Heat Transfer Re-visited Again!

[Doug Flaten]

Re: Jerry: Exchange faster / Better?

Tom, I have to agree with Jerry that more flow rate yields better cooling. I am an engineer and occasionally run into cooling problems on our compressor engines and gas plant heat exchangers. Although I did not pay much attention in my Thermodynamic class, I have re-educated myself enough over the years in order understand my work related purchases. I try to get things explained to me in "bubba-speak" so that I don't have to thouroughly understand all of the equations. Here is my attempt... When you think about it there are two heat exchangers on the cooling system. The engine rejects heat to the cooling water via passages in the block and the radiator rejects coolant water heat to the air. There are two sides to the radiator and the flow rate on either side of the tubes regulates the heat transfer. If you slow down the air movement, you will reduce the amount of heat that is carried away from the radiator. For some reason we are all willing to agree that more air flow at the radiator will cool the water and the engine. The same is true of an air cooled engine. The water cooled engine is no different than the air cooled engine. In order to carry away more heat from an air cooled engine, you circulate more cooling air past the engine or compensate for lower air flow or higher air temp by adding larger cooling fins. To cool the water cooled engine, circulate more water. Why wouldn't more water flow at the engine cool the engine? Higher fluid velocities also increase heat transfer by reducing the fluid film thickness on the tubes. The higher velocity also helps keep the tube clean of scale and dirt buildup.

Some prior postings suggest to decrease the coolant circulation rate so that the water picks up more engine heat in order to maximize the temp differential at the radiator. There has also been mention that the slower rate will give the water more time to cool in the radiator. It is true that the temperature differential is a driving force similar to voltage in an electrical circuit. When the coolant rate is slowed down more heat is added to a unit volume of water via conduction. At the same time, the lower flow rate reduces the heat that is transferred from the block by convection. The heat transferred from the engine to the radiator is a function of the flow rate of the coolant. And is the dominant transfer mechanism. The purpose of the cooling system is to carry heat away from the engine and that cannot be done at lower circulation rates that rely more on conduction as opposed to convection. Technically it is the log mean temperature difference (LMTD) that is used for sizing heat exchangers and we are dealing with two exchangers. The water is the cooling media at the engine block but at the radiator the air is the cooling media. The way the LMTD formula works, the water is the cold fluid at the engine and at the radiator it is the hot fluid. The increased temp differential at the block appears in a different spot in the formula when calculating the LMTD at the radiator. Basically it helps move more heat in one case and decreases heat transfer in the other case. Heat exchangers are always a compromise. There is no substitute for surface area. However larger coolers are more expensive, heavier and increase the frontal area of the vehicle. Higher coolant circulation rates increase the friction which can lead to cavitation. Higher pressure requires more pump horsepower and thicker tubes to contain the coolant which translates into heavier less efficient radiators. Then you are back to increasing cooler size I will end with this... If your engine already tends to overheat, does it make sense to add more heat to the coolant at the engine so that you will have to reject more heat at the radiator?

P.S. I was under the impression that coolant restrictors limited flow to the radiator so that heat was retained in the engine. Higher temperatures lead to higher combustion pressures, efficiency and more power. However, durability decreases with temperature but that is not a huge factor with drag racers who regularly dismantle the engine.

[Doug Flaten]

Re: Jerry: Exchange faster / Better?

