Low-Flow Turbulence induction with vibration?

[aspearin1]

I am investigating the feasibility of attaching a surface mount vibrator on a tubular reactor with heat transfer, to aid in micromixing. Currently, the pilot system is designed with highly laminar flow rates, and the flows necessary for turbulence vs. heat transfer rate will make the tubing far too long, (or heater temperatures too high; no steam utility available). The system works well at laminar conditions, but heat transfer is not homogeneous within the tubing. Does anyone have any experience with implementing vibration to promote mixing within pipes? Are there small surface mount ultrasonic vibrators commercially available? Google searches flood me with "ultrasonic flowmeters." Not quite what I'm looking for.

 

[saxon]

aspearin1, There are devices called "turbulators" (i.e. static mixers) that are designed to fit inside heat exchanger tubes and the like to aid in mixing and heat transfer. See the following site:

http://www.fuelefficiencyllc.com/fehxturb.html

These may be more effective than installing vibration generation equipment.

Hope this helps.
saxon

 

[hacksaw]

vibration cannot help on the tube side.

 

[aspearin1]

hacksaw,
Your statement is blunt. Can you ellaborate on your response or give references that show that vibrating tubes will not transfer vibrations to the contained fluid?
-aspearin1

 

[hacksaw]

bluntness was not my intent, but on second reading i have to agree.

the vibration you indicate is transverse to the direction of flow. to first order, and probably to higher order, there is no transfer of momentum to the fluid stream that would cause improved heat transfer.

outside the tube yes, but with the possibility of undesirable effects in regard to tube stress.

there has been some work done with pulsatile flows that do show improved heat transfer.

 

[235zzzo]

Hi Fellows!

I would like to add some brainstorming:

-Which is the size of your system?

-Has it enough energy to overcome the additional pressure losses, concerning those mentioned turbulators, to be installed inside the tubes-side?

- These elements will use/extract the energy from the mean flow (first order flow) to generate second-order flows like the so-called Von Karman vortices or the spiroval flows, inducing macro-mixing (increasing the turbulence inside the pipes) processes. - Thus inducing uniformed macro heat-transfer increments along the pipes, that's what you want.

- And not a micro-mixing one, as a matter of fact, this one would mean micro increments in the heat-transfer phenomena, which you do not want at all, I suppose!?.

- Then, we will have resulting higher overall mean heat-transfer coefficients along the pipes.

- Vibration phenomena in this context, means in fact, micro-mixing or micro changes in flow momentum. Doesn't help that much, speaking in thermal terms.

- And can imply material fatigue problems depending on the resulting vibration modes/frequencies..., as well.

I hope this can help a bit...
zzzo


Indeed, a very interesting discussion.
zzzo

 

[MJCronin]

aspearin1,

I believe that you are going in the wrong direction....

If your problem is, as I understand it, lack of heat transfer due to tubeside laminar flow, there may be another option.

Properly designed shell and tube heat exchangers, operating at or near their design point have "turbulet flow" within the tubes. They are also designed with a specific tube support span length, to prevent destructive tubing vibration. (see the TEMA software website

http://www.tema.org/software.html

for more information on flow induced vibration)

I have seen the destructive results of S&T HXs that were operated with excessive shellside flow rates and the costly tubing failures......Your idea to promote mixing/tubing vibration sounds like it may promote failure....

If possible, I suggest that you consider plugging some of the tubes in the HX and force more massflow through the remaining tubes. Based on your specific process, you should calculate the minimum number of tubes to be plugged in order to promote turbulent flow. You will, of course increase the tubeside pressure drop by plugging....

Is the heat exchanger operating with a new set of fluid operating conditions or was the heat exchanger purchased "used" ?

Just my opinion....

MJC

 

[aspearin1]

Thank you to everyone for your input. I have found some obscure research that attributes vibration to enhanced heat transfer. This vibration, though, appears to need to be at the resonance frequency of the tubing, which I can see will lead to untimely fatigue in the system. To better explain, this is not a heat-exchanger in its traditional sense. It is more a chemical reactor, partially curing resins at prescribed cure temperatures in continuous flow. The current system is extremely small scale, supplying resin at less than 100 mL/min. The reactants flow through a helical coil immersed in a controlled heat bath. The ID on this tubing is about 0.100". As I said, the system does work in its current condition, I'm only trying to make it more efficient. The flow and scale of this work is so small that acheiving an Re number of 2100 will not happen. I am searching for other means of doing so.

aspearin1

 

[moltenmetal]

Conventionally this is done as 235zzzo described: when you can't get turbulence at a low flow by simply reducing the tube diameter, you need to introduce radial mixing by another means. Have a look at the definition of Reynold's number for a second and you'll soon realize that turbulent Reynolds numbers aren't practical as the tube diameter decreases to a certain point (or viscosity increases to a certain point...).

I'd stay away from vibration for practical reasons, and because I agree with others that it is unlikely to work.

Remember that an infinite number of CSTRs in series gives the same residence time distribution as a plug flow reactor, hence a large number of static mixer elements in series in a tube of much larger diameter can give you results which approximate what the (impossible) ideal PFR may give you, while also enhancing heat transfer performance.

There are heat exchangers with static mixers in the tubes which are commonly used for reactors of this type. But this is a classic pilot reactor design problem, best handled by experts in the design and construction of pilot plants. Try

http://www.zeton.com

as one reference.