Common Terms Used in Fluid Flow Measurement
Common Terms Used in Fluid Flow Measurement
Velocity
This is the speed at which a fluid moves through a pipe. The speed of particles in a fluid flow varies across the flow profile - where the fluid is in contact with the constraining walls (the boundary layer) the velocity of the liquid particles is virtually zero; in the center of the flow the liquid particles will have the maximum velocity. Thus, the average rate of flow is used in flow calculations.
Viscosity
Viscosity is a property of a gas or liquid that is a measure of its resistance to motion or flow.Simply put, it is the ease of flow of a fluid. Viscosity (dynamic) can be measured in poise or centipoise, whereas kinematic viscosity (without force) is measured in stokes or centistokes. Viscosity changes with the temperature of a fluid.
Density
Density is the weight per unit volume.
Reynolds’ Number
Reynolds’ number is a derived relationship combining the density and viscosity of a liquid with its velocity of flow and the cross-sectional dimensions. It defines the flow conditions at a particular point. It is a way of representing fluidity and is a useful indicator of laminar and turbulent flow. Mathematically, the Reynolds’ number is expressed as:
R=VDρ/u
Where:
V = Average velocity of fluid
D = Diameter of the pipe
ρ = Density of the fluid
u = Fluid viscosity
Laminar flow exists if the Reynolds number is less than 2000, and turbulence when the number is above 4000. There is not a clear transition between laminar and turbulent flows, which does complicate flow measurement in this range of operation.
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