Tom, I have to agree with Jerry that more flow rate yields better cooling. I am an engineer and occasionally run into cooling problems on our compressor engines and gas plant heat exchangers. Although I did not pay much attention in my Thermodynamic class, I have re-educated myself enough over the years in order understand my work related purchases. I try to get things explained to me in "bubba-speak" so that I don't have to thouroughly understand all of the equations. Here is my attempt... When you think about it there are two heat exchangers on the cooling system. The engine rejects heat to the cooling water via passages in the block and the radiator rejects coolant water heat to the air. There are two sides to the radiator and the flow rate on either side of the tubes regulates the heat transfer. If you slow down the air movement, you will reduce the amount of heat that is carried away from the radiator. For some reason we are all willing to agree that more air flow at the radiator will cool the water and the engine. The same is true of an air cooled engine. The water cooled engine is no different than the air cooled engine. In order to carry away more heat from an air cooled engine, you circulate more cooling air past the engine or compensate for lower air flow or higher air temp by adding larger cooling fins. To cool the water cooled engine, circulate more water. Why wouldn't more water flow at the engine cool the engine? Higher fluid velocities also increase heat transfer by reducing the fluid film thickness on the tubes. The higher velocity also helps keep the tube clean of scale and dirt buildup.

Some prior postings suggest to decrease the coolant circulation rate so that the water picks up more engine heat in order to maximize the temp differential at the radiator. There has also been mention that the slower rate will give the water more time to cool in the radiator. It is true that the temperature differential is a driving force similar to voltage in an electrical circuit. When the coolant rate is slowed down more heat is added to a unit volume of water via conduction. At the same time, the lower flow rate reduces the heat that is transferred from the block by convection. The heat transferred from the engine to the radiator is a function of the flow rate of the coolant. And is the dominant transfer mechanism. The purpose of the cooling system is to carry heat away from the engine and that cannot be done at lower circulation rates that rely more on conduction as opposed to convection. Technically it is the log mean temperature difference (LMTD) that is used for sizing heat exchangers and we are dealing with two exchangers. The water is the cooling media at the engine block but at the radiator the air is the cooling media. The way the LMTD formula works, the water is the cold fluid at the engine and at the radiator it is the hot fluid. The increased temp differential at the block appears in a different spot in the formula when calculating the LMTD at the radiator. Basically it helps move more heat in one case and decreases heat transfer in the other case. Heat exchangers are always a compromise. There is no substitute for surface area. However larger coolers are more expensive, heavier and increase the frontal area of the vehicle. Higher coolant circulation rates increase the friction which can lead to cavitation. Higher pressure requires more pump horsepower and thicker tubes to contain the coolant which translates into heavier less efficient radiators. Then you are back to increasing cooler size I will end with this... If your engine already tends to overheat, does it make sense to add more heat to the coolant at the engine so that you will have to reject more heat at the radiator?

P.S. I was under the impression that coolant restrictors limited flow to the radiator so that heat was retained in the engine. Higher temperatures lead to higher combustion pressures, efficiency and more power. However, durability decreases with temperature but that is not a huge factor with drag racers who regularly dismantle the engine.

[Tom B.]

Re: Doug: Exchange faster / Better?

I appreciate the information and long effort Doug. It may be time for me to apply all this to the suggestion I was given (a flow restrictor kit) for my particular circumstance.

I have worked with my 68 L89 cooling system the past couple of months. I have the operating temperatures within a parameter that I am satisfied with. In town for stop and go cooling I'm running 205-210 degrees. On the highway, at higher RPM's (and correct clutch fan shut down) I'm running 210-215 degrees.

I was looking for a specific application to correct my higher temperatures at high RPM highway speed, and obviously was told of the "flow reduction idea". It appears to me that I should already have a high rate of coolant flow, and from the information "opposite" of flow-reductiion, should already be experiencing cooler temperatures, which has not been the case.

I'm beginning to wonder if we are discussing a difference in principle that will apply only under specific circumstances OR that either (the "fast" or "slow" principles) have a negligible difference with the design, components, etc, of an automotive cooling system. I'm still pondering Doug, but am re-reading everyone's information over and over. Thanks again. Tom #24014

[Tom B.]

Re: Doug: Exchange faster / Better?

I appreciate the information and long effort Doug. It may be time for me to apply all this to the suggestion I was given (a flow restrictor kit) for my particular circumstance.

