5 replies to this topic

Hello

Im currently looking at compressible flow (AIR) through a pipe section and wish to find out if the flow is either laminar or Turbulent. Is it possibe to use the Reynolds equation to calculate this?

Re = (Rho * Velocity * Diameter) / Viscosity

Any help would be greatly appreciated

James Dean

John Moores University Liverpool England

Mechanical Engineering

[Art Montemayor]

Jim:

That is exactly what is done when handling a fluid flow problem. First, find out if you have turbulent or laminar flow. To determine this, calculate the Reynolds Number for the flow in question, using the Osbourne Reynolds relationship that you have shown.

The biggest problem confronting most students when they calculate the Reynolds Number is that they fail to cancel out all the units. The Reynolds Number is dimensionless; therefore, all units must cancel out. In other words, if you express the numerator units as:

D = diameter in feet;
v = velocity in ft/sec; and,
rho = density in lb/ft³

then, the denominator units must be in lb/ft-sec for the viscosity - NOT in centipoise. You must convert the centipoise units to lb/ft-sec.

All air flow in pipes is usually turbulent; I wouldn't be surprised if the Reynolds Number turns out to be over 100,000.

For an interesting thread on this very subject go to:

http://www.eng-tips.com/viewthread.cfm?qid=180670

and then go to http://katmarsoftware.com and download Harvey Wilson’s great free Units Conversion program, Uconeer. You’ll never get tired of using it in your daily work.


I hope this helps.

Dear Art

Thankyou for replying to my post, its much appreciated as the links are too. I became a little confused by the litrature I'm reading which talks about "real fluid flow in pipes" which I take to be a liquid rather than a gas such as air.

Ovbiously the molecules in water are pact alot tighter together in water, unlike a gas which have more freedom to move around. With this in mind does the gas behave like a liquid at a solid boundary and move in layers?

The problem a Im looking at is the mixing of a primary and secondary stream in the throat of an jet ejector. Although I'm 99.9% sure the flow there is turbulent due to momentum diffusion, Im having problems introducing the subject in my report. Do you know of any literature or links that maybe of use?

Kind regards

James

[joerd]

"Fluid" is generally used to mean "gas and/or liquid".
Gas turbulent flow is not essentially different from liquid turbulent flow, the flow pattern is the same.
If you want to read more about jet ejectors, see http://www.niro.de/j...log_e/index.htm
Hope this helps

[abhi_agrawa]

Jim,
Air flowing in a pipe in not always a compressible flow. In a pipe flow, you encounter compressibility related effects only when the flow speed approaches sonic velocities. I do not think that you would encounter such a situation. Therefore for all practical purposes your flow will be incompressible. And you can and should use the Reynold's number criteria for turbulent flow.
-abhishek

Im currently looking at compressible flow (AIR) through a pipe section and wish to find out if the flow is either laminar or Turbulent. Is it possibe to use the Reynolds equation to calculate this?

[pleckner]

This is unfortunately getting off topic but I need to address a generality given in the last post.

There are only two condions where one can treat a gas/vapor as an incompressible fluid.

(1) The total pressure drop in the system is less than 10% of the upsteam pressure. In this case, the density will be fairly constant and this is why you can treat the gas/vapor as an imcompressible fluid.

(2) The total pressure drop in the system is less than 40% of the upstream pressure. In this case you must use the average density between the upstream and downstream conditions.

What the last post is implying is that you only achieve sonic or choked flow when the pressure drop is 50% or more of the upstream pressure. A rule of thumb yes but only that. You need to always check if this is the case.

My advise is to use the Isothermal gas flow equations whenever dealing with a gas/vapor. These equations are straight forward, as easy to use as incompressible fluid equations (e.g. DARCY) and available in every book on fluid mechanics and thermodynamics.