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Thread: Motor Vehicle Heat Exchanger

  1. #1 Motor Vehicle Heat Exchanger 
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    New memebr hoping for some help on this topic.

    The question is, I have classic cars, and the cooling systems consist of pump driven coolant flowing from the engine into a radiator. The radiator will have variable amounts of air flowing through it dependant on speed, and fan assistance.

    The original pump is engine driven, but some are now being replaced by variable speed electric pumps.

    My question is, using basic priciples where they apply to a large extent, what effect does coolant flow rate have on the cooling of the engine? Can someone suggest a simple model that shows the relationship?

    I was of the opinion that there was an optimum flow rate for maximum heat extraction, but now not so sure. Many colleagues suggest that the greater the coolant flow, the more heat is extracted, hence lower engine temperature.

    Please bear in mind that although degree qualified in electronics, my mechanical engineering is only to 'general engineering' standards.

    Cheers


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    I don't claim to be an expert, but I'll take a shot at it.
    Consider that the coolant will enter the heat exchanger at temperature Th and leave at Tc then the average temperature is Tavg = (Th + Tc)/2.
    The rate of heat flow from the primary coolant to the secondary (air) side of the heat exchanger will be proportional to the difference between Tavg and Tair . The faster the coolant flows through the heat exchanger, the less time it has to lose its heat, and the higher Tc will be. Therfore, higher flow rate means more heat being transferred.

    The only thing I can see that would cause an adverse effect would be the heat input from friction. This will probably negligible compared to the heat input from the engine.


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    Thanks for those thoughts. Your explanation assumes a constant Th with variable flow, whereas this may not be true. Can you perhaps consider the other half of the system which is the heat transfer from the engine block to the coolant? This then seems to suggest the reverse happens, ie faster coolant flow means temp Th would be lower, therfeore less heat transferred.
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  5. #4  
    Cooking Something Good MacGyver1968's Avatar
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    I have classic cars
    I'd like to see pics if you have them. What kind of cars do you own?
    Fixin' shit that ain't broke.
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    Quote Originally Posted by v mad View Post
    Thanks for those thoughts. Your explanation assumes a constant Th with variable flow, whereas this may not be true. Can you perhaps consider the other half of the system which is the heat transfer from the engine block to the coolant? This then seems to suggest the reverse happens, ie faster coolant flow means temp Th would be lower, therfeore less heat transferred.
    We can talk about that but I don't think you need to in order to answer the question. If the engine is adding more heat to the coolant than is being removed from the radiator, then for a given mass flow rate, the temperature will go up. Th will rise, and so will Tc, and this will increase the Tavg and so the rate of heat transfer will increase until a new equilibrium temperature is reached.

    We can consider what happens to the temperature of the coolant as it flows through the engine and radiator. The coolant holds a certain amount of heat, proportional to its mass, average temperature, and heat capacity of water. As the water flows through the radiator, the rate of heat loss is equal to the mass flow rate, multiplied by the change in temperature (i.e., Th-Tc) multiplied by the heat capacity of water. If the temperature is in equilibrium the heat gained while the water is flowing through the engine will be equal to the heat lost in the radiator.
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  7. #6  
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    if you put an electric water pump in, it ain't a classic no more.

    in the hotrod days, we put the biggest radiators in that we could make fit, low scoops to pull in more air, a cowling around the fan to optimize it's ability to move air, and, on an old truck, a couple sweating canvas water-bags hanging on either side of the radiator grill.

    chuck sang of one solution(rain)---



    it would seem that faster is better, unless it causes turbulence which runs air bubbles through the system
    it also seems that a variable speed electric pump could optimize the engine temperature depending on how hard you're running it, and on outside conditions.

    do not all engines have a specific heat window within which they perform the best?
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    Quote Originally Posted by sculptor View Post
    i
    it would seem that faster is better, unless it causes turbulence which runs air bubbles through the system
    it also seems that a variable speed electric pump could optimize the engine temperature depending on how hard you're running it, and on outside conditions.

    do not all engines have a specific heat window within which they perform the best?
    Your thermostat will keep it at the right temperature with a belt driven pump, assuming the radiator is adequate. You're still turning the pump at full speed all the time, so it's not as efficient as a variable speed electric pump, though.
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  9. #8  
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    We used to swap in a higher temperature thermostat for the winter, and a lower temperature one for the summer.
    It'd save a few minutes with a wrench if a computer could do that. It has to do with flow volume.
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  10. #9  
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    Quote Originally Posted by v mad View Post
    New memebr hoping for some help on this topic.

