pedal2themetal45
Famous Member
Bort62":ydwpxxet said:
Tim
Overheating at cruise
- Thread starter Bort62
- Start date
Tim
Well Jack I have to say that is the best arguement I have heard for using some sort of flow restriction in the system.. But it still doesnt quite add up. Nucleate boiling inside the engine would actually increase heat transfer by several orders of magnitude.
Localized nucleate boiling like you describe is actually desirable and is one of the best heat transfer mechanisms in a system like this.
And besides, that heat is then still stored in the water, and as soon as it passes through the restriction and drops to lower pressure, it would flash boil.
So it doesn't quite do it. It may be possible in some engines that having a flow restriction at the Tstat helps because of the design of the cooling passages and it is nesssicary to maintain proper flow in all areas. That is hard to know without closely studying the layout of the engine.
But the statement that the restriction is needed to slow the water down so it has enough time to get the heat out is utter nonsense. I know that makes people mad, but it is simply 100% false.
Localized nucleate boiling like you describe is actually desirable and is one of the best heat transfer mechanisms in a system like this.
And besides, that heat is then still stored in the water, and as soon as it passes through the restriction and drops to lower pressure, it would flash boil.
So it doesn't quite do it. It may be possible in some engines that having a flow restriction at the Tstat helps because of the design of the cooling passages and it is nesssicary to maintain proper flow in all areas. That is hard to know without closely studying the layout of the engine.
But the statement that the restriction is needed to slow the water down so it has enough time to get the heat out is utter nonsense. I know that makes people mad, but it is simply 100% false.
Bort62":3emwz0zn said:
Agreed. That is nonsense, but it's hard to get people to understand. The counterpoint I make is that if it's moving too fast to get rid of heat, it's moving too fast to pick up heat.
As far as the nucleate boiling, I think the explanation is that though active boiling would remove heat, the actual heat transfer from the iron block to gas is far lower than thru liquid, hence the desire to increase pressure to incrrease the boiling point.
There could be many related causes, but removing the restriciton in a Cleveland is a no-no. :nono:
Well it is hard to argue with emperical evidence like "I removed the T-stat and the car started overheating"
But it is important to understand exactly why this is happening. Time spent in the engine or the radiator has nothing to do with it.
The process of phase changing water to steam takes a huge amount of energy (heat). This heat comes out of the engine block. That steam then bubbles through the surrounding water and cools back down and re-condenses. The result is that the overall energy absorbed by the water is increased when boiling occurs.
And yes, while the higher pressure that could be created by the flow restriction (a few PSI at most) will raise the boiling point of the water, if X amount of heat is being dumped in, if it doesnt boil under pressure, as soon as the water passes through the restriction and the pressure drops, it will still contain X amount of heat - and will flash boil upon expansion.
The most likely cause of thermostat related effects on overheating is that the absence of the flow restriction will change the fluild flow properties of the engine's cooling passages, causing higher volumes to flow through certain areas and consequently lower though others. Those lower flow areas will as a result heat up and could cause you problems.
That is however going to be specific to a given engine. It may be the case with the small ford six, but I doubt it.
Worth an experiment I suppose.
But it is important to understand exactly why this is happening. Time spent in the engine or the radiator has nothing to do with it.
The process of phase changing water to steam takes a huge amount of energy (heat). This heat comes out of the engine block. That steam then bubbles through the surrounding water and cools back down and re-condenses. The result is that the overall energy absorbed by the water is increased when boiling occurs.
And yes, while the higher pressure that could be created by the flow restriction (a few PSI at most) will raise the boiling point of the water, if X amount of heat is being dumped in, if it doesnt boil under pressure, as soon as the water passes through the restriction and the pressure drops, it will still contain X amount of heat - and will flash boil upon expansion.
The most likely cause of thermostat related effects on overheating is that the absence of the flow restriction will change the fluild flow properties of the engine's cooling passages, causing higher volumes to flow through certain areas and consequently lower though others. Those lower flow areas will as a result heat up and could cause you problems.
That is however going to be specific to a given engine. It may be the case with the small ford six, but I doubt it.
Worth an experiment I suppose.
Also, increased velocity (IE less restriction) will nearly linearly increase the heat transfer coefficient. (both into the coolant in the engine, and out of the coolant in the radiator)
I have seen before people who observed that removing the thermostat caused their car to run hotter on the gauge - this is true but gives false intuition.
The metal of the engine block at its hottest points is actually at a temperature much hotter than the coolant. The cast iron engine block just isnt that great of a heat exchanger.
