Friction Factor for Extremely Low Renyolds Num.
Friction Factor for Extremely Low Renyolds Num.
(OP)
Hey,
I have a highly viscous flow that has a Re number of only 3.1 (1" Sch 40 SS Pipe, 350 centistokes,0.8 GPH, 60.43 lb/ft^3) I got a large friction factor for this flow using f=64/Re. Is this correct to assume?
Thanks
I have a highly viscous flow that has a Re number of only 3.1 (1" Sch 40 SS Pipe, 350 centistokes,0.8 GPH, 60.43 lb/ft^3) I got a large friction factor for this flow using f=64/Re. Is this correct to assume?
Thanks





RE: Friction Factor for Extremely Low Renyolds Num.
thread378-128004
RE: Friction Factor for Extremely Low Renyolds Num.
So you Darcy-Weisbach f would be correct.
Just a further note, I obtain a Re of about 0.1 for your conditions, assuming that the fluid is Newtonian.
RE: Friction Factor for Extremely Low Renyolds Num.
For Newtonian Flow I Used the Equation in Crane
Re=(50.6*flow*density)/(d*viscosity)
flow=GPM density=lb/ft^3 d=inches viscosity=centistokes
So Re=(50.6*0.35*60.43)/(1.049*338.8)= 3.01
Then f=64/Re=64/3.01=21.3
So should I use the 2 K Method in the laminar flow part? I have access to that. Does everyone agree with my logic? Thanks
RE: Friction Factor for Extremely Low Renyolds Num.
RE: Friction Factor for Extremely Low Renyolds Num.
RE: Friction Factor for Extremely Low Renyolds Num.
The viscosity was 350 centistokes. The SG was 0.968 so I got
350*0.968=338.8 centipoise.
RE: Friction Factor for Extremely Low Renyolds Num.
0.8 GPH is not 0.35 GPM. Given the data in your first post your calculation of Re is wrong and Assumptions' Re calc is correct.
The reason you have laminar flow is not because of high viscosity, but because of very low flowrate. A viscosity of 350 cP is not unusually high. Given the data in your first post, your velocity is only 0.3 ft/minute. Your pressure drop will be around 0.1 PSI per 100 ft, even though your friction factor (Darcy-Weisbach) is about 550.
Under these circumstances pressure drop seems irrelevant, but we do not know the full picture.
RE: Friction Factor for Extremely Low Renyolds Num.
RE: Friction Factor for Extremely Low Renyolds Num.
The 2k method is also mentioned. I understand that you refer to the 2k method for fittings. The usual fixed k method will give too low numbers for the fittings.
You can use the following website: http//:www.engineeringpage.com
Then choose pressure drop > Pipe with fittings
This calculation uses the 2k method.
Regards - Wim
RE: Friction Factor for Extremely Low Renyolds Num.
Re=(50.6*60.43 lb/ft^3*0.35 GPH)/(0.622in*338.8cp)=3.01
f=64/Re=64/3.01=21.3
K=f*L/D=(21.3*51 in)/(0.622 in)=1746.5 (51 in is length of pipe)
delta P= (18*10^-6*K*density*Q^2)/(d^4)
=(18*10^-6*1746.5*60.43 lb/ft^3*(0.35 GPM)^2)/(0.622 in)^4
=1.55 psi
Looks Good? Thanks. The application is for the inlet side of a PSV.
RE: Friction Factor for Extremely Low Renyolds Num.
Your units for flow rate are mixed up.
Your origional post has 0.8 GPH
Your last calculation has 0.35GPH for the Re calculation.
Your delta P calculation has 0.35 GPM.
A good way of checking things is to make sure that the units come out correctly. Otherwise you might not be off by 10% but mabe 1000 times.
RE: Friction Factor for Extremely Low Renyolds Num.
The correct flowrate is 0.35 GPM. Sorry for the confusion.
RE: Friction Factor for Extremely Low Renyolds Num.
I get the same Re and the same f
However, I do not get the same delta P. I looked in Crane and could not find the equation that you have used for your delta P calc.(0.1PSI)
Or list the reference for your delta P calculation.
RE: Friction Factor for Extremely Low Renyolds Num.
It reads:
deltaP=(18*10^-6*K*density*Q^2)/(d^4)
Thanks.
RE: Friction Factor for Extremely Low Renyolds Num.
I checked the Crane reference.
Your value for K contains an error for "d".
You repeated the error for "d" in the calculation for delta P.
Hope that this helps a bit.(My comments pertain only to delta P calcs.) Aparently you have changed to 1/2".
If 1/2" is used Re changes and just by chance f (Fanning) is 3 while f (D-W) is 13.
Anyway. Everything should be on the same basis If 1/2" is used then your Re is incorrect according to my check.
Again. Check dimensions and units and consistency.
RE: Friction Factor for Extremely Low Renyolds Num.
I used 0.622 which is the inner diameter for Schedule 40 1/2" SS Pipe. Check 6 posts above this one to see my whole calculation.
Am I finding the Darcy friction factor incorrectly?
Thanks.
RE: Friction Factor for Extremely Low Renyolds Num.
I do not get the same Re in your calculation.(Assuming the flow was in GPM)
For your own calcs write down the inputs and stay with those same values throughout.
I am not checking all simple calcs. I have been working from your inputs.
In a few years from now if you have to go back to the calculations above do you think that you could show an observer how the inputs have been definitively defined, other than well it seems to be obvious. When you enter something, keep in the back of your mind, is this really correct. In a couple of years from now, will I be able to follow the calculations, that I have assumed is obvious to everyone.
Hope that this helps.
RE: Friction Factor for Extremely Low Renyolds Num.
http:
fluid at very low Reynolds number. In the flow of fluids, a Reynolds number (density • length • velocity/viscosity) describes the relative importance of inertia effects to viscous effects. In creeping flow the Reynolds number is very small (less than 1) such that the inertia effects can be ignored in comparison to the viscous resistance. Creeping flow at zero Reynolds number is called Stokes flow