Pressure Drop Across A Mesh

Depends on the application.

I have done plenty of work using woven polyester mesh (readilly available) in respiratory measurement.

The response will be non linear (similar to an orifice, but not as non linear). experimentation is the best way as there are so many variables.

If this is just a mesh across an otherwise uniform pipe then there are fewer variables and a bit of experimentation will soon set you an the right track.

It also depends on the dynamic range required.

Obviously the finer the mesh the greater the pressure drop, this will limit the range and give greater non linearity. It's a whole bunch of trade offs signal vs back pressure vs linearity .

Have fun (I did!)

Del

The application I am working on is actually on a data rack. This has a 63% open area mesh, with an octagonal shape an orifice size of just over 10mm.

What I am actually trying to show is that with the same open area, a smaller number of larger size holes produces a lower pressure loss than a larger number of smaller size holes. The theory being that the former would have a smaller 'edge' than the later. I thought it would be easy, but I think you are probably right that experimentation may be the way!

Often, with a mesh, the pore size is the crucial thing, its function being to prevent objects larger than a certain diameter to pass through. Making the mesh out of a stronger, and thinner, support material may enable a greater number of pores of the same size to occur in the same area of aperture, thereby lowering the pressure drop.

Another thought: rectangular mesh (two weaves at 90deg) or triangular (three weaves at 60deg)? Or is the mesh to be created from drilled mirco-holes on a triangular pitch? There may be some useful reading in "Perry's Chemical Engineers' Handbook" (any edition) on this subject as some mathematical analysis has certainly been done.

One more point: as well as the 'fineness' of the mesh, the ratio of [mesh-structure-material] to [open-space] across the aperture will have a very significant effect.

Experimentation is a good recommendation.

What size are we talking about?

Small applications: (supply line of washing machines, domestic supply at each house). The first piece of rubble trapped will change any calculated loss. It is therefore customary too allow for an adequate margin. usually determined by experience.

As mentioned above what I am looking at is certainly not a fine mesh. It is a more of a matrix of hexagonal holes with an open area of 63%, and an orifice size of just over 10mm.

What I am actually trying to show is that with the same open area (63%), a smaller number of larger size holes produces a lower pressure loss than a larger number of smaller size holes. The theory being that the former would have a smaller 'edge' than the later. It also make good intuitive sense, but proving it is a different matter.

What I am actually trying to show .

???

At the risk of being extremely pedantic.... ( I'm not 'having a go' ..it's just an observation on scientific method. )

Surely you mean ' What I'm trying to find out' ? or 'hoping to show'

The road to bad science is paved with people predisposed to see the result they are 'trying to show'

maybe a good discussion point...havn't we all been caught out at some point erring on the side of the 'expected' result.... ( My HNC electronics project springs to mind many many years ago.... althouh I did chastise myself in my conclusion!... don't ya love the feeling of that sack cloth on bare skin? )

That is very true, however I do know the result that I will get, in terms of knowing that the smaller number of larger holes will produce a lower pressure drop, as I have made similar calculations before looking at sparge pipes. In fact I would actually expect those to be somewhat worse than this case, as there will be a effect of the proximity of other nozzles that I haven't had previously.

However, I should rephrase my problem perhaps by saying that I am looking to see what reduction in pressure drop I can achieve by increasing the diameter of holes in a matrix, while maintaining the same overall open area.

Having read that it still sounds like an assumption, so I guess you can see my problem!

I do think you have a great point though. An assumption is a problem, and I was not clear enough in my original question, and certainly didn't give enough background.

Have you tried a little experiment with perhaps some cardboard, a fan and a balance?

At one time I had a book on simple desktop modeling experiments which yielded scaleable results. It had a section on modeling and scaling fluid dynamics. I wish I had it to scan and email to you. I found it invaluable when I couldn't find a text book answer or more likely, I couldn't comprehend the text book answer.

I do think your expectations of greater airflow through fewer but larger orifices is probably accurate. Airflow is inhibited by boundry layer drag on the inside surface of each orifice. Therefore more orifices in a given area = more drag which = less airflow.

Too right, can't beat CAD...

(Cardboard Aided Design!)

My only issue with desktop experimentation is of coarse that the act of measuring influences the results. In my case this probably won't be too bad as what I am looking for is a variation across 2 different samples rather than a definitive result, but it would have been nice to be able to give you all a prediction method that worked across other cases too. If you come across the book, let me know and I will give it a try and see if I can get any results.

I was thinking of using CFD originally, but that may be a bit of an overkill on this one!