I have worked with my 68 L89 cooling system the past couple of months. I have the operating temperatures within a parameter that I am satisfied with. In town for stop and go cooling I'm running 205-210 degrees. On the highway, at higher RPM's (and correct clutch fan shut down) I'm running 210-215 degrees.

I was looking for a specific application to correct my higher temperatures at high RPM highway speed, and obviously was told of the "flow reduction idea". It appears to me that I should already have a high rate of coolant flow, and from the information "opposite" of flow-reductiion, should already be experiencing cooler temperatures, which has not been the case.

I'm beginning to wonder if we are discussing a difference in principle that will apply only under specific circumstances OR that either (the "fast" or "slow" principles) have a negligible difference with the design, components, etc, of an automotive cooling system. I'm still pondering Doug, but am re-reading everyone's information over and over. Thanks again. Tom #24014

[John Fowler]

[John Fowler]

[Tom B.]

PS

Also, in my case, I have 4:11 rear gears, higher RPM at "highway" speed. Tom #24014.

[Tom B.]

PS

Also, in my case, I have 4:11 rear gears, higher RPM at "highway" speed. Tom #24014.

[Everett Ogilvie]

Re: Jerry: Exchange faster / Better?

Hello Doug,

Your post has lots of very good information. I think most of us agree with the basics of heat transfer that have been discussed. I especially agree with your first statement that more flowrate yields better cooling. IN AN OPEN SYSTEM.

If the liquid coolant was from an infinite supply, then open her up and let it rip! But we do not have an open system here. Our system is a single pass, crossflow heat exchanger (the hot fluid-the water, and the cold fluid-the air, move perpendicularly to each other). Our system is a CLOSED system from the perspective of the liquid as it recirculates and an open system from the perspective of the air as it is an infinite supply. This means the input temperature of the water at the radiator becomes greater as the system heats up. As the input temperature at the radiator increases, the time required to maintain an equal temperature differential between input and output increases, AT A FIXED AIR FLOWRATE. Because the system is closed, the coolant temperature increases to some maximum, and at that point the radiator may be close to its design limits. Then coolant residence time in the radiator becomes a factor. If this were an open system with an infinite liquid coolant source then increasing coolant flowrate would be beneficial. We don't have that in the closed system and we return hotter and hotter coolant to the engine as it warms up. When you have a 70 degree differential across the radiator as the thermostat starts to open (I chose 70 based on having a 160 thermostat and saying the cold side of the radiator might be at ambient temperature of 90), then the coolant residence time in the radiator is not a factor. As a matter of fact when the thermostat first opens "cold" water flows into the block and the thermostat will close again for a brief time. Right at this point the system is behaving as if it had an infinite source of liquid coolant because the input to the block is "cold". This cycling of the thermostat will continue until the coolant temperature going into the block exceeds the thermostat set point. At that point the temperature differential across the radiator may now be down to only 20 degrees. It is easy for me to imagine "some" high coolant flowrate in a closed system that would not allow enough residence time in the radiator to remove "enough" heat, thus overheating at higher RPM and the temperature slowly coming down with decreasing flowrate to some minimum point where temperature would climb again because of low, or no flow in the system. As the components in the system age, especially due to corrosion and scale, the radiator's efficiency is compromised, and it is even less effective at heat transfer which would require even greater coolant residence time to do the job it did as new.

I have not looked at heat transfer analytically in a long time and my view here is based on a quick review and some personal experience. I hope that our discussions here are fun, and out of interest, and that we all continue to share viewpoints, and information.

[Everett Ogilvie]

Re: Jerry: Exchange faster / Better?

Hello Doug,

Your post has lots of very good information. I think most of us agree with the basics of heat transfer that have been discussed. I especially agree with your first statement that more flowrate yields better cooling. IN AN OPEN SYSTEM.

