Flow in Concentric Pipes

<... 60 mm to 80 mm...> [2.5-3.5in] is not a long piece of pipe and the question remains open as to the length of the pipes feeding it and what happens to them.

Practically, once a pressure test of whatever is needed to determine the structural integrity of the pipe has been passed, it should be subject to an assessment as to the vibration that is experienced at design flowrate and whether the supporting structure is adequate.

Gut feel is that for a pipe of that short length, one bracket ought to be enough, however, were it to be water in the inner pipe, the velocity is higher than the rule-of-thumb economic value by a factor exceeding 2, and increasing the diameter of the inner pipe will aid the reduction of vibration of this whole assembly.

Increasing the diameter of the inner pipe has an impact on the required wall thickness, which is a function of the tensile strength of the unspecified material of which it is made, of course.

its the most easy solution but that is not an option.

Hello, thanks for the input. increasing the inner diameter is not an option. The inner tube has laminar flow where we have a flow rate of maximum 20ml per min. The structure has support at the top(the system has a T connection which forms the outer pipe and the inner cylinder is connected on the top of T connection). But the outer flowrate is very high and we have a turbulent flow in the outer part. The solution I thought was to provide a support to the inner pipe which will be connected to the outer pipe and as the flow in non corrosive and supercritical, it will not be a big problem. What do you think?

<...20ml per min...> differs greatly from <...500kg/h...> as stated in the original posting. It would help the forum greatly were the purpose of this <...60mm to 80mm...concentric pipe...> contraption to become known to it.

Hello, thanks for the input. increasing the inner diameter is not an option. The inner tube has laminar flow where we have a flow rate of maximum 20ml per min. The structure has support at the top(the system has a T connection which forms the outer pipe and the inner cylinder is connected on the top of T connection). But the outer flowrate is very high and we have a turbulent flow in the outer part. The solution I thought was to provide a support to the inner pipe which will be connected to the outer pipe and as the flow in non corrosive and supercritical, it will not be a big problem. What do you think?

300 bar on 1.5 -2 mm of pipe work and you are worried about vibration?

A mass rate without specification of the fluid leaves a blank on the volume flow rate.

There is a couple of issues with this information.

How did you come up with the wall thickness of the pipe?

How are the pipes connected, kept concentric?

What sort of pump are you using, whats the weak point in the system?

Super-critical CO2 at what temperature to what purpose?

Is there a potential for blockage?

Have you factored in erosion? Friction? Phase changes?

Cavitation or hammer effect comes to mind.

There is no phase change in the system. the fluid is supercritical CO2. The inner tube has laminar flow where we have a flow rate of maximum 20ml per min. The structure has support at the top(the system has a T connection which forms the outer pipe and the inner cylinder is connected on the top of T connection). But the outer flowrate is very high and we have a turbulent flow in the outer part. The solution I thought was to provide a support to the inner pipe which will be connected to the outer pipe and as the flow in non corrosive and supercritical, it will not be a big problem. What do you think?

Anonymous Poster #1 has many more questions to answer over this rig, the purpose of which is unclear at this time.

I think high flow in the outer tubing coupled with integrating a support structure for the inner tubing against vibrations is a failure waiting to happen.

High flow rates tend to be obnoxious to obliberate anything in their path, even if deemed not corrosive. CO2 is corrosive in any form but at these flow rates you suggest it seems the least of the problems.

You have a solution there that has problems.

supercritical CO2 is non corrosive so corrosion in not an issue. But you are right about the support so I am thinking of something different. Thank you.

The temperature(s) and the pressure of the <...supercritical CO2...> is of interest, as is the nature of what this <...concentric pipe...> is intended to achieve, particularly in view of its <...60mm to 80mm...> length.

• On an inherent safety basis, could an alternative fluid be used to this <...supercritical CO2...>?
• Why is there a concentric pipe in the first place, given that there is no chemical interaction between the fluids?
• Why is there an inherent mismatch between the flowrate passing through the inner pipe and its diameter, given that there is no chemical interaction between the fluids?

No, there is no alternative fluid .

