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Fluid velocity through a capillary?

Fluid velocity through a capillary?

Fluid velocity through a capillary?

(OP)
I wish to use a capillary tube to dose a fluid and so need to calclate the fluid velocity through the capillary.

How do I do this?

I know the pressure differential, fluid viscosity and the capillary dimension may be set according to need.

Hope you can help

Tom

RE: Fluid velocity through a capillary?


If the flow is considered "laminar", the net ΔP is known, and the velocity (i.e., kinetic energy) of the fluid leaving the capillary is zero, or negligibly small, use the Poiseuille formula:

Volume, m3/s = π r4ΔP/(8 ηL)

r: tube radius, m
ΔP: pressure differential N/m2
η: absolute(=dynamic) viscosity of the fluid, Ns/m2
L: tube length, m

Any consistent set of units may be used.

BTW, this equation serves to measure fluid viscosities by gravity flows through capillary tubes, from the times τ taken by equal volumes of the liquids, that of unknown viscosity when compared with another, say, water, taken as reference, the viscosity of which is known.

Since ΔP is proportional to the density ρ of the fluids:

η = [η/(τ ρ)]ref (τ ρ)

 

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