Is this heat transfer realistic?!

[HarleyE30]

Hi all,

Just after some advice on whether or not some heat transfer values are realistic.

I have a piston, whose upper and lower crown temperatures are shown below:

Underneath the piston, there is a chamber (not shown), filled with engine oil, which has zero flow rate.

I have calculated the heat flow through the crown:

I now want to calculate the rise in temperature of the oil as a result of this heat flow. I have used the following equation, where the mass is equal to the volume of the oil in the chamber x density.

This suggests then that the oil heats up by 284 'C in one second! I can't quite get my head round such an increase in that time-frame....

Any advice/comments greatly appreciated.

Thanks

 

[HarleyE30]

Please note temperatures on the ANSYS screenshot are all in Kelvin

 

[IRstuff]

You are mixing apples and rocks. There is a thermal conductivity involved in both the piston, air, and the oil. You also don't consider how the heat leaves the piston to get to the oil, i.e., is there continuous contact, or momentary contact, or contact only with the oil film around the piston that then flows down into the oil. You are using the thermal mass equation, but that equation assumes infinite time, i.e., how much does the temperature change if each and every molecule of the material has access to the injected heat.

Your heat flow through the crown may be essentially moot, since the oil is not directly in contact with the bottom side of the crown, except, perhaps, through splashing oil. In fact, the low deltaT indicates that your model has air in contact with the bottom of the crown, so the heat flow is buffered by natural convection of air. The heat flow then has to travel through the air, and the convect into the oil. Or something like that, since you've not really defined where everything is and what's touching what and when.

TTFN
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[HarleyE30]

Sorry I should have explained better. The oil is in contact with the underside as shown below.

There is an inner piston and outer piston. The outer piston moves axially relative to the inner, depending on the volume of oil present in the upper chamber. There should be no air present in the upper chamber.

 

[IRstuff]

So, what material is the "fixed" piston? If it's a thermally conducting material, as one might reasonably assume, it's in the thermal circuit, and would be removing heat from the oil, otherwise, your oil would boil and vaporize. You say the piston moves, but by how much physically? Do you get some sort of oil vapor layer when the piston moves up? Oils are generally not particularly compressible.

Assuming that the piston is in intimate, continuous, contact with the oil would mean that bottom piston must remove heat as fast as the heat is introduced from the top piston, otherwise, boiling, vaporized oil would result.

TTFN
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[HarleyE30]

The fixed piston is Aluminium, so yes definitely conductive! This is a very good point which I haven't got round to thinking about yet.

There is a check valve (not shown in diagram), through which oil flows into the upper chamber. There is also a discharge valve (pressure relief valve), through which oil leaves the chamber.

 

[IRstuff]

OK, so that makes more sense, so the entire thermal circuit is crown-oil-Al piston-coolant, I'm guessing the last part, but it's got to be, otherwise there's no way this could be thermally plausible. Since the oil is a very poor thermal conductor, it will be quite hot, nonetheless, with the top of the oil at 432 K, and the bottom of the oil at possibly 400 K ?? assuming the coolant is at 380 K and there's a 20 ºC drop across the bottom piston.

TTFN
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