Increasing the velocity of the coolant will increase it's effectiveness. This will mean that more heat is being transfered into the coolant. As a result, the coolant temperature will rise and the metal's temperature will fall.
The observation is that the engine temperature gauge rises, because this measures coolant temperature.
This gives a false impression because the actual engine itself is running cooler. An oil temperature gauge is a good way to observe this.
It's a tricky subject, and a lot of the time rules of thumb are good enough to get people by... but not always.
I always prefer to understand what is actually going on.
I have seen before people who observed that removing the thermostat caused their car to run hotter on the gauge - this is true but gives false intuition.
The metal of the engine block at its hottest points is actually at a temperature much hotter than the coolant. The cast iron engine block just isnt that great of a heat exchanger.
Increasing the velocity of the coolant will increase it's effectiveness. This will mean that more heat is being transfered into the coolant. As a result, the coolant temperature will rise and the metal's temperature will fall.
The observation is that the engine temperature gauge rises, because this measures coolant temperature.
This gives a false impression because the actual engine itself is running cooler. An oil temperature gauge is a good way to observe this.
It's a tricky subject, and a lot of the time rules of thumb are good enough to get people by... but not always.
I always prefer to understand what is actually going on.
66 Fastback
Famous Member
When you say over-heating, are you going by a gauge reading or is it physically boiling over? Could it be the temp gauge is off?
As far as the great heat transfer debate, I got into one several years ago with several folks on a Corvette discussion board. One guy decided to do a test and bought a set of those restrictors to see if the restrictors would help cooling as they claim. It should not be a surprise that they did not work.
They might work on an engine that is being operated outside of the cooling system's design range, in particular the water pump.
One old hot rodder claimed he used to have to remove half of the impeller blades on his race engines in order to get them to cool giving support to the slower velocity yields better cooling theory. I contended he was operating a stock cooling system outside of its design envelope. He made big horsepower at high rpm's. What likely was happening was his water pump was cavitating and having suction problems at the higher rpm's therby reducing the pump's flow. Removing the blades allowed it to run at the higher rpms and pump enough coolant.
However, the best way to address this nowadays would be to run a high flow water pump such as Stewarts racing pumps. But back in the day of this old rodder, there was not a large selection of after market parts.
Doug
As far as the great heat transfer debate, I got into one several years ago with several folks on a Corvette discussion board. One guy decided to do a test and bought a set of those restrictors to see if the restrictors would help cooling as they claim. It should not be a surprise that they did not work.
They might work on an engine that is being operated outside of the cooling system's design range, in particular the water pump.
One old hot rodder claimed he used to have to remove half of the impeller blades on his race engines in order to get them to cool giving support to the slower velocity yields better cooling theory. I contended he was operating a stock cooling system outside of its design envelope. He made big horsepower at high rpm's. What likely was happening was his water pump was cavitating and having suction problems at the higher rpm's therby reducing the pump's flow. Removing the blades allowed it to run at the higher rpms and pump enough coolant.
However, the best way to address this nowadays would be to run a high flow water pump such as Stewarts racing pumps. But back in the day of this old rodder, there was not a large selection of after market parts.
Doug
66 Fastback
Famous Member
When I got out of school, I don't think I even measured up to half a clue after my thermo classes. I later had to work with gas plants, compressors and generators and was forced to re-visit my thermo texts.
That piece of information is something I never considered. You had me a bit stumped when you said nucleate boiling would increase the heat transfer.
One of the racers on the Corvette website mentioned that he ran external plumbing in an attempt to balance the cylinder temperatures of his engine and optimize the performance. I agree that pulling the thermostat in some engines might change the fluid flow in some engines that could possibly lead to some problems. I also think that it could be affecting the pump curve. I can't say I have ever seen a pump curve for a automobile water pump, nor have I seen a system head curve for an engine block. But I suspect removing the thermostat in some cars changes the system curve enough that the pump could experience problems. But again, that is speculation on my part. But I base it on the fact that centrifugal pumps can pump anywhere from 0 gpm to maximum output at the same fixed pump speed. The governing factor is the system head curve and net positive suction head requirement. The radiator cap does not affect the system head pressure with regards to the pump flow. The reason is that the radiator cap imposes its 15 psig on both the suchtion and the discharge side of the pump, so it essentially cancels out the effect on the system or friction curve of the block.
