What makes a good cooling fluid

[Physics_Kid]

hi all. i am trying to formulate some testing of different motor oils. i am looking to document which oils have better cooling properties. i read some info years back that a straight oil like SAE30 actually cools better than say a 5w30, reason being is because the multi-grade uses long chained polymers which uncoil as they get hotter which affects the cooling abilty of the oil.

but before i design my test apparatus i want to get clarity on what makes for a better cooling fluid. the specific heat is one property of a substance, but that alone to me does not define a better cooling fluid. is there a property of fluids that define endothermic and exothermic rates? in other words, if a fluid flows over a metal channel which property of fluids defines which fluid absorbs the most energy given a fixed flow rate of the fluid?

i am thinking my apparatus will be something like this. a rectangular box, ~12" on the long side, ~4" on the short side, and about ~6" tall to hold fluid around 4"-5" high. this "box" is made of 1" thick rigid foam board with a 1/4thick aluminum plate siliconed to one end of the box. on the opposite end will be an immersion heater. i will measure external temp of the aluminum vs time. i am figuring the better cooling fluid will be the one which shows the rate of temp change to be the fastest regardless of what the equilibrium temp is.

does this make sense?

 

[sophiecentaur]

Important factors will include be viscosity, specific heat capacity, operating temperature, thermal conductivity and even density.
If you want a good test I think you should consider forced circulation. Not many engines use simple convection cooling - the oil is at least 'splashed' around.

 

[CWatters]

What's your definition of "better"?

Have a think about the limits of performance. For example water makes a good cooling media under the right conditions but it gives you other problems to solve at temperatures over 100C. What problems does oil have at different temperatures? Is oil being used for other reasons and are they impacted by heat?

 

[Physics_Kid]

ok,

1) well, i want to see how well the fluid absorbs heat (energy), moves it from one side to the other, and then gives it up to the aluminum heat sink on the other end (similar to the oil in an engine block). do i need to use a circulator for this test? i guess i am testing for thermal conductivity, i cannot find this spec for any motor oil and the makers won't tell me.
2) "better" - i guess my definition is the fluid that acts as the better heat sink.

 

[CWatters]

Google found values for SAE 0W30 here ...

http://in.answers.yahoo.com/question/index?qid=20100904213602AADkuXY

Google can also find what look like good papers on the subject. Perhaps worth a look at their experimental set up...

http://www.sciencedirect.com/science/article/pii/S0196890403003042

 

[Physics_Kid]

ah, thanks much for those links, that helps me quite a bit. however, i do want to compare synthetics.

a few years back i was reading an article about pure water being best for a racing engine (i was investigating those wetter wetter products, etc), in that article it had a snippet on the order of "...and using straight oil grade vs multi-grade also helps cool an engine better".