    The question is, I have classic cars, and the cooling systems consist of pump driven coolant flowing from the engine into a radiator. The radiator will have variable amounts of air flowing through it dependant on speed, and fan assistance.

    The original pump is engine driven, but some are now being replaced by variable speed electric pumps.

    My question is, using basic priciples where they apply to a large extent, what effect does coolant flow rate have on the cooling of the engine? Can someone suggest a simple model that shows the relationship?

    I was of the opinion that there was an optimum flow rate for maximum heat extraction, but now not so sure. Many colleagues suggest that the greater the coolant flow, the more heat is extracted, hence lower engine temperature.

    Please bear in mind that although degree qualified in electronics, my mechanical engineering is only to 'general engineering' standards.

    Cheers
    ANY Engineering background suggests the ability to understand obscure principles, such as: Years back I asked your question, basically, for those who claimed removal of the thermostat allowed "better" engine cooling. The response was: the coolant must be moved through the radiator slowly enough to allow heat extraction. I maintained that idea was bullshit. I felt heat is removed at a much smaller level than the "size" of slugs of fluid passing through the heat exchanger. Some studies of hot-running engines seemed to show that slowing the flow rate through the radiator resulted in lower engine temperatures. Quite a variety of opinions and arguments ensued over the years. A really interesting one was, that without a thermostat, the vehicle's "heater" would not function to warm the interior for comfort purposes! As vehicular emissions considerations became more and more important, control of the engine coolant temperature became a much more concrete variable. Now, emissions take precedence over almost everything. It became obvious over the years that manufacturers knew that the higher the coolant temperatures which could be physically tolerated, the more easily were emissions satisfactorily controlled. So, we went from 140` thermostats, 5 or 6 psi, to 194`, 15 psi, or even higher.

    Higher coolant pressures taxed the ability of water pumps to seal well-enough, and long-enough. Another significant "ringer" was the common introduction of "cross-flow" radiators which worked best when completely full of liquid. Those required either an "expansion tank", or a low-pressure "catch tank" while using a special radiator cap which allowed liquid coolant to "bleed" out of the radiator at atmospheric pressure, be captured in a low-pressure vessel, the cap allowing "suction" to draw the excess liquid back into the radiator as the system cooled down, when the engine was shut off. The "suck-back" system prevailed for many years, had it's own unique problems, and was misunderstood by many degrees of "mechanic".

    50 years ago, or so, all cooling systems used the vertical flow radiator design, filled below capacity, providing a space at the top for vapor, allowing the liquid to expand without the need to leave the radiator. These typically used 7 psi. as the safety relief pressure.

    Late 1950s, cross-flow radiators became common, still using an "unused volume" up at the top, to allow for liquid expansion. About 1958, Ford's "bright idea" produced a design which lasted until the mid-60s: Place a separate, pressurized tank, an "expansion tank", higher up than the top of the cross-flow radiator, thus ensuring the radiator was always full of liquid, connected to the pressurized cooling system by mounting it to the front of the intake manifold. This scheme prevented low hood profiles, and was gone by 1967. But, it worked.