Amen Brother. As Bort mentioned, the higher flow rate will pick up more heat. And it only makes sense that if you are transporting more heat from the block to the radiator, then the temperature of the coolant would have to be raised since it is carrying more heat. It may not seem logical, but a higher coolant temp entering the radiator is able to "drive" off more heat through the radiator than at a lower coolant temp. The reaon is that when there is a greater difference in temperature between two surfaces, then there is a greater driving force to push the heat from the hot side to the cold side.
With respect to higher coolant flow increasing the heat transfer or cooling, think of a wind chill analogy. Say you are outside and it is 40 deg F and there is no wind. Then think of the same 40 deg F temperature but ther is a 30 mph wind. Why do you feel colder with the higher wind? It is because more heat is being removed from your body.
hink about the
If you buy into the argument that the coolant needs to slow down in order to allow it enough time to both absorb the heat from the block and to expel the heat through the radiator, then using that logic, slower and slower flow rates should yield better heat transfer. Why is it that everyone is willing to accept the logic that more air flow results in better cooling, but they don't think the same thing is true on the coolant side of the block. Everyone agrees that putting big electric fans on the radiator should help cooling. Faster coolant flow and faster air flow do the same thing. No one would argue that you need to slow down the air flow around an air cooled engine in order to give the engine more time to transfer the heat. The radiator system is doing the same thing as an air cooled engine but does it in a two step process, shedding heat into the coolant and then into the air.
Addo, I don't know a coefficient for FeO, but I am assuming that you are referring to rust/corrosion products. Bort may be able to explain it much better, but corrosion products and films provide another boundary layer that the heat has to be driven through. It kind of behaves like a dirty connection in an electrical circuit and the net result is increased resistance to the flow of heat. One of the reasons that higher flow rates help increase heat transfer is that the boundary film is reduced.
Sorry for the disertation, but I enjoy a good heat transfer debate.
Doug
Bort, with regards to your car, you might want to verify that timing mark on the balancer ring has not slipped. I had a host of prolems at one time that contributed to my cooling problems. Most of them ended up being ignition timing related due to the slipped balancer timing mark and insufficient ignition advance due to a bad vacuum diaphram on my Loadamatic distributor.
Addo, I have no idea what Iron Alloy the blocks are cast out of, so I can't really begin to make a guess at the differece between the bare iron and it's oxide. Chances are however that the oxide layer has a pretty low thermal conductivity.
However, it's not very thick nominally so it won't have much effect.
If you let a half inch of rust build up, it could act as a pretty good insulator.
However, it's not very thick nominally so it won't have much effect.
If you let a half inch of rust build up, it could act as a pretty good insulator.
The equation for heat transfer is as follows (simplified)
Heat flux = temperature difference / thermal resistivity of the junction.
Its pretty straitforward except for the denominator
That is where velocity, thermal conductivity, turbulence, viscosity, and a bunch of other fun things come into play.
But generally speaking in the case of convection, the following things increase heat transfer (in rough descending order of sensitivity):
Greater difference in temperature
increased contact area
higher velocity
turbulence
higher thermal conductivity of the hot wall material
lower viscosity of the fluid
And the opposite will reduce heat transfer.
Because the idea here is to move as much heat out of the engine and into the air, you want to increase the heat transfer into the coolant as much as possible. So taking all of these steps will help ya out. Altho I don't suggest anyone try altering the specific gravity of water in order to make the car run a little cooler
Nucleate boiling is a different effect that is not strictly part of convection. It is a heat transfer mechanism in of itself.
Heat flux = temperature difference / thermal resistivity of the junction.
Its pretty straitforward except for the denominator
That is where velocity, thermal conductivity, turbulence, viscosity, and a bunch of other fun things come into play. But generally speaking in the case of convection, the following things increase heat transfer (in rough descending order of sensitivity):
Greater difference in temperature
increased contact area
higher velocity
turbulence
higher thermal conductivity of the hot wall material
lower viscosity of the fluid
And the opposite will reduce heat transfer.
Because the idea here is to move as much heat out of the engine and into the air, you want to increase the heat transfer into the coolant as much as possible. So taking all of these steps will help ya out. Altho I don't suggest anyone try altering the specific gravity of water in order to make the car run a little cooler
Nucleate boiling is a different effect that is not strictly part of convection. It is a heat transfer mechanism in of itself.
Addo, if you want to play around with different materials, check out this website.
http://www.matweb.com/search/SearchSubcat.asp
Just look how many Iron Alloy's are listed - with wildly varying properties.
Materials Engineering is a pretty rough subject IMO
http://www.matweb.com/search/SearchSubcat.asp
Just look how many Iron Alloy's are listed - with wildly varying properties.
Materials Engineering is a pretty rough subject IMO
Similar threads
