Pressure fluctuations vs Temperature

In summary, the conversation discusses the effect of pressure fluctuations in a complicated pipe system, specifically in an engine lubrication system, on the net fluid temperature. It is determined that disturbances to the system will generally increase the average fluid temperature, but it is difficult to predict the exact outcome in a complex system. The conversation also touches on the benefits of turbulent flows for cooling and heating, but it is noted that achieving turbulent flow in an engine is unlikely. The role of shear force in heating lubricating oil is also mentioned. The OP is seeking clarification and corrections from a mechanical expert.
  • #1
csiddharthn
9
0
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?

Thanks in advance.
 
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  • #2
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
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
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
Simon Bridge said:
Actually I think that turbulent flows are better for cooling than laminar flows.

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
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
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
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.
 

Related to Pressure fluctuations vs Temperature

1. What are pressure fluctuations and temperature?

Pressure fluctuations refer to changes in the force per unit area exerted by a substance, while temperature is a measure of the average kinetic energy of molecules in a substance.

2. How are pressure fluctuations and temperature related?

Pressure and temperature are directly proportional to each other according to the ideal gas law (PV = nRT). This means that as temperature increases, pressure also increases, and vice versa.

3. What causes pressure fluctuations and temperature changes?

Pressure fluctuations and temperature changes can be caused by a variety of factors, including changes in volume, changes in the number of gas molecules, and external forces such as wind or heat transfer.

4. How do pressure fluctuations and temperature affect each other in a closed system?

In a closed system, an increase in temperature will lead to an increase in pressure, while a decrease in temperature will result in a decrease in pressure. This is because the volume and number of molecules remain constant in a closed system.

5. What are some practical applications of understanding pressure fluctuations and temperature?

Understanding pressure fluctuations and temperature is important in many fields, including meteorology, engineering, and chemistry. It can help predict weather patterns, design efficient machinery, and control chemical reactions.

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