backwards regards flow rate and laminar flow regimes.
I believe several different concepts are being grouped together, helping create confusion.
The initial concept was if coolant moves too fast, it does not have time to pick up the heat from the engine. False.
Next, an Australian slowed the flow and got better cooling, based on the too fast does not pick up heat. Later it was confirmed false, he restricted flow in some places to redirect flow to others, and I would not dispute that may have improved coolant. Different concept.
Then an article says too fast flow may cause laminar flow, not allowing the majority of the coolant to be next to the surfaces. Again, may be true but not because the coolant does not have time to pick up heat, but because the flow is laminar.
Then some mentioned restricting flow may increase pressure at certain places, decreasing vaporization, or the increase pressure may prevent “bubbles” in the cavities, etc. Again, may be true but not a matter of giving the coolant time to pick up more heat.
These cannot all be grouped together, IMO.
Tom
Agree totally.
20 characters…
No! Sorry to repeat myself but laminar flow is when flow is too slow, not too fast. IMHO.
For the engineering inclined.
What is Laminar Flow - Viscous Flow - Definition (thermal-engineering.org)
Apologies for posting the (poorly written and edited?) Motor Trend article. I think the second article from EngineBasics.com is much better.
Robert do not be sorry. I was repeating what the article was saying. I then said “may be true” as my way of saying I was not concerned if it was true or not as that was not my point. I should not have said it that way.
Tom
