Friction factor on very very low reynolds number

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Discussion Overview

The discussion revolves around the behavior of castor oil flowing through a pipe at very low Reynolds numbers, specifically addressing concerns about the friction factor and its implications in laminar flow. Participants explore theoretical aspects, practical applications, and the rheological properties of oils.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant expresses confusion about achieving a Reynolds number of 0.35 in internal flow, questioning if this is typical.
  • Another participant shares an example of a very low Reynolds number (0.0375) encountered with motor oils, suggesting that such values can occur.
  • Concerns are raised about the friction factor calculated using the formula f=64/Re, which yields values greater than 1, prompting questions about its normalcy in this context.
  • Some participants argue that having a friction factor greater than 1 is acceptable for oils, particularly in applications like motor oil in warm engines, where viscosity changes with temperature.
  • One participant seeks information on the rheological properties of oils, specifically viscosity changes under shear, to compare with castor oil.

Areas of Agreement / Disagreement

Participants do not reach a consensus on whether a friction factor greater than 1 is typical for low Reynolds numbers, but some suggest it may be normal for certain oils. The discussion remains unresolved regarding the implications of these findings.

Contextual Notes

Participants note that the viscosity of oils can vary significantly with temperature, which may affect the flow characteristics and friction factor. The specific conditions under which the measurements were taken, such as temperature and pressure, are not fully detailed.

Who May Find This Useful

Researchers and practitioners in fluid dynamics, lubrication engineering, and those studying the rheological properties of fluids may find this discussion relevant.

han168
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hai all,
I'm now doing a research about castor oil. I've a got a few a confusing problems..

1. to flow castor oil in a 12 mm acrylic pipe i use a reservoir 1 metres from the outlet. Because castor oil is very thick (dyn visc. = 0.5 Pa-m), the velocity of the flow is very low just about 0.02 m/s, therefore the reynolds number i got is very small just about 0.35. do u guys think it's normal? coz' I've never found any case that has a reynolds number below 1 in an internal flow.

2. related to the first question, i got the friction factor for laminar flow using the eg f=64/Re, so i got about 140-150. do u guys think it's normal to have a friction factor > 1.

that's my confusing problem, btw i don't want to use pump, because pump will break the oil's molecule.

please help me here. =)

Thanks.
 
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Very low Reynolds numbers are encountered with motor oils. I have an example of oil leakage past a cylinder head/bore in which Re = 0.0375.

The formula I have for volumetric flow rate is

Q = [tex]\frac{\pi\,\Delta{p}D^4}{128\,\mu\,L}[/tex], where D is pipe/tube diameter and L is pipe/tube length, p is pressure, and [itex]\mu[/itex] is dynamic or absolute viscosity.

Q = V A where V is mean fluid velocity.

The f is probably correct for the Re, but I am not familiar with the application at such low Re.

What is the temperature of application?
 
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so u think it's normal to have a friction factor > 1 with such Reynolds number?
the temperatur is ambient about 30 deg celsius
Thanks btw
 
han168 said:
so u think it's normal to have a friction factor > 1 with such Reynolds number?
the temperatur is ambient about 30 deg celsius
Thanks btw

The friction factor for laminar flows can be derived analytically and has a value of 64/Re. There's no further problem if it is larger than one.
 
han168 said:
so u think it's normal to have a friction factor > 1 with such Reynolds number?
the temperatur is ambient about 30 deg celsius
Thanks btw
It would appear to be normal for oils, e.g. motor oil in a warm engine. At some point, the oil temperature increases to around 100°C or slightly higher, so viscosity will vary considerably. The dimensions used in lubrication are generally very small (parallel surfaces).

As Clausius2 mentioned it should not be a problem. I believe that f = 64/Re is derived from the formula I posted by virtue of the definition of the Darcy friction factor - http://en.wikipedia.org/wiki/Darcy_Friction_factor

See also - http://web.umr.edu/~wlf/MW/HagPoi.html

http://en.wikipedia.org/wiki/Hagen-Poiseuille_law
 
Last edited by a moderator:
thank you very much for the answers..i really really appreciates it. =)
 
continuing my research about oil for lubricants, could you guys tell me where can i find the rheology overview of oil (viscosity change under shear) chart?
i want to compare it with castor oil..
thanks
 

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