Ya, you are right. I have already designed one system with two different pipe connection and I just wanted to explore if I can make a concentric connection. I work in the fluid domain and vibration is proving to be a bit tricky to me, so I just wanted to understand more about it.

Yes there is a huge difference between the flowrate in the inner and outer cylinder and there is no chemical interactions between the fluids but i dont want the fluid to mix before.

A paper mache pipe will not contain 300 bar supercritical CO2 for very long at all. It is foolish and ridiculous for you to propose using such a flimsy material on a public form. I don't care at all how easy paper mache is to paint.

We are not your free engineering service. Hire somebody to do this work.

A word of advice, if you want something to not vibrate then you should secure it with better fasteners than loose springs.

thank you for your not so constructive comment. I am a good enough engineer to know what is foolish and stupid. If you can't offer anything constructive, there is no need to comment please. Thank you very much.

I disagree. It seems to me that you are not a good enough engineer to work outside of your own narrow discipline. My sarcastic suggestions were an attempt to make you think about the sparse information you brought us and where I suspect you probably should be reexamining your system. When simple harmonic motions (vibrations) are not desired then identifying where the springs, dampers and frictions reside in your system.

You do remember your undergraduate kinematics class, don't you?

<...of length between 60 mm to 80 mm...> and <...the springs, dampers and frictions reside...> seem, from here, to be in different universes.

What is this contraption supposed to achieve once it is operating? Please help the forum with <...constructive comment...>.

lots of possibilities with this. I am still working on some of them. Could be used for drying things for example

<...have a concentric pipe...>, which implies it is already built, differs from <...Could be used for...>, which suggests the contraption is not past the concept stage and its purpose is currently indeterminate.

Please get back to the forum, on this thread, when its purpose has matured.

Sounds dangerous...

http://iopscience.iop.org/article/10.1016/0169-5983(94)00038-2/meta

You need to calculate the Reynolds number to begin with...

https://www.uio.no/studier/emner/matnat/math/MEK4450/h11/undervisningsmateriale/modul-5/Pipeflow_intro.pdf

I'm looking for this thing...and it's blue.

So are others.

I don't understand your math. A 60mm long cylinder with a diameter of 9 mm contains a volume of 1.909 mL.

0.5*pi*(4.5 mm)^2*60 mm≈1909 mm^3= 1.909 cm^3= 1.909 mL

Thus in one minute just over ten inner tube volumes must flow. That is nowhere near the speed of sound.

The supercritical CO2 is contained in the outer pipe. We know nothing about the outer pipe diameter. We do know the mass flow of the CO2, so the volume per second can be calculated but with an unknown cross sectional area the velocity is unknown.

We don't have anywhere near enough information here to help but what we do know should not be misapplied.

To make these erratic vibrations I suspect one of the two fluids is actually in transitional flow (Reynolds number between 2300 and 4000) or turbulent flow (Re > 4000) instead of the proposed Laminar flow. This might be happening at a boundary layer next to a pipe wall.

I tried to extract more information from the OP by making some absurd assumptions I hoped they would correct and hopefully expound.

In short, I cannot trust anything proposed by the OP.

The external tube must be the one with 500kg/hr mass flow rate because the fluid in internal tube, in the volumetric flow rate described would have to exceed densities available on Earth.

We do know something about the maximum size of the outer pipe diameter because we are given the tubing material (stainless), the tubing wall thickneas (1.5 to 2 mm) the and the max system pressure (300 bar).

Available stainless seamless tubing will be limited to around ~15 mm OD w/ 2mm wall (or ~12mm OD w/ 1.5mm wall) OD max to handle 300 bar unless temperature extreems further reduce possible OD.

The internal tube ia given at 5mm OD. Annular cross section must be equal to a 12 mm (or a 9mm) diameter circle minus a 5 mm circle for the internal tube.

Supercritical CO2 at 300 bar or less has a density around 1 g/cc. The speed of sound for sc co2 in the conditions described, I am taking a guess is close to /less than 500 m/s.

The length of the tube doesn't matter, just the cross section, density and mass flow rate compared to the speed of sound.