If the liquid coolant was from an infinite supply, then open her up and let it rip! But we do not have an open system here. Our system is a single pass, crossflow heat exchanger (the hot fluid-the water, and the cold fluid-the air, move perpendicularly to each other). Our system is a CLOSED system from the perspective of the liquid as it recirculates and an open system from the perspective of the air as it is an infinite supply. This means the input temperature of the water at the radiator becomes greater as the system heats up. As the input temperature at the radiator increases, the time required to maintain an equal temperature differential between input and output increases, AT A FIXED AIR FLOWRATE. Because the system is closed, the coolant temperature increases to some maximum, and at that point the radiator may be close to its design limits. Then coolant residence time in the radiator becomes a factor. If this were an open system with an infinite liquid coolant source then increasing coolant flowrate would be beneficial. We don't have that in the closed system and we return hotter and hotter coolant to the engine as it warms up. When you have a 70 degree differential across the radiator as the thermostat starts to open (I chose 70 based on having a 160 thermostat and saying the cold side of the radiator might be at ambient temperature of 90), then the coolant residence time in the radiator is not a factor. As a matter of fact when the thermostat first opens "cold" water flows into the block and the thermostat will close again for a brief time. Right at this point the system is behaving as if it had an infinite source of liquid coolant because the input to the block is "cold". This cycling of the thermostat will continue until the coolant temperature going into the block exceeds the thermostat set point. At that point the temperature differential across the radiator may now be down to only 20 degrees. It is easy for me to imagine "some" high coolant flowrate in a closed system that would not allow enough residence time in the radiator to remove "enough" heat, thus overheating at higher RPM and the temperature slowly coming down with decreasing flowrate to some minimum point where temperature would climb again because of low, or no flow in the system. As the components in the system age, especially due to corrosion and scale, the radiator's efficiency is compromised, and it is even less effective at heat transfer which would require even greater coolant residence time to do the job it did as new.

I have not looked at heat transfer analytically in a long time and my view here is based on a quick review and some personal experience. I hope that our discussions here are fun, and out of interest, and that we all continue to share viewpoints, and information.

[Doug Flaten]

Re: Jerry: Exchange faster / Better?

Hi Everett, I agree with you that the liquid side is closed and would continually heat up if the heat was not removed adequately. I also agree that the air is an open system with a fixed air rate AT a given engine and road speed. The liquid coolant is primarily transporting the heat to the radiator. The faster that it is transported the more heat is removed from the engine. Even if the coolant had all vaporized to steam at 260 degrees, it would still carry some heat away from the engine because there is a very large temp differential between the combustion chamber and the water jacket. (Granted the pressure would have blown the radiator cap by that time). I think we sometimes tend to forget that either some engineer years ago went through the iterations on his slide rule or the shop foreman built and tested a cooling system by trial and error and together they designed a system that essentially worked. At low speed or idle the air flow is fixed at that engine speed. However, the radiator is supposed to have been designed to dissipate the heat at that low fan speed. It's ability to do that decreases as the air around the car heats up and you recycle hotter air. (Today's cars employ the supplemental electric fans to provide greater air flow at engine idle) As the cars speed increases, the fan pulls in more air in addition to the "ram air" that is supplementing the air. So as engine and heat load increase, the air flow increases and in the properly designed system, the radiator sufficiently cools the liquid to keep it from continually building heat and melting the engine. Of course, without that additional "ram air" the system overheats rapidly. (Just ask my coworker's teenage son who was waiting for his sister and fell asleep in the car with the AC and radio on and his foot planted on the gas pedal). The system was in all likelihood designed properly for both the low speed and high speed operation although at either extreme, performance may have been marginal. But corrosion and scale take their toll and we often do not adequately address radiator size when we hot rod the cars. Unfortunately the cooling systems were not over-engineered to work properly after 30+ years of decreasing efficiency or the addition of more power. Unless we increase the radiator's size, we are left with circulating more air or water via larger fans or pumps. Today's cars benefit from better longer lived coolants that do not foul the tubes as much, electric fans, and more air tight systems. I too think the discussion is fun and I wonder how things work as well as they do. Not being a mechanical engineer, I marvel at the work of those people that designed and built these machines.