    Lo and behold, I found by sheer accident, the industry adopted what I thought much earlier was the best scheme, in the '90s. The cross-flow radiator remains full at all times, it has NO pressure relief cap, but allows it's expansion of liquid to flow (via a pressurized hose) to a pressurized "catch tank", which has on it's top, a normal, usually-encountered pressure-relieving fill-cap. This "secondary" reservoir is the location at which determination of adequate quantity of coolant present in the engine is made. This system is basically identical to the expansion tank used by Ford long ago, though it utilizes plastic components instead of the expensive brass used formerly. jocular
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    Well, let me be the first to say the most obvious of all things related to this. The heat exchange system in a vehicle is a bit more intricate than that. Types of materials used, length of the pipes and so forth. For instance, let's take my little Nissan 1400 bakkie ( a little pick-up truck for our American friends. ). The core material used in the radiator is normally copper. I think. Regardless, it's not really suitable for the type of weather I live in. Being so close to the coast, it corrodes away very quickly. So, I had to replace it with a stainless steel type radiator. In the copper core radiator, I had to use a coolant solution. Even for a brand new one. In the steel core one, I only need to use water. Plain ol' tap water. And it still runs much cooler than the copper core radiator. (Can someone explain why? I actually have no idea). Now, as for the idea of faster or slower flow of the coolant/water, I would like to propose an experiment. Take an ice cube and touch it very quickly. Just for a moment, like for a split second( if that's possible). Now, repeat the experiment, but only this time touch the ice cube for about ten seconds. Which one was colder? I know this is from extreme to the other, but I believe the basic concept can still be transferred to the heat exchange system of the vehicle. If the coolant/water flows to quickly or to slowly, it won't be able to cool the engine efficiently enough. However, I do stand to be corrected on this. Any comments or corrections will be welcomed.
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  12. #11  
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    I'm tempted to model a simplified version of this as a differential equation, but it'd probably be too complicated to really offer much in the way of answers.
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    Quote Originally Posted by WaterWalker View Post
    Take an ice cube and touch it very quickly. Just for a moment, like for a split second( if that's possible). Now, repeat the experiment, but only this time touch the ice cube for about ten seconds. Which one was colder? I know this is from extreme to the other, but I believe the basic concept can still be transferred to the heat exchange system of the vehicle. If the coolant/water flows to quickly or to slowly, it won't be able to cool the engine efficiently enough. However, I do stand to be corrected on this. Any comments or corrections will be welcomed.
    This isn't a fair comparison. You are just looking at what happens to a fixed mass of material, but the water flowing through the radiator is a continuous stream. If you follow a particular slug of water, say a cubic centimeter of water, through the cooling coils, it's true. If the cc of water starts at a particular temperature, it will be cooler at the end if it goes more slowly. But that's not the only cc of water carrying heat away from the engine. The greater the number of cc's of water that go through in a given time, the more heat is transferred.

    The rate at which the cc of water transfers its heat is not constant. It's greater when it has a higher temperature. So, the higher its average temperature, the more heat it will transfer within a given time in its trip through the cooling coils.
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  14. #13  
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    as/re stainless steel vs copper
    (wild guess du jour) The stainless is a thinner material which transfers the heat faster, and retains less heat in the material.

    ...............
    Why are pots and pans copper clad on the bottom?
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  15. #14  
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    Quote Originally Posted by MacGyver1968 View Post
    I have classic cars
    I'd like to see pics if you have them. What kind of cars do you own?
    I have a Sunbeam Tiger, 1964 with original 260 V8 engine. I also have a Triumph Stag 1972, again original 3.0 V8 engine. WHY? I can post pics later if you want but I want to keep on topic for the moment.

    So back to the original question, whilst I know that some subjective discussion is necessary to make progress and end up with the right model, I want to try to get a simple model that we can bash around until its considered rperesentative and useful to show how flow affect engine temp.

    Just the process of raising this topic and seeing your replies has already helped me make progress in my thinking. So far I have a model with two heat exchangers, one is the engine block, the other is the radiator. The input to the system is a constant heat source q (approx 70 kW). We assume a constant ambinet temp Ta. The coolant flow is the only variable, and we want to solve for engine temp, Te. I am looking at using a simple formula based on Fouriers Law. Am I on the right track?
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    Thanks! I was just curious what your "babies" looked like.
    Fixin' shit that ain't broke.
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  17. #16  
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    Quote Originally Posted by MacGyver1968 View Post
    Thanks! I was just curious what your "babies" looked like.
    I would but I get an erro on upload. Is there a file size limit?
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    Quote Originally Posted by v mad View Post
    Just the process of raising this topic and seeing your replies has already helped me make progress in my thinking. So far I have a model with two heat exchangers, one is the engine block, the other is the radiator. The input to the system is a constant heat source q (approx 70 kW). We assume a constant ambinet temp Ta. The coolant flow is the only variable, and we want to solve for engine temp, Te. I am looking at using a simple formula based on Fouriers Law. Am I on the right track?
    Yes, I think this is the general idea. Of course, you will need to know or assume a value for the thermal conductivity of the radiator.
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    sell it fast
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