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Reynolds Number

Reynolds Number

Reynolds Number

In any engineering text, it states that a flow with a reynolds number of less than 2000 is of the Laminer flow and anything over 4000 is Turbulent flow. These figures are used for calcultion reasons. What i do not understand is when making calculations, figures of 40000, 60000 and 100000 and higher arise.
What do these figures tell me about the system and how do i know if the Reynolds Number calculated is an acceptable figure? I would of thought the aim was to keep the figure below 2000.

Hope somebody can give me a simple explanation.

J Boy.

RE: Reynolds Number

Laminar flow is very difficult to obtain in normal systems. This because of the very very low velocities involved. To give you an idea how the experiment of Reynolds was conducted:

You have a glass tank with water with an outlet (glass tube) with very smoot walls. You introduce a coloured stuff (water), with a small tube in this outlet. If you can see clearly two different streams (a line of the coloured stuff) in the outlet, you will have laminar flow.
 Try to put coca cola together with sprite, without the two substances mixing, pumping them to a line, and at the end separating them again. I you can do that you are dealing with laminar flow.
In real life you will encounter laminar flow with very sticky (viscous) substances (crude oils,  butter, etc)  and at very low flows. But the cases where very rare.

Hope this helps you out

Steven van Els

RE: Reynolds Number

Your explanation of laminer flow is good. But i still do not understand what is to be an acceptable Reynolds number. Do we just accept any number. If this is so , why do we make the calculation and what is it telling us?

J Boy

RE: Reynolds Number

A laminar flow Reynolds < 2000 is very hard to achieve. So you will allways deal with big numbers.
When you are sizing pump systems, you will need the Reynolds number to determine the pressure drop (loss caused by friction (resistance) in your system. There are also other factors like material and smootness of your conducts, which will influence the pressure drop.
The physical difference between laminar and turbulent flow is that the pressure drop in laminar systems can be put in a very simple formula which depends on the reynolds number, which is a composition of the fluid velocity, the diameter and the viscosity of the fluid. Unfortunately this allmost never happens.
In the real world if you have a 2" pipe, of pvc and a 2" rusted steel pipe with 1/8" of scale, its evident that you will need more energy to pump the same amount of water for the steel pipe (greater pressure drop)

For calculating the pressure drop you will need the Reynolds Number (or  charts with deductions based on the Re). Also in heat transfer (heat exchangers) you will encounter Re. There is no advantage in keeping a laminar flow. (You would not wait for 2 days to fill a cup of water)

Steven van Els

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