[Doug Flaten]

Re: Jerry: Exchange faster / Better?

Hi Everett, I agree with you that the liquid side is closed and would continually heat up if the heat was not removed adequately. I also agree that the air is an open system with a fixed air rate AT a given engine and road speed. The liquid coolant is primarily transporting the heat to the radiator. The faster that it is transported the more heat is removed from the engine. Even if the coolant had all vaporized to steam at 260 degrees, it would still carry some heat away from the engine because there is a very large temp differential between the combustion chamber and the water jacket. (Granted the pressure would have blown the radiator cap by that time). I think we sometimes tend to forget that either some engineer years ago went through the iterations on his slide rule or the shop foreman built and tested a cooling system by trial and error and together they designed a system that essentially worked. At low speed or idle the air flow is fixed at that engine speed. However, the radiator is supposed to have been designed to dissipate the heat at that low fan speed. It's ability to do that decreases as the air around the car heats up and you recycle hotter air. (Today's cars employ the supplemental electric fans to provide greater air flow at engine idle) As the cars speed increases, the fan pulls in more air in addition to the "ram air" that is supplementing the air. So as engine and heat load increase, the air flow increases and in the properly designed system, the radiator sufficiently cools the liquid to keep it from continually building heat and melting the engine. Of course, without that additional "ram air" the system overheats rapidly. (Just ask my coworker's teenage son who was waiting for his sister and fell asleep in the car with the AC and radio on and his foot planted on the gas pedal). The system was in all likelihood designed properly for both the low speed and high speed operation although at either extreme, performance may have been marginal. But corrosion and scale take their toll and we often do not adequately address radiator size when we hot rod the cars. Unfortunately the cooling systems were not over-engineered to work properly after 30+ years of decreasing efficiency or the addition of more power. Unless we increase the radiator's size, we are left with circulating more air or water via larger fans or pumps. Today's cars benefit from better longer lived coolants that do not foul the tubes as much, electric fans, and more air tight systems. I too think the discussion is fun and I wonder how things work as well as they do. Not being a mechanical engineer, I marvel at the work of those people that designed and built these machines.

[Everett Ogilvie]

Re: Jerry: Exchange faster / Better?

Hi Doug,

I too marvel at how these old things still function. It may be that some of the cooling system designs back then were closer to the absolute limits of their performance than others, like 300 HP vs 375 HP, etc. And then if anything changes, like old slipping fan clutches or scale in the system or bad timing etc., then problems will manifest themselves, maybe in different patterns on different system configurations. I can't believe that my 33 year old radiator, which LOOKS like it has never been out of the car or gone into, still works. I am nervous about it on these hot days...

Anyway, these discussions are fun and interesting. Keep up the info sharing.

[Everett Ogilvie]

Re: Jerry: Exchange faster / Better?

Hi Doug,

I too marvel at how these old things still function. It may be that some of the cooling system designs back then were closer to the absolute limits of their performance than others, like 300 HP vs 375 HP, etc. And then if anything changes, like old slipping fan clutches or scale in the system or bad timing etc., then problems will manifest themselves, maybe in different patterns on different system configurations. I can't believe that my 33 year old radiator, which LOOKS like it has never been out of the car or gone into, still works. I am nervous about it on these hot days...

Anyway, these discussions are fun and interesting. Keep up the info sharing.

[Doug Flaten]

Re: Jerry: Exchange faster / Better?

Everett, ou bring up an excellent point in that these cars essentially had a one size fits all cooling system regardless of the engine power rating which may vary by 100 HP. By the time that the Hi-Po engine and AC was added, I'm sure that if the cooling system was marginal and would have required some kind of retooling to upgrade it, the bean counters sent the car out of the door.