Coolant flowspeed

Hi Muzza,

Dont fall into that trap...

Faster is definately not better,same for to slow...there is a happy medium, it is all relevant to the time in which it takes for heat to displace from the coolant to the radiator tubes and fins and then from the tubes and fins to the air travelling through them. In a conventional cooling system, the thermostat provides the restriction to flow when coolant is cold by closing, but when fully open it is still providing a restriction to flow due to the cross sectional area of its opening. The only time where more is better is when you are talking cfm of air movement through the radiator.

If you have ever had a car where the thermostat was removed...you will probably have noticed how long it took to warm up on the guage...what was really happening was the bores and combustion chambers were wickedly hot because the coolant was travelling to fast through the block to be as effective as it is capable of being when travelling at a slower pace.

To solve your cooling issues the answer is in the cross sectional and cubic area of the radiator(and its efficiency) and the airflow through the radiator...As long as the pump and thermostat are in good working order[;^)]

One of the guys who first fitted the FJ to your car ran an FJ20DET in a 120Y successfully for many years without cooling issues...the engine had the std water pump and thermostat.

Cheers,

Phil.

Like this

If the coolant moves to fast through the radiator, the majority of heat will stay in the coolant and travel back into the engine. The radiator and coolant need to be able to perform their heat transfer as in the 'cold engine no thermostat' example. If the coolant travels to fast, you will have the same effect but at the opposite end of the process, the heat exchange(coolant to radiator) part of the process cant occur effectively and the heat is retained in the coolant and therefore in the cooling system.

quote:

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Originally posted by muzza



In creating good thermal heat dissipation is to get the radiator as hot as possible (closest to engine temp) then flow maximum ambient air through it thus creating the highest heat transfer. (Radiator temp - Ambient temp = differential temp) The higher the differential temp the better cooling of cousre suface area plays a big part too.

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Agree'd...this is why aircon systems run higher pressures on a hot day, in order to create a temp/pressure differential. Your cooling system operating pressure also comes into it

ok I believe that cavitation occurs when the impeller spins at high speeds and creates occsilations rendering the impeller inefficient.

But consider this, remove cavitation from the equation, What you are saying is that it takes time for the coolant to radiate heat into the rad core. But if this takes time then the theory of the faster the coolant flows thru the core the cooler the radiator will get cause the coolant won't have time to transfer heat, but imagine coolant flowing say 50000 litres per minute this would mean the radiator would stay the cool cause heat transfer would be way inefficent, but I say the core would = the source temp.

Haha - I do like a good technical debate!

I did a lot of work on engine cooling systems in 2008 as part of my final year engineering project and really enjoy this subject so my thoughts are as follows:

From the graph I have attached you can see that at normal atmospheric pressure, ~101 KPa or just under 15 psi, water will boil at 100 deg C. If the pressure is increased, the temperature at which it boils increases. Whilst if the pressure decreases, the boiling temperature also decreases.



Cavitation occurs when the pressure of a liquid drops enough to cause it to boil at whatever temperature it is at. In engines, the motion of the pump impeller in the coolant creates both high and low pressure regions around it, in much the same way an object, such as a car, passing through air does. Cavitation will occur in the low pressure zones of the coolant if the temperature is high enough to cause it to boil. So, for example, if the pressure of the coolant at one point near the impeller is ~70 KPa, than the temperature at that point only needs to be ~90 deg C for cavitation to occur. Cavitation around the water pump will show as pitting on the pump housing and impeller.

The reason race engines such as Danny's usually feature larger water pump pulleys when using otherwise standard water pumps is to reduce the impeller speed. Ford did not design the YB engine to run at 10,000 rpm or whatever Danny's does, so if the impeller speed was not reduced either the coolant pressure would be too great (possibly causing things like heater core failures as mentioned by Phil) and/or causing cavitation at the impeller due to it rotating at a speed higher than it was designed to.

With regard to the location of the fill point, if the cooling system fill point is lower than any of the coolant passages in the engine the system may be difficult to bleed and the possibility of pockets of air being trapped at high points exists. Cavitation is not the correct term here, rather this can cause air locks in the system which hinder coolant flow, or the formation of pockets of steam at very high temperatures and pressures which can cause damage to the engine.

Back to the flow rate question, I believe Phil is correct with regard to coolant flow speed. Heat transfer is not an instant process, and water takes a fair amount of time to heat up and cool down relative to the engine block and radiator. This means that if the coolant flow is too great then not only will it not be taking as much heat away from the engine as it potentially could, it will not be losing as much of its heat to the air passing through the radiator as it could. If coolant flow is too slow the engine will produce more heat than the coolant can carry away. Both would result in an over heating engine.

So the ideal point is indeed somewhere in the middle...

Sorry about the length there, I hope you find it useful.

Regards,
Wagin

So the ideal situation is as much flow as possible without the pump cavitating, but no more flow than can produce an efficient exchange of heat[%]