Pressure fluctuations vs Temperature

1. May 31, 2013

csiddharthn

How do pressure fluctuations in a fluid flowing through a complicated pipe system (such as an engine lubrication system) affect its temperature?

Does the net fluid temperature increase with increase in the magnitude of pressure fluctuations?

Or does it decrease?

2. Jun 1, 2013

Simon Bridge

Generally disturbances to the system that give rise to random pressure fluctuations will increase the average fluid temperature - however, you are dealing with a complex system so there is no way to predict what will do which or how much.

Automobile cooling systems are usually designed to accommodate a wide variation in transient pressures.
Is there a particular system you are thinking of?

3. Jun 1, 2013

csiddharthn

Actually there is a system that I am looking at. I need to see if reducing the fluctuations in pressure of lubricating oil in a motorcycle engine can reduce the oil's average temperature. Atleast by 2 or 3 degrees.

4. Jun 1, 2013

Simon Bridge

Actually I think that turbulent flows are better for cooling than laminar flows.
Turbulence would produce random pressure fluctuations.

I don't think it is possible to predict what would happen for your system from the information available - you'll just have to do the experiment.

5. Jun 1, 2013

They are also better at heating things up than laminar flows. Really, what they are better at is transferring heat to and from a surface, so whether or not you want turbulent flow in this case depends on whether your fluid is hotter or colder than the object you are trying to cool.

6. Jun 1, 2013

Simon Bridge

Which is a good point - here the coolant has to be a conduit between a hot reservoir and a cold one ... so being turbulent should help at both ends right?

7. Jun 1, 2013

It all depends on the OP's setup. To me it sounded like he was hoping that simply reducing pressure fluctuations in the oil would reduce the oil temperature. I don't think that makes much sense, personally. For one, the oil is almost certainly incompressible, so there is not likely a link between pressure and temperature like there is in a gas, so the pressure fluctuations themselves wouldn't likely be able to change the temperature of the oil. Given, I am not all that familiar with the equations of state of oils, but I do know that for other liquids like water, when an equation of state is needed (e.g. modeling an underwater nuclear explosion, which actually can compress water slightly), there is no temperature involved, just pressure and density.

As far as linking the pressure fluctuations to turbulence goes, I would imagine that the engine would be hotter than the oil to begin with, so making that flow turbulent would likely just cause the oil to get hotter. Further, the viscosity of oil is so high while the density and the velocity are so low that I can't imagine a scenario where you could actually get a turbulent oil flow in an engine. For example, 10W-40 oil at 100°C has a kinematic viscosity of 0.0000143 m²/s, so to even reach the transitional Reynolds number in pipe flow you would need like 2.74 m/s going through a 15 mm pipe (corresponding roughly to 1/2 NPS SCH 40 pipe, which I believe is a little large for oil flow). A smaller pipe would require more velocity to get to the proper Reynolds number. Typically, oil flow into engines is measured in drops per minute, so I highly doubt you would ever see that kind of velocity through your pipes, so I really doubt there is any hope of having a turbulent oil flow through an engine in the first place.

8. Jun 2, 2013

jim hardy

Will a genuine Mechanical type correct me here if i'm wrong ?

Heating of lubricating oil is mostly from shear force inside the thin film of oil in the bearings.
Look at the velocity profile across that thin layer of oil - a few ten-thousandths of an inch?

Friction work in the plumbing pales in comparison.

Corrections are solicited.

9. Jun 2, 2013

SteamKing

Staff Emeritus